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Behavioral interactions between red-winged blackbirds and long-billed marsh wrens and their role in the… Picman, Jaroslav 1980

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BEHAVIORAL INTERACTIONS BETWEEN RED-WINGED BLACKBIRDS AND LONG-BILLED MARSH WRENS AND THEIR ROLE IN THE EVOLUTION OF THE REDWING POLYGYNOUS MATING SYSTEM JAROSLAV PICMAN Diploma, Charles' University i n Prague, Czechoslovakia, 1972 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE FACULTY OF GRADUATE STUDIES Department of Zoology We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1980 by THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n Jaroslav Pieman, 1980 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l1 owed w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f Zoology  The U n i v e r s i t y o f B r i t i s h Co lumbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date November 2, 1980 i i ABSTRACT Behavioral interactions between red-winged blackbirds and l o n g - b i l l e d marsh wrens are an important s e l e c t i v e force influencing redwing reproductive t a c t i c s - Marsh wrens, which destroy redwing eggs and k i l l redwing nestlings, were the most important nest mortality factor i n my redwing population and possibly also i n other marshes throughout the range of sympatry between these species- This nest mortality factor probably selected for four d i f f e r e n t adaptations through which redwings reduce the impact of marsh wrens on t h e i r nesting. F i r s t , redwings of both sexes aggressively exclude marsh wrens from the v i c i n i t y of t h e i r nests. Second, redwings avoid marsh wrens by breeding i n sparser vegetation which i s more e a s i l y defended against wrens. Marsh wrens, on the other hand, seem to prefer denser vegetation where they are more l i k e l y to avoid redwing aggression. These differences i n habitat selection by redwings and marsh wrens re s u l t i n a s p a t i a l segregation of t h e i r breeding areas. Third, female redwings reduce the impact of marsh wrens by clumping t h e i r nests and hence improving t h e i r nest defense through cooperation with t h e i r neighbors. Fourth, females p r e f e r e n t i a l l y join older males and harems with older females probably because such older birds are more e f f i c i e n t i n nest defense against marsh wrens. Breeding i n a harem with experienced birds increases a female's chance of reproductive success. Redwings exhibit two forms of polygyny. True harem polygyny has probably evolved as a compromise between two sel e c t i v e forces: (1) high nest predation rates which drive the evolution of the clumping tendency by females; and (2) seasonally abundant, predictable, and r e l a t i v e l y uniformly distributed food resources i n marshes and nearby uplands, which presumably favor the evolution of t e r r i t o r i a l i t y . As a r e s u l t , females breed c o l o n i a l l y within male t e r r i t o r i e s . The most important prediction from t h i s hypothesis i s that harem size selected for in various populations should be correlated with the i n t e n s i t y of predation. This i s because females should be selected to adjust t h e i r degree of clumping and hence their degree of cooperation i n nest defense to the average nest predation pressure. This prediction i s supported by data from various redwing populations throughout North America. In addition, redwings also exhibit resource defense polygyny. The presence of older experienced birds appears to be the most important factor responsible for a great degree of variation i n the s i z e of a harem attracted to i n d i v i d u a l male t e r r i t o r i e s . Thus t e r r i t o r i e s with older experienced males and females attr a c t large harems, whereas those without experienced birds acguire small harems. Preferences of females for older males and for harems with older females could be explained in terms of a greater contribution to nest defense by such experienced birds as compared with young, inexperienced individuals- The pattern of d i s t r i b u t i o n of older experienced in d i v i d u a l s i n the marsh, therefore, determines the mating pattern i n a breeding redwing population. The d i s t r i b u t i o n of older experienced individuals of both sexes i s determined by i v t h e i r strong tendency to return to the same t e r r i t o r i e s year after year. Consequently, mating success of newly established males i s i n i t i a l l y determined by the number of females acquired by previous t e r r i t o r y holders and their overwinter s u r v i v a l rates. These observations indicate that mating success of any given male i s a function of: (1) the number of old, experienced females breeding on his t e r r i t o r y ; (2) attractiveness of h i s t e r r i t o r y to new females as r e f l e c t e d by the number of older females; and (3) attractiveness of his t e r r i t o r y to new females in terms of his age-related experience. Evidence from t h i s study suggests that to explain the adaptive value of redwing polygyny, we have to consider both the cooperative and competitive interactions between females. A model combining adaptive values of true harem polygyny and resource defense polygyny i s presented. V TABLE OF CONTENTS ABSTRACT - , ............ i i TABLE OF CONTENTS V LIST OF TABLES , .... . ... .. xiv LIST OF FIGURES .., xxi ACKNOWLEDGEMENTS - XXV GENERAL INTRODUCTION » . 1 GENERAL METHODS .. 7 SECTION. I. Behavioral interactions between red-winged blackbirds and l o n g - b i l l e d marsh wrens and th e i r influence on redwing nesting success ....... 11 CHAPTER 1. Nest-destroying behavior of l o n g - b i l l e d marsh wrens .................................... 12 Introduction ............................................. 12 Methods . . 14 Results 16 1. Size of eggs .............. 16 2. Color and shape of eggs . 19 3. Response to nestlings ................................ 22 4. Types of nest placement 25 5. Sex and age of marsh wrens 25 Discussion .............................................. 29 CHAPTER 2. I n t e r s p e c i f i c aggression by red-winged blackbirds towards l o n g - b i l l e d marsh wrens 34 Introduction , 34 Methods 36 Results and Discussion 36 v i 1. Frequency of interactions between redwings and marsh wrens 36 2. Frequency of redwing - wren interactions at different times of day 38 3. Influence of marsh wren density on the rate of redwing - wren interactions 40 4. Adaptive value of redwing aggression towards marsh wrens .....................^ .......... ... . 43 CHAPTER 3. Response of red-winged blackbirds to nests of l o n g - b i l l e d marsh wrens 45 Introduction 45 Methods „• , .... 47 Results „ 49 1. Types of redwing responses to marsh wren nests .... 49 2. Control t r i a l s 51 3. Influence of experience of marsh wren nest destruction on redwing responses to wren nests .... 51 4. Relationship between redwing responses to wren nests and proximity to marsh wrens ., 56 5. Are females which responded p o s i t i v e l y to marsh wren nests more, successful breeders? .......... 56 6. Response of redwings to nests, with eggs ........... 58 Discussion . . . 60 1. The role of previous experience of marsh wrens on redwing responses 61 2. Effect of marsh wren density on redwing responses to marsh wren nests 64 3. Implications of redwing aggression 67 CHAPTER 4. Impact of l o n g - b i l l e d marsh wrens on reproductive success of red-winged blackbirds Introduction ......................................... Methods Results ...........—................................. 1. General breeding information .................. 2. D i s t r i b u t i o n of redwings and marsh wrens ..... 3. Impact of marsh wrens on redwing nesting success A. Impact of marsh wrens on fledging success of successful redwing nests B. Impact of marsh wrens on redwing nestling losses C. Indirect impact of marsh wrens through redwing aggressive neglect D. The extent of redwing exposure to wren nest destruction Discussion 1. Impact of marsh wrens on redwing nesting success 2. Interactions between redwings and marsh wrens as a possible cause of segregation of t h e i r nesting s i t e s -CHAPTER 5 . Nest s i t e quality and i t s influence on reproductive success of red-winged blackbirds Introduction v i i i Methods 98 Results and Discussion 100 1. Nest height - ... .. 100 2. Nest concealment . 103 A. Why did nest concealment play different roles in the three seasons? ........................... 107 (a) Nest concealment i s always important but i t s role may sometimes be masked by other factors . 107 (b) Role of nest concealment varies with marsh wren density ................................... 114 CONCLUSIONS FROM SECTION I . 119 SECTION I I . The adaptive value of redwing reproductive t a c t i c s i n terms of redwing-marsh wren interactions 123 CHAPTER 6. The adaptive value of the clumped pattern of nesting by female red-winged blackbirds .... 125 Introduction 125 Methods . 126 Results .- ,. 127 1. The pattern of d i s t r i b u t i o n of redwing nests i n the marsh 127 2. The adaptive value of female clumping ............. 129 3. Do redwings adjust distances from conspecific neighbors i n response to marsh wrens? - 135 4. Effect of clumping by redwing females on marsh wren d i s t r i b u t i o n ................................. 136 Discussion 138 ix CHAPTER 7. The s p a t i a l and temporal organization of nesting, harem s i z e , and reproductive success of red-winged blackbirds ............... 142 Introduction - 142 Results and Discussion —.,. 143 1. Temporal organization of nesting 143 2. Spatial organization of nesting 147 ,A. How i s a greater degree of clumping i n larger harems achieved? ................................. 154 3. The relationship between the s p a t i a l and temporal d i s t r i b u t i o n of nesting 162 4. The influence of temporal and s p a t i a l organization of nesting on reproductive success of females 168 CHAPTER 8. The influence of previous breeding experience of red^winged blackbirds on female choice of a breeding s i t u a t i o n ......... 169 Introduction . -. .. ..... 169 Methods ...... 172 Results and Discussion 174 1. Influence of age of males on female choice of mates 174 A. Mating success as a function of age of males .... 175 B. Nesting success as a function of age of males ... 180 2. How does the female choice of mates operate? ...... 184 A. How do females discriminate? 184 B. How do females assess quality of males? ......... 188 (a) Direct assessment of quality of males from X their behavior .. 1 9 0 (b) Assessment of males from t e r r i t o r y quality ... 1 9 6 T e r r i t o r y size ................... i . . 1 9 7 The a v a i l a b i l i t y of nest s i t e s 2 0 0 Quality of vegetation on redwing t e r r i t o r i e s ... 2 0 0 The a v a i l a b i l i t y of food resources ............. 2 0 2 The abundance of predators 2 0 2 3 . What other factors might influence mating success of males? 2 1 0 CONCLUSIONS FROM SECTION II ......... . . . 2 2 3 SECTION I I I . The evolution of polygyny i n red-winged blackbirds 2 2 6 CHAPTER 9 . The e a r l i e r proposed hypotheses on the evolution of polygyny i n red-winged blackbirds and the i r evaluation 2 2 7 1 . Heterogeneous d i s t r i b u t i o n of food as a factor selecting for polygyny i n marsh-nesting passerines 2 2 7 2 . Different predation rates i n marshes and uplands as a factor selecting for polygyny i n marsh-nesting redwings 2 3 1 3 . The "sexy son" hypothesis .......................... 2 3 5 4 . Cooperation between females as a force selecting for polygyny i n redwings ................ 2 3 8 CHAPTER 1 0 . An alternative model on the evolution of polygyny i n red-winged blackbirds ............ 2 4 2 1 . Nest destruction by marsh wrens as a se l e c t i v e x i force - ,. ........ 242 2. Marsh wrens as a factor selecting f o r cooperation between female redwings 243 3. The nature of cooperation between female redwings 251 4. The influence of male nest defense behavior on female clumping tendency ......................... 253 5. Negative effects of clumping as a force setting an upper l i m i t to harem size .............. 254 A. T e r r i t o r i a l behavior of females ................. 255 (a) The theory of habitat d i s t r i b u t i o n and a test of the role of female t e r r i t o r i a l i t y 257 B. Males as a possible factor setting an upper l i m i t to harem size 273 6. The role of nest predation and competition for lim i t e d resources i n the evolution of polygyny i n redwings 274 A. The i n t e n s i t y of nest predation 277 B- The role of food resources 281 C. I n t r a s p e c i f i c competition for limited breeding space ..... 287 D. Contribution of males to nest defense ........... 289 E. I n t e r s p e c i f i c interactions 289 7. A model on the role of predation in the evolution of polygyny and i t s implications 291 A. Evidence from the comparative method 297 (a) Nest defense behavior ........................ 300 (b) The degree of polygyny 302 x i i (c) S p a t i a l organization of nests ................. 303 B. Problems associated with the model ............... 305 (a) Are lo n g - b i l l e d marsh wrens the only agent selecting f or cooperation between female redwings? 305 (b) Why have redwings evolved harem polygyny as opposed to c o l o n i a l i t y ? 307 (c) What i s the influence of upland-nesting blackbirds? . . 314 8. The role of previous breeding experience in the evolution of polygyny i n redwings 316 9. Model combining competitive and cooperative interactions between females 319 CHAPTER 11. Overlap i n breeding d i s t r i b u t i o n of red-winged blackbirds, yellow-headed blackbirds, and l o n g - b i l l e d marsh wrens; test of the major assumption of the proposed theory 328 Introduction 328 Methods ... 330 Results . 330 1. Overlap i n breeding d i s t r i b u t i o n of redwings and marsh wrens 330 2. Sympatry among yellowheads, redwings, and marsh wrens 335 3. Do marsh wrens and blackbirds i n t e r f e r e throughout their range of sympatry? - 335 Discussion 340 x i i i CONCLUSIONS FROM SECTION. I l l .... 344 BIBLIOGRAPHY . .............. 349 APPENDIX I. Technique for trapping female red-winged blackbirds. 359 APPENDIX II. Technique for trapping l o n g - b i l l e d marsh wrens. ........................................363 APPENDIX I I I . Summary of data on reproductive rates, sex r a t i o and t e r r i t o r y size of red-winged blackbirds in various marshes throughout. North America. ............... 367 xiv LIST OF TABLES Table 1. Predation on r e a l eggs of diff e r e n t sizes offered to lo n g - b i l l e d marsh wrens i n redwing and robin nests. . 17 Table 2. Offering a r t i f i c i a l polystyrene eggs of di f f e r e n t sizes to male l o n g - b i l l e d marsh wrens i n redwing and a r t i f i c i a l nests. 20 Table 3. Results of t r i a l s with eggs of diff e r e n t colors placed in redwing nests. .................... 21 Table 4. Results of t r i a l s with eggs of di f f e r e n t shape and redwing and marsh wren nestlings placed i n redwing and marsh wren nests. ...................... 23 Table 5. Results of t r i a l s with eggs offered in dif f e r e n t types of nests placed high i n c a t t a i l and on the ground. 26 Table 6. Summary of t r i a l s , by sex. ........................ 28 Table 7. Summary of t r i a l s p o s i t i v e l y responded to by juvenile marsh wrens. 28 Table 8. Summary of data on aggression by male and female redwings towards marsh wrens obtained by three observers. 37 Table 9. Rate of redwing-wren agonistic interactions at dif f e r e n t times of a day. ............................ 39 Table 10. Agonistic interactions between redwings and marsh wrens i n areas with d i f f e r e n t densities of marsh wrens. 41 Table 11. Summary of responses by redwing females to XV marsh wren nests. 50 Table 12. Responses by female redwings to a marsh wren nest during early and l a t e t r i a l s . ................ 53 Table 13. Responses by banded and unhanded female redwings i n early t r i a l s . .......................... 55 Table 14. Effect of proximity of marsh, wren nests on responses by female redwings. 57 Table 15. Nesting success of female redwings that responded p o s i t i v e l y or negatively to a marsh wren nest. 59 Table 16. Evaluation of distances of redwing nests from the nearest marsh wren nest. ....................... 65 Table 17. General information on redwing breeding. .......... 74 Table 18. Nesting success of redwings related to the time i n a season. ...................................... 76 Table 19. I n t e r s p e c i f i c association between redwing and marsh wren nest locations. ......................... 77 Table 20. Nesting success of redwing nests located at various distances from nearest marsh wren. nests. 81 Table 21. Mean distances between redwing and marsh wren nests i n 1976 and 1977 seasons. ................... 83 Table 22. Losses from successful redwing nests i n r e l a t i o n to t h e i r distance from nearest marsh wren nests. ........................................ 84 Table 23. Effect of short distances between redwing and marsh wren nests on redwing clutch s i z e . 87 Table 24. Success of redwing nests i n r e l a t i o n to nest xvi height ------ • ................101 Table 25- Nesting success of red-winged blackbirds with marsh wrens close and farther away, as related to nest height. 102 Table 26. Role of nest concealment i n reproductive success of red-winged blackbirds. ................ 105 Table 27. Relationship between redwing nesting success and vegetation density within 5m of. redwing nests. ........................................... 106 Table 28. Comparison of proportions of redwing nests which had the nearest marsh wren nest within 15m and farther away for 1977-1979 seasons. ... 108 Table 29. Nesting success of red-winged blackbirds with marsh wrens close and farther away, as related to nest concealment. .......110 Table 30. Comparison of proportions of redwing, nests which had the nearest conspecific neighbor within 30m or farther away for 1977-1979 seasons. ......................................... 111 Table 31. Density of marsh vegetation i n 1977-1979- ........ 113 Table 32. D i s t r i b u t i o n of redwing nests i n study. quadrats i n 1976 and 1977. 128 Table 33. Summary of distances between the. nearest simultaneously active and contributing redwing nests in 1976 and 1977. .,. 130 Table 34. Redwing nesting success i n relationship to distances between nearest redwingrredwing and. . redwing-marsh wren nests. ........................ 131 x v i i Table 35- Effe c t of the exclusion of l a t e redwing nests on the analysis pf redwing nesting success as related to redwing-redwing and redwing-wren nest distances. ................................... 133 Table 36. Success of redwing nests as related to (1) the number of conspecific nests within 30m, and. (2) the distance from the nearest marsh wren nest 134 Table 37. Effect of distances between redwing and marsh wren nests on clumping of redwing nests. ......... 137 Table 38. Relationship between clump siz e and the density of vegetation around nest. 141 Table 39. Mean i n t e r v a l between i n i t i a t i o n of nesting by successive females i n harems of various s i z e s . ... 146 Table 40. Comparison of hypothetical and observed i n t e r -nest distances. 150 Table 41. Comparison of distances of sequentially s e t t l i n g females from the f i r s t female to s e t t l e i n small, medium, and large harems. ........ 153 Table 42. Influence of conspecific neighbors from the same or neighboring harem on nesting success of female redwings. 159 Table 43. Comparison of mean distances of nests of early and l a t e females from the nearest conspecific nest i n small, medium, and large harems. ...... 161 Table 44. Nesting success of redwing nests whose nearest conspecific neighbor exhibited either low or high degree of asynchrony in nesting. ............ 165 x v i i i Table 45. Influence of age of male redwings on t h e i r harem s i z e . 176 Table 46. Harem size of redwings i n 1976-1979. ............. 177 Table 47. Changes i n mating success of i n d i v i d u a l male redwings. ......................................... 179 Table 48..Influence of harem size on fledging success of inexperienced and experienced male redwings and t h e i r females. ... 183 Table 49. Proportion of females which renested with t h e i r o r i g i n a l male or with a dif f e r e n t male i n one breeding season. 186 Table 50. T e r r i t o r y tenacity of female red-winged blackbirds as related to the presence of th e i r o r i g i n a l male. 187 Table 51. Correlation analyses of the re l a t i o n s h i p between harem size and t e r r i t o r y s i z e . 198 Table 52. Relationship between changes in harem s i z e and t e r r i t o r y size of i n d i v i d u a l male redwings i n subsequent years. ................................ 198 Table 53. Correlation analyses.of the rela t i o n s h i p between density of vegetation on redwing t e r r i t o r i e s and harem s i z e . ...................... 201 Table 54. Correlation analyses of the re l a t i o n s h i p between density of marsh wren nests on . t e r r i t o r i e s of male redwings and t h e i r harem s i z e . ............................................. 201 Table 55- Relationship between date of laying the f i r s t egg on t e r r i t o r i e s of inexperienced and xix experienced males and t h e i r harem s i z e . .......... 216 Table 56. Number of females attracted to the same t e r r i t o r i e s i n successive years by di f f e r e n t males. , 219 Table 57. Nesting success of female redwings as related to the mating status of t h e i r male and the distance from the nearest conspecific neighbor. 250 Table 58. Hatching success of red-winged blackbirds and yellow-headed blackbirds related to t h e i r distance from marsh wren courting centres. ....... 301 Table 59. Comparison of distances between the nearest simultaneously active nests of red-winged blackbirds and yellow-headed blackbirds. ......... 304 Table 60. Di s t r i b u t i o n of breeding populations of red-winged blackbirds and l o n g - b i l l e d marsh wrens i n d i f f e r e n t marshes. . .............. 331 Table 61. Overlap i n d i s t r i b u t i o n of red-winged blackbirds and l o n g - b i l l e d marsh wrens i n western, c e n t r a l , and eastern North America. 333 Table 62. Degree of overlap between red-winged blackbirds and l o n g - b i l l e d marsh wrens as related to the size of redwing male populations. ..................................... 334 Table 63. Summary of data on the abundance of marsh wrens as related to redwings and the s i z e of redwing male populations 336 Table 64. Temporal s t a b i l i t y of marsh wren populations XX as related to the si z e of redwing male populations. 337 Table 65. Degree of sympatry of breeding populations of yellow-headed blackbirds, red-winged blackbirds, and l o n g - b i l l e d marsh wrens. 338 Table 66. Summary of information on d i f f e r e n t types of int e r a c t i o n s between marsh wrens and redwings or yellowheads. ,.,., 339 Table 67. Degree of polygyny, t e r r i t o r y s i z e , and nesting success of redwings from various marshes of North America 367 xxi LIST OF FIGURES Figure 1- Brackish water marsh i n Delta, B. C., where the study was conducted............... 8 Figure 2. Long-billed marsh wren pecking (a) . red-winged blackbird and (b) pheasant eggs. 18 Figure 3. Male lo n g - b i l l e d marsh wren removing nestling red-winged blackbird previously k i l l e d t>y pecking. 24 Figure 4. Number of simultaneously active redwing nests throughout 1976 and 1977 seasons. ................ 73 Figure 5. Distribution of redwing and wren nests on May 8-10, 1976 and 1977. 79 Figure 6. Proportion of redwing nests b u i l t and proportion of redwing young fledged at various distances from marsh wren nests. 89 Figure 7. Frequency d i s t r i b u t i o n of redwing nests of diff e r e n t degree of concealment i n 1977-1979. ... 115 Figure 8. Relationship between date of laying the f i r s t egg on a t e r r i t o r y and size of a harem attracted to that t e r r i t o r y . .................... 145 Figure 9. Mean distance of redwing nests from the nearest conspecific neighbor as related to harem s i z e . ........................................ 148 Figure 10. Mean distances between the nearest redwing simultaneously active nests from various size harems of inexperienced and experienced males. 156 x x i i Figure 11. Temporal pattern of formation of various size harems as seen from the average dates of i n i t i a t i o n of nesting by successive females from i n d i v i d u a l harems 158 Figure 12. Selection of conspecific neighbors by s e t t l i n g female redwings i n terms of the degree of asynchrony i n t h e i r nesting -. 167 Figure 13. Adult male, immature male, and female red-winged blackbird. 173 Figure 14. Relationship between mean number of fle d g l i n g s per male or female and male age. ..... 181 Figure 15. Age d i s t r i b u t i o n of t e r r i t o r i a l male redwings i n a study population. 189 Figure 16. Relationship between mean number of fledglings per male or female and harem si z e . ... 193 Figure 17. Relationship between date laying the f i r s t egg i n a t e r r i t o r y and male age. ................^195 Figure 18. Frequency d i s t r i b u t i o n of harems of d i f f e r e n t s i z e s , and proportion of females breeding, i n harems of different sizes 213 Figure 19. Relationship between harem size of previous adult t e r r i t o r y holders and new. adult, males. , . that replaced them. ............................. 221 Figure 20. Mean harem siz e as a function of mean. t e r r i t o r y size for d i f f e r e n t redwing populations. ............... 230 Figure 21. Relationship between success of redwings and their distance from marsh wrens and other x x i i i conspecifics. ....................................... 244 Figure 22. Influence of addition of females to a harem on the average f i t n e s s per female as predicted by the competitive female choice model and cooperative female choice model. ...... 247 Figure 23. Non-Allee type i d e a l free d i s t r i b u t i o n . ......... 260 Figure 24. Non-Allee type i d e a l despotic d i s t r i b u t i o n . ..... 262 Figure 25. Allee type i d e a l free d i s t r i b u t i o n . . . . . . . . . . . . . . . 264 Figure 26. Allee type i d e a l despotic d i s t r i b u t i o n . .......... 266 Figure 27. Average f i t n e s s per female as a function of harem s i z e as related to marsh wren nest, destruction and competition for food. ............. 276 Figure 28. Average f i t n e s s per female as a function of harem s i z e as related to marsh wrens. 279 Figure 2 9. Number of fledglings per female and male as a function of harem siz e . ........................... 285 Figure 30. Model of the role of marsh wrens and limited resources in the evolution of polygyny i n marsh-nesting red-winged blackbirds. , 292 Figure 31. Nesting success of various redwing populations as a function of t h e i r degree of polygyny. 295 Figure 32. Presumed roles of d i f f e r e n t patterns of d i s t r i b u t i o n of food resources and predation pressures in the evolution of various reproductive strategies i n passerines. .......... 310 Figure 33. Model of the adaptive value of redwing polygyny combining the cooperative and xxiv competitive interactions between females. ....... 320 Figure 34. Number of active redwing nests as a function of time i n the f i r s t part of the breeding season. 322 Figure 35. Trap for capturing l o n g - b i l l e d marsh wrens (a) , and d e t a i l s of the. trigger mechanism (b). ... 360 Figure 36. a. Trap for capturing redwing females; b. Details of the trigger mechanism. ............... 365 XXV ACKNOWLEDGEMENTS I would l i k e to thank my supervisor. Dr. C. L. Gass, for discussions and advice throughout this study and for h e l p f u l comments on my thesis. Dr. . C. J . Krebs kindly provided s t a t i s t i c a l advice. I am grateful to members of my research committee. Dr. C. J. Krebs, Dr. J. D. McPhail, Dr. W. E. N e i l l , and Dr. J. N. M. Smith, for many valuable suggestions on t h i s manuscript. In addition, a number of other individuals have i n d i r e c t l y contributed to the improvement of my thesis by providing c r i t i c a l comments on e a r l i e r papers from th i s work. I am p a r t i c u l a r l y grateful to Dr. D. Chitty, Dr. A. A. Dhondt, Dr. G. H. Orians, Dr. M. T a i t t , Dr. R. H. Wiley, and P. Thompson. I greatly appreciate f i e l d assistance from D. Goodwin, A. K. Pieman, Dr. M. T a i t t , and P. Thompson. The management of George C. R e i f e l Migratory Bird Sanctuary, the Canadian W i l d l i f e Service, and Mrs. V. Robertson kindly allowed me to work on th e i r property. I acknowledge f i n a n c i a l support through the NRCC grant and the University Research Support Fund from the Canadian W i l d l i f e Service to Dr. C. L. Gass, and the University of B r i t i s h Columbia f o r support i n the form of a Graduate Student Fellowship and a teaching assistantship. F i n a l l y , I would l i k e to express special thanks to my wife, Anna K. Pieman, for her encouragement and invaluable help i n a l l aspects of my work. 1 GENERAL INTRODUCTION The ways i n which male and female animals come together for breeding have been referred to as th e i r mating system (Brown 1975). Three generally recognized mating systems are monogamy, polygamy, and promiscuity (e.g. Barash 1977). In a monogamous system one male mates with one female, whereas in a polygamous system a member of one sex mates with several members of the opposite sex. According to whether the sex which acquires multiple mates i s male or female, th i s mating system i s c a l l e d polygyny or polyandry, respectively. In a promiscuous system, in d i v i d u a l s of one sex may copulate with a number of in d i v i d u a l s of the opposite sex; in th i s system, however, long-lasting pair bonds between members of the opposite sex are not established (e.g.. Brown 1975, Barash 1977). Mating systems of birds were reviewed by Lack (1968), who found that monogamy i s by f a r the most prevalent avian mating system. Polygyny has been documented only i n two percent of species, i n which careful studies of breeding systems have been carried out, and polyandry i s even rarer. Promiscuity or polybrachygamy (Selander 1972) i s more frequent than polygyny. The evolution of different mating systems has been one of the most frequently studied s o c i o b i o l o g i c a l topics, which has stimulated many th e o r e t i c a l considerations (e.g. Verner and Willson 1966, Orians 1969, Wittenberger 1976, Altmann et a l . 2 1977, Emlen and Oring 1977). The conditions that select for d i f f e r e n t mating patterns in animals can be best summarized in terms of Trivers' (1972) theory of parental investment. In animals the i n i t i a l investment i n offspring by females i s larger than that by males because female gametes are much larger and thus energetically more expensive. Because males produce a large number of small gametes, they can f e r t i l i z e eggs of several females with l i t t l e cost. Hence males w i l l compete for the opportunity to f e r t i l i z e eggs of more females and t h i s w i l l increase t h e i r variance i n reproductive success. As a consequence, males w i l l be exposed to more intense sexual s e l e c t i o n . Thus, with regard to a greater i n i t i a l parental investment by females, animals should be fundamentally polygamous (Wilson 1975).. However, another important factor that may influence the amount of investment by the two sexes i s t h e i r l a t e r contribution . to reproduction, which may include building the nest, feeding the female, feeding the young, and protecting them from predators (Alcock 1975). If t h i s investment through parental care i s greater by females than that by males, i t should further increase the difference between the parental investment of both sexes and thus promote the evolution of polygamy. On the other hand, selection for equal parental care by males and females w i l l favor monogamy (Wilson 1975).. In those animals where females invest more i n th e i r offspring than males and thus have more to lose by making a bad choice, females should protect t h e i r greater investment by 3 choosing the best possible s i t u a t i o n for t h e i r reproduction (Orians 1969). Thus, for example, females might increase their chances of reproductive success by breeding in a habitat with abundant food, no predators, and suitable vegetation providing both nest support and cover. In addition, females might p r e f e r e n t i a l l y mate with high quality males which should increase the f i t n e s s of their offspring because 50% of t h e i r offspring's genes w i l l be from the male. This s i t u a t i o n thus w i l l favor the evolution of polygyny. However, i n order for polygyny to evolve, differences in the quality of males and/or their t e r r i t o r i e s , among which females choose, must exceed the "polygyny threshold" (this i s the minimum difference i n quality of males or t h e i r t e r r i t o r i e s that would make polygynous matings in better quality situations advantageous to females than i f they mated monogamously in a less suitable s i t u a t i o n ; Verner and Willson 1966, Orians 1969). Therefore, in polygynous species females should be selected to exercise a stronger preference when choosing a mate. In monogamous species, on the other hand, female choice of a mate should be r e l a t i v e l y weak because of s i m i l a r investment by both sexes. Thus to explain the evolution of polygyny of any particular species, we must discover a l l features that influence female choice of a breeding s i t u a t i o n . In this study I w i l l investigate the adaptive value of the polygynous mating system i n the red-winged blackbird, Agelaius phoeniceus (Icteridae), which i s a widespread North American marsh-nesting passerine. Male redwings acquire, on the average, "between 1.3 and 7.6 females per male in different marshes 4 (Orians 1980), but i n d i v i d u a l males may acquire as many as 13 females (Orians 1980). The nesting system of marsh nesting redwings was described as c o l o n i a l (Mayr 1941, Smith 1943). However, the great v a r i a b i l i t y i n t e r r i t o r y size between marshes suggests that a system of grouped t e r r i t o r i e s (Lack 1968) or neighborhoods (Crook 1964) better describes the s p a t i a l arrangement of a breeding population. The adaptive value of the redwing nesting system has been the subject of many investigations. One group of studies, examining the role of s o c i a l stimulation i n redwing colonies of dif f e r e n t sizes, indicated that the "Darling e f f e c t " (Darling 1938) does not apply to redwings because variations in timing of nesting, synchrony, and clutch s i z e were induced e c o l o g i c a l l y rather than s o c i a l l y (Mayr 1941, Smith 1943, Orians 1961). Two studies, however, indicated that the survival value of large colonies may l i e i n reduced predation rates (Smith 1943, Robertson 1973a). Therefore, the evolution of the c o l o n i a l nature of nesting by redwings may have been driven by i t s advantages in terms of reduced nest predation rates. Several other studies, that concentrated on the environmental features influencing the female choice of a breeding s i t u a t i o n , yielded four major hypotheses: (1) the heterogeneity in d i s t r i b u t i o n of food within marshes could select for polygynous matings on t e r r i t o r i e s of the highest quality (Verner and Willson 1966, Orians 1969); (2) differences i n predation rates i n upland and marsh habitats could result i n polygynous matings on more suitable marsh t e r r i t o r i e s (Wittenberger 1976); (3) polygyny 5 could have evolved as a re s u l t of selection for increased female density that improves f i t n e s s of individuals through some form of cooperative interactions (Altmann et a l . 1977) ; and (4) polygyny i n redwings could have evolved as a consequence of female preferences for highly a t t r a c t i v e males which might increase female f i t n e s s ultimately (rather than through th e i r immediate reproductive success) through their "sexy sons" which w i l l , presumably, be also highly polygynous (Weatherhead and Robertson 1977a, 1979). This indicates that, i n spite of the fact that the evolution of polygyny in redwings has been inte n s i v e l y studied, conclusions on the adaptive value of redwing reproductive strategy reached by various authors are inconsistent and sometimes even appear to be i n d i r e c t contradiction (individual hypotheses and their p l a u s i b i l i t y w i l l be discussed in Chapter 9) . An important feature of redwing biology which has not been examined in d e t a i l by previous authors i s the nature of nest predation and i t s influence on redwings. The fact that marsh-nesting birds suffer from the highest nest mortality rates among a l l temperate zone passerines (Ricklefs 1969) indicates that predation could s i g n i f i c a n t l y influence redwing reproductive strategy. In t h i s study I examined the impact of one p a r t i c u l a r predator, the lo n g - b i l l e d marsh wren, Cistothorus p a l u s t r i s , on redwing reproductive success i n a single marsh. Marsh wrens have previously been observed destroying eggs of blackbirds (e.g. Allen 1914). This, and the fact that the breeding ranges 6 of redwings and marsh wrens greatly overlap (Bent 1948, 1958), therefore, indicate that marsh wrens might present an important mortality factor reducing success of redwings perhaps i n most marshes and hence an important sel e c t i v e force. The major objectives of my study were: (1) to investigate behavioral interactions between red-winged blackbirds and l o n g - b i l l e d marsh wrens and the i r influence on redwing reproductive success (Section I ) ; (2) to examine the adaptive value of redwing reproductive t a c t i c s i n terms of redwing-wren interactions (Section II) ; (3) to examine the adaptive value of redwing polygyny (Section III) . I 7 GENERAL METHODS The study was conducted during the 1 9 7 6 - 1 9 7 9 breeding seasons on an extensive brackish water marsh at George C. R e i f e l Migratory Bird Sanctuary on Westham Island, Delta, B r i t i s h Columbia (Fig. 1 ) . Relatively homogeneous c a t t a i l (£l£ha l a t i f o l i a ) vegetation covers the higher grounds i n t h i s marsh, while sedge (Carex spp.) and bulrush (Scirp_us spp. ) are dominant species on the lower grounds near the sea. The only passerine species breeding i n t h i s marsh are red-winged blackbirds and l o n g - b i l l e d marsh wrens. Females of both species star t laying eggs early i n A p r i l but the breeding season of marsh wrens i s longer (the l a s t redwing and wren clutches were l a i d in l a t e June and early August, r e s p e c t i v e l y ) . The length of the breeding season thus allows redwings and marsh wrens i n t h i s marsh to p o t e n t i a l l y rear two and three broods, respectively. The potential nest predators common i n the marsh are: western t e r r e s t r i a l garter snakes (Thamnophis elecjans) , marsh hawks (Circus cy_aneus), and deer mice (Peromyscus maniculatus) . Other potential mammalian predators such as racoons and mink are controlled by the management of the W i l d l i f e Refuge and are e s s e n t i a l l y absent. Early i n March, I established a grid system by dividing the study area into 20 x 20 m squares. Accurate maps of the study F i g . 1. Brackish water marsh i n Delta, B. C., where the study was conducted. 9 area were drawn from an a e r i a l photograph taken by Burnett Resource Surveys Ltd., Vancouver, B. C. and the established grid system. In 1976 and 1977, a l l redwing males were captured in traps baited with a redwing male "intruder" and color-banded in early A p r i l , but 8 of 43 males remained unhanded i n 1978 and 1979. Boundaries of males' t e r r i t o r i e s were determined by observing their singing and other t e r r i t o r i a l a c t i v i t i e s . To study redwing nesting, I systematically searched the entire study area by walking transects at 10 m i n t e r v a l s once every three days i n 1976 and twice a week i n 1977-1979. I marked new nests with white numbered labels and estimated the l o c a t i o n of each nest within 20 x 20 m squares. I also noted: the stage of nesting, number of eggs or young, and, i f the nest f a i l e d , I searched for possible causes. I considered a nest to have been destroyed by predators i f there were punctured or broken eggs in or near i t , s p i l t egg contents in the nest or on nearby vegetation, or dead young with wounds in or below the nest. I considered wet nests with unbroken eggs inside or on the ground underneath them, or nests with dead, drowned young i n or near them to have been destroyed by high tides. Completed nests for which I do not have any evidence that eggs were l a i d i n them or nests that were never completed I considered to have been abandoned. In the 1977 season, to f i n d out i f there was any consistent age-dependent disappearance of redwing young from successful nests, I marked i n d i v i d u a l nestlings on t h e i r legs using a waterproof 'Felt Tip Marking Pen'. I weighed a l l marked nestlings on each v i s i t , except during adverse weather. 10 To evaluate the role of vegetation structure i n redwing nesting, between May 5 and May 15 (1977-1979) I measured the density of vegetation i n a l l t e r r i t o r i e s of male redwings. I used a v e r t i c a l , 2 m high s t i c k with three horizontal, 50 cm long, white s t i c k s attached to i t at heights of 50, 100, and 150 cm above the ground. On each of the three short s t i c k s I painted 20 red spots (1 cm in diameter) i n a l i n e and with equal distances between them. To estimate the vegetation density, I counted the number of red spots I could see 50, 100, and 150 cm above the ground through the vegetation from a distance of 10 meters. These readings were taken at regular 20 m distances along the transects throughout the whole study area, and at the same s i t e s i n three consecutive years. I added the number of spots seen at three d i f f e r e n t heights for i n d i v i d u a l readings (the range i s 0-60 spots) and used these t o t a l s for analyses. lOA S E C T I O N I. 11 Behavioral interactions between red-winged blackbirds and l o n ^ - b i l l e d marsh wrens and the i r influence on redwing nesting success The f i r s t observation of a lo n g - b i l l e d marsh wren puncturing eggs of red-winged blackbirds was made almost 70 years ago (Allen 1914). In spite of that, no study has yet been conducted which would allow us to evaluate the nature of marsh wren egg-pecking behavior, i t s frequency within a marsh wren population, and i t s influence on reproduction of redwings or other marsh-nesting birds. In t h i s section I w i l l , therefore, investigate the nest-destroying behavior of marsh wrens, and attempt to esta b l i s h how widespread i t i s within the wren population of my study marsh (Chapter 1). Then I w i l l examine agonistic behavior of redwings towards marsh wrens (Chapter 2) and marsh wren nests (Chapter 3). In the f i n a l part of t h i s section I w i l l quantitatively evaluate the influence of behavioral interactions between redwings and marsh wrens on redwing reproductive success i n terms of the s p a t i a l pattern of d i s t r i b u t i o n of these species i n my study marsh (Chapter 4), and also i n r e l a t i o n to the quality of redwing nesting s i t e s , as determined by the interference from marsh wrens (Chapter 5 ) . 12 CHAPTER 1 Nest-destroying behavior of l o n g - b i l l e d marsh wrens Introduction Breeding birds and t h e i r eggs are subject to several types of predation. The predators are generally larger than the birds upon which they prey (e.g. magpies, jays, and crows). In a few cases, however, very small birds may be able to destroy nests of larger b i r d species as noted i n the house wren. Troglodytes aedon (Kendeigh 1941), l o n g - b i l l e d marsh wren, Telmatodytes p a l u s t r i s p a l u s t r i s (Allen 1914), cactus wren, Campylorhynchug brunneicapillus (Anderson and Anderson 1973), and s h o r t - b i l l e d marsh wren, Cistothorus platensis (Pieman and Pieman 1980), There i s a limited amount of information on possible egg destruction by marsh wrens. Chapman i n 1900 (in Bent 1948) f i r s t described marsh wren destruction of eggs, i n t h i s case eggs of the l e a s t b i t t e r n , Ixobrychus e x i l i s , The destruction of red-winged blackbird eggs was f i r s t observed by Allen (1914), who in addition found that eggs i n 14 of 51 red-winged blackbird nests were probably destroyed by marsh wrens. Most other authors studying marsh birds either have not considered marsh wrens as potential egg destroyers (e.g. Smith 1943) or else have suggested that this type of predation i s rare and involves only a few birds within a population (e.g. Welter 1935). Three more recent studies, however, indicate that marsh wrens may be 13 responsible for much of the nesting mortality i n some marsh birds- Orians and Willson (1964) noted that nesting and feeding areas of yellow-headed blackbirds, Xanthocephalus xanthoce£halus, red-winged blackbirds, and long- b i l l e d marsh wrens are strongly segregated, and that a l l redwing nests located within marsh wren t e r r i t o r i e s f a i l e d . Burt (1970) concluded that redwing nests within 100 f t of marsh wren courting centers suffered higher egg mortality than did nests farther away. Verner (1975) suggested that marsh/ wrens destroyed yellow-headed blackbird eggs i n a nest 5 m from a marsh wren nest. However, a l l three studies lack any dir e c t evidence. During a study of redwing nesting i n 1976 I found many eggs with small punctures in them. Later I saw a male l o n g - b i l l e d marsh wren removing a red-winged blackbird egg s h e l l from a nest. This finding, coupled with contradictory opinions on the extent of marsh wren egg destruction, indicated the need for more intense investigation. In t h i s Chapter I describe experiments done that f i r s t year to investigate certain aspects of nest destruction by marsh wrens. The main questions I asked were: (1) How common i s egg destruction by marsh wrens? (2) What kinds of eggs w i l l marsh wrens attack? (3) Do marsh wrens attack nestlings? (4) How do the c h a r a c t e r i s t i c s of nests a f f e c t the responses of marsh wrens to eggs placed i n them? (5) How do the age and sex of marsh wrens affect t h e i r responses 14 to eggs? (6) Hhy do marsh wrens destroy eggs and nestlings? Methods This study was conducted during spring and summer 1976, mostly i n bulrush and sedge areas of the marsh, where wren nests are easy to f i n d and wrens are more observable than i n the denser c a t t a i l . Seven series of experiments based on of f e r i n g eggs (or nestlings) i n d i f f e r e n t nests were designed to test the responses of marsh wrens to the following variables: (1) type of eggs (size, color, and shape) ; (2) nestlings; (3) type and position of the nest; (4) sex of the marsh wrens; and (5) age of the marsh wrens. I used rea l eggs taken from nests of several species of birds: l o n g - b i l l e d marsh wren; red-winged blackbird; robin. Turdus migratorius; s t a r l i n g , Sturnus vulgaris; r i n g -necked pheasant, Phasianus colchicus; mallard, Anas platyrhynchos; and domestic hen, Galius domesticus. I also used a r t i f i c i a l eggs of polystyrene, wood, and p l a s t i c ; eggs of d i f f e r e n t s i z e s , colors, and shapes (both r e a l and polystyrene); and nests of redwings and robins. The largest eggs were placed in large a r t i f i c i a l nests b u i l t of c a t t a i l stems and leaves. I also offered redwing and marsh wren nestlings of d i f f e r e n t ages in redwing and marsh wren nests. To study the r o l e of nest type and po s i t i o n , I offered eggs of d i f f e r e n t types i n redwing, marsh wren, robin, and a r t i f i c i a l nests placed 50-100 cm high in c a t t a i l or bulrush vegetation, and on the ground. Nests with 15 eggs or nestlings were usually fastened to standing c a t t a i l or bulrush vegetation near the singing perches of t e r r i t o r i a l male wrens or within 2 m of wren nests with eggs. To investigate the role of marsh wren age, I offered the nests with eggs to juvenile wrens, which occur i n small groups after fledging. Responses of birds in May and June were d i r e c t l y observed from about 7-10 m, but i n July and early August the birds became more cautious and had to be observed from at least 15 m-Altogether 56 adult marsh wrens (40 males, 16 females) were involved in the experiments. In most cases only 1 nest with one to eight (usually two to three) eggs was offered each time to each wren (92 t r i a l s ) , but some wrens were offered several nests (2-3) simultaneously (30 nests during 11 t r i a l s ) . Each o f f e r i n g of a single nest w i l l be referred to as one t r i a l . Because of the small numbers of t r i a l s performed i n i n d i v i d u a l experiments and because of the comparatively uniform r e s u l t s , marsh wren responses to i n d i v i d u a l variables were evaluated independently of other variables. A positive response consisted of the bird breaking or try i n g to break the egg by pecking i t with i t s b i l l , or pecking the nestlings. A negative response consisted of the bird a r r i v i n g on the nest but not pecking the eggs or nestlings.. As defined here, small eggs are those up to the si z e of s t a r l i n g eggs; large eggs are those down to the si z e of pheasant eggs. 16 Results 1- Size of e_g_s Two series of experiments with r e a l and a r t i f i c i a l eggs of dif f e r e n t sizes were performed. Marsh wrens responded p o s i t i v e l y to a l l sizes of r e a l eggs, from marsh wren eggs to extra large chicken eggs (Table 1, Fig. 2). Seventy-one percent of a l l responses were p o s i t i v e , and of the 13 negative responses, 12 were from females. Success of marsh wrens i n breaking eggs depends on the size of eggs. The largest eggs wrens broke were s t a r l i n g eggs (78% of eggs offered in f i v e positive t r i a l s ) . Marsh wrens broke 97% of a l l other smaller eggs during pos i t i v e t r i a l s , but none of 21 larger eggs, although they responded p o s i t i v e l y to 11 clutches with large eggs. Marsh wrens responded s i m i l a r l y to a l l small eggs. After a r r i v i n g at the nest the wrens started pecking them (Fig. 2). During short pauses they removed some nest material (usually l i n i n g ) , and males also sang. After breaking the eggs wrens usually removed them from the nest (unbroken eggs were not removed) by either dropping them from the nest edge or carrying them several metres away. Some birds then continued to remove nest material. In t h i s experiment 70% of a l l r e a l , broken eggs were Table 1. Predation on r e a l eggs of dif f e r e n t sizes offered to long-billed marsh wrens i n redwing and robin nests. Response No. eggs offered Frequency of Size of egg, Number positive negative i n t r i a l s responded No. eggs removing nest Type of egg * mm t r i a l s o V ?$ Of p o s i t i v e l y to broken removed material Long-billed marsh wren 15.8x12.5 7 6 1 0 0 19 18 15 2 Red-winged blackbird 25.5x18.6 10 6 1 0 3 13 13 7 2 Robin 27.6x19.2 2 2 0 0 0 5 5 2 1 Sta r l i n g 29.0x22.7 13 5 0 0 8 18 14 11 1 Pheasant 42.0x34.3 6 6 ^ 0 0 0 12 . 0 0 3 Mallard 52.4x40.0 5 3 0 1 1 6 0 0 0 Chicken 57.5x45.6 2 2 0 0 0 3 0 0 0 Total 45 30 2 1 12 76 50 35 9 * Dimensions of one measured egg of each species are given. 19 removed from the nests by the wrens. Twenty percent of a l l involved birds ( a l l males) removed some nest material (Table 1). Marsh wrens responded p o s i t i v e l y but more weakly to the largest eggs, and more weakly to large eggs after f a i l i n g to break them i n e a r l i e r t r i a l s . The same male wren responded very strongly on the f i r s t day ( i t pecked vigorously one chicken egg for about 2 min), weakly on the next day ( i t pecked only once one of two mallard eggs and then l e f t ) , and not at a l l on the t h i r d day (the bird arrived on the nest but l e f t without pecking the two mallard eggs). However, th i s bird broke a l l of three redwing eggs offered l a t e r on the t h i r d day. In the second series of experiments male marsh wrens were offered polystyrene eggs of four d i f f e r e n t s i z e s . Marsh wrens responded p o s i t i v e l y to a l l sizes of a r t i f i c i a l eggs (Table 2). Eighty-two percent of the responses were po s i t i v e ; three negative responses were to the largest eggs. Three times i n one t r i a l a male pecked a polystyrene egg nearly twice as large as a chicken egg. The behavior of wrens toward polystyrene eggs was s i m i l a r to that toward r e a l eggs. In positive t r i a l s 75% of the polystyrene eggs were removed from the nests. During 71% of the t r i a l s male marsh wrens removed some nest material (Table 2). 2- Color and shape of egjjs Marsh wrens responded p o s i t i v e l y to eggs of any color (Table 3). Eighty percent of the t r i a l s were p o s i t i v e , most of the negative t r i a l s were with females. In 40% of the t r i a l s Table 2. Offering a r t i f i c i a l polystyrene eggs of different sizes to male long-billed marsh wrens i n redwing and a r t i f i c i a l nests. Number eggs i n Number Frequency of Size of egg, Number Response clutches p o s i t i v e l y eggs removing nest Type of egg mm t r i a l s positive negative responded to removed material 1 31x23 9 9 0 19 14 7 2 54x39 2 2 0 2 2 2 3 72x54 4 2 2 2 2 0 4 101x72 2 1 1 1 0 1 Total 17 14 3 24 18 10 One egg of each size category was measured. 2 1 Table 3. Results of t r i a l s with eggs of different colors placed in redwing nests. Color and type of eggs Number tr i a l s Response positive negative 9? do* No. t r i a l s with removing nest material White (styrene eggs) Red (styrene eggs, cubes) Blue (styrene, starling eggs) Brown (marsh wren eggs) Yellow (wooden coral) Black (plastic b a l l , cubes) 12 15 10 0 0 0 0 8 0 0 0 0 Total 50 38 20 22 wrens removed some nest material. Different colors of eggs did not seem to af f e c t the egg-destroying behavior of marsh wrens in any way. In 11 t r i a l s i n which eggs of three d i f f e r e n t shapes were offered to marsh wrens in redwing nests, a l l birds responded p o s i t i v e l y (Table 4). Behavior of marsh wrens on nests with eggs of d i f f e r e n t shapes was similar to that of birds on nests with eggs of the usual shape. Marsh wrens removed from the nests a l l cubes and S-shaped eggs which they could grasp with their b i l l s . T h i r t y - s i x percent of the birds also removed some nest material (Table 4) . 3. Response to nestlings Marsh wrens attacked not only eggs but nestlings as well (Table 4). Two t e r r i t o r i a l male wrens responded strongly to two redwing nestlings of di f f e r e n t ages (Fig 3). The smaller redwing (5 g) was f i r s t vigorously pecked and k i l l e d and then carried about 5 m from the nest. The larger nestling (20 g) was vigorously pecked for about 6 min, but survived after I chased the wren away and returned i t to i t s own nest. Offering marsh wrens conspecific nestlings i n 15 t r i a l s resulted in s ix positive responses (Table 4)._ A l l these t r i a l s were performed with marsh wren nests except one with a redwing nest (in thi s case a male wren did not peck the young, but removed some nest material from the nest). Marsh wrens which responded negatively to the young usually l e f t the nest soon Table 4. Results of t r i a l s with eggs of different shape and redwing and marsh wren nestlings placed i n redwing and marsh wren nests. Response No. eggs (young) No. eggs No. t r i a l s Shape of 'eggs' Size of 'eggs' Number positive negative i n t r i a l s with (young) with removing (nestlings) (mm) t r i a l s oV oV $9 positive response removed nest material B a l l (plastic) 33.5 4 3 1 0 0 4 - 2 Cube (polystyrene) 12x12x26 4 4 0 0 0 7 7 0 S-shaped polysty- length,47 rene bar diameter,13 3 2 1 0 0 6 6 2 Young redwing 5; 20 g 2 2 0 0 0 2 1 0 Young marsh wren 1.75-8.5 g 15 6 0 9 0 12 8 3 24 F i g . 3. Long-billed marsh wren removing n e s t l i n g red-winged blackbird. 25 after a r r i v i n g , but some removed some nest material before leaving (five of nine b i r d s ) . A l l nestlings younger than 4 days were removed from the nests i n positive t r i a l s , but nestlings 5-7 days old (5.5, 8.5 g) were not removed and survived the attacks. 4. Types of nest placement Marsh wrens responded p o s i t i v e l y to eggs i n a l l four types of nests (Table 5). Suprisingly, marsh wrens also pecked eggs of conspecific nests, Two male marsh wrens responded p o s i t i v e l y to marsh wren and pheasant eggs placed in a r t i f i c i a l nests on the ground (Table 5) . 5. Sex and age of marsh wrens Seventy-five t r i a l s with males and females with r e a l or a r t i f i c i a l eggs are summarized in Table 6. Ninety-three percent of a l l males responded p o s i t i v e l y to eggs of any type. A l l responses of males to small eggs were positive, the negative responses were to large eggs. Only 25% of a l l females responded p o s i t i v e l y to offered eggs. The difference between males and females i s s t a t i s t i c a l l y s i g n i f i c a n t (Table 6 ) . The frequency of egg-pecking behavior in the study marsh wren population was also examined using a trap which I designed s p e c i f i c a l l y for capturing marsh wrens. This trap (see Appendix I) uses a redwing nest with eggs as a b a i t . Any bird that pecks the eggs placed on a trigger sets off the trap. Using t h i s 26 Table 5. Results of t r i a l s with eggs offered i n d i f f e r e n t types of nests placed high i n c a t t a i l s (bulrushes) and on the ground. Size of nest Number * Type of nest (cm) t r i a l s Eggs offered Response p o s i t i v e negative Redwing Robin 14; 14 15; 9 Marsh wren 10; 18 40 wren, redwing, s t a r l i n g , pheasant, mallard, chicken 5 pheasant, robin 16 wren 27 11 13 0 5 A r t i f i c i a l high i n vegetation 30; 5 2 l a r g e s t styrene A r t i f i c i a l on the ground 2 wren, pheasant Dimensions of one nest measured are i n sequence outside diameter and outside height. 27 technique i n 1977-1979 I was able to capture almost a l l (approximately 95%) male marsh wrens i n my study area. In 1978 I used th i s trap with a s i m i l a r success f o r capturing the breeding female marsh wrens i n one part of my study area. This suggests that probably a l l adult l o n g - b i l l e d marsh wrens exhibit the nest-destroying behavior. To see whether young marsh wrens exhibit the same behavior as adults, I offered them marsh wren eggs (one or two at a time) in redwing nests. Usually a group of fledglings was observed near the offered nest. Some juveniles quickly l e f t the nest without pecking the eggs, whereas others pecked them. Because these juveniles frequently chased each other around the nest and i n d i v i d u a l f l e d g l i n g s could not be i d e n t i f i e d , the proportion of birds not pecking eggs i s not known. The r e s u l t s on f l e d g l i n g s which pecked the eggs are summarized i n Table 7. Out of 14 observations of juveniles pecking eggs 3 were recently fledged wrens with neossoptiles s t i l l on t h e i r heads. Only three birds succeeded i n breaking an egg; none of the younger f l e d g l i n g s succeeded. Only one young b i r d removed the broken egg from the nest, and two f l e d g l i n g s removed some nest material. Other evidence that juvenile wrens attack eggs i s that 52 were taken in traps in which a redwing nest with one redwing egg was placed (two of these juveniles were recently fledged birds with neossoptiles s t i l l on t h e i r heads). 28 Table 6. Summary of t r i a l s , by sex. Sex group No. t r i a l s No. positive t r i a l s negative % positive t r i a l s Males 59 55 4 93 Females 16 4 12 25 Total 75 59 16 Note : X2=34.95; p 0.001. Table 7. Summary of t r i a l s positively responded to by juvenile marsh wrens. No. birds seen Age of Number No. eggs No. eggs No. eggs removing nest fledglings t r i a l s offered broken removed material * Younger 3 3 0 0 0 ** Older 11 13 3 1 2 Total 14 16 3 1 2 Birds with neossoptiles on the tips of feathers on occipital region. Birds without neossoptiles on the tips of feathers on occipital region. 29 Discussion These experiments show that wrens pecked any offered eggs regardless of t h e i r s i z e , c olor, and even shape, and also regardless of d i f f e r e n t nest types and nest locations. However, i t appears that wrens learn to discriminate between d i f f e r e n t egg types. This i s supported by the described offering of large eggs to a male wren i n 3 successive days. Thus i t seems possible that many positive responses by wrens were due to their lack of experience with eggs which they normally could not or could very r a r e l y f i n d i n the marsh. Marsh wrens broke most of the small r e a l eggs, but none of the large eggs, and usually removed broken eggs from the nests. It i s possible that by removing broken eggs marsh wrens remove any signs of nesting a c t i v i t i e s from attacked nests, thus presumably making nest abandonment by the nest owner more l i k e l y . This habit makes i t d i f f i c u l t to estimate the impact of marsh wrens on nesting success of other species ( I assume that i t i s not possible to distinguish between predation by marsh wrens and other small nest predators that leave no or s i m i l a r signs of t h e i r attacks) . Wrens also pecked redwing and conspecific nestlings. Younger nestlings of both species were f i r s t k i l l e d and then removed, though the frequency of t h i s behavior under natural 30 conditions i s unknown. However, the strong positive responses in t h i s study, finding f i v e redwing nestlings probably injured by wrens, and two add i t i o n a l observations of marsh wrens removing redwing nestlings i n eastern Washington (C. Monnett, personal communication) indicate that this behavior may be common. Marsh wrens also remove nest material (usually lining) from attacked nests, perhaps more often i n nature than they did i n these t r i a l s , which were terminated after the wrens destroyed the eggs and l e f t . Four observations made in t r i a l s with wren nests indicated that wrens may use removed nest material for building t h e i r own nests.. But also, the nest destruction might make nest abandonment more l i k e l y . This could be p a r t i c u l a r l y important i n case of nests in which females might continue to lay eggs. Nearly a l l male wrens responded p o s i t i v e l y to a l l offered eggs, but only one-fourth of females reacted p o s i t i v e l y , perhaps because females are more sensi t i v e to any disturbance near t h e i r nests (Welter 1935). Thus the presence of the introduced nest and the observer near the active nest may have affected normal behaviors of marsh wren females. This i s supported by the fact that i n 1979 I was able to capture, in one part of my study area, a l l breeding female marsh wrens using a trap baited with a redwing nest and eggs. What i s the adaptive si g n i f i c a n c e of nest destruction by marsh wrens? 31 (1) Nest predation could provide a s i g n i f i c a n t energy or nutrient resource for breeding birds. This i s supported by Allen's observation of a marsh wren drinking the contents of a broken egg (Allen 1914) and by several similar observations made by C.. Monnett (personal communication) i n eastern Washington. However, in only 1 of my 35 observations of egg-destruction did a wren seem to have eaten something from the broken egg. Also, marsh wrens are not equipped to eat the f l e s h of the nestlings they attack. Thus predation for food i s unlikely to be important. (2) Nest destruction could reduce interference from other species. Competition, which has been considered as a demand by indivi d u a l s of one or several species for some common l i m i t i n g resource, i s i n general d i f f i c u l t to i d e n t i f y (Miller 1967). In the study situation there are, however, two indications of exi s t i n g competition between marsh wrens and red-winged blackbirds: f i r s t , the two species i n t e r a c t . Wrens destroy redwing nests, and redwings chase wrens (e.g. Welter 1935, Verner 1975). Second, their nest s i t e s are i n general s p a t i a l l y segregated (see Chapter 4). In addition, i t i s my experience that where these species breed close to each other, redwings are in the sparser c a t t a i l vegetation, while marsh wrens are i n the denser vegetation. This seems to result from d i f f e r e n t e f f i c i e n c i e s of the described interference mechanisms i n c a t t a i l vegetation of d i f f e r e n t densities. Hypothetically, small wrens can destroy redwing nests and avoid redwing aggression better i n dense vegetation. On the other hand, larger blackbirds should 32 be able to more e f f i c i e n t l y protect t h e i r nests from marsh wrens i n sparse vegetation. At present, there are not s u f f i c i e n t data to i d e n t i f y the l i m i t i n g resource competed f o r , so the competition hypothesis must be considered speculative. However, in the study area, nest s i t e s and singing perches of both species seem to be superabundant and thus competition for space seems to be l e s s l i k e l y . But there i s some information available which indicates the p o s s i b i l i t y of competition for food. Both species feed on a variety of marsh insects (Bent 1948, 1958; Orians and Willson 1964) and thus a s i g n i f i c a n t overlap may be expected. Moreover, in eastern Washington both species eat damselflies, which are the mainstay of breeding blackbirds there (G. H. Orians, personal communication). The p o s s i b i l i t y that food might be i n short supply i s indicated by many data on starvation of redwing nestlings (e.g. Holcomb and Twiest 1968, Robertson 1972, Holm 1973) and also by the finding that the sizes of redwing populations correspond to the available energy supply (Brenner 1966, 1968). The evolution of nest-destroying behavior i n marsh wrens might have also been promoted by the following facts. F i r s t , marsh wrens are small, inconspicuous birds with short wings, and are highly manoeuvreable (Welter 1935, Hamilton 1961), which should f a c i l i t a t e easy escape i n dense c a t t a i l vegetation. Secondly, marsh wrens breed i n closed domed nests that provide places for nesting out of sight of the species that chase them. And t h i r d l y , r e l a t i v e homogeneity and s i m p l i c i t y of marshes and 33 their l i m i t e d area probably prevent further resource s p e c i a l i z a t i o n by these species. To conclude, I suggest that nest destruction by marsh wrens has most l i k e l y evolved as an interference mechanism to reduce probable i n t e r s p e c i f i c competition with some other marsh nesting birds. Through t h i s behavior marsh wrens probably exclude other birds from those parts of the marsh to which marsh wrens are best adapted under given conditions. 34 CHAPTER 2 Ag r e s s i o n by red-winged blackbirds towards l o n g - b i l l e d marsh wrens Introduction Marshes are highly productive, s t r u c t u r a l l y simple, two-dimensional habitats (e.g. Verner and Willson 1966), which support a simple passerine community. Because marshes are r e l a t i v e l y uniform, they should, t h e o r e t i c a l l y , provide l i t t l e opportunity for ecological s p e c i a l i z a t i o n for di f f e r e n t species of birds l i v i n g i n them (Orians and Willson 1964, Cody 1968, Tramer 1969). Environmental l i m i t a t i o n thus r e s t r i c t s the p o s s i b i l i t y of avoidance of competition for resources by evolution of ecological divergence. This should favor the evolution of i n t e r s p e c i f i c aggression and hence i n t e r s p e c i f i c t e r r i t o r i a l i t y as a means of reducing i n t e r s p e c i f i c competition (Orians and Willson 1964). This view i s supported by observations on i n t e r s p e c i f i c aggression and complete or p a r t i a l i n t e r s p e c i f i c t e r r i t o r i a l i t y between the following North American marsh-nesting passerines: red-winged blackbirds and yellow-headed blackbirds (Burt 1970, Fautin 1940, Linsdale 1938, Orians and Willson 1964); red-winged blackbirds and t r i - c o l o r e d blackbirds, Aqelaius t r i c o l o r (Orians and C o l l i e r 1963); red-winged blackbirds and common grackles, J Q - H i s q u i s c u l a (Nero 1956b, Wiens 1965, Willson 1967) ; red-35 winged blackbirds and l o n g - b i l l e d marsh wrens (Burt 1970, Nero 1956b, Orians and Willson 1964, Verner 1975, t h i s study); yellow-headed blackbirds and l o n g - b i l l e d marsh wrens (Burt 1970, Orians and Willson 1964, Verner 1975, Willson 1966); l o n g - b i l l e d marsh wrens and swamp spparrows, Melospiza ggorgiana (Willson 1967); and swamp sparrows and song sparrows, MelosEiza melodia (Willson, 1967). In addition to the above species known to interact frequently, Willson (1967) reported occassional interactions between song sparrows and l o n g - b i l l e d marsh wrens; yellow warblers, Dendroica fietechia, and swamp sparrows; and between yellowthroats, Geothlyeis t r i c h a s , and swamp sparrows. In t h i s chapter, I w i l l examine i n t e r s p e c i f i c aggression by breeding red-winged blackbirds towards l o n g - b i l l e d marsh wrens. The main purposes of this study are to: (1) quantitatively evaluate the freguency of interactions between male.and female redwings and l o n g - b i l l e d marsh wrens; (2) investigate the frequency of redwing-wren interactions at diffe r e n t times of day; (3) examine the relationships between the rate of redwing-wren interactions and the density of marsh wrens i n the area; (4) discuss the significance of the reported r e s u l t s i n terms of the interference competition between redwings and marsh wrens, and the adaptive value of the redwing reproductive t a c t i c s . 36 Methods Data on redwing aggression towards marsh wrens were co l l e c t e d on May 16, 17, and 28, 1978, when both redwings and marsh wrens were at theic peak of breeding a c t i v i t i e s . Observations were made i n the morning (6:00-11:30), early i n the afternoon (12: 00-15:30), and l a t e i n the afternoon (16:00-20:30), by three observers from chairs 2-4 meters high. Each observer kept track of 1-2 male and 3-8 female redwings, simultaneously. Data on redwing aggression towards marsh wrens include: (1) a l l observations when both the redwing aggressor and the marsh wren "victim" were seen; and (2) chases during which only the redwing aggressor was observed. Since redwing aggression towards marsh wrens t y p i c a l l y consists of a short, quick, and vigorous chase through the vegetation, during which marsh wrens generally avoid redwings by escaping into dense c a t t a i l s and hiding, these agonistic interactions cannot be easi l y mistaken for any other redwing a c t i v i t y . Results and Discussion 1. Frequency of interactions between redwings and marsh wrens The redwing-wren interactions recorded by the three observers i n a t o t a l of 40.5 hours are summarized i n Table 8. A great majority of these interactions involved male redwings. This f i n d i n g i s even more s i g n i f i c a n t when i t i s considered that the sex r a t i o within the study group of redwings was 37 Table 8. Summary of data on aggression by male and female redwings towards marsh wrens obtained by three observers. Total number (%) of chases by > -Observer males females T o t a l 1 10 (76.9) 3 (23.1) 13 2 53 (82.8) 11 (17.2) 64 3 58 (79.5) 15 (20.5) 73 Total 121 (80.7) 29 (19.3) 150 Note: X2=0.38; df=2; p>0.8. 38 approximately three females to one male. Higher rates of agonistic interactions between male redwings and marsh wrens are also evident from observations of Nero (1956b) and Burt (1970), and i n the case of yellow-headed blackbirds, from the data of Burt (1970) and Verner (1975). This evidence, therefore, strongly suggests that male red-winged and probably also yellow-headed blackbirds play an important role i n defense of t h e i r females 1 nests against l o n g - b i l l e d marsh wrens, which are the most important cause of redwing nesting mortality in t h i s marsh (see Chapter 4)-. However, because marsh-nesting redwings are usually highly polygynous (e.g. Orians 1980), the contribution of males to defense of each female's nest should decrease with increasing number of simultaneously active nests on their t e r r i t o r i e s . Therefore, the capacity of males to e f f i c i e n t l y defend only a certain maximum number of nests against marsh wrens could play an important r o l e i n the evolution of the female clumping tendency and i n setting an upper l i m i t to harem size (these problems w i l l be discussed in d e t a i l in Chapter 10). 2- Frequency of redwing-wren interactions at different times of day The rate of interactions between redwings of both sexes and marsh wrens i s highest i n the morning, lowest early i n the afternoon, and i t i s again higher l a t e in the afternoon (Table 9). This i s consistent with a general pattern of t e r r i t o r i a l and foraging a c t i v i t i e s of redwings and marsh wrens in t h i s marsh, which also have two peaks, one i n the morning, 39 Table 9. Rate of redwing-wren agonistic interactions at different times of a day. Data from May 16 and 17 were combined. Number of chases per hour by (total number of chases) m e °f males and Number of hours day males females females of observations 6:00-11:30 4.8 (58) 1.1 (13) 5.9 (71) 12 12:00-15:30 1.3 (4) 0.7 (2) 2.0 (6) 3 16:00-20:30 3.3 (40) 0.8 (10) 4.2 (50) 12 Total 3.8 (102) 0.9 (25) 4.7 (127) 27 40 and the other one l a t e i n the afternoon. Therefore, the rate of interactions between redwings and marsh wrens throughout the day i s apparently determined by the i r a c t i v i t i e s . 3• Influence of marsh wren density on the rate of redwing-wren interactions The rate of interactions between i n d i v i d u a l redwings and marsh wrens should also be determined by the density of marsh wrens within or near the redwing breeding area. Table 10 presents data on the frequency of interactions between redwing males and marsh wrens from three areas with d i f f e r e n t wren densities (in the calculation of the area per marsh wren, which I used as an index of wren density, both male and female marsh wrens occuring in the area observed were included). The rate of redwing-wren interactions increases with increasing density of marsh wrens. This i s probably a consequence of: (1) a smaller number of marsh wrens i n a low marsh wren density area; (2) greater s p a t i a l segregation between redwings and marsh wrens i n a habitat with low densities of marsh wrens (see Chapter 4), which should further reduce the freguency of encounters between redwings and marsh wrens; and (3) a lower l e v e l of responsiveness by redwings towards marsh wrens under low wren density conditions, which i s probably associated with t h e i r d i f f e r e n t amount of experience of marsh wren nest destruction (Chapter 3). This correlation between the rate of redwing-wren inter a c t i o n s and the density of marsh wrens implies that any negative effects of marsh wrens on redwing reproduction through 41 Table 10. Agonistic interactions between redwings and marsh wrens i n areas with different densities of marsh wrens. Data from May 16 and 17 were combined (9 hours of observations were made at each study s i t e ) . Area per Area per No. chases No. male Number chases wren male redwing by male redwings per male redwing 2 2 (m ) (m ) redwings involved per hour 600 3600 45 1 5.0 1000 3000 54 2 3.0 2300 4600 3 2 0.2 42 the so c a l l e d "aggressive neglect" (see Hutchinson and MacArthur 1959, Ripley 1961) should increase with the increasing density of marsh wrens i n a habitat. The rates of redwing-wren interactions, as estimated from data on male redwings (Table 10), most l i k e l y present a r e l i a b l e estimate for the study marsh. Male redwings are conspicuous, spend most of their time on a t e r r i t o r y on high perches, and, therefore, the observers could not miss many of their agonistic encounters with marsh wrens. Female redwings, on the other hand, are much l e s s conspicuous, especially while incubating. Therefore, i f females chased marsh wrens d i r e c t l y from t h e i r nests, i t i s possible that the observers might have missed a number of t h e i r interactions with marsh wrens- I do not have data to examine t h i s p o s s i b i l i t y . I believe, however, that the observers could have missed only a small number of female-wren interactions- Data on female redwings thus probably only s l i g h t l y underestimate the actual frequency of female interactions with wrens.. In addition, the evidence on a higher rate of interactions between male redwings and marsh wrens i s consistent with a theoreti c a l expectation based on d i f f e r e n t roles of male and female redwings i n nesting. Females which are f u l l y involved i n breeding have less time available for interactions with marsh wrens than males that do not generally a s s i s t t h e i r females with most of the nesting duties (e.g. Orians 1961). In addition, males probably can see marsh wrens better from high perches. 43 4. Adaptive value of redwing aggression towards marsh wrens The evolution of redwing (and probably also yellowhead) aggression towards marsh wrens could be explained i n terms of interference competition between blackbirds and marsh wrens. Long-billed marsh wrens destroy blackbird eggs (e.g. Allen 1914, Willson 1966) and also k i l l blackbird nestlings (see Chapter 1), and they present an important source of redwing nesting mortality i n various marshes (Allen 1914; Orians and Willson 1964; Burt 1970; Verner 1975; Runyan 1979; th i s study, see Chapter 4). Therefore, i t i s l i k e l y that aggression by blackbirds towards marsh wrens has evolved i n response to marsh wren nest-destroying behavior (see also Orians and Willson 1964, Verner 1975). Hence, through t h e i r aggression towards marsh wrens redwings probably reduce interference from marsh wrens. This hypothesis i s supported by the fact that, presumably as a consequence of redwing-wren agonistic interactions, nesting areas of redwings and marsh wrens are s p a t i a l l y segregated i n t h i s and other marshes (Orians and Willson 1964; Burt 19 70; Verner 1975; t h i s study, see Chapter 4). In addition, redwings appear to r e s t r i c t the d i s t r i b u t i o n of marsh wrens, as indicated by the fact that marsh wrens expand the i r t e r r i t o r i e s into the redwing breeding areas after redwings have fi n i s h e d nesting and l e f t the marsh (Burt 1970). Similar evidence on the role of aggression by yellow-headed blackbirds towards marsh wrens i n r e s t r i c t i n g marsh wrens mainly outside the yellowhead breeding areas i s also available (Burt 1970, Verner 1975). To conclude, blackbird aggression towards marsh wrens i s 44 l i k e l y to play an important role i n biologies of these species. Since the available evidence on the exclusion of marsh wrens by redwings i s circumstantial, t h i s problem w i l l reguire a more direct t e s t . This could be achieved by removing breeding blackbirds from some parts of a marsh and studying the e f f e c t s of t h i s removal experiment on the d i s t r i b u t i o n of marsh wrens. Implications of redwing aggression towards marsh wrens on redwing nesting t a c t i c s w i l l be examined in the second and t h i r d sections. 45 CHAPTER 3 Response of red-winged blackbirds to nests of lo n g - b i l l e d marsh wrens Introduction Long-billed marsh wrens are known to attack eggs (Allen 1914; Bent 1948; Orians and Willson 1964; Burt 1970; Verner 1975; t h i s study, Chapter 1) and nestlings (see Chapter 1) of other small sympatrically nesting birds. Because the breeding ranges of the red-winged blackbirds and yellows-headed blackbirds overlap with that of l o n g - b i l l e d marsh wrens (Bent 1948, 1958), the nests of both blackbird species are l i k e l y to be attacked by marsh wrens. Both blackbird species have been observed acting aggressively towards marsh wrens. Yellow-headed blackbirds systematically chase adult marsh wrens (Burt 1970, Verner 1975). Burt (1970) observed 5 chases i n which male yellow-headed blackbirds captured marsh wrens and pecked them vigorously. Yellowheads also land on and search wren nests (Verner 1975). Verner (op. c i t . ) suggested that yellow-headed blackbirds may have destroyed several active wren nests that were located near the i r own nests. On the other hand, information on redwing aggression towards marsh wrens (Burt 1970, Nero 1956b, Orians and Willson 1964, Verner 1975) i s r e s t r i c t e d to aggressive chases of wrens rather than attacks on t h e i r nests. It i s not known whether the lack of reported nest attacks indicates a r e a l 46 low l e v e l of aggression or a f a i l u r e of observers to detect i t in redwings. Aggression of blackbirds towards marsh wrens and destruction of wren nests could influence reproductive success of marsh wrens breeding near blackbirds. Moreover, t h i s aggression could also depress the reproductive success of blackbirds through "aggressive neglect" (see Hutchinson and MacArthur 1959, Ripley 1961), because redwing females wasted time and energy by chasing marsh wrens or examining th e i r nests (see Chapter 4). In t h i s study I examined the responses of female red-winged blackbirds to marsh wren nests by o f f e r i n g wren nests experimentally. The experiments were designed to answer four questions: (1) Do redwings respond to marsh wren nests, and i f they do, how do d i f f e r e n t females respond? (2) If female redwings respond to marsh wren nests, do they respond s p e c i f i c a l l y to these nests, or do they respond to any strange objects placed near th e i r own nests? (3) Does previous experience of having their nests destroyed by marsh wrens affect redwing females' response to marsh wren nests? (4) Is the response by female redwings to marsh wren nests influenced by the l o c a l density of marsh wrens? (5) How do redwings respond to nests containing eggs? 47 Methods I studied the response of redwing females to marsh wren nests by offering courtship (non-breeding) nests b u i l t by male wrens (see e.g. Verner 1965) to redwing females near t h e i r own nests. The marsh wren nest i s a domed structure with a single entrance near the top (Verner 1965). The wren nests were cut from t h e i r o r i g i n a l s i t e s and then fastened with e l a s t i c bands, along with some of the o r i g i n a l c a t t a i l supporting the nest, to a one meter bamboo s t i c k . The s t i c k was then positioned at either 2 m or 5 m from redwing nests, i n a position that was v i s i b l e from the redwing nest,. Responses of redwing females were observed for 30 minutes from a range of 20-60 m. I offered wren nests to redwings at two di f f e r e n t times during the 1978 and 1979 breeding seasons i n order to observe the ef f e c t of previous experience of destruction of t h e i r own nests by marsh wrens,. I assumed that most birds involved i n early t r i a l s (April 30 - May 15) were breeding for the f i r s t time that year and thus had not experienced nest destruction by marsh wrens that year. However, because the number of simultaneously active redwing nests reached i t s maximum around May 8 i n both seasons (see Chapter 10, F i g . 34), both old females that had already bred i n previous year(s), and young birds breeding f o r the f i r s t time were involved i n the early t r i a l s . On the other hand, nests of redwings involved in l a t e r t r i a l s (May 31 - June 8) probably represented second or t h i r d nesting attempts f o r a l l females. Because nest destruction by marsh wrens was the most important cause of redwing nest f a i l u r e 48 (see Chapter 4), many redwing females used i n late t r i a l s probably had experienced harassment by marsh wrens e a r l i e r i n the year. Twenty t r i a l s with di f f e r e n t redwing females were conducted at each 2 m and 5 m distances, both early and l a t e in 1978 and 1979. In 1978, thirteen females were tested twice during early t r i a l s (these birds were f i r s t offered nests at 5 m and then, at least a day l a t e r , at 2 m) . During l a t e t r i a l s 10 females were offered nests at both 2 m and 5 m. In 1979, 10 and 19 females were offered a marsh wren nest at both 2 m and 5 m i n early and la t e t r i a l s , respectively. The number of females used i n both early and l a t e t r i a l s i s not known. A l l t r i a l s were done between 7 a.m. and 12 noon. Bedwing nests containing eggs were selected for a l l but f i v e t r i a l s . Because there were not enough nests with eggs, those 5 nests contained eggs and/or nestlings less than 3 days of age. To determine whether redwing females respond s p e c i f i c a l l y to marsh wren nests or just to a strange object placed near their nests, i n 1978 and 1979 several female redwings were offered a bundle of c a t t a i l and sedge of approximately the same diameter as a marsh wren nest but three times longer. This bundle was fastened to a one meter bamboo s t i c k that was placed 2 m from the redwing nests. On the next day, or at least one day before these t r i a l s , the same females had been presented with a marsh wren nest, also at a distance of 2 m. These t r i a l s were conducted as previously described. 49 Results 1. Types of redwing responses to marsh wren nests Female redwings responded to marsh wren nests i n 5 ways (Table 11): (1) The strongest response was shown by two agitated females. They perched on the wren nest, hopped about on top of the nest, and pecked at i t several times. (2) In 14 t r i a l s female redwings landed on a wren nest and searched i t (none of the females entered the nest). These birds were evidently agitated and some vigorously hopped about on top of the wren nest. (3) The most common type (62 t r i a l s ) of positive response was when a female arrived near the nest and perched on c a t t a i l s usually within 0.5-1.0 m of the wren nest. The birds responding i n t h i s way stayed nearby for a short time and examined the nest area from t h e i r perches. (4) The weakest and less common positiv e response (5 t r i a l s ) was when a female flew around the wren nest (usually within 1m). The females responding i n t h i s way presumably approached the wren nest to examine i t and i t s v i c i n i t y for marsh wrens. (5) Some females ( 7 7 t r i a l s ) did not respond i n any way to the wren nest. These birds l e f t t h e i r nests to forage and returned without apparently paying any attention to the wren nest,. I considered responses 1-4 as positive and 5 as negative. 50 Table 11. Summary of responses by redwing females to marsh wren nests offered during early and late t r i a l s at 2m and 5m from redwing nest in 1978 and 1979. Number (%) of responses during stance . 6 from redw. Type of early late combined early * nest response t r i a l s t r i a l s and late t r i a l s 2 m 1 0 (0) 1 (2.5) 1 (1.3) 2 4 (10) 6 (15) 10 (12.5) 3 15 (37.5) 22 (55) 37 (46.2) 4 2 (5) 1 (2.5) 3 (3.8) 5 19 (47.5) 10 (25) 29 (36.2) 5 m 1 0 (0) 1 (2.5) 1 (1.3) 2 1 (2.5) 3 (7.5) 4 (5) 3 6 (15) 19 (47.5) 25 (31.2) 4 1 (2.5) 1 (2.5) 2 (2.5) 5 ) 32 (80) 16 (40) 48 (60) 1 Female pecked at the nest 2 Female landed on the nest and searched i t 3 Female perched near the nest 4 Female flew around the nest 5 No response 51 2. Control t r i a l s In 1978 I offered 6 females the bundle of c a t t a i l and sedge, but none of these birds responded to i t in any way. However, 3 of 4 of these females (the nests of the other two had already f a i l e d ) responded p o s i t i v e l y to the marsh wren nest on the next day (one female perched near the nest and two females landed on the nest and examined i t ) . . This difference i s s i g n i f i c a n t (Fisher exact probability test, one-tailed, p=0.03), The second set of control t r i a l s was conducted i n 1979 with 8 female redwings that had responded p o s i t i v e l y to a wren nest i n previous t r i a l s . When these birds were offered the bundle of c a t t a i l and sedge, only one responded p o s i t i v e l y to the bundle by perching nearby, while the other 7 females ignored t h i s object (Fisher exact probability test, one-tailed, p=0.0007). Therefore, I conclude that female redwings respond s p e c i f i c a l l y to nests of marsh wrens rather than to a strange object such as the bundle of c a t t a i l and sedge. 3- Influence of experience of marsh wren nest destruction on redwing responses to wren nests As female redwings become more experienced with marsh wren predation of their nests, they might learn to recognize marsh wrens as potential nest predators and become aggressive toward them. I f t h i s i s true, then renesting female redwings that f a i l e d as a r e s u l t of wrens early i n the season should respond more p o s i t i v e l y to marsh wren nests than females nesting f o r the 52 f i r s t time. To test t h i s , I compared the incidence of a l l positi v e and negative responses during early and late t r i a l s for both distance categories. In 1978, s i g n i f i c a n t l y more female redwings responded p o s i t i v e l y to a marsh wren nest during l a t e t r i a l s than during early t r i a l s , i n both the 2 m and 5 m distance categories (Table 12). In 1979, however, the proportion of positive responses i n early and l a t e t r i a l s at 2 m and 5 m does not d i f f e r s i g n i f i c a n t l y (Table 12). The influence of previous experience with marsh wrens on redwing responses may also be examined by comparing responses by old (experienced) and young (inexperienced) females. Many old females, that bred i n previous year(s) i n this marsh, had presumably experienced nest destruction by marsh wrens. Therefore, a higher proportion of old females should respond p o s i t i v e l y i n early t r i a l s to nests of marsh wrens as compared with young females breeding f o r the f i r s t time. I evaluated responses by color-banded females (those that bred in t h i s marsh in at least one previous season) and those that were not banded (many of these birds are presumably young females breeding for the f i r s t time). In both the 1978 and 1979 seasons there were more po s i t i v e responses by banded than unhanded females (Table 13). These differences are not s i g n i f i c a n t for i n d i v i d u a l seasons but when data from the two seasons are combined, the difference becomes s i g n i f i c a n t (Table 13) . 53 Table 12. Responses by female redwings to a marsh wren nest during early and late t r i a l s at 2m and 5m from redwing nest (a), and s t a t i s t i c a l evaluation of these results (b). (a) Number (%) redwing responses Year Distance Trials positive negative Total 1978 2 m early 13 (65) 7 (35) 20 late 19 (95) 1 (5) 20 Total 32 8 40 5 m early 5 (25) 15 (75) 20 late 17 (85) 3 (15) 20 Total 22 18 40 1979 2 m early 8 (AO) 12 (60) 20 late 11 (55) 9 (45) 20 Total 19 21 40 5 m early 3 (15) 17 (85) 20 late 7 (35) 13 (65) 20 Total 10 30 40 54 Table 12b: Comparison 2 X p (1-tail test) 1978 early vs. late, 5m 12.22 <0.0005 early vs. late, 2m * <0.025 early, 2m vs. 5m 4.95 <0.025 late, 2m vs. 5m * >0.3 1979 early vs. late, 5m 1.2 >0.1 early vs. late, 2m 0.4 >0.25 early, 2m vs. 5m 2.01 >0.05 late, 2m vs. 5m 0.91 >0.15 1978 early, 2m 1.6 >0.1 vs. early, 5m 0.16 >0.7 1979 late, 2m 6.53 <0.01 late, 5m 8.44 <0.005 Fisher exact probability test used 55 Table 13. Responses by banded (old) and unbanded (mostly young) female redwings during early t r i a l s . Number (%) redwing responses Year status positive negative Total 1978 banded 6 (86) 1 (14) * 7 unbanded 9 1 (50) 9 (50) 18 Total 15 10 25 1979 banded 4 (57) 3 (43) 7 ** unbanded 6 (27) 16 (73) 22 Total 10 19 » 29 1978 banded 10 (71) 4 (29) 14 *** and unbanded 15 (37) 25 (63) 40 1979 Total 25 29 54 Fisher exact probability test (one-tailed), p>0.1 Fisher exact probability test (one-tailed), p>0.15 2 X = 3.53, one-tailed test, p<0.05 56 4. Influence of proximity to marsh wren nests on response of redwings to marsh wrens The presence of marsh wrens close to breeding redwings should res u l t i n a higher frequency of redwing-wren int e r a c t i o n s , a lower tolerance of redwing females for marsh wrens, and, therefore, more posit i v e responses by female redwings to marsh wren nests. To test t h i s hypothesis, I examined female responses to experimental marsh wren nests during early t r i a l s at 2 m and 5 m i n 1978 and 1979 as a function of the distance of these females from the nearest natural wren nest in the i r breeding areas. In 1978, females that responded p o s i t i v e l y to a marsh wren nest had the nearest wren nest nearer than females that responded negatively (this i s s i g n i f i c a n t for 2 m t r i a l s ; Table 14). In 1979, however, the mean distance from the nearest wren nest was s i m i l a r for females that responded p o s i t i v e l y and negatively during both 2 m and 5 m t r i a l s (Table 14). 5- Are females which responded positively, to marsh wren nests more successful breeders? Female redwings might increase t h e i r chances of nesting success through t h e i r aggression toward marsh wrens. I would also predict that female redwings that examine marsh wren nests should be more successful than females that ignore wren nests. To test t h i s , I compared the nesting success of females that Table 14. Effect of proximity of marsh wren nests on responses by female redwings to experimentally offered wren nests during early t r i a l s in 1978 and 1979. 1978 1979 Mean distance (m) Mean distance (m) of redw. nest from °f redw. nest from marsh wren nest marsh wren nest Trials at Trials ±SD (N) T-value D.F. p ±SD (N) T-value D.F. 2 m positive 11.8+4.5 (13) 20.3±5.8 (8) -2.423 7 <0.05 0.904 18 >0.35 negative 23.1+11.9 (7) 17.7±6.4 (12) A l l t r i a l s 15.8+9.4 (20) 18.7±6.1 (20) 5 m positive 10.4±4.0 (4) 20.9+5.6 (3) -1.408 17 >0.15 0.123 18 >0.8 negative 17.3±9.4 (15) 20.4+7.3 (17) A l l t r i a l s 15.8±8.9 (19) 20.5±7.0 (20) 58 responded p o s i t i v e l y and negatively to a wren nest offered to them in early t r i a l s (either at 2 m or 5 m) i n 1978 and 1979,. In both seasons, more p o s i t i v e l y responding females i n each distance category were successful i n fledging at least one young than females that ignored the marsh wren nest (this difference i s 23 % and 21 % i n 1978 and 1979, respectively; Table 15). The difference i s not, however, s i g n i f i c a n t probably because the sample siz e was too small (Table 15). 6. Response of redwings to nests with eggs In general, the breeding and courtship marsh wren nests are s i m i l a r in t h e i r appearance; the only difference I noticed i s a small amount of l i n i n g i n the entrance of some breeding nests. This l i n i n g consists of dry s t r i p s peeled from dead c a t t a i l stalks and feathers of various species (Verner 1965) and i s evidently d i f f e r e n t from the outside structure of the nest which consists mainly of dead c a t t a i l leaves and a varying amount of fuzz from c a t t a i l heads. If redwings can distinguish the breeding from courtship wren nests then i t i s possible that they might respond more strongly to them than to the courtship nests which I used during my t r i a l s . To examine t h i s p o s s i b i l i t y and redwing behavior towards eggs placed i n open-topped nests, i n June and July 1976 I offered breeding female redwings: (1) breeding marsh wren nests with marsh wren eggs; (2) open-topped marsh wren nests with marsh wren eggs (I cut o f f the upper half of the nest); (3) redwing nests with redwing eggs; and (4) redwing nests with marsh wren eggs. In these t r i a l s 12 agitated 59 Table 15. Nesting success of female redwings that responded positively or negatively to a marsh wren nest. Number (%) redwing responses Year success positive negative Total 1978 successful 11 (73) 5 (50) 16 failed 4 (27) 5 (50) 9 Total 15 10 25 1979 successful 10 (100) 15 (79) 25 ** failed 0 (0) 4 (21) 4 Total 10 19 29 1978 successful 21 (84) 20 (69) 41 and *** 1979 failed 4 (16) 9 (31) 13 - Total 25 29 54 * X2=0.586 (1-tailed), p>0.15 ** Fisher exact probability test (1-tailed), p>0.15 *** ? X =0.940 (1-tailed), p>0.15 6 0 redwings (10 females, 2 males) arrived on top of the offered nests, examined them and t h e i r contents (except domed marsh wren nests, which they never t r i e d to enter), but none of these birds t r i e d to break the eggs or destroy the nests. Discussion Female red-winged blackbirds, l i k e yellow-headed blackbirds, respond to marsh wren nests that are located near thei r own nests. Two kinds of information suggest that female redwings respond s p e c i f i c a l l y to wren nests. F i r s t , a higher proportion of females responded p o s i t i v e l y to wren nests during late t r i a l s when, presumably, many of them had experienced marsh wren nest destruction. Second, during the control t r i a l s redwings ignored the bundles of c a t t a i l and sedge offered as a strange object near the i r nests, but they responded p o s i t i v e l y to marsh wren nests. Most of the posit i v e responses by female redwings consisted of searches of wren nests and the area around them. Only i n 2 (out of 160) t r i a l s did female redwings peck at the nests, though without causing any damage. This suggests that female redwings do not normally purposefully destroy nests of marsh wrens. This conclusion i s also supported by observations showing that, unlike marsh wrens, redwings do not destroy nests with eggs. The f a i l u r e of redwings to evolve the nest destroying 61 behavior as an e f f i c i e n t way of excluding marsh wrens from the v i c i n i t y of redwing nesting areas may be explained as follows. F i r s t , marsh wrens build domed nests that have a r e l a t i v e l y firm structure with a single small entrance. Second, multiple nests b u i l t by male marsh wrens (e.g. Verner 1965), may function as "decoys" to reduce the impact of redwings on the success of breeding wren nests. And t h i r d , because redwings are much larger than marsh wrens, they can probably exclude marsh wrens from the v i c i n i t y of t h e i r nests d i r e c t l y by aggressive i n t e r a c t i o n s . In f a c t , redwing and marsh wren nests are s p a t i a l l y segregated in th i s marsh (see Chapter 4). Redwing response to marsh wren nests i s influenced by the distance between wren and redwing nests. In the early t r i a l s more redwings responded to wren nests at 2 m than at 5 m (this i s s i g n i f i c a n t only for 1978 data; Table 12), in d i c a t i n g that the response of female redwings decreases with increasing redwing-wren internest distance. Also yellowheads seem to respond more strongly to nearby marsh wren nests because only marsh wren nests located near a yellowhead nest were damaged (Verner 1975). 1. The ro l e of previous experience of marsh wrens on redwing responses The response of redwings to wren nests increases with the progress of the season. This was p a r t i c u l a r l y clear i n 1978, when almost a l l females responded to wren nests during l a t e t r i a l s at 2 m and 5 m. This observation might be explained i n 62 terms of increasing experience of redwings of marsh wren nest destruction with the progress of the breeding season. A l t e r n a t i v e l y , i t i s also possible that more responses l a t e r i n a season might r e s u l t from generally increasing aggression of redwings with the progress of the breeding season. But there are two reasons to believe that t h i s alternative explanation i s unl i k e l y . F i r s t , there were s i g n i f i c a n t differences i n responses of females in l a t e t r i a l s i n 1978 and 1979, when females should have responded s i m i l a r l y i f the alternative hypothesis were rig h t . Second, as w i l l be discussed l a t e r i n thi s chapter, these differences i n female responses are related to various densities of marsh wrens in the two years. Thus I suggest that previous experience of marsh wrens plays an important role i n the redwing response to wren nests (and presumably also to marsh wrens). This could also explain why some females responded to wren nests during early t r i a l s . It i s possible that these positive responses were by old females that may have experienced nest destruction by marsh wrens i n a previous year(s). This idea i s supported by the fact that i n both seasons there were more positive responses by banded (old) than by unhanded females (Table 13). Most unbanded females were probably young, inexperienced birds breeding for the f i r s t time. Additional evidence supporting the idea that previous experience plays an important role i n the redwing response to marsh wren nests comes from the 1978 data on distances of breeding female redwings from the nearest marsh wren nests. Because p o s i t i v e l y responding females (probably mainly 63 experienced birds) had marsh wren nests closer than birds that did not respond (Table 14) , the suggestion that old birds that i have marsh wrens nearby w i l l more l i k e l y respond to wrens and their nests because of their previous experience with marsh wrens i s reasonable. (The f a i l u r e to obtain si m i l a r r e s u l t s i n 1979 w i l l be discussed later.) This res u l t also o f f e r s an alternative explanation on more posit i v e responses by banded than unbanded females. If banded females had the nearest marsh wren nests closer, then more posit i v e responses by these birds might be a consequence of th e i r greater immediate exposure to marsh wrens as compared to unbanded females. However, the mean redwing-wren internest distances for banded and unbanded females were s i m i l a r i n both 1978 and 1979 seasons (Table 16). Therefore, more positive responses by old females are more l i k e l y associated with their previous experience with marsh wrens. As female redwings become older and more experienced, they should become more e f f i c i e n t i n defending their nests against marsh wrens. This i s supported by the higher breeding success recorded for females that responded p o s i t i v e l y to a wren nest (Table 15). A l t e r n a t i v e l y , the higher success of p o s i t i v e l y responding females might be explained by their nest location being farther from marsh wrens as compared with females that did not respond, especially since i t has been reported that redwings with marsh wrens nearby have a reduced breeding success (Burt 1970; Runyan 1979; t h i s study, see Chapter 4). But the redwing-wren internest distances of t r i a l females whose nests were 64 successful or were destroyed by predators were not s i g n i f i c a n t l y d i f f e r e n t (Table 16). Therefore, the higher success of p o s i t i v e l y responding females i s probably the re s u l t of their more e f f i c i e n t nest defense. 2. Effect of marsh wren density on redwing responses to marsh wren nests The r e s u l t s of t h i s study indicate that redwings have to learn to associate wren nests with marsh wrens. It seems l i k e l y that redwings also have to learn to recognize marsh wrens as potential nest predators. I t i s possible that t h i s learning process i n redwings could be influenced by marsh wren density such that at high wren densities ( i . e . high encounter rates) redwings might learn faster. On the other hand, at low wren densities, the rate of redwing-wren interactions should be low, and redwings should respond l e s s to marsh wrens and the i r nests. This hypothesis i s supported by the re s u l t s from 1978 and 1979-The density of marsh wrens was lower i n 1979 and the mean redwing-wren internest distance was larger (Table 16). In 1978 almost a l l redwings responded to wren nests i n l a t e t r i a l s , whereas i n 1979 the proportion of positive responses did not increase s i g n i f i c a n t l y from early to l a t e t r i a l s (Table 12). The proportion of positive responses i n early t r i a l s i n the two years are s i m i l a r (Table 12), so the differences i n the l a t e t r i a l s may be attributed to the degree of learning by female redwings i n the two seasons. So i n 1979 with low wren densities redwings did not have as much opportunity to learn to associate T a b l e 1 6 . E v a l u a t i o n o f d i s t a n c e s o f r e d w i n g n e s t s i n c l u d e d i n 1978 a n d 1979 t r i a l s f r o m t h e n e a r e s t m a r s h w r e n n e s t . C a t e g o r y M e a n d i s t a n c e (m) +SD N T - v a l u e D . F . T - p r o b a b i l i t y 1 9 7 8 , e a r l y t r i a l s n e s t s o f b a n d e d f e m a l e s n e s t s o f u n b a n d e d f e m a l e s 1 9 7 9 , e a r l y t r i a l s n e s t s o f b a n d e d f e m a l e s n e s t s o f u n b a n d e d f e m a l e s 1 9 7 8 , a l l n e s t s 1 979 , a l l n e s t s 1 9 7 8 , s u c c e s s f u l n e s t s 1 9 7 8 , p r e d a t e d n e s t s 1 9 7 9 , s u c c e s s f u l n e s t s 1 9 7 9 , p r e d a t e d n e s t s N e s t s i n c l u d e d i n 1 9 7 8 , e a r l y t r i a l s 1 9 7 8 , l a t e t r i a l s 1 9 7 9 , e a r l y t r i a l s 1 9 7 9 , l a t e t r i a l s 1 3 . 8 1 + 6 . 2 4 7 1 6 . 4 7 ± 9 . 8 8 19 * 1 9 . 4 9 ± 7 . 2 3 7 2 0 . 2 2 ± 5 . 9 8 23 1 4 . 4 8 ± 8 . 2 3 51 2 0 . 0 8 ± 1 0 . 3 46 1 3 . 2 3 ± 6 . 4 0 30 1 5 . 9 1 ± 1 0 . 6 5 19 2 1 . 1 1 ± 1 1 . 2 3 33 1 7 . 2 6 ± 7 . 2 3 11 1 5 . 7 6 + 9 . 0 1 26 1 3 . 1 5 ± 7 . 2 8 25 2 0 . 0 5 ± 6 . 1 7 30 2 0 . 1 3 ± 1 5 . 6 4 16 - 0 . 6 6 24 0 . 5 2 - 0 . 2 7 28 0 . 7 7 - 2 . 9 7 95 0 . 0 0 - 0 . 9 8 26 0 . 3 3 1 . 06 42 0 . 2 9 1 . 1 3 49 0 . 2 6 - 0 . 0 1 18 0 . 9 3 66 their nesting f a i l u r e s with marsh wrens and consequently were less responsive to marsh wren nests. An alternative explanation could be that the differences between the two years might be the resul t of smaller redwing-wren internest distances during l a t e t r i a l s i n 1978. However, in both years, redwing-wren internest distances were not s i g n i f i c a n t l y d i f f e r e n t for redwing nests included i n early and l a t e t r i a l s (Table 16). The high density of marsh wrens i n 1978 resulted i n a small mean redwing-wren internest distance (Table 16). Also, i n 1978 female redwings that responded p o s i t i v e l y to an experimental wren nest had a natural wren nest closer to t h e i r own nest than did females that responded negatively (Table 14). By contrast, i n 1979 the redwing-wren internest distance was larger (Table 16) and t h i s year there was no relationship between response to marsh wren nests and the distance from the nearest wren nest (Table 14). These results can be interpreted i n terms of di f f e r e n t responses by old experienced female redwings under low and high wren density conditions. At low wren densities, when distances between redwing and marsh wren nests are generally large, the rate of interactions between old, experienced females and marsh wrens should be low, and hence the response of these females to marsh wren nests i s l i k e l y to decrease. This can be seen from a smaller proportion of positive responses by banded (old) females i n 1979 (Table 13). On the other hand, at high wren densities, with decreasing redwing-wren distance, old experienced females should be able to quickly adjust to marsh wrens. This i s probably why almost a l l banded females responded 67 p o s i t i v e l y to wren nests i n 1978 (Table 13) and p o s i t i v e l y responding females (presumably old, experienced birds) had the nearest marsh wren nests s i g n i f i c a n t l y closer than females that ignored the marsh wren nests (Table 14). This explanation i s also supported by two additional f a c t s . F i r s t , more females responded p o s i t i v e l y to wren nests i n 1978 than i n 1979 early t r i a l s (60% of 25 females and 35% of 29 females responded p o s i t i v e l y i n 1978 and 1979, respectively; X 2 = 2.565, one-t a i l e d , p>0-05), presumably as a conseguence of generally smaller redwing-wren internest distances in 1978 (Table 16). Second, the rate of redwing-wren agonistic interactions was higher i n 1978 than i n 1979 (Pieman, unpublished data). Learning by redwings to respond to marsh wren nests may be influenced by two factors. F i r s t , i n dense vegetation, marsh wrens might destroy redwing nests without being, detected. Thus reduced v i s i b i l i t y could slow down redwing learning. Second, inexperienced female redwings might learn that marsh wrens are potential nest predators by seeing other experienced males or females chasing wrens. This should be presumably more important under high wren density conditions when the rate of redwing-wren int e r a c t i o n s should be high. 3. Implications of redwing aggression The examination of marsh wren nests by redwings and agonistic interactions between female redwings and marsh wrens should have negative e f f e c t s on redwing f i t n e s s through "aggressive neglect". This i s p a r t i c u l a r l y because such 68 a c t i v i t i e s a f f e c t female time and energy budgets. Moreover, by leaving t h e i r nests unguarded more frequently, female redwings provide more opportunities for marsh wrens to destroy t h e i r nests. However, because under high wren density conditions female redwings seem to respond to marsh wren nests more strongly and also chase marsh wrens more freguently (see Chapter 2), negative effects of these a c t i v i t i e s on th e i r reproductive output through the "aggressive neglect" are l i k e l y to become more pronounced in s i t u a t i o n s with high densities of marsh wrens. In addition, by v i s i t i n g and examining marsh wren nests and their v i c i n i t y , female redwings might influence the nesting success of marsh wrens in several ways. F i r s t , by chasing breeding female wrens away from t h e i r nests, redwings might encourage other wrens to destroy the unguarded wren nests ( i n t r a s p e c i f i c nest destruction i n marsh wrens was reported by Pieman 1977). Second, by frequently chasing marsh wrens, redwings may also increase the chances of nest abandonment by wrens. Third, redwings might destroy s t r u c t u r a l l y weak wren nests by landing on them. And fourth, redwing aggression towards breeding female marsh wrens may have a s i g n i f i c a n t impact on the time and energy budgets of female marsh wrens. This i s supported by the observed higher f a i l u r e rates of marsh wren nests with redwings nearby (Pieman, unpublished data; Bunyan 1979). Marsh wrens could reduce the impact of redwing aggression by: (1) building non-breeding "decoy" nests; (2) locating the 69 breeding nest i n dense vegetation where they might escape redwing attention; and (3) locating the breeding nest as far as possible from active redwing nests. Further work i s neccessary to test these hypotheses. 70 CHAPTER 4 Impact of l o n g - b i l l e d marsh wrens on reproductive success of red-winged blackbirds introduction A number of previous studies showed that predation i s the most important cause of nesting mortality in red-winged blackbirds (Smith 1943; Orians 1961, 1980; Robertson 1972, 1973a, b; Holm 1973; Caccamise 1976; Weatherhead and Robertson 1977). Most of the nesting mortality of redwings has been attributed to a number of various mammalian and avian predators (see Allen 1914, Bent 1958, Robertson 1972),, However, there i s a lack of any guantitative information on the impact of i n d i v i d u a l predator species. In the f i r s t Chapter I showed that i n my study marsh the majority, i f not a l l , l o n g - b i l l e d marsh wrens w i l l attack redwing nests with eggs and probably also small nestlings. In t h i s Chapter I w i l l examine the impact of lon g - b i l l e d marsh wrens on redwing nesting and show that, i n the study marsh at lea s t , marsh wrens are l i k e l y to be responsible for much of the redwing nesting mortality. Four major objectives of t h i s study are: (1) to i d e n t i f y various nest mortality factors reducing redwing reproductive output, and estimate their r e l a t i v e importance in two consecutive years; (2) to examine the d i s t r i b u t i o n of redwings and marsh wrens i n 71 the study marsh in r e l a t i o n to redwing-wren behavioral interactions; (3) to guantitatively evaluate the impact of marsh wrens on redwing reproductive success i n two years with high and low densities of marsh wrens; (4) to evaluate the extent of redwing exposure to marsh wren nest destruction,. Methods Direct observations of marsh wrens destroying eggs or k i l l i n g young of other species are rare under natural conditions. The fa c t that I observed many cases of egg pecking by marsh wrens during my experiments (Chapter 1) and that most wrens can be captured in traps baited with eggs (Chapter 1) demostrates that the response of marsh wrens to eggs i s well developed. However, t h i s shows nothing of the b i o l o g i c a l s i g n i f i c a n c e of t h i s behavior. I therefore studied the impact of marsh wren nest destruction on redwing reproductive success i n d i r e c t l y by evaluating success of redwing nests as a function of t h e i r distance from marsh wren nests (only successful nests; i . e . ones that fledged at least one young; and nests destroyed by predators were included i n th i s analysis, while nests that f a i l e d due to high tides or nest abandonment were excluded). I used marsh wren nests as an indicator of the presence of wrens i n the area. This method i s sa t i s f a c t o r y for the period between A p r i l and the f i r s t half of June, when marsh wrens generally retain the same t e r r i t o r i e s . Nearly a l l redwing nesting occurs 72 during t h i s period (Fig. 4). I conducted three surveys of marsh wren nests i n the study area at 3-week in t e r v a l s i n May and June (the peak of redwing nesting) 1976 and 1977. To reduce the time spent near active nests during these surveys, two observers simultaneously searched the marsh. In 1976 the regular checks of redwing nests were usually done by one observer, while in the 1977 season a l l checks were done by two observers. However, because marsh wren nests were also studied in 1977, the checks required approximately the same time as i n 1976. To measure the distance between a redwing nest and the nearest wren nest (either breeding or courtship nest), I used data from the marsh wren nest survey that had been done closest to the most vulnerable stages of redwing nests (laying, incubation, and f i r s t 5 days of l i f e of nes t l i n g s ) . I measured these distances on the maps with a rule r . Results 1. General breeding information In 1976 sixteen redwing males established t e r r i t o r i e s on an area of approximately 14 ha, and in 1977 seventeen males established t e r r i t o r i e s on 16 ha. Table 17 shows that there were no s i g n i f i c a n t differences i n mean t e r r i t o r y size and harem size between 1976 and 1977. Redwing females b u i l t the f i r s t nests early i n A p r i l and Fig. 4. Number of simultaneously active redwing nest throughout 1976 and 1977 seasons. 74 Table 17. General information on redwing breeding. two years 1976 1977 combined 2 Mean t e r r i t o r y s i z e (m ) 9202 8986 9080 (±SD; N) (±4357, 13) (±3988, 17) (±4080, 30) Range of t e r r i t . s i z e (m2) 3122-16932 3171-16196 3122-16932 Mean harem s i z e 4.9 4.7 4.8 (±SD; N) (±1.6, 16) (±1.9, 17) (±1.8, 33) Range of harem s i z e 2-8 1-8 1-8 Tot a l number of nests 211 188 399 Percent (N) abandoned nests 21.8 (46) 20.8 (39) 21.3 (85) Percent (N) nests destroyed by high tides 18.5 (39) 0 (0) 9.8 (39) Percent (N) nests destroyed by predators 40.8 (86) 50.5 (95) 45.4 (181) Percent (N) successful nests 18.9 (40) 28.7 (54) 23.5 (94) Number young fledged 91 124 215 75 the l a s t nests i n l a t e June,. The number of active nests present i n the study area throughout the 1976 and 1977 i s shown i n Fig. 4. The major factor reducing reproductive success of redwings i n both years was nest predation (Table 17). High tides were responsible for a s i g n i f i c a n t number of nesting f a i l u r e s i n 1976 but not in 1977 (Table 17). This factor also p a r t i a l l y explains the higher figures for the percentage of successful nests and number of young fledged i n 1977 (Table 17). The o v e r a l l nesting success increased as the season progressed in 197 6, but i t decreased i n May and then again increased i n June 1977 (Table 18). 2- Distrib ution of redwings and marsh wrens In both years blackbirds defended a l l areas i n the study marsh that contained vegetation suitable for nesting. Because nearly a l l marsh wren nests (both courtship and breeding nests) were b u i l t within the vegetation defended by male redwings, t e r r i t o r i e s of wrens overlapped with those of male redwings. In 1976, however, three surveys of marsh wren nests, conducted during peak redwing breeding a c t i v i t i e s , showed a pronounced s p a t i a l segregation of nesting s i t e s of the two species. Using the information on the presence or absence of redwing and marsh wren nests on quadrats 20 x 20 m, I evaluated the d i s t r i b u t i o n of nesting s i t e s of the two species (Table 19). The nests of redwings and marsh wrens were not di s t r i b u t e d randomly with respect to each other. Hurlbert's c o e f f i c i e n t (Hurlbert 1969) computed for the same sets of data showed a s i g n i f i c a n t negative 76 Table 18. Nesting success of redwings related to the time i n a season (dates r e f e r to the i n i t i a t i o n of nest b u i l d i n g ) . Only successful nests and nests destroyed by predators were were included i n t h i s analysis. Number (%) nests 1976 1977 Month successful predated t o t a l successful predated t o t a l A p r i l 4 (17 .4) 19 (82 • 6) 23 22 (46.8) 25 (53.2) 47 May 17 (30 .4) 39 (69, • 6) 56 17 (24.6) 52 (75.4) 69 June 19 (40 .4) 28 (59, • 6) 47 15 (45.5) 18 (54.5) 33 Total 40 86 126 54 95 149 Table 19. I n t e r s p e c i f i c a s s o c i a t i o n between redwing and marsh wren nest locations. No . quadrats with nests of Chi-square Date of both no No. with Yates' survey wrens redwings species species quadrats correction * C8 1976 May 1 111 38 6 119 274 ** 16.71 -0.681 May 15 105 44 5 121 275 ** 20.57 -0.745 June 1 145 33 8 114 298 ** .17.05 -0.615 1977 May 7 192 29 24 123 368 *** 3.97 -0.228 Hurlbert's c o e f f i c i e n t of i n t e r s p e c i f i c association p<0.001 *** p<0.05 78 association for the d i s t r i b u t i o n of redwing and marsh wren nests in a l l time periods. The s p a t i a l segregation of nests of redwings and marsh wrens was much less pronounced i n 1977 (Table 19), when the maximum number of marsh wren nests was nearly twice that i n 1976 (within the same area there were 610 marsh wren nests (both breeding and courtship nests) as compared with 349 nests i n 1976). The higher density of marsh wrens i n 1977 resulted i n their presence i n nearly a l l parts of the marsh and consequently also i n the reduced s p a t i a l segregation of nesting s i t e s of the two species (Fig. 5). However, i n spite of th i s higher density of marsh wren nests, there was s t i l l a s i g n i f i c a n t negative i n t e r s p e c i f i c association i n d i s t r i b u t i o n of redwing and wren nests during the period of the highest redwing density (Table 19) . The degree of s p a t i a l segregation of redwing and marsh wren nesting s i t e s appears to be influenced by vegetation structure. I noticed that the greatest s p a t i a l segregation occurred i n areas of the marsh where density and height of c a t t a i l vegetation varied greatly. Redwing nests were b u i l t i n sparse c a t t a i l and wren nests i n denser vegetation. Where there were r e l a t i v e l y homogeneous dense stands of c a t t a i l , the segregation of nesting s i t e s between the two species was much le s s pronounced. 79 Fig. 5. Di s t r i b u t i o n of redwing and wren nests on May 8-10, 1976 (low wren density) and 1977 (high wren density). 1977 80 3. Impact of marsh wrens on redwing nesting success The impact of marsh wrens on redwing nesting success can be evaluated i n d i r e c t l y from the success of redwing nests i n r e l a t i o n to distance from the nearest wren nests. I therefore grouped redwing nests into 5 or 10 m i n t e r v a l s according to thei r distance from wren nests. Because there were few nests more than 50 m from the nearest wren neighbor, I assigned a l l such nests to one group. I calculated the mean inter-nest distance for each group and correlated t h i s with the proportion of successful nests per group (a successful nest was one from which at least one young was fledged). Nesting success of redwings decreased s i g n i f i c a n t l y with proximity to marsh wren nests in both 1976 and 1977 (Table 20). However, slopes of the regression l i n e s for the two years d i f f e r s i g n i f i c a n t l y (ANCOVA, test of common slopes, F=7.79; d.f.=1,7; p<0.05; see also Fig. 6). Thus, in spite of the fact that wren nests were more abundant i n 1977, the probability of redwing nesting success increased at a greater rate with increasing distance from wren nests, as compared with 1976.. Data on the average distances between redwing and wren nests provide an opportunity to compare nests with various h i s t o r i e s as well as the 1976 and 1977 seasons. In both years successful nests were s i g n i f i c a n t l y farther from wrens than were nests destroyed by predators, presumably marsh wrens (Table 21). The difference, however, was les s pronounced in 1977 than i n 1976, apparently because of the higher density of marsh wren nests in t h i s season. 81 Table 20. Nesting success of redwing nests located at various distances from nearest marsh wren nests. Ranges of Proportion of redw . - wren Mean successful Number of Year distances (m) distance (m) nests nests 19 76 1 - 10 8.02 0.13 30 11 - 20 15.78 0.32 38 21 - 30 23.75 0.32 28 31 - 40 37.42 0.50 12 41 - 50 47.67 0.67 6 51 - 100 66.67 0.67 6 1977 1 - 5 4.06 0.28 18 6 - 10 8.03 0.16 38 11 - 15 13.00 0.39 57 16 - 20 17.50 0.48 25 21 - 35 23.73 0.73 11 Note: For 1976 data r=0.95, df=4, p<0.01. For 19 77 data r=0.92, df=3, p<0.05. 82 Abandoned redwing nests were closer to the nearest wren neighbors as compared with a l l other nests in which eggs were l a i d (this difference i s nearly s i g n i f i c a n t ; Table 21). This indicates that nest abandonment i s more l i k e l y i n less s u i t a b l e locations near wrens, The main difference between the two seasons i s much smaller average distance between redwing and wren nests i n 1977 (Table 21). This i s apparently a conseguence of higher density of marsh wren nests i n 1977 than i n 1976 (Fig.. 5) -A. Impact of marsh wrens on fledging success of successful redwing nests Most nests that fledged at least one young suffered p a r t i a l nesting losses (Table 22). We might therefore expect such nests to lose fewer young or eggs the farther they were located from the nearest wren nests. In 1976 p a r t i a l nesting losses per nest decreased with increasing redwing-wren distance (Table 22). The c o r r e l a t i o n between the proportion of eggs fledging young and the mean distance of redwing nests from wren nests i s highly s i g n i f i c a n t (r= 0.96, d.f.= 4, p<0.005). The relationship i n 1977 (Table 22), however, was not s i g n i f i c a n t (r=0.47, d.f.=3, p>0.2). In t h i s season only redwing nests b u i l t farther than 20 m from wren nests apparently suffered less from p a r t i a l nesting losses than did those located closer to wren nests. Table 21. Mean distances between redwing and marsh wren nests i n 1976 and 1977 seasons. Mean distance (m) 1SD (sample size) Year a l l nests with eggs abandoned * nests ** P successful nests depredated nests *** P 19 76 22.5115.5 18.6113.9 >0.1 29.3118.9 18.9112.4 < 0.005 (155) (43) (39) (82) 1977 12.015.5 10.215.1 > 0.05 14.116.1 11.214.8 <0.005 (156) (32) (54) (95) Eggs were not l a i d i n these nests Level of s i g n i f i c a n c e for differences between a l l nests with eggs and abandoned nests attained using t - t e s t . Level of s i g n i f i c a n c e for differences between successful and predated nests attained using t - t e s t . 00 u> Table 22. Losses from successful redwing nests i n r e l a t i o n to t h e i r distance from nearest marsh wren nests. Observed frequency Proportion Redw. - wren Mean No. eggs (nestlings) l o s t Mean fledged of Sample Year distances (m) distance (m) 0 1 2 3 4 reduction a l l eggs s i z e 1976 1 - 10 8.25 2 1 1 1.75 0.53 4 1 1 - 2 0 16.46 3 5 1 2 1 1.42 0.59 12 21 - 30 23.72 2 5 - 2 - 1.22 0.67 9 31 - 40 37.33 1 3 2 - - 1.17 0.67 6 41 - 50 47.75 - 3 1 - - 1.25 0.69 4 51 - 100 70.63 2 2 - - - 0.50 0.83 4 1977 1 - 5 4.10 1 3 1 - - 1.00 0.71 5 6 - 1 0 8.25 2 4 - 1 - 1.00 0.70 7 11 - 15 12.66 3 12 3 3 - 1.29 0.59 21 16 - 20 17.29 4 4 4 1.00 0.69 12 21 - 31 23.79 2 5 - - - 0.71 0.81 7 85 B. I§£act of marsh wrens on redwing nestling losses Asynchronous hatching of redwing nestlings (Holcomb and Twiest 1971) r e s u l t s in s i g n i f i c a n t body weight differences within broods (the weight of the youngest nestling within i t s f i r s t 5 days of l i f e i s usually 20 - 50 % less than that of the oldest one at that time; Pieman, unpublished data). Because older and thus larger nestlings may survive wrensf pecking (see Chapter 1), the younger, more vulnerable nestlings should more frequently be the victims of wrens* attacks. This can be tested from the 1976 information on p a r t i a l losses from successful redwing nests with i n d i v i d u a l l y marked nestlings. When one or more young were l o s t , the younger, smaller nestlings were the victims i n 13 of 14 nests. In the fourteenth nest the second smallest young disappeared while the other nestlings fledged. C. Indirect impact of marsh wrens through redwing aggressive neglect Red-winged blackbirds chase marsh wrens from the v i c i n i t y of t h e i r nests. During the two seasons, while searching the study area f o r redwing nests, I observed accidentally a t o t a l of 37 chases; 17 by female and 20 by male redwings. The chases usually covered l e s s than 7 meters and lasted a few seconds (the length and duration of these chases may have been influenced by the presence of an observer). In addition, a large number of redwing-wren interactions were observed by three d i f f e r e n t observers i n 1978 (see Chapter 2). Redwing aggression towards marsh wrens was also observed by Nero (1956b), Orians and 86 Willson (1964), Burt (1970), and Verner (1975). This information indicates that, i n addition to the d i r e c t destruction of nests, marsh wrens might also a f f e c t redwing reproductive success through "aggressive neglect". By chasing marsh wrens, redwing females reduce the time available for brooding and foraging and increase t h e i r own energy demands and those of t h e i r nestlings (heat l o s s ) . This i n t e r s p e c i f i c aggression should have negative e f f e c t s on reproductive rates of birds breeding near wrens. To test for t h i s p o s s i b i l i t y , I compared clutches i n redwing nests that had the nearest wren nests nearby or farther away. In 1976 redwing nests with complete clutches that were more than 20 m from wren nests had, on the average, 8% larger clutches than the rest (Table 23). In 1977, however, the difference (12%), though larger, was not s i g n i f i c a n t , probably because of the small number of nests farthest from wrens (Table 23). D. The extent of redwing exposure to wren nest destruction Evaluation of nest destruction by marsh wrens as a possible s e l e c t i v e force acting upon redwing reproductive strategy requires knowledge of the proportion of redwing females that are exposed to wren predation. Furthermore, the o v e r a l l production of redwing young at various distances from wrens should also provide useful information on the i n t e n s i t y of s e l e c t i o n . In 1976, 80% of a l l redwing nests that contained eggs (N = 120) were b u i l t within 30 m of the nearest wren nests, where their 87 Table 23. E f f e c t of short distances between redwing and marsh wren nests on redwing c l u t c h s i z e . Observed frequency Year Redw. - wren distances (m) 2 Clutch 3 s i z e 4 5 Mean clutch Sample si z e 1976 1 - 2 0 6 28 27 1 3.37 62 21 - 100 2 18 36 1 * 3.63 57 1977 1 - 2 0 21 - 31 13 46 3 40 6 1 3.29 ** 3.67 100 9 * t=2.22, df=117, p<0.05 ** t=1.57, df=107, p>0.1 88 success was most affected by marsh wrens. Fifteen percent of redwing nests were located between 30 and 50 m, where the incidence of wren nest destruction was s i g n i f i c a n t l y lower, and only 5% were farther than 50 m, where the impact of wrens was very small (Fig. 6) . S i m i l a r l y , from a t o t a l of 89 young, 61% were fledged from nests located within 30 m of the nearest wren nests, 28% fledged from nests with wrens 30 to 50 m f a r , and only 11% fledged from nests with suitable locations that were farther than 50 m from wrens (Fig. 6) . In 1977, when the average redwing-wren distances were much smaller (see Table 21), the pattern was similar. In this year, 77%, 21%, and 2% of the redwing nests that contained eggs were located within 1-15 (highly unsuitable locations) , ,16-25 (intermediate quality l o c a t i o n s ) , and 26-32 (suitable locations) meters, respectively, of the nearest wren nests (Fig. 6). Also out of 124 young most (60%) fledged from nests located within 15 m of the nearest wren nests, but only 8% fledged from nests with wren nests farther than 25 m (Fig. 6). 89 Fig. 6. Proportion of redwing nests b u i l t and proportion of redwing young fledged at various distances from the nearest wren nests i n 1976 and 1977 seasons. Quality of nest locations i n each season Is indicated by the regression l i n e that shows proportion of successful redwing nests at different distances from marsh wren nests. (A c W V) u u c o o o. o Y- 0.009 00* 0.124 V. of all nests % of all fledglings \ \ \ •xQ-~ 0 10 20 30 40 50 60 70 80 90 Ho 30 20 V) cn c • o at o 10 W c H0 =§ Distance (m) c .8 .6 V) o u u D c o .4 .2 J o S .0 1977 Y= 0.026 ( X ) * 0.065 U0 h 3 0 £ x V. of all nests o V. of all fledglings 10 15 20 25 30 — i — 35 in cn c r 2 0 o .»-• w c ho I a o Distance (m) 90 Discussion 1. Impact of marsh wrens on redwing nesting success This study strongly suggests that in the study marsh long-b i l l e d marsh wrens are responsible for most of the redwing nesting mortality. This i s supported by several kinds of information: (1) In both seasons there was a s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n between redwing nesting success and the distance of redwing nests from the nearest wren nest. (2) In 1976 the proportion of young fledged from successful nests s i g n i f i c a n t l y increased with increasing distance from marsh wren nests. Lack of significance of t h i s c o r r e l a t i o n i n 1977 was probably caused by higher density of marsh wrens that resulted i n low v a r i a b i l i t y i n exposure of most redwing nests to wrens i n that year. (3) Nestling losses usually a f f e c t the smallest young that are most vulnerable to marsh wrens. A s i g n i f i c a n t c o r r e l a t i o n between the number of young fledged from successful nests and t h e i r distance from wren nests i n 1976 suggests that redwing nestlings that disappeared were either k i l l e d by wrens or died of starvation, possibly i n conseguence of aggressive neglect by redwing females. The e a r l i e r suggestion that p a r t i a l nestling losses are caused by starvation because predators usually take a l l nestlings from a nest 91 (Orians 1973) thus probably does not hold for redwings and other small passerines nesting sympatrically with marsh wrens that can k i l l and remove small nestlings one at a time, but that are e a s i l y expelled from the nest i f the mother returns. Marsh wrens reduce redwing nesting success d i r e c t l y by breaking eggs and k i l l i n g nestlings (see Chapter 1) and probably also i n d i r e c t l y through aggressive neglect by redwing females that chase wrens. Because the reduced success of redwing nests located near wren nests i s presumably the result of both d i r e c t and i n d i r e c t impacts of marsh wrens, i t i s not possible from available data to quantitatively evaluate the r e l a t i v e s i g n i f i c a n c e of these factors. The i n d i r e c t impact of wrens, however, appears to be r e f l e c t e d i n smaller clutches l a i d i n redwing nests with wren nests nearby than in those with wren nests farther away (Table 23). But since females breeding for the f i r s t time lay smaller clutches than older birds (Crawford 1977), an alt e r n a t i v e explanation could be that females breeding near wrens are young, inexperienced birds. The se l e c t i o n of unsuitable nest s i t e s by these birds then might r e s u l t from their l a t e r nesting when a l l or most suitable s i t e s are defended by older females and lack of previous experience with marsh wrens. But the fa c t that a majority of a l l females had their nests near wrens (Fig- 5) suggests that the observed differences i n clutch size were more l i k e l y the r e s u l t of aggressive neglect by redwing females. Another possible explanation requiring further investigation i s that the observed 92 smaller clutches i n redwing nests with wren nests nearby may have been caused by undetected egg-losses from these nests. The comparison of the impact of marsh wrens on redwing nesting success i n the two seasons indicated that i n 1977, i n spite of t h e i r much higher densities, marsh wrens had a s l i g h t l y smaller impact on redwing nesting than i n 1976 (of a l l nests, excluding those abandoned and destroyed by high tides, 32% and 36% were successful i n 1976 and 1977, respectively; i n both seasons 2.3 young fledged, on the average, per successful nest). Possible explanations of this observation are: (1) In spite of higher marsh wren nest density i n 1977 the number of wrens could be s i m i l a r to that in 1976 (the distance from the nearest wren nest thus would not adeguately indicate the distance from marsh wrens). My f i e l d experience suggests that the density of wren nests may be used as an index of the density of marsh wrens. These data are now being analyzed,. (2) The vegetation may have been more suitable for redwings i n 1977. The presence of patches of sparse vegetation, presumably more defendable for redwings, may have increased redwing nesting success. I do not have guantitative data to examine t h i s , however, I did not notice any s t r i k i n g differences i n \ vegetation structure in the marsh i n 1976 and 1977.. (3) The mild 1976/77 winter and generally lower nesting success i n 1976 could have resulted i n a higher proportion of older redwing females i n the 1977 breeding population. 93 These birds could have had higher nesting success because of their previous experience with marsh wrens. Testing of t h i s p o s s i b i l i t y requires data on i n d i v i d u a l l y marked females from successive years and i s impossible here. (4) A higher degree of clumping by redwing females in 1977 could r e s u l t i n their r e l a t i v e l y high success. However, the degree of redwing clumping, as indicated by distances between the nearest simultaneously active nests (see Chapter 6; Table 33), was sim i l a r i n both seasons. (5) The e f f i c i e n c y of redwing nest defense might increase with increasing wren density,. I t i s possible that redwing aggression may become more s p e c i f i c a l l y directed towards marsh wrens under high wren density conditions when redwing - wren encounters should become more freguent. This i s supported by data on stronger responses of redwings to wren nests i n a year with a higher wren density (see Chapter 3). (6) The rates of i n t r a s p e c i f i c and i n t e r s p e c i f i c nest destruction by wrens may change with marsh wren density. I f redwing density i s constant, then redwing interference should present a stronger disturbance to the breeding wrens at low wren density (the rate of i n t r a s p e c i f i c i nteractions between wrens should be low at low wren density) . At high wren density, interference from redwings may be r e l a t i v e l y unimportant and i n t r a s p e c i f i c nest destruction by marsh wrens (Pieman 1977) might reduce i n t r a s p e c i f i c 94 competition. This r e l a t i v e s h i f t from i n t r a s p e c i f i c to i n t e r s p e c i f i c nest destruction at low wren density might s i g n i f i c a n t l y influence redwing nesting success even under very low wren density conditions. Theoretical implications of t h i s s h i f t are discussed in Chapter 10. The proposed hypotheses are not mutually exclusive. The most l i k e l y explanation i s that the s h i f t from i n t e r s p e c i f i c to i n t r a s p e c i f i c nest destruction i n marsh wrens, more e f f i c i e n t redwing nest defense under high wren density conditions, and possibly also the higher proportion of older, experienced females within the population may have resulted i n a r e l a t i v e l y high redwing nesting success i n 1977. These hypotheses reguire testing. 2- Interactions between redwings and marsh wrens as a possible cause of segregation of t h e i r nesting s i t e s The s p a t i a l segregation of redwing and marsh wren nesting s i t e s i s associated with differences i n nest s i t e s e l e c t i o n . Redwings seem to be more abundant i n sparse vegetation while marsh wrens defending t e r r i t o r i e s near redwings prefer denser c a t t a i l s (but i n situations without blackbirds, marsh wrens also commonly occur i n sparse vegetation). The nest s i t e segregation of these species along the vegetation density gradients apparently re s u l t s from: (1) Destruction of redwing nests by wrens (also suggested by Orians and Willson 1964, Verner 1975); 95 (2) Redwing aggression towards marsh wrens (also observed by Nero 1956b, Orians and Willson 1964, Burt 1970, Verner 1975) ; (3) Inconspicuousness, small s i z e , and other adaptations of marsh wrens such as shortened wings and high manoeuvreability (Welter 1935, Hamilton 1961) that f a c i l i t a t e the approach to and destruction of nests, and easy escape of wrens i n dense vegetation; (4) Large size of blackbirds that can aggressively exclude marsh wrens but that also makes blackbirds l e s s e f f i c i e n t i n excluding small wrens from dense vegetation; (5) Presumably more e f f i c i e n t defense of nests by redwings i n sparse vegetation (higher success of redwing nests i n sparse vegetation has been reported by Weatherhead and Robertson 1977; see also Chapter 5) . although I have no d i r e c t evidence for competition, i t i s plausible that interference between red-winged blackbirds and lo n g - b i l l e d marsh wrens has evolved to reduce competition between these species for some l i m i t i n g resource,. This interference r e s u l t s i n p a r t i a l i n t e r s p e c i f i c t e r r i t o r i a l i t y between redwings and marsh wrens ( i . e . male t e r r i t o r i e s overlap but nesting s i t e s of the two species are s p a t i a l l y segregated) . In addition, interference between redwings and marsh wrens could be i n t e n s i f i e d by the presence of yellow-headed blackbirds that aggressively exclude redwings from some parts of the habitat i n some marshes (Orians and Willson 1964, Weller and Spatcher 1965, Voigts 1973, Verner 1975). The fac t that within one habitat 96 overlap i n nest s i t e selection of redwings and marsh wrens i s much greater than that of yellowheads and marsh wrens (Burt 1970) indicates that yellow-headed blackbirds may force redwings into habitats near wrens. Yellowheads thus might reduce the degree of s p a t i a l segregation between redwing and marsh wren nesting s i t e s . 97 CHAPTER 5 West s i t e quality and i t s influence on reproductive success of red-winged blackbirds Introduction One of the most important factors influencing the evolution of reproductive strategies i n birds i s predation (e.g. Crook 1964, Lack 1968). High predation rates might drive the evolution of c o l o n i a l nesting i n some birds (e.g. Brown and Orians 1970, Alexander 1974, Hoogland and Sherman 1976), and i n other species they might favor avoidance through nest dispersion and camouflage (e.g. Lack 1968). The evolution of these a l t e r n a t i v e strategies i s most l i k e l y determined by the nature of the predation and the r e l a t i v e e f f i c i e n c i e s of various reproductive t a c t i c s i n reducing the impact of predators on the reproductive success of a given species. Predation i s the most important nest mortality factor i n marsh-nesting red-winged blackbirds (e.g. Ricklefs 1969). In my study marsh, long-billed marsh wrens are the major cause of redwing nesting f a i l u r e s and hence probably present an important force driving the evolution, of various adaptations i n redwing reproductive t a c t i c s (see Chapter 4). In t h i s Chapter I w i l l examine the nest s i t e s e l e c t i o n by redwings and i t s adaptive value in terms of interference between redwings and marsh wrens. The main purpose i s to evaluate reproductive success of female 98 redwings in r e l a t i o n to: (1) guality of nest placement as r e f l e c t e d by the height of nests above the ground, and nest concealment; (2) gu a l i t y of nest s i t e s (as r e f l e c t e d by the nest height and nest concealment) i n terms of redwing int e r a c t i o n s with marsh wrens. Methods During 1977-1979 I measured two nest s i t e parameters for a l l redwing nests: (1) nest height from the ground l e v e l to the nest base (nest height was measured to the nearest 5 cm); and (2) nest concealment, as ref l e c t e d by the density of vegetation immediately surrounding the nest. Data on nest concealment I obtained using a 50 cm long and 3 cm wide white s t i c k , on which I painted, in a l i n e , 20 red spots (diameter 1 cm) evenly d i s t r i b u t e d on the s t i c k . I measured nest concealment by placing this s t i c k on the top of redwing nests and counting the number of red spots which were not covered by the vegetation from a 2 m distance (only complete spots were counted). Because the density of vegetation around redwing nests frequently varies, I always measured nest concealment from the sparsest and hence presumably the most vulnerable side to predators. Counts of 0-4, 5-9, and 10 or more spots were considered to indicate good, intermediate, and poor nest concealment, respectively. To establish whether the data on nest concealment which were obtained in a way described above provide a good estimate 99 of the vegetation density, i n 1979 I took 2 kinds of measurements on each nest. During one I counted the number of spots on a 50 cm s t i c k (maximum 20 spots), and, in order to obtain the other one, I used 150 cm long s t i c k with 60 spots. Using t h i s longer sti c k I, therefore, measured the density of vegetation located also farther away from the nest. To evaluate the accuracy of the estimates on nest concealment i n terms of the density of vegetation farther from the nest, I correlated the number of spots counted using the two techniques. Because there i s a highly s i g n i f i c a n t positive c o r r e l a t i o n between the number of spots on 50 cm and 150 cm long s t i c k s for the same nests (r=0.75, d.f.=157, p<0.0001), I conclude that the data on nest concealment obtained using the 50 cm s t i c k adequately r e f l e c t the density of nearby vegetation. In 1977 and 1978 I also estimated the density of vegetation in an area of approximately 5 m around redwing nests. Vegetation density around nests was estimated as sparse, medium, or dense when they were f i r s t found. To eliminate possible va r i a t i o n i n nesting success of redwings due to di f f e r e n t types of vegetation used as a nesting substrate, i n the following analyses I included data on redwing nests b u i l t in c a t t a i l s only. 100 Results and Discussion 1. Nest height Redwing nests b u i l t i n c a t t a i l s were placed 25-90 cm above the ground, with most nests (62,3 %) being 50-65 cm above the ground (Table 24). In spite of t h i s great degree of v a r i a t i o n , there i s no s i g n i f i c a n t relationship between nesting success and nest height for i n d i v i d u a l years or for for a l l years combined (Table 24). I conclude that nest height i s not important feature influencing redwing nesting success i n t h i s marsh. However, these data do not provide a s u f f i c i e n t proof that nest height does not play an important role i n redwing nesting i n terms of redwing-marsh wren intera c t i o n s . This i s because any re l a t i o n s h i p between nest height and success might have been masked by increased redwing nest success with distance from marsh wrens. To examine this p o s s i b i l i t y , I compared success of redwing nests, which had the nearest marsh wren nest within 15 m or farther away, i n r e l a t i o n to nest concealment. It appears that, regardless of the distance of redwing nests from the nearest marsh wren nest, there i s no relationship between nest height and nesting success (Table 25). Hence, t h i s r e s u l t supports the conclusion that nest height i s not an important factor influencing nesting success of redwings i n my marsh. Nest height has also been examined as a parameter possibly influencing redwing reproductive success by several other 101 Table 24. Success of redwing nests i n r e l a t i o n to nest height. Only successful nests and nests destroyed by predators were included. Nest Number (%) nests height 2 Year (cm) successful f a i l e d Total X P * 1977 25-45 5 (35.7) 9 14 50-55 13 (29.6) 31 44 60-65 23 (44.2) 29 52 3.39 >0.15 70-80 9 (27.3) 24 33 Total 50 93 143 1978 25-45 10 (43.5) 13 23 50-55 21 (46.7) 24 45 60-65 11 (47.8) 12 23 0.65 >0.9 70-90 9 (56.3) 7 16 Total 51 56 107 1979 30-45 15 (65.2) 8 23 50-55 30 (62.5) 18 48 60-65 23 (54.8) 19 42 2.52 >0.3 70-90 22 (48.9) 23 45 Total 90 68 158 A l l 25-45 30 (50.0) 30 60 years 50-55 64 (46.7) 73 137 60-65 57 (48.7) 60 117 1.10 >0.7 70-90 40 (42.6) 54 94 Total 191 217 408 two-tailed test 102 T a b l e 2 5 . N e s t i n g s u c c e s s o f r e d - w i n g e d b l a c k b i r d s w i t h m a r s h w r e n s c l o s e a n d f a r t h e r a w a y , a s r e l a t e d t o n e s t h e i g h t . D a t a o n s u c c e s s f u l . n e s t s a n d n e s t s d e s t r o y e d b y p r e d a t o r s f r o m 1 9 7 7 - 1 9 7 9 w e r e c o m b i n e d . N e s t N e a r e s t h e i g h t N u m b e r (%) n e s t s w r e n n e s t ( cm) s u c c e s s f u l f a i l e d T o t a l 2 * X P l - 1 5 m 4 0 - 4 5 9 ( 4 2 . 9 ) 12 21 5 0 - 5 5 2 8 ( 3 5 . 0 ) 5 2 80 6 0 - 6 5 2 8 ( 4 0 . 0 ) 42 70 0 . 6 4 > 0 . 8 7 0 - 9 0 2 4 ( 3 7 . 5 ) 4 0 6 4 T o t a l 89 146 2 3 5 b e y o n d 15m 2 5 - 4 5 2 1 ( 5 3 . 9 ) 18 39 5 0 - 5 5 36 ( 6 3 . 2 ) 2 1 57 6 0 - 6 5 29 ( 6 1 . 7 ) 18 4 7 1 . 3 8 > 0 . 5 7 0 - 9 0 16 ( 5 3 . 3 ) 14 30 T o t a l 102 71 1 7 3 t w o - t a i l e d t e s t 103 studies. Goddard and Board (1967) and Weatherhead and Robertson (1977a) reported a negative c o r r e l a t i o n between nest height and redwing nesting success, but Meanley and Webb (1963), Holcomb and Twiest (1968), and Holm (1973) reported a p o s i t i v e c o r r e l a t i o n . However, since some of the above authors included in t h e i r analyses nests b u i l t i n d i f f e r e n t types of vegetation (e.g. Holcomb and Twiest 1968, Weatherhead and Robertson 1977a), i t i s possible that i n those studies redwing nest success might be related to nest substrate rather than to height se (Francis 1971). In any case, the issue i s not clear from the l i t e r a t u r e . 2. Nest concealment The presence of large predators, whose impact marsh-nesting redwings could not a c t i v e l y reduce through cooperation i n nest defense, should be reflected i n higher success of well concealed nests that are r e l a t i v e l y l e s s accessible to such predators. On the other hand, i f predators were generally small and/or did not present a danger to the breeding birds themselves, they should, t h e o r e t i c a l l y , select f o r cooperation i n nest defense between the breeding birds. One way of improving such cooperation i n nest defense between females would be to breed i n more open, and hence less concealed situations, which would allow better v i s u a l contact between neighbors, thereby improving mutual nest protection. Therefore, i f the nature of predation favored the evolution of the cooperative strategy between females, nests b u i l t i n more open situations should be more successful than 104 well concealed nests. Because female redwings apparently reduce nest predation by marsh wrens through t h e i r cooperation i n nest defense with neighbors (see Chapter 6), I predict that they should have higher success i n more open and hence more defendable s i t u a t i o n s . In my marsh, there was a s i g n i f i c a n t negative r e l a t i o n s h i p between nest concealment and nesting success i n 1977 (Table 26). Thus redwing nests which were b u i l t in more open sit u a t i o n s , and hence could be more e a s i l y found by any predator, were more freguently successful than well concealed nests, This r e s u l t i s consistent with the hypothesis that predation pressures have been favoring cooperation between females as a means of reducing nest predation rates on their nests. This rela t i o n s h i p , though not s i g n i f i c a n t , i s also evident from the 1979 data (Table 26). In 1978, however, there was no r e l a t i o n s h i p between nest concealment and success. Evaluation of redwing nesting success i n r e l a t i o n to the density of vegetation i n the area within 5 m from nests yielded s i m i l a r r e s u l t s (Table 27). In 1977, success of female redwings whose nests were b u i l t i n sparse vegetation was s i g n i f i c a n t l y higher than that of females whose nests were i n dense vegetation. But there was no relationship between nesting success and the density of vegetation around redwing nests i n 1978 (Table 27). 105 Table 26. Role of nest concealment i n reproductive success of red-winged blackbirds. Only successful nests and nests destroyed by predators were included. „ , Number (%) nests Number 2 * Year spots successful f a i l e d Total X p 1977 0-4 10 (22.7) 34 44 5-9 29 (37.7) 48 77 5.33 <0.05 10-13 11 (50.0) 11 22 Total 50 93 143 1978 0-4 13 (50.0) 13 26 5-9 28 (45.9) 33 61 0.17 >0.9 10-16 9 (50.0) 9 18 Total 50 55 105 1979 0-4 47 (56.6) 36 83 5-9 34 (54.8) 28 62 0.92 >0.25 10-12 9 (69.2) 4 13 Total 90 68 158 A l l years 0-4 70 (45.8) 83 153 combined ^ 91 (45.5) 109 200 1.54 >0.15 10-16 29 (54.7) 24 53 Total 190 216 406 one-tailed test 106 Table 27. Relationship between redwing nesting success and vegetation density within 5 m of redwing nests. Only data on successful nests and nests destroyed by predators were included. Because the density of vegetation was estimated when a nest was f i r s t found, the obtained estimates are not biassed by the outcomes of nesting attempts. Number (percent) nests Year density Successful Destroyed T o t a l 1977 Sparse 15 (51.7) 14 (48.3) 29 Medium 31 (36.9) 53 (63.1) 84 Dense 4 (13.3) 26 (86.7) 30 Tot a l 50 93 143 1978 Sparse 30 (47.6) 33 (52.4) 63 Medium 13 (52.0) 12 (48.0) 25 Dense 7 (41.2) 10 (58.8) 17 Tot a l 50 55 105 Note: For 1977 data : 2 * =9.89; p<0.005 ( 1 - t a i l e d t e s t ) . For 1978 data: X 2=0.48; p> 0.15 ( 1 - t a i l e d t e s t ) . 107 A. Why. did nest concealment play d i f f e r e n t roles i n the three seasons? The fact that the density of vegetation around nests and nest concealment played a s i g n i f i c a n t role in nest success of redwings i n 1977 suggests that vegetation structure i s an important parameter determining quality of a nest s i t e . Therefore, i t i s necessary to explain the f a i l u r e to obtain a si m i l a r s i g n i f i c a n t r e l a t i o n s h i p i n 1978 and 1979. The higher success of poorly concealed redwing nests cannot be explained i n terms of food resources (almost a l l foraging i s done away from t e r r i t o r i e s ) , or guality of nest support (there were no f a i l u r e s due to poor support). Hence I conclude that the role of vegetation structure, as seen i n 1977, can probably be explained only i n terms of redwing interactions with marsh wrens. In the following part I w i l l discuss two situations which might be responsible for di f f e r e n t r e s u l t s on the ro l e of nest concealment i n redwing nesting in the three years. (a) Nest concealment i s always important but i t s role may sometimes be masked by other f a c t o r s Three factors could have masked the predicted e f f e c t s of nest concealment i n 1978 and 1979. F i r s t , there may have been a higher proportion of redwing nests with marsh wrens farther away which had generally high chances of success, regardless of their concealment. This idea i s plausible because i n 1977, 1978, and 1979, as a consequence of decreasing density of marsh wrens (Pieman, unpublished data), approximately 78, 61, and 37 percent 108 Table 28. Comparison of proportions of redwing nests which had the nearest marsh wren nest within 15 m and farther away for. 1977-1979 seasons. A l l successful nests and nests destroyed by predators were included. Number (%) nests with wren nest Year 1 - 15 m 16 - 100 m Total 1977 111 (77.6) 32 143 1978 64 (61.0) 41 105 1979 58 (36.7) 100 158 Total 233 173 406 Note: X2=52.12; p<0.001 (2-tailed test). 109 of a l l redwing nests had the nearest marsh wren nest within 15 m (Table 28). Hence to test t h i s hypothesis, I divided a l l redwing nests into two groups; one with the nearest marsh wren nest within 15 m and the other one with marsh wren nests farther away. The proposed hypothesis predicts that poor nest concealment should result in a higher success of redwings which had marsh wrens close, but differences i n nest concealment should not play an important role i n si t u a t i o n s with marsh wrens farther away. The obtained r e s u l t s (Table 29), however, do not support t h i s hypothesis,. Second, d i f f e r e n t roles of nest concealment i n the three years might be explained by various proportions of redwing nests which had the nearest conspecific neighbor farther away. This i s because females from such nests could not have increased t h e i r success through cooperation i n nest defense with t h e i r neighbors. Since these s o l i t a r i l y nesting birds have to leave their nests, and usually also t h e i r t e r r i t o r i e s , i n order to forage, t h e i r absence makes th e i r unguarded nests vulnerable to predators. Moreover, because poorly concealed nests are easier to discover, they ought to be even more vulnerable i n such s i t u a t i o n s . Therefore, there should be no r e l a t i o n s h i p between nest concealment and nesting success of s o l i t a r i l y nesting females. But because the proportion of female redwings that had a conspecific neighbor within 30 m has not s i g n i f i c a n t l y changed in the three years (Table 30), I suggest that t h i s i s not a l i k e l y explanation. Third, differences in the structure of vegetation between 1 1 0 Table 29. Nesting success of red-winged blackbirds with marsh wrens close and fa r t h e r away, as re l a t e d to nest concealment. Data on successful nests and nests destroyed by predators from 1977-1979 were combined. Nearest Number Number (%) nests f a i l e d '2 * wren nest spots successful T o t a l X P l-15m 0-4 28 (34.6) 53 81 5-9 43 (37.1) 73 116 2.64 > 0.2 10-16 18 (50.0) 18 36 Total 89 144 233 beyond 15m 0-4 42 (58.3) 30 72 5-9 48 (57.1) 36 84 0.33 > 0.5 10-16 11 (64.7) 6 17 Total 101 72 173 one-tailed test I l l Comparison of proportions of redwing nests which had the nearest co n s p e c i f i c neighbor within 30 m or fa r t h e r away f o r 1977-1979 seasons. A l l successful nests and nests destroyed by predators were included. Number (%) nests with conspecific neighbor Year 1 - 30 m 31 - 100 m To t a l 1977 118 (82.5) 25 143 1978 78 (74.3) 27 105 19 79 136 (86.1) 22 158 Total 332 74 406 Note: X2=5.97; p>0.05 ( 2 - t a i l e d t e s t ) . 0 112 years might have an important e f f e c t on the placement of redwing nests and hence on the examined r e l a t i o n s h i p between nest concealment and nest success. I assume that i n a year with variable vegetation redwings could build t h e i r nests i n a broad range of s i t u a t i o n s , i n terms of nest cover density. The presence of patches of sparse and dense vegetation should also promote the s p a t i a l segregation of redwings and marsh wrens, which ought to occupy habitats most suitable for each species i n terms of t h e i r i n t e r s p e c i f i c i n t e r a c t i o n s . This idea i s supported by my observations that redwings were generally more abundant i n sparser c a t t a i l , whereas marsh wrens were more common i n denser c a t t a i l . Consequently, i f nest cover played a role i n redwing reproductive success, the relationship should be most pronounced under the conditions with a great degree of variation i n vegetation structure i n a habitat. On the other hand, i f the vegetation was generally sparse and thus offered a narrow range of nesting s i t e s , the predicted r e l a t i o n s h i p between nest cover and nesting success should become les s s i g n i f i c a n t . Therefore, i t i s possible that the differences i n the r o l e of nest concealment between 1977-1979 seasons could have been the r e s u l t of d i f f e r e n t densities of c a t t a i l between these years. To test t h i s hypothesis, I compared the density of c a t t a i l s in the three years (data were collected between May 5 and May 15 each year). The vegetation density s i g n i f i c a n t l y varied between these years, being sparsest i n 1978 (Table 31) when there was no rela t i o n s h i p between nest concealment and nesting success 113 T a b l e 3 1 . D e n s i t y o f t h e m a r s h v e g e t a t i o n i n 1 977 - 1 9 7 9 , a s r e f l e c t e d b y t h e n u m b e r o f 1 cm s p o t s ( 6 0 i s t h e max imum) s e e n t h r o u g h c a t t a i l s f r o m a d i s t a n c e o f 10 m a t h e i g h t s o f 5 0 , 1 0 0 , a n d 1 5 0 cm a b o v e t h e g r o u n d . T h e d e n s i t y o f v e g e t a t i o n w a s m e a s u r e d e v e r y y e a r b e t w e e n May 5 a n d May 15 a t t h e same s i t e s i n t h e m a r s h . M e a n n u m b e r Y e a r s p o t s ±SD N 1 9 7 7 3 3 . 4 ± 1 2 . 8 322 19 78 3 6 . 9 ± 9 . 7 3 7 8 19 79 2 2 . 4 ± 1 1 . 2 390 N o t e : 1 -way a n a l y s i s o f v a r i a n c e ; F = 1 7 4 . 2 4 ; d . f . = 2 , 1 0 8 7 ; p < 0 . 0 0 0 1 . 114 (Table 26). In 1978, there was also a smaller amount of va r i a t i o n (as indicated by smaller standard deviation) i n nest concealment of the examined nests (Table 31). Moreover, i n 1979 when the density of the vegetation was highest (Table 31), the largest proportion of redwing nests were well concealed (Fig. 7). But i n 1978, with the sparsest vegetation, there were proportionally more redwing nests which had intermediate concealment (Fig. 7; 1-way analysis of variance; F=13.37; d.f.=2, 424; p<0.0001). This indicates that the choice of a nesting substrate by female redwings i s influenced by the o v e r a l l q u a l i t y of marsh vegetation available for nesting. These r e s u l t s support the hypothesis that differences i n the role of nest concealment between the three years could be p a r t i a l l y explained by d i f f e r e n t densities of the vegetation available in t h i s marsh in these years. (b) Role of nest concealment varies with marsh wren density Different roles of nest concealment between years (see Table 26) might be a consequence of d i f f e r e n t e f f i c i e n c y of nest defense by female redwings i n those years. In 1977 the density of marsh wrens was extremely high i n my marsh (approximately 7.5 male marsh wrens/ha), but i n 1978 i t was 25% lower, and i n 1979 i t was only 50% of that i n 1977 (Pieman, unpublished data). Because i n redwings learning seems to play an important r o l e i n the defense of nests against marsh wrens (the rate of learning by i n d i v i d u a l birds i s probably determined by the density of F i g . 7. Frequency d i s t r i b u t i o n of redwing nests of d i f f e r e n t degree of concealment i n 1977-1979. In 19 78 the vegetation was extremely sparse. 0-1 2-3 4-5 6-7 8-9 10-11 12-13 K-15 16-17 18-19 Number spots 116 marsh wrens and hence by the freguency of redwing-wren encounters; see Chapter 3), in the two years with lower densities of marsh wrens redwings probably were generally l e s s e f f e c t i v e in defense of t h e i r nests against marsh wrens. Because t h i s reduced effectiveness nest defense by i n d i v i d u a l females should also reduce the e f f i c i e n c y of mutual nest protection by females breeding close to each other, the predicted negative r e l a t i o n s h i p between nest concealment and nest success should become weaker with decreasing density of marsh wrens. Under extreme conditions, when marsh wrens are absent, there should be, t h e o r e t i c a l l y , no rel a t i o n s h i p between the degree of nest concealment and success of redwing nests, This assumes, however, the absence of other predators. On the basis of these considerations I suggest that the lack of a s i g n i f i c a n t r e l a t i o n s h i p between nest concealment and success of redwings i n 1978 and 1979 could be a consequence of at l e a s t two factors: (1) d i f f e r e n t structure of the marsh vegetation which might r e s t r i c t female redwings to a narrower range of nesting s i t e s , thereby masking the predicted rel a t i o n s h i p ; and (2) lower density of marsh wrens which should resu l t i n reduced e f f i c i e n c y of redwing nest defense against wrens and hence also reduced role of vegetation structure. However, an experimental manipulation of the s t r u c t u r a l features of marsh vegetation ( i t s density and patchyness) and the density of marsh wrens are necessary f o r establishing the importance of these factors. One problem with the above conclusions i s why so few female 117 redwings placed t h e i r nests i n poorly concealed places i f these present, i n terms of nesting success, high guality s i t u a t i o n s (see Table 26)? A possible answer to t h i s i s that the s e l e c t i o n of a suitable nesting substrate by female redwings should also be determined by physical features of the vegetation ( i t s sturdiness) which has to provide an adeguate support to r e l a t i v e l y heavy redwing nests, Therefore, t h i s probably sets a l i m i t as to how sparse the vegetation a female can choose as a suitable substrate for her nest. This idea i s supported by my observations that female redwings breeding in areas of my marsh with extremely sparse vegetation generally located t h e i r nests in denser clumps of c a t t a i l s providing good support. Hence i t i s possible that the nest concealment data which r e f l e c t the density of vegetation i n the immediate v i c i n i t y of redwing nests underestimate the role of vegetation structure i n redwing nesting. Weatherhead and Robertson (1977a) also reported a higher success of redwing nests i n l e s s concealed si t u a t i o n s (sparse vegetation) than i n dense vegetation. In another study Holm (1973) observed that redwings had highest success on a lake with sparser vegetation, where the incidence of nest predation was lower. Holm (op. c i t , ) , therefore, suggested that t e r r i t o r i e s with sparser c a t t a i l may be most suitable for female redwings. This view i s further supported by observations of Lenington (1980) that i n her marshes, most redwing nests were located i n sparse vegetation. The conclusions I reached on the basis of my data thus probably also apply to other marshes. However, i t i s 118 possible that the predicted r e l a t i o n s h i p between nest cover density (or vegetation density) and nesting success of redwings might be weak or even non-existent i n marshes which also support yellow-headed blackbirds and/or possibly t r i - c o l o r e d blackbirds. This i s because yellowheads aggressively exclude redwings from areas with deeper water and sparse vegetation (Orians and Willson 1964). Therefore, i n such marshes redwings would generally be forced into denser vegetation.. The role of i n t e r s p e c i f i c aggression by yellowheads towards redwings i n terms of redwing nest s i t e s e l e c t i o n , and i t s influence on the interference between redwings and marsh wrens requires further i n v e s t i g a t i o n . 119 CONCLUSIONS FROM SECTION I Long-billed marsh wrens of both sexes, as well as juveniles, attacked various types of eggs in d i f f e r e n t nests, regardless of nest placement. In addition, marsh wrens also pecked nestlings of redwings and conspecifics. However, marsh wrens could destroy only small eggs and k i l l only small nestlings of redwings and conspecifics. Because almost a l l marsh wrens could be captured i n traps baited with redwing nests and eggs, the egg-pecking behavior i s widespread among ind i v i d u a l s from the study wren population. As a response to the interference from marsh wrens, redwings of both sexes are i n t e r s p e c i f i c a l l y aggressive towards marsh wrens. However, male redwings chase marsh wrens much more freguently than females, probably because they have more time available f o r such interactions and because they can also see marsh wrens better from high perches. Therefore, males apparently play an important role i n defense of nests of their females against marsh wrens- The rate of redwing-wren inter a c t i o n s i s highest during the peak redwing and marsh wren a c t i v i t i e s in the marsh, and i s also p o s i t i v e l y correlated with the density of marsh wrens near the breeding area of redwings. Female redwings freguently responded to nests of marsh wrens, which were placed near t h e i r own nests, by f l y i n g towards 120 them and examining them. Except possibly i n two cases, female redwings did not attempt to destroy the offered marsh wren nests. However, because females probably responded s p e c i f i c a l l y to nests of marsh wrens (as opposed to a strange object), i t seems reasonable to conclude that redwings associate wren nests with marsh wrens. Therefore, redwings probably go to wren nests to search f o r marsh wrens. The response of redwings to marsh wren nests i s influenced by t h e i r previous experience with marsh wrens. This i s supported by the fact that: (1) individuals become more responsive through the breeding season; and (2) experienced females are more responsive than inexperienced females. In addition, redwing response to wren nests increases with increasing wren density between years, and decreasing redwing-wren internest distance within a year, These r e s u l t s are consistent with the finding that the rate of agonistic interactions between redwings and marsh wrens increases with increasing density of marsh wrens. Marsh wrens are the major cause of redwing nesting mortality i n the study marsh. Redwing nesting success increases with increasing distance of redwing nests from the nearest marsh wren nest. But, i n spite of a higher density of marsh wrens i n 1977, marsh wrens had a r e l a t i v e l y smaller impact on redwing nesting success i n 1977 than i n 1976. This could be probably explained by: (1) more e f f i c i e n t nest defense by breeding redwings i n a year with high wren density when redwing aggression probably becomes more s p e c i f i c a l l y directed towards 121 marsh wrens; and (2) by r e l a t i v e l y lower rates of i n t e r s p e c i f i c nest destruction by marsh wrens i n 1977 due to strong i n t r a s p e c i f i c compettion between wrens in that year. Nest height had no e f f e c t on nesting success of redwings but nest concealment played a s i g n i f i c a n t role i n one of three years. In t h i s year poorly concealed nests were more successful than well concealed nests. This finding i s consistent with the nature of redwing-wren interactions-. Nests placed i n sparse vegetation are probably more defendable because here blackbirds can presumably more e f f e c t i v e l y observe marsh wrens and chase them away from the v i c i n i t y of t h e i r nests. However, the role of nest concealment i n r e l a t i o n to the interference from marsh wrens i s 'apparently also influenced by the density of marsh wrens. Under low wren density conditions, when redwing effectiveness of nest defense against marsh wrens i s probably lower, nest cover does not seem to play an important r o l e in redwing success. Another important factor influencing the r e l a t i o n s h i p between nest cover and nesting success i s the o v e r a l l density of the marsh vegetation, which may r e s t r i c t female redwings to a narrower range of nesting s i t e s . These factors were probably responsible for the lack of predicted r e l a t i o n s h i p between nest concealment and nest success i n two years. Behavioral interactions between redwings and marsh wrens resul t i n the s p a t i a l segregation of t h e i r nesting s i t e s . These inte r a c t i o n s thus probably reduce interference between the two species for some limited resource (possibly food). In addition. 122 as a consequence of the d i f f e r e n t e f f i c i e n c i e s of their interference mechanisms i n vegetation of various d e n s i t i e s , marsh wrens are more common i n dense c a t t a i l , whereas redwings are more abundant i n sparser vegetation. However, the degree of s p a t i a l segregation between the nesting s i t e s of these birds decreases with the increasing density of marsh wrens. I l l A S E C T I O N I I . 123 lk§. adaptive value of redwing reproductive t a c t i c s i n terms of redwing-marsh wren interactions Nesting success of birds i s influenced by various factors such as predation, nestling starvation, adult death, and nest parasitism. In most passerines, however, nest predation i s responsible f o r most nesting mortality (Ricklefs 1969). Since predation and other mortality factors must select for adaptations increasing reproductive success of birds (Ricklefs 1969), they should, presumably along with the type of s p a t i a l and temporal d i s t r i b u t i o n of food resources (Orians 1961, 1980; Crook 1965; Horn 1968; Lack 1968), present the most important dri v i n g forces influencing the evolution of various reproductive strategies i n birds (Crook 1965). The r e s u l t i n g reproductive strategy of a given species, therefore, should represent a compromise between various s e l e c t i v e forces (Ricklefs 1969). In previous chapters I have shown that i n my study area l o n g - b i l l e d marsh wrens are probably responsible for most of the red-winged blackbird nesting mortality- Marsh wrens, therefore, should present, at least in this marsh, an important s e l e c t i v e agent influencing redwing reproductive t a c t i c s . The purpose of t h i s section i s to examine the adaptive value of the redwing reproductive strategy i n terms of redwing-wren interactions, and thereby to estimate the role of marsh wrens as a s p e c i f i c s e l e c t i v e agent. In p a r t i c u l a r , I w i l l examine the clumped 1 2 4 pattern of d i s t r i b u t i o n of the breeding redwing females and i t s effect on the rates of redwing nesting f a i l u r e s due to marsh wrens (Chapter 6). Then I w i l l investigate the adaptive value of the temporal and s p a t i a l organization of nesting i n harems of various sizes (Chapter 7). F i n a l l y , i n Chapter 8, I w i l l evaluate the influence of previous breeding experience of male and female redwings on the mating pattern and i t s value i n terms of marsh wren nest destruction. 125 CHAPTER 6 The adaptive value of the clumped pattern of nesting by. female red-winged blackbirds Introduction North American marshes usually support high density redwing populations. For this reason some researchers have described the nature of redwing nesting system as c o l o n i a l (Mayr 1941, Smith- 1943). However, t h i s description i s inappropriate for my study marsh where male redwings establish, as compared with other previously studied marshes, extremely large t e r r i t o r i e s (see Appendix I I I ) . But I noticed that, within i n d i v i d u a l male t e r r i t o r i e s , neighboring female redwings were freguently breeding close to one another. Because c o l o n i a l nesting i n birds may play an important role i n reducing predation rates (e.g. Lack 1968, Brown 1975, Hoogland and Sherman 1976), i t i s possible that clumping tendency of females might reduce the impact of marsh wrens on redwing nesting success. Therefore, i n t h i s chapter I w i l l examine the pattern of d i s t r i b u t i o n of breeding female redwings and i t s possible role i n reducing the impact of marsh wrens on redwing nesting. The major objectives of t h i s study are to: (1) establish whether the breeding female redwings are di s t r i b u t e d i n a clumped pattern i n a study area, regardless of boundaries of males' t e r r i t o r i e s ; 126 (2) evaluate the role of distance between breeding females i n terms of interference from marsh wrens; (3) determine whether female redwings adjust t h e i r distances from other females i n response to marsh wrens; (4) examine whether clumping by females has any effect on marsh wren d i s t r i b u t i o n . Methods The evaluation of the value of clumped nesting by redwing females against marsh wrens required measuring distances between each redwing nest and the nearest conspecific nests. In t h i s case I measured two kinds of distances: (1) distance from the study nest to the nearest conspecific nest that was simultaneously active at any time when the study nest was active; and (2) distance from the study nest to the nearest conspecific nest that was active during the time when the study nest was vulnerable to wrens ( i t contained eggs or nestlings not older than 5 days). I measured these distances on the maps with a r u l e r . Henceforth I w i l l refer to distances between the study nest and (1) the nearest simultaneously active and (2) contributing redwing nests. 127 Results 1 - The pattern of d i s t r i b u t i o n of redwing nests i n the marsh To evaluate the pattern of d i s t r i b u t i o n of redwing nesting s i t e s in 1976, I surveyed active redwing nests four times at 15-day i n t e r v a l s during the peak of breeding a c t i v i t i e s i n the marsh. The observed frequencies of redwing nests per quadrat (50x50 or 60x60 m) and s t a t i s t i c a l analyses of these data are shown in Table 32. Few guadrats had only one nest (between 15 and 29 % of a l l nests); most had 2 to 6 nests per guadrat. The test of the goodness of f i t of the Poisson d i s t r i b u t i o n to observed data by Chi-sguare demonstrated that redwing nests were not d i s t r i b u t e d randomly i n any of the four surveys. During the surveys on May 1, May 15, and June 1 the redwing nests were di s t r i b u t e d contagiously (variance/mean r a t i o > 1). The d i s t r i b u t i o n of active redwing nests surveyed on June 15 was rather uniform (variance/mean r a t i o < 1). There are three factors that could have reduced the degree of clumping of redwing nests on June 15. F i r s t , there were fewer active redwing nests in the marsh at the end of the season. Second, redwings used a larger area f o r breeding later i n t h i s season (Table 32). And t h i r d , the density of marsh wrens i n the c a t t a i l vegetation was lower (most male marsh wrens es t a b l i s h new t e r r i t o r i e s i n bulrush areas l a t e r i n a season; Pieman, unpublished data). Redwing nests were also d i s t r i b u t e d contagiously on May 7, 1977, during the peak of redwing breeding Table 32. Distribution of redwing nests in study quadrats in 1976 and 1977. Date Observed frequency Number of nests per quadrat 1 2 3 4 5 Total Total nests area (ha) (df) Variance to mean p ratio 1976 May 1 26 8 6 3 2 3 52 12.0 May 15 24 12 2 5 1 3 1 56 12.0 June 1 25 10 7 6 June 15 22 11 12 1977 May 7 40 15 10 42 12.0 38 16.6 71 15.0 44.65 (4) 55.77 (5) 8.10 (2) 6.69 (2) 123.66 (5) <0.0005 2.154 <0.0005 2.310 <0.05 1.343 <0.05 0.963 <0.0001 2.040 129 a c t i v i t i e s (Table 32) « The clumping tendency of redwings may also be evaluated from mean distances from the nearest conspecific nests. The average distance from the nearest simultaneously active nest was si m i l a r i n both seasons (Table 33). But the mean distance from the nearest contributing redwing nest was s i g n i f i c a n t l y larger in 1977 than i n 1976 (Table 33). 2. The adaptive value of female clumping A l i k e l y function of the clumped pattern of nesting by redwing females could be to improve nest defense against wrens. If so, the pr o b a b i l i t y of success of redwing nests with marsh wren nests nearby should increase with decreasing distance from conspecific neighbors. In 1976, among female redwings which had the nearest wren nest within 30 m from their own nests, those with conspecific nests within 30 m had s i g n i f i c a n t l y higher nesting success than females with conspecific neighbors farther than 30 m (Table 34). But conspecific neighbors had l i t t l e e ffect on success of nests b u i l t farther than 30 m from wren nests, where nesting success was higher in a l l cases (Table 34). In 19 77 the presence of conspecific neighbors also increased nesting success of redwing nests that had marsh wren nests nearby (this i s almost s i g n i f i c a n t ; Table 34) . As i n 1976, the redwing nests which had marsh wren nests farther away had higher success independent of the distance from conspecific neighbors. The above analysis probably underestimates the s i g n i f i c a n c e Table 33. Summary of distances between the nearest simultaneously active and contributing redwing nests in 1976 and" 1977 seasons ( a l l nests laid in included). r Distances between Year Mean distance ±SD N t P Nearest simultaneously 19 76 20.7 ± 11.2 156 active redw. nests 0.07 >0.9 1977 20.6 ± 15.5 156 Nearest contributing 1976 24.6 ± 15.3 156 redw. nests -2.22 <C0.05 1977 29.7 ± 24.0 156 CO o Table 34. Redwing nesting success i n r e l a t i o n to distances between nearest redwing - redwing and redwing - marsh wren nests. Number (%) of Number (%) of Total number Redw. - wren Redw. - redw. successful destroyed (% < Df a l l Year distances (m) distances (m) nests nests nesi ts) 1976 1 - 30 1 - 30 23 (32) 49 (68) 72 (60) 31 - 80 2 (8) 22 * (92) 24 (20) 31 - 100 1 - 30 11 (61) 7 (39) 18 (15) 31 - 80 3 (50) . 3 ** (50) 6 (5) 1977 1 - 15 1 - 30 26 (37) 44 (63) 70 (47) 31 - 100 8 (19) 35 *** (81) 43 (29) 16 - 31 1 - 30 14 (61) 9 (39) 23 (15) 31 - 100 6 (46) 7 **** (54) 13 (9) * X 2=5.21, p<0.05 ** Fisher exact'probability t e s t , p>0.3 *** X 2=3.52, p?-0.05 **** X2=0.25, p>0.5 1 3 2 of clumping of redwing nests i n reducing the impact of marsh wrens. For example, marsh wren nests need not indicate the presence of wrens i n the area l a t e r i n a season, when marsh wrens generally establish new t e r r i t o r i e s i n the seashore bulrush areas. This suggests that some of the late redwing nests may have a high probability of success i n spite of being close to wren nests. The 1977 data (Table 35), from which I excluded nests in which eggs were l a i d after June 15, show that the difference between the proportion of successful nests with conspecific neighbors within 30 m or farther i s larger than i f these l a t e nests are included (see Table 34). This exclusion of late redwing nests results in a s i g n i f i c a n t difference i n the success of nests with conspecific neighbors nearby as compared with those with conspecific neighbors farther away. In addition, the analyses performed (Table 34) probably also underestimate the value of female clumping because they considered only the effect of the nearest conspecific neighbor on success of breeding female redwings. However, i t i s l i k e l y that success of females should increase even more i f there were several conspecific neighbors breeding nearby. To evaluate t h i s hypothesis, I examined success of females which had various numbers of simultaneously breeding conspecifics nearby (within 30 m) as a function of t h e i r distance from the nearest marsh wren nest. In both 1976 and 1977 seasons the proportion of successful redwing nests with marsh wrens nearby increased with increasing number of conspecific nests within 30 m (Table 36). But when marsh wren nests were farther away, female redwings 133 Table 35. Effect of the exclusion of lat e redwing nests (nests i n which eggs were l a i d after June 15) on the analysis of redwing nesting success as related to redwing - redwing and redwing - wren nest distances. No. (%) of No. (%) of Tot a l No. Redw. - wren Redw. - redw. successful destroyed (% of a l l distances (m) distances (m) nests nests nests) 1 - 1 5 1 - 3 0 25 (37) 43 (63) 68 (63) 31 - 100 6 (15) 34 (85)* 40 (37) * X2=4.82, p<0.05 Table 36. Success of redwing nests as related to (1) the t o t a l number of other, simultaneously active, conspecific nests which were b u i l t within an area of 30 m around a given nest, and (2) the distance from the nearest marsh wren nest. Only successful nests and nests destroyed by predators were included i n this analysis. The two ranges of distances of redwing nests from the nearest marsh wren nest were chosen for 1976 and 1977 data i n such a way, so that the short distances would include a l l redwing nests whose chances of success, as related to marsh wren nest predation, would approximately be 50% or less , whereas chances of success of redwing nests with marsh wrens farther away would be greater than 50%. Percent successful nests ( t o t a l number of nests i n a given category) when the Redwing- number of conspecific nests within 30 m of a given nest was Year -wren distance 0 1 2 3 4 5 6 7 8 Total 1976 1-41.5m 16.7 20.0 29.6 27.8 36.4 60.0 66.7 (18) (25) (27) (18) (11) (5) (3) (107) 42.Om-more 66.7 (6) 50.0 (8) - - - - -(14) 1977 1-16.5m 9.5 19.1 38.1 32.0 30.8 50.0 50.0 80.0 (21) (21) (21) (25) (13) (8) (6) (5) (120) V 17.Om-more 60.0 (10) 20.0 (5) 60.0 (5) 77.8 (9) (29) 135 were generally more successful, regardless of the number of conspecific neighbors breeding within 30 m (Table 36). Therefore, I conclude that natural selection should favor those i n d i v i d u a l s which exhibit a stronger clumping tendency. 3- Do redwings adjust distances from conspecific neighbors i n response to marsh wrens? The clumped pattern of nesting by female redwings can reduce impact of marsh wrens. But the clumped pattern of nesting should also increase i n t r a s p e c i f i c competition (probably for food, as indicated by freguently reported starvation of nestlings; e.g. Holm 1973, Robertson 1972). These p o t e n t i a l l y c o n f l i c t i n g conseguences of aggregation suggest that there may be an optimum degree of clumping for any marsh wren density, and that females should be able to adjust t h e i r nest placement to th i s optimum. This would, however, be possible only i f redwings could estimate the guality of a habitat i n terms of wrens and i f the density of marsh wrens varied greatly within marshes thus of f e r i n g redwings more and l e s s suitable areas for breeding. But i f redwings could not adeguately estimate the presence and density of marsh wrens, i f marsh wrens could destroy redwing nests without being detected, or i f wren populations were usually r e l a t i v e l y uniformly d i s t r i b u t e d throughout marshes, then selection for grouped nesting by redwing females could result i n a clumping tendency that would be generalized throughout the population. To distinguish between these hypotheses, I examined 136 distances between redwings as a function of t h e i r distance from the nearest marsh wren nest. I grouped redwing nests containing eggs according to their degree of aggregation with other redwing nests (nests with conspecific neighbors within 30 m and farther than 30 m) and redwing - wren distances (short redwing - wren distances represent unsuitable nest locations). As can be seen from Table 37, most redwing nests had conspecific neighbors nearby, independent of the distance from the nearest marsh wren nests. This indicates that redwings did not adjust distances from conspecific neighbours i n r e l a t i o n to marsh wrens. 4- Effect of clumping by redwing females on marsh wren d i s t r i b u t i o n Since redwings are aggressive towards marsh wrens, the clumping of redwing females may tend to exclude marsh wrens from the v i c i n i t y of redwing nesting s i t e s . I f t h i s were true, then, with decreasing distance between redwing nests, the distance between them and the wren nests should increase. To test t h i s p o s s i b i l i t y , I evaluated distances of redwing nests from (1) the nearest simultaneously active conspecific nest and (2) the nearest marsh wren nest. I selected redwing nests that contained eggs and computed the co r r e l a t i o n c o e f f i c i e n t between redwing - redwing and redwing - wren distances. In 1976 there was no c o r r e l a t i o n (r= 0.02, d.f.= 152, p>0.8), which indicates that redwing clumping had no additional e f f e c t on the distances between redwing and wren nests. But in 1977 there was a s i g n i f i c a n t negative co r r e l a t i o n (r= -0.20, d,.f.= 154, p<0.05), Table- 37. E f f e c t of short and long distances between redwing and marsh wren nests on clumping of redwing nests (as indicated by short and long distances between the nearest simultaneously active redwing nest s ) . Redw. - wren Year distance (m) 1976 1 - 30 107 (87) 16 (13) 123 31 - 100 24 (77) 7 (23) * 31 To t a l 131 (85) 23 (15) 154 1977 1 - 1 5 75 (63) 45 (37) 120 ** 16 - 31 23 (64) 13 (36) 36 Total 98 (63) 58 (37) 156 Number (%) of redwing nests Redwing - redwing distance (m) Total No. 1 - 3 0 31 - 100 of nests * X 2 =1.11, p>0.2 ** X 2=0.002, p>0.9 138 which indicates that redwing females with conspecific neighbors farther away may have been less e f f i c i e n t in excluding wrens than were females i n clumps. However, t h i s c o r r e l a t i o n i s not strong. Discussion The clumped pattern of nesting by female redwings (also noticed by Mayr (1941) and Caccamise (1977)) could be the most s t r i k i n g adaptation evolved by redwings i n response to the interference from marsh wrens. My re s u l t s suggest that clumped nesting by redwing females reduces the impact of marsh wrens on the i r nesting success. S o l i t a r y redwing females that attempted to breed near wrens had a lower probability of nesting success as compared with females that nested contagiously near wrens. By clumping t h e i r nests redwings may reduce the impact of marsh wrens i n two ways: (1) by increasing the e f f i c i e n c y of the i r nest defence; and (2) by excluding marsh wrens from the v i c i n i t y of their nesting areas. The l a t t e r idea i s supported by data from 1976 that showed that small distances between redwing nests were associated with a higher degree of s p a t i a l segregation of nesting s i t e s of redwings and marsh wrens. The fact that i n 1977 there was no relationship between redwing clumping and the degree of s p a t i a l segregation of nesting s i t e s of these species may be a consequence of d i f f e r e n t marsh wren densities i n the two years. I assume that i n 1977, when the density of marsh wren nests was much higher than i n 1976, marsh i i wrens were exposed to a strong i n t r a s p e c i f i c competition for 139 space, and many wrens were forced to breed near redwings. Under these conditions any difference between the impact of redwing females breeding near and far from other conspecifics on the d i s t r i b u t i o n of marsh wrens should be more pronounced. On the other hand, when marsh wren density i s low and marsh wrens have the opportunity to avoid redwing aggression by moving into more suitable parts of the marsh (farther from redwings), then the impact of a sing l e redwing female may be similar to that of several females. There are two other plausible explanations for the evolution of clumped nesting by redwing females, F i r s t , the clumping may have evolved as an adaptation to unpredictable and unevenly d i s t r i b u t e d food resources (clumping by females might result i n more e f f i c i e n t u t i l i z a t i o n of food resources through some form of cooperation in foraging). However, th i s does not appear to be the case i n t h i s marsh because i n d i v i d u a l redwing females usually forage independently of other birds (Pieman et a l . , research i n progress). Second, the clumped pattern of nesting may be a consequence of redwing tendency to clump i n highly favorable areas - such as areas with superabundant food, areas with highly suitable vegetation for nesting, or areas without marsh wrens. However, the available information on foraging by redwing females, mainly away from t h e i r t e r r i t o r i e s (e.g. Holm 1973, Orians 1961, Pieman et a l . , research i n progress), suggests that the quality of nesting s i t e s i n terms of food resources i s not important. The guality of vegetation does not seem to play an important r o l e because i n 2 of 3 years 140 redwings bred contagiously in vegetation of any density (Table 38). In addition, i n a year when larger clumps of females were established i n sparse vegetation (as indicated by poor nest concealment), the obtained c o r r e l a t i o n was weak and could account for only 4 percent of the t o t a l v a r i a b i l i t y in clump size (Table 38)- Thus even i n t h i s year the density of vegetation played only a minor role i n female clumping. Also marsh wrens had no observable, immediate e f f e c t s on the degree of redwing clumping (this was si m i l a r i n areas near and far from wren nests and also in the two successive years with d i f f e r e n t densities of marsh wrens). The above information suggests that clumped nesting by redwing females represents a rather generalized tendency that has not evolved as a consequence of the concentration of females in highly suitable areas, though preferences of females f o r sparse vegetation might promote i t . Preliminary r e s u l t s of the continuing research also indicate that cooperation among females in foraging i s not an important f a c t o r s e l e c t i n g for clumped nesting by female redwings. Because clumped nesting by redwing females plays a s i g n i f i c a n t r o l e i n reducing the impact of marsh wrens on redwing nesting success i t i s reasonable to suggest that marsh wrens may have s i g n i f i c a n t l y influenced i t s evolution. The clumping tendency by redwing females could i n turn present an important force selecting f o r the redwing polygynous mating system (see Chapter 10). 141 T a b l e 3 8 . R e l a t i o n s h i p b e t w e e n c l u m p s i z e ( n u m b e r o f s i m u l t a n e o u s l y a c t i v e c o n s p e c i f i c n e s t s w i t h i n 30 m f r o m a g i v e n n e s t ) a n d t h e d e n s i t y o f v e g e t a t i o n a r o u n d t h e n e s t . Y e a r r d . f . P 1 977 0 . 2 0 3 136 < 0 . 0 2 1 9 7 8 0 . 1 2 5 107 > 0 . 1 1979 0 . 1 2 8 159 > 0 . 1 142 CHAPTER 7 The s p a t i a l and temporal organization of nesting, harem s i z e , and reproductive success cjf red-winged blackbirds Introduction In a previous Chapter I discussed the role of the clumped pattern of d i s t r i b u t i o n of female redwings i n my study marsh i n reducing marsh wren nest predation rates. However, to better understand the reproductive t a c t i c s of redwings, we need detailed information on the pattern of settlement of females on t e r r i t o r i e s of males acguiring harems of various sizes.. Such information should allow us to determine the factor(s) that influence d i s t r i b u t i o n of females i n a breeding population and hence mating success of i n d i v i d u a l males. By examining these factors in the l i g h t of the adaptive value of redwing reproductive t a c t i c s , we should i n turn be able to get a better insight into selective forces that have been driving the evolution of polygyny i n t h i s species. In t h i s study I w i l l examine: (1) the intensity of competition among females for the best guality nesting s i t u a t i o n s , as r e f l e c t e d by the temporal pattern of settlement of females on t e r r i t o r i e s a t t r a c t i n g harems of various sizes; (2) the s p a t i a l d i s t r i b u t i o n of females i n various size harems 143 and factors that influence i t ; (3) the influence of the s p a t i a l and temporal organization of nesting on redwing reproductive success. Besults and Discussion 1. Temporal organization of nesting The theory of habitat d i s t r i b u t i o n i n birds (Fretwell and Lucas 1969) predicts that, in an environment o f f e r i n g a range of habitats of various g u a l i t i e s , the f i r s t individuals should s e t t l e i n the best habitat i n order to maximize t h e i r fitness,. But as the density of a population increases, new ind i v i d u a l s should s t a r t s e t t l i n g also i n poorer habitats,. This could re s u l t from the lowered f i t n e s s of birds s e t t l e d i n best habitat down to the i n i t i a l s u i t a b i l i t y of a poorer habitat due to competition f o r limited resources. A l t e r n a t i v e l y , new indi v i d u a l s might be prevented from s e t t l i n g i n the best habitat by t e r r i t o r i a l behavior of e a r l i e r s e t t l e d birds (Fretwell and Lucas 1969). T e r r i t o r i e s of male redwings which eventually acguired large harems are (by definition) most a t t r a c t i v e to females i n some respect and conseguently should present the best guality breeding situations for female redwings. Therefore, i n terms of the theory of habitat d i s t r i b u t i o n , female redwings should s e t t l e f i r s t on these t e r r i t o r i e s , and l a s t on t e r r i t o r i e s acguiring small harems. In addition, because male t e r r i t o r i e s which eventually acguire large harems should be most a t t r a c t i v e . 144 I also predict that settlement of females and consequently t h e i r i n i t i a t i o n of nesting ought to be more synchronous on these t e r r i t o r i e s . To test the f i r s t hypothesis, I correlated the date on which the f i r s t egg was l a i d on each male's t e r r i t o r y with i t s eventual harem s i z e (the maximum number of simultaneously breeding females on a t e r r i t o r y ) . On average, egg-laying started approximately 10 days e a r l i e r on t e r r i t o r i e s with large harems than on t e r r i t o r i e s with small harems (Fig. 8). The date on which the f i r s t egg was l a i d on males' t e r r i t o r i e s i s s i g n i f i c a n t l y negatively correlated with harem s i z e , but t h i s c o r r e l a t i o n i s mainly a conseguence of differences between harems of 1-3 females and larger ones (Fig- 8). Orians (1980) obtained a si m i l a r relationship between the i n i t i a t i o n of egg-laying on t e r r i t o r i e s of male redwings and the i r mating success. To test the second hypothesis, I correlated the time i n t e r v a l between the i n i t i a t i o n of nesting by subsequent females of a harem with harem s i z e (only nests of the maximum number of females nesting simultaneously on a t e r r i t o r y were included). The larger the harem, the shorter i s , on the average, the time i n t e r v a l between the i n i t i a t i o n of nesting by subsequent females (Table 39). This i s true, however, for harems of up to 6 females. Harems of 6 and larger ones appear to exhibit a si m i l a r degree of synchrony (Table 39). These results thus support the hypothesis that female redwings s t a r t nesting f i r s t and s e t t l e faster on t e r r i t o r i e s that attracted large harems, whereas females s e t t l e l a t e and more asynchronously on Fig. 8. Relationship between date of laying the f i r s t egg on a t e r r i t o r y and size of a harem attracted to that t e r r i t o r y (1976 1979 data were combined). a 2 30 20 1 10 (June 4) (June 19) t t r = -0.469 p < 0.0005 30 i a. 20 10 «•» 8 9 H a r e m s i z e 146 Table 39. Mean time Interval between the I n i t i a t i o n of nesting by successive females i n harems of various sizes (1976 - 1979 data were combined). Harem s i z e Mean number of days between nesting by successive females from a harem ±SE N 2 9.57±4.04 7 3 6.3610.88 28 4 5.50±0.77 36 5 4.91±0.50 76 6 3.65±0.59 40 7 3.67±0.49 42 8 2.3610.69 14 9 3.5010.60 16 Note: Correlation between the time i n t e r v a l between the i n i t i a t i o n of nesting by successive females and harem s i z e : r=-0.27, d.f.=257, p<0.0005. 147 t e r r i t o r i e s a t t r a c t i n g small harems. These r e s u l t s support the idea that t e r r i t o r i e s a t t r a c t i n g large harems present high guality breeding situ a t i o n s . 2- S p a t i a l organization of nestings By nesting close to conspecific neighbors, female redwings can reduce the impact of l o n g - b i l l e d marsh wrens on th e i r reproductive success (see Chapter 6). Because females from larger harems fledge, on average, more young than females from small harems (see Chapter 8, Fig. 16), i t i s possible that t h i s could be a conseguence of smaller distances between redwing nests i n male t e r r i t o r i e s acquiring large harems and hence better cooperation between females i n nest defense against marsh wrens. To examine t h i s p o s s i b i l i t y , I w i l l now evaluate the s p a t i a l d i s t r i b u t i o n of nests of females from harems of various sizes and attempt to establish what factor (s) control the degree of clumping i n di f f e r e n t harems. To examine the relat i o n s h i p between harem size and the degree of female clumping, I measured distances of redwing nests that contained eggs from the nearest simultaneously active conspecific nest, calculated mean inter-nest distances for a l l redwing nests from i n d i v i d u a l harems, and correlated these mean inter-nest distances with harem s i z e . A highly s i g n i f i c a n t negative correlation between these variables shows that the mean distance between the nearest redwing nests decreases with increasing harem siz e u n t i l i t reaches minimum f o r harems of 6 (Fig. 9 ) . I t appears, however, that the mean inter-nest Fig. 9. Mean distance of redwing nests from individual harems from the nearest conspecific neighbor as related to harem size (1976-1979 data were combined). 80 70 60 r = -0.41 d.f. = 75 p < 0.001 <-> c a _ 50 40 30 20 10 0 3 t H a r e m s i z e oo 149 distance s l i g h t l y increases f o r larger harems (Fig. 9), but this might be a conseguence of a small number of the largest harems i n my sample. Decreasing distances between the nearest redwing nests with increasing harem s i z e could be explained in two ways. F i r s t , i t i s possible that for some b i o l o g i c a l reason females i n large harems exhibit a stronger clumping tendency. A l t e r n a t i v e l y , this e f f e c t might be a s t a t i s t i c a l artefact. Because mean density of females increases with increasing harem size (Table 40), smaller distances between females could simply r e s u l t from a higher density of females that might be di s t r i b u t e d more or less uniformly throughout the males' t e r r i t o r i e s . To distinguish between these two explanations, I compared mean distances between the nearest simultaneously active redwing nests calculated for harems of various sizes (data from 1976 -1979 were combined) with calculated t h e o r e t i c a l distances between nests i f their d i s t r i b u t i o n on male t e r r i t o r i e s was i d e a l l y uniform. For these comparisons I chose the t h e o r e t i c a l distances for the uniform d i s t r i b u t i o n of nests because th i s type of d i s t r i b u t i o n of breeding i n d i v i d u a l s i n r e l a t i v e l y uniform habitats l i k e marshes should f a c i l i t a t e most e f f i c i e n t u t i l i z a t i o n of food resources (see Orians 1980). This assumes, however, the absence of selection for grouping tendency. I used the r a t i o of the mean observed distances to the hypothetical distances for the uniform d i s t r i b u t i o n as an index of the degree of clumping by females on t e r r i t o r i e s with harems of various sizes (a r a t i o of 1 indicates the i d e a l l y uniform d i s t r i b u t i o n ) . Table 40. Comparison of density of females, hypothetical inter-nest distances i f nests were distributed uniformly, and observed distances of redwing nests from the nearest simultaneously active conspecific nests for harems of various sizes. The theoretical distances between uniformly distributed nests (assuming that female redwings defend regular hexagonal areas around their nests) were calculated as a distance between centres of neighboring hexagons. Mean t e r r i t o r y Theoretical d i s t . Ratio of mean Harem size (mZ) Mean area per for uniform Mean observed observed to size (No. t e r r i t o r i e s ) 2 female (m ) di s t r i b u t i o n (m) distance (m) theoretical d i s t . 1 4765 (6) 4765 74.2 36.5 0.49 2 " 6207 (7) 3103.5 59.9 34.9 0.58 3 8678 (14) 2892.7 57.8 26.6 0.46 4 8425 (11) 2106.3 49.3 21.0 0.43 5 8712 (19) 1742.4 44.9 20.6 0.46 6 8553 (8) 1425.5 40.6 16.4 0.40 7 9551 (7) 1364.4 39.7 18.5 0.47 8 15094 (2) 1886.8 46.7 22.9 0.49 9 14168 (2) 1574.2 42.6 22.7 0.53 Note: Correlation between harem size and the ratio of observed to theoretical distances for the uniform d i s t r i b u t i o n of nests: r=-0.10, d.f.=7, p>0.5. 151 The index of clumping i s consistently less than 1 and does not vary systematically with harem size (Table 40), suggesting that females clump their nests regardless of the density of breeding birds. Therefore, I reject the hypothesis that the smaller inter-nest distances on t e r r i t o r i e s with large harems could be a conseguence of a high density of females and hence of a greater degree of crowding on these t e r r i t o r i e s . Another way of evaluating the degree of clumping by female redwings i n r e l a t i o n to harem s i z e i s to examine distances of subseguently s e t t l i n g females from the female which s e t t l e d f i r s t on that t e r r i t o r y . If clumping plays an important role i n redwing nesting then i t i s l i k e l y that, i n general, new females joining an already existing harem should s e t t l e close to e a r l i e r s e t t l e d birds. However, with regard to the t e r r i t o r i a l behavior of female blackbirds (e. g. Nero 1956b) there are two possible patterns of settlement by consecutive birds with regard to the f i r s t s e t t l e d female, which should r e s u l t i n d i f f e r e n t s p a t i a l d i s t r i b u t i o n of females in large and small harems, F i r s t , i f the l e v e l of aggression by t e r r i t o r i a l females remains constant throughout the breeding season, the addition of new birds to a harem should r e s u l t i n the increasing distance of subseguently s e t t l e d i n d i v i d u a l s from the f i r s t breeding female (this i s i f we assume that a l l birds s e t t l e as close to the center of the clump as possible). This i s because afte r the density of birds within a clump reached a certain maximum, new indiv i d u a l s should be forced to s e t t l e on the periphery of the clump, In t h i s case the density of birds, expressed as a number of i n d i v i d u a l s 152 within a clump per unit area, should remain constant with increasing harem s i z e . Second, i f females become generally less aggressive towards other conspecifics with the progress of their breeding (see Nero 1956b), late a r r i v i n g i n d i v i d u a l s joining large harems might succeed i n s e t t l i n g closer to already breeding birds (see also Orians 1980) and hence inside the clump. Consequently, the distance between early and l a t e s e t t l e d i n d i v i d u a l s from the f i r s t female may remain s i m i l a r . This si t u a t i o n should, therefore, r e s u l t i n increasing density of females within a clump with increasing harem s i z e . To distinguish between these two p o s s i b i l i t i e s , I measured distances of nests of consecutively s e t t l i n g females from the nest of the f i r s t female breeding i n harems of various s i z e s . The fact that there are no s i g n i f i c a n t differences between distances of nests of consecutively s e t t l e d birds from the f i r s t female i n small, medium, and large harems (Table 41) suggests that t h i s case approaches the second s i t u a t i o n described above. This view i s also consistent with the e a r l i e r discussed finding that the average distance between the nearest conspecific neighbors decreases with increasing harem size (Table 41). But, the interpretation based on a generally decreasing l e v e l of aggression by s e t t l e d birds towards other females with the progress of t h e i r breeding i s complicated by the fact that the early nesting i n d i v i d u a l s , which are from large harems (Fig. 8, see also F i g . 11), are old experienced birds (Crawford 1977, Jackson 1971) that might exhibit a stronger clumping tendency (I Table 41. Comparison of distances of sequentially s e t t l i n g females from the f i r s t female to s e t t l e i n small, medium, and large harems. The date of i n i t i a t i o n of nesting was used as an index of s e t t l i n g time. Data from 1976 - 1979 were combined. Mean distance (m) ±SD (N) of sequentially s e t t l i n g females from the f i r s t breeding female Harem from a given harem si z e 2nd <j> 3rd £ 4 th £ 5th £ 6th £ 7th o_ 8th £ 9 th £ A H remaies combined 2-3 34.4118.0 46.0±29.3 39.1123.6 (22) (15) (37) 4-6 29.5±16.3 33.1119.4 29.4120.5 32.9+18.1 23.6113.5 _ _ 30.7118.4 (38) (38) (38) (27) (8) (149) 7-9 24.5±15.7 38.9129.7 39.3119.8 34.5118.0 32.0127.9 34.9119.4 41.6128.5 37.213.8 34.5122.0 (11) (11) (11) (11) (11) (11) (4) (2) (72) Note: Two-way analysis of variance; (1) H„: There i s no e f f e c t of sequence on the distance from the f i r s t female; F=0.88; d.f.=7, 243; p=0.52. (2) H„ : There i s no e f f e c t of harem s i z e on the distance from the f i r s t female; F=2.92; d.f.=2, 243; p=0.06. (3) H, : There i s no i n t e r a c t i o n of sequence and harem size on the distance from the f i r s t breeding female; F=0.66; d.f.=5, 243; p=0.66. 154 w i l l return to t h i s problem l a t e r ) . In addition, the nest s i t e s e l e c t i o n of consecutively s e t t l i n g females might be influenced by the stage of nesting of e a r l i e r s e t t l e d birds. For example, i f new in d i v i d u a l s preferred neighbors with whom they would be most synchronous i n nesting, then t h i s would r e s u l t i n a d i f f e r e n t s p a t i a l arrangement of birds in a clump (harem) than i f they simply concentrated around the f i r s t s e t t l e d (experienced) female. In spite of these problems the obtained re s u l t s (Table 41) show that the degree of clumping (as r e f l e c t e d by the density of females within a clump) increases with increasing harem siz e because: (1) distances of successively s e t t l i n g females from the f i r s t female i n medium and large harems are smaller than those for small harems (this i s close to the l e v e l of sig n i f i c a n c e ; Table 41) , and (2) there are more females breeding i n medium and large harems. A. How i s a greater degree of clumping i n larger harems achieved? There are three possible explanations: F i r s t , females might aggregate i n the most suitable microhabitats (for example places with abundant food, without predators, or with sparse vegetation), and t e r r i t o r i e s a t t r a c t i n g large harems have better patches of good habitat. However, I reject t h i s hypothesis because there i s no correlation between harem size (and hence the degree of clumping by females) and various features determining t e r r i t o r y guality (see Chapter 8) . Second, because old experienced male redwings acguire the 155 largest harems (see Chapter 8), the greater degree of clumping by females on t h e i r t e r r i t o r i e s might be a conseguence of their greater influence on the nest s i t e selection of t h e i r females compared to young inexperienced males. I f t h i s were true, then distances between females should be smaller on t e r r i t o r i e s of older experienced males, independently of the size of the harem. To test t h i s hypothesis, I divided male redwings into those breeding in t h i s marsh for the f i r s t time (inexperienced males) and those that have bred here i n at least one previous year (experienced males), and compared mean distances between the nearest simultaneously active nests from harems of the same siz e of inexperienced and experienced males. Mean internest distances are s i m i l a r f o r the same size harems of inexperienced and experienced males (Fig. 10). This impression i s supported by the f a c t that slopes of the regression l i n e s for inexperienced and experienced males do not d i f f e r s i g n i f i c a n t l y (ANCOVA, test of common slopes, F=0.58; d.f.=1, 55; p>0.4). Therefore, I r e j e c t the hypothesis that the higher degree of clumping i n large harems i s a conseguence of the greater influence of experienced males on the d i s t r i b u t i o n of breeding females i n t h e i r t e r r i t o r i e s . Third, the fact that the mean inter-nest distances decrease with increasing harem size for harems of both inexperienced and experienced males (Fig. 10) suggests that females themselves determine the degree of clumping of t h e i r nests, Females that sta r t nesting f i r s t are old experienced birds (Crawford 1977, Jackson 1971) which are also more e f f i c i e n t breeders that fledge 156 F i g . 10. Mean distances between the nearest redwing simultaneously active nests from various s i z e harems of (a) inexperienced and (b) experienced males (1976 - 1979 data combined). (a) 80 60 r=-0.52 d.f. = 33 p< 0.002 (b) a> o c o w b 40 20 0 60 i 40 ~l I I 1 1 1 j 1 1 1 2 3 4 5 6 7 8 9 r = -0.44 d.f. = 22 p<0.05 20 0 " I 1 1 r 2 3 4 5 6 7 8 9 H a r e m size 157 more young (Crawford 1977). The smaller mean inter-nest distances i n large harems, which include most early nesting experienced females (Fig. 11), might therefore be explained by the greater degree of clumping exhibited by these experienced birds (females with previous experience i n breeding might clump more to reduce impact of marsh wrens on t h e i r nesting). In addition, i t i s also possible that the higher degree of clumping in large harems i s a conseguence of the tendency of young inexperienced females to p r e f e r e n t i a l l y join harems with experienced i n d i v i d u a l s , build t h e i r nests close to them, and i n this way benefit from the more e f f i c i e n t cooperation i n nest defense with these birds. To test these hypotheses, I measured distances between i n d i v i d u a l nests and the nearest conspecific nest that was active at a time when a female set t l e d i n the area and i n i t i a t e d building her nest. I divided a l l females into early and l a t e breeders (the dashed l i n e in Fig. 11 shows t h i s d i v i s i o n of females according to their mating status into the two categories) and a l l harems into large (7-9 females), medium (4-6 females), and small (2-3 females) harems. I did not include data on females of monogamous males and the f i r s t females that s e t t l e d on t e r r i t o r i e s of polygynous males because nest s i t e selection by these birds could not have been influenced by other females from the same t e r r i t o r y . In addition, because cooperation between females from neighboring harems i s poorer (Table 42), I excluded a l l nests whose nearest conspecific neighbor was a female from the neighboring harem. Fig. 11. Temporal pattern of formation of various size harems as seen from the average dates of i n i t i a t i o n of nesting by successive females from individual harems (1976-1979 data were combined). Harem size plotted on Y axis denotes (1) temporal sequence of females as they s e t t l e d on a t e r r i t o r y and started nesting (thus, for example, 1 always represents the f i r s t nesting female i n a harem of any par t i c u l a r s i z e ) , and (2) the maximum harem size achieved (this i s also indicated by numbers near the f i r s t and l a s t female i n i t i a t i n g nesting). The dashed l i n e indicates d i v i s i o n of females into early (mostly older birds) and late (mostly young birds) breeders. 15 20 25 30 5 10 15 April May 159 Table 42. Influence of conspecific neighbors from the same or neighboring harem on nesting success of female red-winged blackbirds. Only successful nests and nests destroyed by predators which had the nearest marsh wren nest within 15m were included. Data from 1976-1979 were combined. O r i g i n of neighboring female Distance between nearest redwing neighbors Number (%) nests successful depredated Total The same harem 0 - 30m 30.5 - more Tota l 64 (40.3) 13 (21.0) 77 95 (59.7) 159 * 49 (79.0) 144 62 221 Neighboring 0 - 30m 11 (37.9) 18 (62.1) 29 harem 30.5 - more 13 (30.2) 30 (69.8) ** 43 Total 24 48 72 * X2=6.48, p<0.02. ** X2=0.18, p>0.5. 160 The f i r s t females i n small harems were l a t e nesting birds (Fig. 11) which b u i l t , on average, s i g n i f i c a n t l y farther from the nearest active conspecific nest than the f i r s t nesting females from medium size and large harems (Table 43). The fa c t that early nesting females (presumably older birds that have most l i k e l y already experienced nest destruction by marsh wrens) s e t t l e closer to each other thus indicates that previous breeding experience s i g n i f i c a n t l y influences the nest s i t e s e l ection of females in terms of t h e i r distance from the nearest conspecific neighbor. This higher degree of clumping by females in large harems might be explained by the fact that females generally return to the same t e r r i t o r i e s i n subseguent years (see Chapter 8; Table 50) and hence renest with the same fa m i l i a r neighbors. Because such birds presumably developed mutual tolerance towards one another i n a previous year(s), t h e i r lowered l e v e l of aggression to each other might allow the reduction of the distance between t h e i r nest s i t e s . Two kinds of evidence support th i s view. F i r s t , Nero and Emlen (1951) demonstrated by experimentally moving active redwing nests within and between male t e r r i t o r i e s that the resident females w i l l , i n time, develop tolerance towards "intruders" whose nests were relocated. Second, data from my marsh show that the distance between the f i r s t two breeding females decreases from small to large harems (Table 41), presumably as a conseguence of increasing proportion of experienced females with increasing harem s i z e . However, another explanation could be that as females become older and more experienced, their l e v e l of aggression towards conspecifics might, i n general, decrease. 161 Table 43. Comparison of mean distances of nests of early and late females from the nearest conspecific nest in small, medium, and large harems (1976 - 1979 data were combined). Mean distance ±SE (N) of redwings from the nearest conspecific nest at time of building Harem size Early females Late females 2-3 (small) - 31.2±2.9 (37) 4-6 (medium) 24.9±1.7 (65) 19.2±1.3 (84) 7-9 (large) 21.711.8 (41) 18.9±2.8 (31) Note: S t a t i s t i c a l comparison of: (1) late females, small harems vs. early females, medium harems; t=2.0, d.f.=100, t-probability= 0.045. (2) late females, small harems vs. early females, large harems; t=2.78, d.f.=61, t-probability= 0.007. (3) late females: small vs medium harems; t=3.8, d.f.=50, t-probability= 0.0005. (4) late females: small vs. large harems; t=3.05, d.f.=66, t-probability= 0.003. (5) late females: medium vs. large harems; t=0.13, d.f.=43, t-probability= 0.87. 162 There are no data available to examine these p o s s i b i l i t i e s . In addition, a higher degree of clumping i n medium and large harems can also be explained by the fact that l a t e females s e t t l i n g i n these harems bred s i g n i f i c a n t l y closer to the nearest neighbor than l a t e females from small harems (Table 43). But l a t e females from medium and large harems se t t l e d at s i m i l a r distances from the nearest conspecific (Table 43). This suggests that the early breeding (older) females, which are i n medium and large harems, a t t r a c t l a t e nesting birds to s e t t l e near them. Therefore, the increasing degree of clumping with increasing harem size may be due both to: (1) a stronger clumping tendency exhibited by early nesting females which are mostly older experienced birds s e t t l i n g on t e r r i t o r i e s a t t r a c t i n g larger harems, and (2) the tendency of l a t e females (presumably young inexperienced birds) to s e t t l e closer to the early nesting females. 3. The relationship between the s p a t i a l and temporal d i s t r i b u t i o n of nesting The timing of nest i n i t i a t i o n might have important e f f e c t s on the s p a t i a l arrangement of simultaneously active redwing nests. Female redwings defend small breeding t e r r i t o r i e s around the i r nests from which they exclude any other intruding females (Nero 1956b). This behavior i s most intense during t e r r i t o r y establishment. But, according to Nero (1956b), i t then decreases with the progress of nesting, presumably because of the demands of nesting (females alone build the nest, incubate, 163 and feed nestlings; e.g. Bent 1958, Orians 1961). Conseguently new females might more l i k e l y succeed in s e t t l i n g closer to the resident birds when they are more asynchronous. However, evidence i s also available that the l e v e l of female aggression towards intruding conspecifics may remain si m i l a r throughout the breeding season. O'Connor (1976) examined aggression of breeding female redwings to a caged "intruder" and found no s i g n i f i c a n t differences i n responses of resident females at various times during their nesting,. These data thus suggest that the degree of synchrony i n nesting should not have any ef f e c t s on distances between adjacent female neighbors. To examine the r o l e of nesting asynchrony i n the s p a t i a l arrangement of female redwings, I correlated distance of females when they started building from the nearest active redwing nest with the time i n t e r v a l between the i n i t i a t i o n of t h e i r nesting. For t h i s analysis I selected nests of the maximum number of simultaneously breeding females on a male t e r r i t o r y whose nearest neighbor was a nest of a female from the same harem. I excluded data on nests of females which were mated with monogamous males or were the f i r s t birds to s e t t l e on t e r r i t o r i e s of polygynous males. There i s a weak but s i g n i f i c a n t negative c o r r e l a t i o n indicating that females s e t t l i n g more asynchronously ( r e l a t i v e to their e a r l i e r established neighbors) b u i l t t h e i r nests, on the average, closer to t h e i r conspecific neighbors than females s e t t l i n g more synchronously (r=-0.17, d.f.=211, p<0.01). Because small distances between redwing nests res u l t in better defense against 164 predators (see Chapter 6 ) , i t might be concluded that nesting asynchrony by females from a harem should be selected f o r . On the other hand, i t could be argued that the e f f i c i e n c y of cooperation among females i n nest defense should decrease with increasing asynchrony in nesting. This i s because i f a female s e t t l e d near a resident female with nestlings, then a new female would not benefit as much from cooperation i n nest defense with the resident female who i s l i k e l y to be foraging away from her nest most of the time.. To examine t h i s hypothesis, I evaluated 1979 data on nesting success of female redwings whose nearest conspecific neighbors i n i t i a t e d their nests within 15 days or more than 15 days from the f o c a l female-I selected only nests which had the nearest marsh wren nest within 20 m and the nearest redwing neighbor within 30 m because nesting synchrony should have the greatest impact on success of such nests. Nests whose neighbors were more synchronized were s i g n i f i c a n t l y more freguently successful in fledging any young than nests with highly asynchronous neighbors (Table 44). This supports the hypothesis that synchronous nesting by female neighbors increases t h e i r chances of reproductive success, presumably through the increased e f f i c i e n c y of cooperation i n defense of nests against marsh wrens. Hence, natural s e l e c t i o n should favor nesting synchrony or a minimum degree of nesting asynchrony. The in t e n s i t y of selection for either synchronous or asynchronous nesting by redwing neighbors can be estimated by examining the proportion of females s e t t l i n g near or away from 165 Table 44. Nesting success of redwing nests whose nearest conspecific neighbor exhibited either low or high degree of asynchrony i n nesting. Only 1979 successful and depredated nests that had the nearest marsh wren nest within 20 m and the nearest conspecific simultaneously active nest within 30 m were included. Number days Number (%) nests between females Successful Depredated Total 0 - 1 5 36 (59.0) 25 (41.0) 61 16 - 30 6 (28.6) 15 (71.4) 21 Total 42 40 82 Note: x =4.64, d . f . - l , p<0.025 ( 1 - t a i l t e s t ) . 166 t h e i r neighbors at various time i n t e r v a l s . I f asynchrony was selected f o r , then most females should choose neighbors incubating eggs or feeding young. Consequently, neighboring females should generally be separated by more than approximately 11 days (this i s i f we assume that a female: (1) st a r t s building roughly 3 days aft e r s e t t l i n g i n a given t e r r i t o r y ; (2) needs 4 days for the construction of a nest; (3) needs 4 days for laying the most common clutch of 4 eggs). On the other hand, i f most females s e t t l e d near neighbors that i n i t i a t e d nesting at a s i m i l a r time, t h i s would indicate that nesting synchrony has been favored. I tested t h i s from 1976-1979 data, using nests whose nearest neighbor was a female from the same harem. Because choice preferences should be most evident when the density of females i s high, I analyzed data on nests of the maximum number of simultaneously nesting females on male t e r r i t o r i e s . I excluded data on monogamously mated females of monogamous and polygynous males. Host females i n i t i a t e d nesting within 0-11 days a f t e r their adjacent neighbors (Fig- 12). Thus, i n most cases (71.9 % of 213 cases) new females s e t t l e d near conspecifics which started nesting i n that area at the same time, were building a nest, were laying eggs, or just started incubating t h e i r clutch. The test of the goodness of f i t of the Poisson d i s t r i b u t i o n to observed data shown i n Figure 12 demonstrated that female redwings s i g n i f i c a n t l y more freguently selected conspecific neighbors which i n i t i a t e d nesting at a more si m i l a r time than would be expected i f females were not influenced by the stage of nesting of e a r l i e r s e t t l e d i n d i v i d u a l s (X* = 246.63; d.f. = 10; p<0.0001; see Fig. 12). 167 Fig. 12. Selection of conspecific neighbors by settling female redwings in terms of the degree of asynchrony in their nesting (213 nests from 1976 - 1979 were included). The dotted bars indicate the expected Poisson distribution frequency. 25 20 1 to ~ 15 H 10 CU C c <D 10 CL 5 rxi L O I i O ro CT> o ro 1" 'i Cvl LO co I ' — rsi CO K CXI un ro O ro ro ro (NI ro Number days between neighbors 168 Therefore, I conclude that nesting synchrony has been favored by natural s e l e c t i o n , probably because i t r e s u l t s , on average, i n higher reproductive rates (Table 44) . 4. The influence of temporal and s p a t i a l organization of nesting on reproductive success of females Results from my marsh suggest that by breeding polygynously female redwings increase t h e i r chances of nesting success through cooperation in nest defence (see Chapter 6, 10). Fledgling success per female increases with increasing harem size and reaches a maximum around harems of 4-6 females, Lower success i n larger harems i s probably a conseguence of increasing negative e f f e c t s of high density of breeding females (see Chapter 8, F i g . 16). The increasing reproductive success of females with increasing harem size can be explained i n terms of differences i n the temporal and s p a t i a l arrangement of nesting i n harems of various sizes, Thus higher reproductive success of females from larger harems could be attributed to higher e f f i c i e n c y of t h e i r nest defense which i s most l i k e l y a conseguence of: (1) a larger number of females i n a clump; (2) smaller distance between the nearest nests; and (3) a greater degree of nesting synchrony. In addition, as w i l l be shown i n Chapter 8, also the presence of older experienced females and males i n large harems contribute to the greater reproductive success of highly polygynous females. 169 CHAPTER 8 The influence of previous breeding experience of red-winged blackbirds on female choice of a breeding s i t u a t i o n lJ3i£i2d]12ti2l_ The most obvious and most freguently studied c h a r a c t e r i s t i c s that are l i k e l y to influence females' choice of mates i n polygynous species are various ecological factors determining guality of males' t e r r i t o r i e s , c h a r a c t e r i s t i c s of males' behavior that influence reproductive output of females (e.g.. parental care and nest defense behavior), and the genetic guality of males that influences f i t n e s s of females through guality of their offspring (see reviews by Orians 1969, Wittenberger 1976, Emlen and Oring 1977, Searcy 1979). However, another important feature known to influence reproductive success of many monogamous species of birds i s the phenotypic va r i a t i o n i n behaviors related to reproduction as a conseguence of t h e i r d i f f e r e n t age (e.g. Lehrman and Wortis 1967, Burley and Moran 1979),. In general, older birds with previous experience are more e f f i c i e n t i n breeding than young inexperienced i n d i v i d u a l s . Because experience i s also l i k e l y to influence reproductive success of polygynous species, females should p r e f e r e n t i a l l y mate with older, experienced males. But because the evolution of polygyny has been generally associated 170 with great reduction of male parental care ( p a r t i c u l a r l y i n highly polygynous species usually females alone build the nest, incubate, and feed nestlings) i t would appear that the r o l e of experience of males should be r e l a t i v e l y less important. It i s possible, however, that i n sp i t e of this males of some polygynous species might s t i l l have an important influence on the f i t n e s s of females. Males might a s s i s t their females at least in two ways: (1) by defending nests against predators; and (2) i n highly polygynous species with intense intrasexual competition among males, a male might reduce the amount of disturbance to his females by e f f i c i e n t l y excluding conspecific intruders. In addition, i f older experienced males were capable of defending t e r r i t o r i e s of best gual i t y , then t h i s might also influence t h e i r mating success. This idea i s supported by the following evidences. F i r s t , male black grouse, Ly.rurus t e t r i x , appear to be f u l l y successful in attr a c t i n g females only after gaining necessary experience ( K r u i j t and Hogan 1967/ K r u i j t et a l . 1972),. Second, Wiley (1974) proposed that i n the sage grouse. Centrocere us urophasianus, older experienced females, which may return to the same mating centres i n consecutive years, might a t t r a c t young inexperienced indiv i d u a l s to these s i t e s . Thus female choice of a mate might be influenced not only by the behavior of males but also by other females. The l a s t evidence comes from the study of the indigo bunting, Passerina cyanea (Carey and Nolan 1975), where older males were more successful in acguiring and keeping mates than yearlings. In t h i s Chapter I w i l l investigate the influence of 171 previous age-related experience of male red-winged blackbirds on females' choice of breeding s i t u a t i o n s . Several features make redwings p a r t i c u l a r l y suitable for t h i s study. F i r s t , t h i s species i s highly polygynous and intrasexual competition among male redwings f o r limited breeding space i s intense (as shown by removal experiments, only a proportion of a l l males succeed i n establishing a t e r r i t o r y and breed; Beer and Tibbits 1950, Orians 1961, Peek 1971). Therefore, disturbance by non-t e r r i t o r i a l conspecific intruders might s i g n i f i c a n t l y influence reproductive success of females. Second, because nesting mortality due to predation i s very high i n marsh nesting redwings (Ricklefs 1969) and male redwings generally a s s i s t i n defense of t h e i r females* nests (Nero 1956b; t h i s study, see Chapter 2), th i s male contribution might increase nesting success of females. And t h i r d , because female redwings breed i n a clumped pattern (see Chapter 6), male contribution to defense of nests should not decrease subs t a n t i a l l y with increasing harem si z e . The purpose of th i s Chapter i s : (1) to test i f age-related experience of male red-winged blackbirds i s an important factor influencing females' choice of breeding s i t u a t i o n s ; (2) to investigate how female choice of a mate might operate i n terms of p r e f e r e n t i a l mating of females with older experienced males; (3) to examine what other factors might influence mating success of males. 1 7 2 Methods To test the possible impact of male age (experience) on female choice of a mate, I divided male redwings into 5 age categories. The youngest birds were yearling males that can be recognized from a l l older males because of their orange epaulets (adult males have bright red epaulets) and a varying amount of brown coloration on the rest of t h e i r plumage (see F i g . 13) . This i s the only category of birds i n which the age of a l l birds i s known. The second age category includes adult males that had either bred i n the marsh as yearlings i n a previous year or that appeared i n t h i s marsh for the f i r s t time. Individual males defend approximately the same t e r r i t o r i e s in subseguent years (only i n 2 of 24 occasions did a male defend a new t e r r i t o r y that did not overlap with the o r i g i n a l one; see also Nero 1956a, Searcy 1979, Lenington 1980). Therefore, the males that did not return to t h e i r t e r r i t o r i e s had probably died and the newcomers probably were surplus young males that had been waiting f o r the opportunity to breed. The following 3 age categories include males that had defended t e r r i t o r i e s i n this marsh as adults i n 1, 2, or 3 previous years. Males known to have been present i n th i s marsh the f i r s t year and at least the second year are considered (in terms of their breeding experience) as inexperienced and experienced, respectively. F i g . 13. Adult male, immature male, and female red-winged blackbird. 174 Results and Discussion 1. Influence of age of males on female choice of mates Since male redwings do not a s s i s t their females i n various nesting duties, the only other way they might increase female nesting success i s through nest defense and by reducing interference from intruding birds. This i s supported by observations of redwings mobbing various predators (Nero 1956b, Bent 1958, Robertson 1972), and by information on redwing-wren inter a c t i o n s (see Chapters 2, 3). Redwings are aggressive toward marsh wrens because wrens destroy their clutches and k i l l small redwing nestlings (Chapter 1). Moreover, i n my marsh male redwings chase marsh wrens more freguently than females do (Chapter 2). Also high rates of intrusions by non-breeding (surplus) males into t e r r i t o r i e s of resident males and aggressive chases of such intruders by t e r r i t o r y owners have been noted (e.g. Nero 1956b, Orians 1961). Therefore i t i s reasonable to conclude that male redwings may s i g n i f i c a n t l y influence reproductive success of th e i r females through t h e i r t e r r i t o r y and nest defense. However, the e f f i c i e n c y of t e r r i t o r y and nest defense by male redwings might be also influenced i n d i r e c t l y through t h e i r foraging e f f i c i e n c y . I assume that experienced birds foraging more e f f i c i e n t l y should have more time available f o r other a c t i v i t i e s related to reproduction. Therefore, I predict that more experienced males should a t t r a c t more mates and that t h e i r females should have, on 175 average, higher reproductive success compared to younger, l e s s experienced birds. A. Mating success as a function of age of males The influence of age (experience) of males on harem size i s shown i n Table 45. The youngest, yearling males attracted only 1 or 2 females (mean = 1.44) to th e i r t e r r i t o r i e s , whereas older males attracted as many as 9 females (mean = 4.72). Mating success of males i s s i g n i f i c a n t l y p o s i t i v e l y correlated with their previous breeding experience (Table 45). This re l a t i o n s h i p may be even more s i g n i f i c a n t than the above analysis suggests. Because I do not know the actual age of most males, I probably underestimated the age and experience of some of them. The p o s i t i v e correlation between harem s i z e and age of males could r e s u l t from two si t u a t i o n s . F i r s t , increases i n habitat guality might account for increasing harem size of males in successive years. I f this were true, then the mean harem size should increase from 1976 through 1979. However, t h i s i s not the case because the mean harem size was s i m i l a r i n the four years (Table 46). Second, harem s i z e of i n d i v i d u a l males might increase with t h e i r age as a conseguence of t h e i r increasing attractiveness to females, due to t h e i r increasing experience i n a c t i v i t i e s related to reproduction. This i s supported by the following analysis. If experience linked with age of i n d i v i d u a l males played no s i g n i f i c a n t role in their mating success then the t o t a l number of females for which some harems increased t 176 Table 45. Influence of age of male redwings on their harem size. Age category Mean harem ± SE Range of harems N Yearling males 1.44±0.18 1-2 9 Males present as adults first year 4.24±0.28 1-7 29 Males present as adults second year 4.87±0.46 3-8 15 Males present as adults third year 5.4010.93 4-9 5 Males present as adults fourth year 6 6 1 Note: Correlation between age of males and harem size: r=0.54; d.f.=57; p<0.0005. Table 46. Harem size of redwings in 1976 - 1979. Year Mean harem ± SE Range of harems N 1976 4.69±0.44 2-8 16 1977 4.5010.49 1-8 18 1978 4.1110.46 1-9 18 1979 4.2010.40 1-9 25 A l l years combined 4.35+0.22 1-9 77 178 should be s i m i l a r to that for which other harems decreased. But the sum of increases i n the number of females i s s i g n i f i c a n t l y greater than the sum of decreases (Table 47). Therefore, I conclude that, on average, i n d i v i d u a l males at t r a c t more females in subseguent years when they are more experienced. However, i f t h i s was a general trend, why did harems of some males become smaller i n subsequent years (see Table 47)? I t i s possible that these males were very old and that t h e i r decreasing attractiveness to females was a conseguence of deterioration of some of t h e i r q u a l i t i e s important f o r female reproduction. This idea i s supported by two f a c t s . F i r s t , mean harem size of these males i n a year before the decrease i n their harems (5.75 + 0.63) i s s i m i l a r to the mean harem of the oldest, most a t t r a c t i v e males (see Table 45). Second, i n 4 out of 5 cases the reduction i n harem size was also accompanied by the reduction i n t e r r i t o r y s i z e . However, when harem size of other males increased (14 cases) size of t e r r i t o r i e s of these males increased and decreased in the same number of cases, therefore, suggesting no consistent r e l a t i o n s h i p between changes i n harem and t e r r i t o r y s i z e . This indicates that as the males become too old, they become also l e s s e f f i c i e n t i n both female a t t r a c t i o n and the a b i l i t y to defend t e r r i t o r i e s of a similar s i z e . Therefore, i t i s possible that mating and possibly reproductive success of male redwings increases with th e i r age only up to a certain age ("most reproductive age") after which i t may s t a r t declining. Because such decline i n reproductive success of old i n d i v i d u a l s has been observed i n r i n g ^ b i l l e d g u l l s , Larus 179 Table 47. Changes in mating success of individual male redwings that were defending a territory in the marsh in at least two subsequent years. Number of females acquired Sum of changes in number of females when harem Male 1976 1977 1978 1979 Increased decreased 1 3 5 5 6 3 2 3 3 - - - -3 5 5 - - - -4 8 8 - - - -5 2 3 4 5 3 - ' 6 2 3 - - 1 -7 - 6 3 - - 3 8 - 3 3 4 1 -9 - 5 7 9 4 -10 - 7 6 4 - 3 11 - 1 4 5 4 -12 - 6 4 - - 2 13 - - 3 7 4 -14 - - 2 5 3 -15 - 5 7 2 -16 - - - 4 3 - 1 Total 25 9 * Comparison of total increase and decrease In number 2 females: X =7.53, 1-tail test, p<0.005. 1 8 0 delawarensis (Haymes and Blokpoel 1980) , f e r a l pigeons, Collimba l i v i a (Burley and Moran 1979), and great t i t s , Parus major (Dhondt 1971), t h i s explanation i s plausible. However, i f females generally returned to t h e i r o r i g i n a l t e r r i t o r i e s i n consecutive years, another explanation could be that fl u c t u a t i o n s i n overwinter s u r v i v a l of females between harems might r e s u l t i n reduced harems of some males. Because female redwings exhibit a strong t e r r i t o r y tenacity i n t h i s marsh, as w i l l be shown l a t e r in t h i s Chapter, t h i s l a t t e r hypothesis i s plausible. Data are not available to examine the two hypotheses further. B. Nesting success as a function of acje of males The amount of previous experience of males also has a s i g n i f i c a n t impact on mean reproductive success (number of young fledged) of both sexes (Fig. 14) . The older the males are the more young they fledge (r=0.61, d.f.=57, p<0.0005). This i s a conseguence of: (1) t h e i r increasing attractiveness to females re s u l t i n g i n increasing harem size as they become older (Table 44); and (2) the increasing mean fledging success of t h e i r females as the males become older (r=0.42, d.f-=57, p<0.001; Fig. 14). The increasing fledging success per female with age of males may have a complex causation. F i r s t , as a male becomes older, the number of females he acquires also increases. Because redwing nests are distributed i n a clumped pattern and the degree of clumping of redwing nests increases with F i g . 14. Q. to cn C "cn o 6 Relationship between mean number of f l e d g l i n g s per male or female and male age. Data from 1976 - 1979 were combined. 2 0 15 10 # — • f ledglings per male x — x f ledglings per female - 4.0 • 3.5 • 3.0 • 2.5 2.0 r 1.5 1.0 o c_» Q_ to cn c "o> <D o 6 5 Age of males (years) 00 182 increasing harem s i z e (see Chapter 6), the higher success of females i n large harems of more experienced males may be a consequence of t h e i r more e f f i c i e n t cooperation i n nest defense-Second, higher mean fledging success of females breeding with old males might be a r e s u l t of a greater contribution of such males to nests of th e i r females. This idea i s supported by the finding that, when harems of the same size of inexperienced and experienced males are compared, mean fledging success per female i s always higher for females from harems of experienced males (Table 48) . Third, i t i s possible that old experienced males may at t r a c t p a r t i c u l a r l y old females that are also experienced and, therefore, might be more e f f i c i e n t i n nest defence. This hypothesis i s supported by two facts: (1) old experienced females, that s t a r t nesting on average two weeks before inexperienced birds (Crawford 1977, Jackson 1971), are from large harems (Fig. 8, 11); and (2) old females are more successful i n nesting than young inexperienced birds (Crawford 1977). I do not have data to evaluate the r e l a t i v e s i g n i f i c a n c e of these three situations guantitatively, however, I suggest that they a l l have contributed to the higher reproductive success of females from harems of experienced males. 183 i Table 48. Influence of harem size on fledging success of Inexperienced and experienced male redwings and their females. Mean number of young fledged (sample size) Harem size per male per female of Inexp. experienced inexp. o* experienced cf 1 1.00 (6) - 1.00 (6) _ 2 3.33 (6) - 1.67 (12) -3 5.43 (7) 7.60 (5) 1.81 (21) 2.53 (15) 4 8.17 (6) 9.75 (4) 2.04 (24) 2.44 (16) 5 10.00 (8) 11.00 (5) 2.00 (40) 2.20 (25) 6 11.00 (2) 17.50 (2) 1.83 (12) 2.92 (12) 7 6.33 (3) 18.33 (3) 0.91 (21) 2.62 (21) 8-9 - 16.50 (2) - 1.94 (17) 184 2- How does the female choice of mates operate? Because age of males has a s i g n i f i c a n t influence on the reproductive success of their females, female redwings should have been selected to discriminate between males of d i f f e r e n t age (experience). Females that can discriminate and p r e f e r e n t i a l l y mate with older experienced males w i l l have the maximum return (measured i n number of young fledged) and t h e i r genotypes w i l l be favored by natural sele c t i o n . In the following part I w i l l discuss two guestions dealing with the mechanism of female choice of breeding situation,. A- How do females discriminate? The a b i l i t y of females to discriminate among males of d i f f e r e n t ages must be at least p a r t i a l l y innate. This i s indicated by the finding that yearling males always attracted only one or two females in spite of the fact that there were many naive (yearling) females present in the marsh every year. Therefore, most of the naive females could discriminate between males of d i f f e r e n t ages and, despite t h e i r lack of previous experience and presumably more intense competition among females for older males, p r e f e r e n t i a l l y mated with adult males. This suggests that females have been selected to respond more to s t i m u l i provided by older experienced males. Presumably even inexperienced females can e a s i l y discriminate between yearling and older males because of differences in their plumage (Fig. 185 13) and probably also differences i n behavior correlated with t h e i r experience-Learning may also be an important component of female choice of mates because i t would allow for adjustments throughout the l i f e of an i n d i v i d u a l . Because some females change t h e i r mates (even when the o r i g i n a l male i s alive) between subseguent breeding attempts during one or successive breeding seasons (Fankhauser 1964; Nero 1956 a; t h i s study, see Tables 49, 50), they might learn to associate t h e i r reproductive success (or lack of i t ) with cert a i n q u a l i t i e s of their males or their t e r r i t o r i e s and subsequently mate with males possessing such g u a l i t i e s . I f t h i s were true, then inexperienced females should more freguently make bad choices and conseguently have lower reproductive success but they should improve as they become older and more experienced. This i s supported by the fact that yearling inexperienced females have lower reproductive success than older birds (Crawford 1977). However, evidence i s also available which indicates that learning may not play an important role i n female choice of a breeding s i t u a t i o n . Because most female redwings renest i n the same t e r r i t o r y within a breeding season (Table 49) and also i n subseguent seasons, regardless of whether the o r i g i n a l male returned or not (Table 50), experience gained by females after they had made t h e i r f i r s t choice of a t e r r i t o r y does not seem to affect t h e i r subseguent choices. But the f i n a l test of the r o l e of the previous breeding experience on female choice of a breeding situation w i l l reguire the evaluation of females which Table 49. Proportion of females which renested with their original male or with a different male(a) during the same breeding season. Only data on color-banded females known to have nested at least twice during a given breeding season were included. Female renested with 1976 Percent (number) cases in 1977 1978 1979 A l l years combined Original male Another male 73.0 (27) 74.1 (20) 94.4 (17) 100.0 (9) 24.3 (9) Another two males 2.7 (1) 25.9 (7) 0.0 (0) 5.6 (1) 0.0 (0) 0.0 (0) 0.0 (0) 80.2.(73) 18.7 (17) 1.1 (1) Total 100.0 (37) 100.0 (27) 100.0 (18) 100.0 (9) 100.0 (91) 187 Table 50. Summary of data on t e r r i t o r y tenacity of color-banded female red-winged blackbirds between consecutive years i n si t u a t i o n s when the o r i g i n a l male returned and reestablished h i s t e r r i t o r y , or when the o r i g i n a l male was replaced by a new t e r r i t o r y holder. Percent (number) cases when female redwing O r i g i n a l renested i n a renested i n a t e r r i t o r y returned to the t e r r i t o r y of the fart h e r located holder o r i g i n a l t e r r i t . nearest neighbor t e r r i t o r y T o t a l Present 62.5 (20) 31.3 (10) 6.2 (2) 32 Absent (replaced) 62.3 (33) 30.2 (16) 7.5 (4) 53 Tota l 62.3 (53) 30.6 .(26) 7.1 (6) 85 188 switched males and moved into another t e r r i t o r y i n terms of th e i r previous breeding history ( p a r t i c u l a r l y with regard to th e i r previous success, the guality of the o r i g i n a l male and t h e i r new mate, and the return rates of other females from th e i r o r i g i n a l harem). There are no data available to evaluate the two components of female choice guantitatively. However, i t i s possible that both innate and learned a b i l i t y to discriminate may be involved. But whatever the mechanism i s , females do discriminate between males of d i f f e r e n t age and p r e f e r e n t i a l l y mate with older experienced males that a t t r a c t , on average, the largest harems (Table 45) . B. How do females assess guality of males? Any c h a r a c t e r i s t i c of a male might influence female choice of a mate only i f i t has a s i g n i f i c a n t impact on f i t n e s s of females, i f i t i s s u f f i c i e n t l y variable, and i f i t i s accurately assessable by a female p r i o r to mating (Searcy 1979). I have shown that age of males i s strongly correlated with f i t n e s s of females through fledging success (Fig 14). The second condition i s also f u l f i l l e d because i n a my marsh age of breeding males varies from one year to at least f i v e years (Fig- 15). The t h i r d condition, however, i s more d i f f i c u l t to test because female choice of a male might be influenced by a number of features, some of which might not be assessable p r i o r to mating. I assume that females can e a s i l y discriminate between yearling and older males p a r t i c u l a r l y because of differences in their 189 F i g . 1 5 . A g e d i s t r i b u t i o n o f t e r r i t o r i a l m a l e r e d w i n g s i n a s t u d y p o p u l a t i o n ( e s t i m a t e s a r e b a s e d o n 1 9 7 6 - 1 9 7 9 d a t a ) . to ~6 e c <D O 1_ D L 50 40 30 2 0 10 0 A g e c a t e g o r y 190 ! plumage (Fig. 13). However, how can a female assess the guality of adult males i n terms of th e i r future contribution to reproduction? In the following part I w i l l examine two possible ways. (a) Direct assessment of guality of males from t h e i r behavior In some birds previous experience in reproduction has a s i g n i f i c a n t e f f e c t on subseguent nesting. Thus experienced birds s t a r t nesting sooner (Coulson and White 1958, 1960; Lehrman and Wortis 1967; Kallander 1974; Crawford 1977; Haymes and Blokpoel 1980), lay larger clutches (Kluyver 1951, Coulson and White 1958, Perrins and Moss 1974, Crawford 1977, Haymes and Blokpoel 1980), and are more e f f i c i e n t parents that fledge more young (Richdale 1957, Crawford 1977) than young, inexperienced birds. Previous experience i n nesting might thus also play an important role i n redwings. With increasing age a male should become more experienced and thus more e f f i c i e n t i n a l l a c t i v i t i e s related to reproduction. Therefore, assuming that changes i n a l l such a c t i v i t i e s are d i r e c t l y correlated, a female might assess the guality of a male i n terms of his future contribution to her nesting from any features of male behavior that develop with his age. This hypothesis i s supported by a positive c o r r e l a t i o n between mating success of male redwings and the i n t e n s i t y of t h e i r courtship (Weatherhead and Robertson 1977b). Because i t has been shown i n other species of birds that for males with previous mating experience the time between the i n i t i a t i o n of courtship and laying i s shorter (e.g. Lehrman 191 and Wortis 1967), i t would seem reasonable to interpret t h i s c o r r e l a t i o n between harem size and courtship behaviour of redwings i n terms of differences i n courtship experience of old birds (those attracting on average large harems) and young birds (those a t t r a c t i n g on average small harems).. This int e r p r e t a t i o n i s supported by the fact that egg-laying starts sooner on t e r r i t o r i e s of males that exhibit more intense courtship (Weatherhead and Robertson 1977b). Because r e s u l t s of my study indicate that most of the early nesting females are from large harems ( i . e . mostly harems of older males (Fig. 11, Table 45), i t might be concluded that the i n t e n s i t y of courtship of males and consequently t h e i r mating success may at l e a s t p a r t i a l l y be determined by t h e i r previous breeding experience, In addition to evaluating the performance of male behavior, female redwings might also assess guality of males simply from the i r a v a i l a b i l i t y early in the season. Presumably, a mere presence of a male on the breeding grounds early i n the season integrates a great deal of information about his physical condition and age-related experience. In many species of birds old experienced males: (1) a r r i v e sooner at t h e i r breeding grounds (Coulson and White 1958, 1960; Coulson 1965, 1972; Potts 1966; Darley et a l . 1971); (2) usually return to th e i r o r i g i n a l t e r r i t o r i e s (Richdale 1957; Coulson and White 1958, 1960; Potts 1965; Krebs 1971); and (3) freguently remate with the same females or mate with females of s i m i l a r experience (Richdale 1957, Coulson and White 1958, Darley et a l . 1971, Richdale and Warham 1973). 192 Consequently the f i r s t breeding attempts occur on t e r r i t o r i e s of experienced males-Available evidence on redwings also suggests that previous experience influences their nesting. Testes growth s t a r t s e a r l i e r i n the spring i n adult males than in yearlings (Payne 1969). Old males usually return to t h e i r o r i g i n a l t e r r i t o r i e s (Nero 1956a, Searcy 1979, t h i s study),. Also older females develop ovaries sooner than yearlings (Payne 1969) and s t a r t nesting e a r l i e r (Crawford 1977, Jackson 1971). These differences between yearling (inexperienced) and older (experienced) birds, therefore, might have an important impact on mating pattern i n a breeding redwing population. Given t h i s , the early, experienced females should p r e f e r e n t i a l l y mate with experienced males to increase t h e i r f i t n e s s . The fact that older birds of both sexes are ready to breed f i r s t should f a c i l i t a t e t h i s . Young, inexperienced females that s t a r t nesting l a t e r choose among males of various ages but they should prefer experienced males because these males w i l l increase their chances of nesting success. However, because there i s apparently selection against very large harems that produce fewer fl e d g l i n g s per female (see Fig. 16), probably most inexperienced females are forced to mate with inexperienced males. I f t h i s i s true, then the guality of a male i n terms of his previous experience should be re f l e c t e d i n the date of laying the f i r s t clutch on his t e r r i t o r y . Further, i f the a b i l i t y of a male to breed early i n a season was correlated with his other g u a l i t i e s important for females, then there should be Fig. 16. Relationship between mean number of fledglings per male or female and harem size. Numbers i n brackets are sample sizes (1976 - 1979 data were combined). •—• f l e d g l i n g s p e r m a l e x — x f l e d g l i n g s p e r f e m a l e (8) ^ ( A ) H a r e m s i z e C O 194 a negative co r r e l a t i o n between the date of laying the f i r s t clutch and the size of a harem a male w i l l acguire. To te s t these hypotheses, I correlated the date on which the f i r s t egg was l a i d on t e r r i t o r i e s of i n d i v i d u a l males with (1) t h e i r age and (2) the eventual size of their harems. F i r s t , there i s a highly s i g n i f i c a n t negative c o r r e l a t i o n between the date of egg-laying and age of the male (Fig. 17) showing that older more experienced males attracted the f i r s t breeding females. This supports the idea that female choice may be influenced by age of males as r e f l e c t e d by t h e i r a b i l i t y to s t a r t breeding early in a season. However, there are two alte r n a t i v e explanations: (1) The e a r l i e r egg-laying on t e r r i t o r i e s of older males might be a r e s u l t of t h e i r greater attractiveness to f i r s t breeding females rather than th e i r p h y s i o l o g i c a l l y determined a b i l i t y to s t a r t nesting early. This i s supported by the fact that sperm development i n testes of males begins weeks before the f i r s t nestings (Payne 1969). However, because sperm reguires ad d i t i o n a l time for maturation outside of the testes (van Tienhoven 1961), and because there i s a great degree of variation in gonad development among i n d i v i d u a l males (Payne 1969), the f i n a l test w i l l require accurate data on the development of gonads of males of d i f f e r e n t ages and the i n i t i a t i o n of nesting on t h e i r t e r r i t o r i e s . (2) Because female redwings exhibit a strong t e r r i t o r y tenacity (Table 49, 50), the early breeding on t e r r i t o r i e s of older males may be a conseguence of the presence of old experienced females that s t a r t nesting early. I w i l l examine t h i s idea l a t e r i n 195 Fig. 17. Relationship between date of laying the f i r s t egg in a t e r r i t o r y and male age. Data from 1976 - 1979 were combined. O 3 0 A 2 0 1 0 r = - 0 . 3 7 3 p < 0 . 0 0 5 < 3 0 2 0 .a 3* 1 0 2 3 4 ( / a g e c a t e g o r y 1 9 6 t h i s Chapter. However, I suggest that t h i s hypothesis provides a probable explanation to the e a r l i e r nesting on t e r r i t o r i e s of older males. The test of the second hypothesis showed that there i s a highly s i g n i f i c a n t negative c o r r e l a t i o n between the date of f i r s t egg-laying and harem size (Fig- 8 ) . Therefore, i t would appear reasonable to conclude that the a b i l i t y of males to breed early (or to a t t r a c t early nesting females) i s also correlated with other male c h a r a c t e r i s t i c s that make such birds successful in a t t r a c t i n g more females. However, another explanation could be that large harems are established on male t e r r i t o r i e s with early nesting experienced females because these females a t t r a c t a d d i t i o n a l i n d i v i d u a l s to s e t t l e near them. These two explanations are not mutually exclusive. (b) Assessment of males from t e r r i t o r y guality The second way females might evaluate g u a l i t y of males i s i n d i r e c t l y by assessing guality of t h e i r t e r r i t o r i e s . If better males defended better t e r r i t o r i e s , then by choosing a better t e r r i t o r y a female would also choose a better mate. I f t h i s were true then males' mating success would be predictable from guality of their t e r r i t o r i e s . Because the term t e r r i t o r y guality includes several d i s t i n c t c h a r a c t e r i s t i c s , I w i l l examine them i n d i v i d u a l l y i n the following part. 1 9 7 T e r r i t orY s i z e ; T e r r i t o r y size might influence mating success of males i n d i f f e r e n t ways. For example, larger t e r r i t o r i e s might contain more food resources and conseguently attract larger harems. On the other hand, because t e r r i t o r y size might be determined also by the i n t e n s i t y of competition among males, t e r r i t o r i e s are l i k e l y to be smaller i n optimum habitats that should a t t r a c t more females (Holm 1973),. If female choice was influenced by t e r r i t o r y size i n any way, then harem s i z e and t e r r i t o r y s i z e should be correlated. In 3 out of 4 years there was a s i g n i f i c a n t positive c o r r e l a t i o n between the two factors but in one year there was no r e l a t i o n s h i p (Table 51). However, when data from a l l 4 years are combined, the c o r r e l a t i o n becomes highly s i g n i f i c a n t (Table 51). However, as calculated from the c o e f f i c i e n t of determination, this correlation accounts for only 19 % of the t o t a l v a r i a b i l i t y i n harem size between t e r r i t o r i e s . This indicates that t e r r i t o r y size plays a role i n female choice of breeding s i t u a t i o n but t h i s factor by i t s e l f may only p a r t i a l l y explain the great degree of variation between mating success of i n d i v i d u a l males. Two additional facts support t h i s conclusion; F i r s t , i f harem size were simply a function of a t e r r i t o r y s i z e then the density of females on male t e r r i t o r i e s of d i f f e r e n t sizes should be approximately the same,. This i s not the case because mean area per female decreases with increasing harem size (r=-0.56, d.f.=57, p<0.0005). Thus males with large t e r r i t o r i e s attracted r e l a t i v e l y more mates per unit area (female density on t h e i r t e r r i t o r i e s was greater) than males with small t e r r i t o r i e s . Second, because harem s i z e i s highly s i g n i f i c a n t l y correlated with age (experience) of males, 198 Table 51. Correlation analyses of the relationship between harem si z e and t e r r i t o r y s i z e . Year r P N 1976 0.16 >0.5 15 1977 0.62 <0.01 18 1978 0.57 <0.02 18 1979 0.40 <0.05 25 A l l years combined 0.43 <0.0005 76 Table 52. Relationship between changes i n harem size and t e r r i t o r y size of indiv i d u a l male redwings i n subsequent years. Number of cases when t e r r i t o r y s i z e harem size increased decreased Total increased decreased Total 7 7 14 1 4 5 8 11 19 Note: Fisher exact p r o b a b i l i t y , p=0.53 ( 2 - t a i l t e s t ) . 199 the c o r r e l a t i o n between harem size and t e r r i t o r y size might also be a r e s u l t of greater experience of older males that might defend larger t e r r i t o r i e s . Therefore, older males might acguire larger harems by acguiring larger t e r r i t o r i e s . However, males that acquired larger harems i n subsequent years had larger or smaller t e r r i t o r i e s i n the subseguent year in an egual number of cases (Table 52). P a r t i c u l a r l y those cases of males whose harems became larger while t h e i r t e r r i t o r i e s became smaller (7 out of 14 males) suggest that female choice i s influenced more by c h a r a c t e r i s t i c s of a male and possibly also by the guality of other females on his t e r r i t o r y . The positive correlation between harem siz e and t e r r i t o r y size (Table 51) i s consistent with the findings on female t e r r i t o r y tenacity. Because most females return to t h e i r o r i g i n a l t e r r i t o r i e s even when t h e i r o r i g i n a l male disappeared (Table 50), mating success of a new male i s l i k e l y to be determined not only by the number of females acguired by a previous male but also by the size of a t e r r i t o r y he i s defending (this may include the breeding"grounds of a l l or only a part of the resident females). In addition to t h i s study Howard (1977) also reported a positive c o r r e l a t i o n between harem size and t e r r i t o r y size for redwings. On the other hand, the lack of any r e l a t i o n s h i p between mating success of male redwings and the s i z e of t h e i r t e r r i t o r i e s was reported by Nero (1956b), Orians (1961, 1980), Case and Hewitt (1963), and Holm (1973). 200 Th§. a v a i l a b i l i t y of nest s i t e s ; In the study marsh a great majority of redwing nests were b u i l t i n c a t t a i l . Because the c a t t a i l vegetation i s r e l a t i v e l y uniform within redwing t e r r i t o r i e s , although i t s density may vary greatly among t e r r i t o r i e s , there i s ' no reason to believe that nesting s i t e s are l i m i t e d . Thus i t i s unlikely that females would discriminate among males on the basis of the a v a i l a b i l i t y of nest s i t e s on t h e i r t e r r i t o r i e s . Quality of vegetation on redwing t e r r i t o r i e s ; Density of vegetation i n the v i c i n i t y of redwing nests should influence th e i r success. I assume that females breeding i n sparser vegetation should have greater success because here they probably can protect t h e i r nests from marsh wrens more e f f i c i e n t l y . This i s supported by results on greater nesting success of redwing nests i n sparser vegetation (Weatherhead and Robertson 1977a; this study, see Chapter 5 ) . Therefore, males capable of securing t e r r i t o r i e s with sparser vegetation might at t r a c t more females. To test t h i s hypothesis, I correlated the mean vegetation density score obtained for i n d i v i d u a l t e r r i t o r i e s with the size of harems attracted to those t e r r i t o r i e s . There i s no c o r r e l a t i o n between these f a c t o r s , either i n 3 i n d i v i d u a l years or for a l l years combined (Table 5 3 ) . Therefore, I conclude that the density of vegetation on t e r r i t o r i e s does not influence mating success of males. 2 0 1 Table 53. Correlation analyses of the relationship between the density of vegetation on redwing territories and size of harems attracted to those te r r i t o r i e s . Year r P N 1977 0.36 >0.1 18 1978 -0.25 >0.2 18 1979 -0.14 >0.2 25 A l l years combined -0.00 >0.5 61 Table 54. Correlation analyses of the relationship between the density of marsh wren nests on territories of male redwings and their harem size. Year r P N 1976 -0.17 >0.5 13 1977 -0.46 >0.05 18 1978 0.22 >0.2 18 1979 0.00 >0.5 25 A l l years combined 0.10 >0.2 74 202 The a v a i l a b i l i t y of food resources; In the study marsh redwings generally forage outside t h e i r t e r r i t o r i e s , either i n adjacent seashore areas or on nearby uplands (Pieman et a l . , unpublished data). This i s also the case i n many other redwing populations (e.g. Orians 1961, Holm 1973). Therefore, differences i n food a v a i l a b i l i t y among t e r r i t o r i e s probably are not important. However, food a v a i l a b i l i t y on foraging grounds might influence females' choice of breeding s i t u a t i o n s through the location of t e r r i t o r i e s r e l a t i v e to the foraging grounds. T e r r i t o r i e s closer to foraging grounds should be more a t t r a c t i v e to females than those located farther away, But i n my marsh almost a l l t e r r i t o r i e s bordered with seashore and/or upland foraging grounds. Therefore, t h i s factor should be r e l a t i v e l y unimportant to females of t h i s marsh. The abundance of predators; By choosing a male defending a t e r r i t o r y with very few or no predators females could increase their chances of nesting success. Because marsh wrens are the most important nest mortality factor i n my marsh (see Chapter 4), female redwings should discriminate between t e r r i t o r i e s of d i f f e r e n t guality i n terms of marsh wrens. Using the density of marsh wren nests as an index of density of marsh wrens, I evaluated mating success of male redwings with regard to the density of marsh wren nests on t h e i r t e r r i t o r i e s (a survey of a l l marsh wren nests was conducted every year during the peak of redwing breeding), There i s no rel a t i o n s h i p between these factors (Table 54). This i s probably because females cannot assess marsh wren density at the c r i t i c a l time when they choose 203 t h e i r mates early i n the season. At t h i s time there are only a few marsh wren nests (both redwings and marsh wrens s t a r t nesting at approximately the same time), and a large proportion of females are inexperienced birds that are l i k e l y to make bad choices. In addition, because under low wren density conditions marsh wrens can probably avoid redwings better (see Chapter 3, 4), the assessment of habitat guality i n terms of marsh wrens may be d i f f i c u l t for female redwings under such circumstances. This i s supported by the fact that only i n the 1977 season, when the density of marsh wrens was higher compared to four other years (Pieman, unpublished data), was there a negative c o r r e l a t i o n between harem size and the density of marsh wren nests on male t e r r i t o r i e s which was close to the l e v e l of si g n i f i c a n c e (Table 54). Therefore, I cannot exclude the p o s s i b i l i t y that, under extremely high wren density conditions, the mating pattern i n a breeding redwing population might be influenced by the s p a t i a l pattern of d i s t r i b u t i o n of marsh wrens. To conclude, there i s no apparent r e l a t i o n s h i p between the examined c h a r a c t e r i s t i c s that determine t e r r i t o r y guality and the number of females that were attracted to t e r r i t o r i e s . Of the examined features only t e r r i t o r y size played a s i g n i f i c a n t role in the mating success of male redwings and t h i s role could be explained i n terms of a strong s i t e tenacity of females. Weatherhead and Robertson (1977a) also found no s i g n i f i c a n t r e l a t i o n s h i p between mating success of male redwings and t h e i r estimate of t e r r i t o r y guality. However, contrary to t h i s , 204 Searcy (1979) concluded that females* choice of breeding si t u a t i o n s i n redwings should be influenced by t e r r i t o r y guality, i n p a r t i c u l a r by the v u l n e r a b i l i t y of t e r r i t o r i e s to predation and by the a v a i l a b i l i t y of food resources on t e r r i t o r i e s . Searcy (1979) further concluded that female choice of a mate should not be influenced by h i s nest defense behavior, his contribution to feeding young, or by the v u l n e r a b i l i t y of a t e r r i t o r y to cowbird parasitism, which i s r e l a t i v e l y unpredictable. But I suggest that Searcy's conclusions, which are in dire c t contradiction to those of Weatherhead and Robertson's (1977a) and my study, were made on the basis of i n s u f f i c i e n t evidence. To show t h i s , I w i l l now summarize the most important evidence obtained by Searcy (1979) and discuss the problems associated with i n t e r p r e t i n g i t i n a way Searcy did. F i r s t , Searcy suggested that i f male nest defense e f f o r t influences female f i t n e s s and hence presumably female choice of a mate, then (1) strongly aggressive males should have lower predation rates, and (2) males which defend th e i r nests most vigorously should attr a c t most females. To test the f i r s t prediction, Searcy examined responses of males to human intruders. Results of his analysis indicated that males with greater nest defense e f f o r t had higher (nearly s i g n i f i c a n t ) predation losses than males with smaller nest defense e f f o r t . Therefore, Searcy concluded that male nest defense behavior against predators had l i t t l e e f f e c t on f i t n e s s of females. 205 Two major problems with t h i s conclusion are: (1) the study of male nest defense e f f o r t i s based on an assumption that redwing response to various intruders i s s i m i l a r . This assumption, however, may be wrong since I observed that redwings responded d i f f e r e n t l y to, for example, crows, peregrine falcons, and marsh wrens, which present danger of d i f f e r e n t degrees to the adult birds. Conseguently, response of redwings to human intruders might d i f f e r from that towards marsh wrens or other types of predators. (2) Because response of redwings to various predators may be determined by the rate of previous encounters with those predators (see Chapter 3), the nest defense vigor, as measured by Searcy (1979), may r e f l e c t simply the in t e n s i t y of predation on i n d i v i d u a l t e r r i t o r i e s . This p o s s i b i l i t y i s supported by Searcy's data showing that predation rates were higher on t e r r i t o r i e s of males with high nest defense ratings and by my observations that female redwings which have marsh wrens nearby are more l i k e l y to respond to nests of wrens (see Chapter 3). Conseguently, Searcy's data probably say l i t t l e about the s i g n i f i c a n c e of the actual male contribution to female nesting because they do not allow us to evaluate t h i s male contribution in r e l a t i o n to nest predation pressures. Therefore, an adeguate test w i l l reguire the comparison of success of redwing nests i n two d i f f e r e n t s i t u a t i o n s : (1) i n t e r r i t o r i e s with male contribution through nest defense; and (2) in t e r r i t o r i e s without t h i s male contribution (this might be achieved, for example, by removing the males after t h e i r females completed laying). But the two groups of t e r r i t o r i e s would have to have s i m i l a r predation 206 pressures, harem si z e s , and age composition of females. To test the second prediction, Searcy (1979) correlated the nest defense vigor of in d i v i d u a l males with the number of females they acguired. There was no relat i o n s h i p between these factors. Hence Searcy concluded that females do not assess male nest defense behavior when choosing a mate. But t h i s i n t e r p r e t a t i o n i s complicated by two f a c t s . F i r s t , as discussed above, the nest defense vigor of males may be determined by the rate of interactions with potential predators and hence by the density of predators in an area. Therefore, to estimate the actual g u a l i t y of a male, i n terms of his nest defense behavior, one would have to experimentally or s t a t i s t i c a l l y control for predator density i n various t e r r i t o r i e s . Without t h i s control, the data on male nest defense vigor, as collected by Searcy, probably cannot be used as a r e l i a b l e index of the guality of males. Second, Searcy (1979) has not considered the p o s s i b i l i t y that female redwings may not be able to assess the guality of habitats in terms of future nest predation rates. Assessment could be d i f f i c u l t , e s pecially i f marsh wrens are the most important nest predators, and i f t h e i r density i s low and wrens are thus r e l a t i v e l y inconspicuous. Because marsh wrens are present i n the area where Searcy conducted his study and have been seen pecking yellow-headed blackbird eggs there (Willson 1966), t h i s p o s s i b i l i t y has to be considered and examined before f i n a l conclusions can be made. On the basis of these considerations and evidence from my study (see Chapter 2) I 207 suggest that Searcy has underestimated, the role of male redwings in nest defense.. Conseguently, h i s conclusions concerning the role of male redwings in nest defense and the influence of the gual i t y of males on female choice of breeding situations are probably incorrect. In addition to his nest defense vigor t e s t , Searcy (1979) presented another kind of evidence supporting his prediction that t e r r i t o r y guality should be an important factor influencing female choice of breeding s i t u a t i o n s . Searcy speculated that i f females choose the breeding s i t u a t i o n on the basis of habitat guality ( i . e . male t e r r i t o r i e s ) , then d i f f e r e n t areas of a marsh should have consistent attractiveness to females i n di f f e r e n t years. In order to test t h i s hypothesis, Searcy correlated the number of females breeding i n " a r b i t r a r y t e r r i t o r i e s " i n three consecutive years (arbitrary t e r r i t o r i e s were defined as s t r i p s of equal length of shoreline, whose boundaries were drawn a r b i t r a r i l y ; i . e. regardless of actual boundaries of male redwing t e r r i t o r i e s ) . He obtained highly s i g n i f i c a n t p o s i t i v e correlations between harem sizes on these a r b i t r a r y t e r r i t o r i e s in the three years. This r e s u l t i s consistent with Searcy's hypothesis.. However, as noted also by Searcy, these correlations could also re s u l t from preferences of females to mate with the same males i n consecutive years,. But the fact that correlations between mating success of i n d i v i d u a l males i n consecutive years were weaker than those f o r the ar b i t r a r y t e r r i t o r i e s has led Searcy to conclude that t h i s a l t e r n a t i v e explanation i s un l i k e l y . 208 To examine t h i s problem further, Searcy correlated harem size on a r b i t r a r y t e r r i t o r i e s i n the f i r s t year with that i n the t h i r d year of his study. He hypothesized that i f female choice of breeding s i t u a t i o n was influenced primarily by male g u a l i t y , then the correlation between harems in the f i r s t year and the t h i r d year should become much lower because of higher losses of the o r i g i n a l males as compared to correlations between harems on a r b i t r a r y t e r r i t o r i e s i n one-year comparisons. Because the c o r r e l a t i o n f o r the two-year comparison was just as high as those for one-year comparisons, Searcy concluded that i t must be t e r r i t o r y guality and not the guality of males which influences female choice of breeding s i t u a t i o n . I suggest, however, that t h i s Searcy's conclusion i s probably wrong for the following reasons. F i r s t , an important aspect of redwing ecology, not examined by Searcy (1979), i s the fact that as males become older, they attract larger harems (see Table 45). Therefore, I would expect to find greater differences i n mating success of i n d i v i d u a l males between years, as a r e s u l t of th e i r increasing attractiveness to females, as compared with that of a r b i t r a r y t e r r i t o r i e s where such age-rel a t e d effects of male attractiveness are l i k e l y to be reduced by the incl u s i o n of parts of several male t e r r i t o r i e s . To examine t h i s p o s s i b i l i t y , I used data given by Searcy (1979) to calculate the mean harem siz e of actual and a r b i t r a r y t e r r i t o r i e s for the f i r s t two years of Searcy's study. In those years the density of female redwings was low, hence the e f f e c t s of male age on female choice of breeding s i t u a t i o n should have 209 been more pronounced.. Harems of 14 color-banded males increased from 1.21 females per male i n 1974 to 3.79 females i n 1975 (the average increase i s 2.58 females per male). But harems on 41 a r b i t r a r y t e r r i t o r i e s increased from 2.68 females per t e r r i t o r y to only 4.00 females i n the same years (the average increase i s 1.32 females per a r b i t r a r y t e r r i t o r y ) . This indicates that age also played a role i n mating success of male redwings i n Searcy's study marsh (harems of i n d i v i d u a l males increased more from 1974 to 1975 than harems of arbi t r a r y t e r r i t o r i e s ) . Therefore, differences i n the strength of correlations between harems i n consecutive years i n r e a l and a r b i t r a r y t e r r i t o r i e s cannot be interpreted as a support for Searcy's prediction on the influence of habitat guality on female choice of a breeding s i t u a t i o n . On the contrary, i n s p i t e of the fact that the e f f e c t s of age-related experience of males probably are greatly masked by a general increase in female population density, Searcy's r e s u l t s seem to indicate that male guality influences female choice of breeding s i t u a t i o n . To conclude, I suggest that the evidence presented by Searcy (1979) as a support to the view that female choice of breeding s i t u a t i o n i s influenced by t e r r i t o r y quality and not by male guality i s inadeguate. 3. What other factors might influence mating success of males? I have shown that age (experience) of males s i g n i f i c a n t l y influences female choice of breeding s i t u a t i o n . However, great differences i n mating success among males of any pa r t i c u l a r age 210 (Table 45) suggest that differences i n their age (experience) alone cannot explain a l l observed variation i n mating success of male redwings. P a r t i c u l a r l y the f a c t that some inexperienced males acguired large harems (up to 7 females) suggests that i n addition to t h e i r age some other factor(s) influencing female choice of breeding s i t u a t i o n must be involved. In the following part I w i l l discuss three hypotheses that might explain the observed differences^ The f i r s t hypothesis i s that because the exact ages of most males were not known, i t i s possible that in each age category the males that acguired the largest harems were older and thus more experienced birds (these birds may have been breeding somewhere else i n a previous year). This i s supported by the fact that mean harem size of adult males of known age (birds i n the f i r s t adult plumage) i s s l i g h t l y smaller (3.86 + 1.86) than that of a l l adult males that appeared in the marsh for the f i r s t time (4. 24 + 1. 53). On the other hand there i s strong evidence suggesting that differences i n mating success of males from any p a r t i c u l a r age category are not usually a conseguence of differences i n t h e i r previous experience: (1) Range of harems acguired by inexperienced adult males of known age (birds i n the f i r s t adult plumage) was the same as that of a l l adult males breeding in t h i s marsh for the f i r s t time (1-7 females). (2) Surviving males usually return to t h e i r o r i g i n a l t e r r i t o r i e s year after year (Nero 1956a, Searcy 1979, t h i s study). I f t h i s i s a general trend i n redwings, then new males establishing a t e r r i t o r y must generally be inexperienced birds that have been 211 waiting for the opportunity to reproduce- (3) I f the largest harems within i n d i v i d u a l age categories were acguired by more experienced males then these males and t h e i r females should have, on average, r e l a t i v e l y greater reproductive success than other males i n the same age category (mean success per male and female increases with increasing age of males; Fig- 14). This can be tested by comparing differences i n mean fledging success of harems of the same siz e of experienced and inexperienced males. I f the inexperienced males that acguired large harems were actually experienced birds, then they should have s i m i l a r average fledging success as experienced males that acguired the same number of females. But there should be s i g n i f i c a n t differences i n the mean fledging success of experienced and inexperienced males that acquired small harems because of t h e i r d i f f e r e n t contribution to nests of t h e i r females. Table 48 shows that, contrary to t h i s prediction, the largest differences i n mean fledging success per either sex were between the largest harems of experienced and inexperienced males. Therefore, I reject the hypothesis that i n i n d i v i d u a l age categories males with the largest harems were older and more experienced birds. The second hypothesis i s that variation i n mating success among males of the same age might be the r e s u l t of differences i n their genetically controlled attractiveness. If attractiveness and consequently mating success of males was determined by th e i r genetic quali t y , then th e i r mating success r e l a t i v e to other males of the same age from a given population should remain similar i n subseguent years. I have no data to 212 test t h i s hypothesis, but suggest that the g e n e t i c a l l y controlled attractiveness of males should not be an important c h a r a c t e r i s t i c that females evaluate when choosing mates. There are two reasons for t h i s suggestion. F i r s t , i f mating success of males was larg e l y determined genetically, then males should be selected to deceive the females when possible by signaling that they possess better g u a l i t i e s than they actually do (Searcy 1979). But because males have a s i g n i f i c a n t input i n reproduction, as can be seen from a larger number of young fledged i n harems of experienced than inexperienced males (Table 48), females should be selected to assess guality of males from those c h a r a c t e r i s t i c s that males cannot feign. Second, males' and females' reproductive strategies c o n f l i c t i n terms of the relat i o n s h i p between harem siz e and t h e i r fledging success. The maximum number of young the males can fledge on average are from harems larger than i s optimum for females (Table 48, Fig. 16). Because the most freguent harem siz e i s the optimum harem size i n terms of female reproductive output but l e s s than the optimum in terms of male reproductive output (Figs. 16, 18) , i t i s reasonable to conclude that females control harem s i z e . The evolution of harem s i z e thus appears to have been driven by female strategy that sets the upper l i m i t to the degree of clumping rather than by the optimum strategy of males. The innate capacity of males to a t t r a c t females (males should be selected for the a b i l i t y to a t t r a c t as many females as possible), therefore, should not be an important feature influencing female choice. F i g . 18. (1) Frequency d i s t r i b u t i o n of harems of d i f f e r e n t s i z e s ; and (2) proportion of females breeding i n harems of d i f f e r e n t s i z e s . 1 2 3 4 5 6 7 8 9 H a r e m s i z e ro 214 This view i s also supported by evidence discussed l a t e r that i t i s the previous breeding history of a t e r r i t o r y rather than the attractiveness of males that i n i t i a l l y determines their mating success. However, I cannot exclude the p o s s i b i l i t y that male attractiveness and mating success may be to a c e r t a i n degree genetically controlled. The c r u c i a l test that would show that v a r i a t i o n i n mating success may p a r t i a l l y be a conseguence of genetic differences among males would be to demonstrate that male attractiveness i s heritable. The t h i r d hypothesis i s that differences i n mating success of males of the same age might be a conseguence of the tendency of females to clump at cert a i n places regardless of male guality. Two factors that might influence the degree of female clumping on males' t e r r i t o r i e s are: (1) guality of a t e r r i t o r y , and (2) the presence of old experienced females. I already concluded that t e r r i t o r y guality i s not important i n determining mating success of males. On the other hand, there are two reasons to believe that older experienced females might determine the mating success of males. F i r s t , females i n t h i s marsh, reduce the impact of marsh wrens on t h e i r nesting success by breeding close to each other and thus by cooperating i n nest defense (see Chapter 6). Because old females st a r t nesting f i r s t , young females should p r e f e r e n t i a l l y join already established older females to increase their nesting success. Second, older females are more successful in nesting than young birds (Crawford 1977), presumably because of experience gained in previous breeding. Thus by joining these experienced birds, inexperienced females might further increase th e i r nesting 215 success through more e f f i c i e n t cooperation i n nest defence, Therefore, differences i n mating success of males of the same age might be explained i n terms of differences i n d i s t r i b u t i o n of old experienced females among t h e i r t e r r i t o r i e s . This view i s supported by two kinds of evidence. F i r s t , i f the hypothesis on the role of experience of females i n mating success of males was r i g h t , then there should be a p o s i t i v e c o r r e l a t i o n between harem size and the presence of old experienced females i n a harem. I tested t h i s hypothesis i n d i r e c t l y from dates of i n i t i a t i o n of egg-laying (early laying females are old experienced birds; Crawford 1977, Jackson 1971) by females from d i f f e r e n t size harems of inexperienced (only males i n adult plumage were included) and experienced males. The presence of early laying (old) females i s correlated with large harems for both inexperienced and experienced males (Table 55) . The second support comes from the fact that the patterns of s e t t l i n g of females (as indicated by t h e i r i n i t i a t i o n of nesting) on t e r r i t o r i e s with large and small harems d i f f e r greatly. In large harems successive females i n i t i a t e nesting more synchronously and tend to s e t t l e closer to each other than females i n small harems (see Chapter 7). This and the fact that the date of i n i t i a t i o n of egg-laying i s negatively correlated with harem size (Fig. 8, Table 55) suggest that t e r r i t o r i e s with f i r s t breeding (experienced) females are more a t t r a c t i v e and therefore more f i e r c e l y competed for by females. On the other hand, t e r r i t o r i e s a t t r a c t i n g the smallest harems probably 216 Table 55. Relationship between date of laying the f i r s t egg on territories of inexperienced and experienced males and their harem size. (Only males in adult plumage were included in the analysis on inexperienced males.) Date of laying the f i r s t egg on territories of Inexperienced males experienced males Harem s i z e mean ±SE N mean ±SE N 1 32 - 1 - - -2 2 8 . 5 ± 9 . 5 2 - - -3 2 3 . 1 ± 1 . 2 7 2 8 . 2 ± 2 . 0 5 4 2 4 . 0 ± 2 . 1 6 2 7 . 3 ± 1 . 4 4 5 2 1 . 9 ± 1 . 8 8 2 2 . 6 ± 1 . 4 5 6 2 4 . 0 ± 6 . 0 2 2 1 . 0 ± 7 . 0 2 7 2 0 . 3 ± 2 . 2 3 18.7 ± 6 . 2 3 8 - - 22 - 1 9 - - - 21 - 1 Day 1= A p r i l 1 s t N o t e : C o r r e l a t i o n between d a t e o f e g g - l a y i n g and harem s i z e : (1) f o r i n e x p e r i e n c e d m a l e s ; r = - 0 . 3 5 1 , df=27, p < 0 . 0 5 ; (2) f o r e x p e r i e n c e d m a l e s ; r = - 0 . 5 1 8 , df=19, p < 0 . 0 1 . 217 are l e a s t desirable because redwings (probably young females which did not succeed i n joining harems with experienced birds) s e t t l e there very l a t e (Fig- 8, Table 55). Because these differences between harems of various sizes cannot be explained by d i f f e r e n t guality of t e r r i t o r i e s or males (in the next section I w i l l show that the pattern of d i s t r i b u t i o n of experienced females i s not usually determined by males), i t i s reasonable to conclude that the early nesting (experienced) females s i g n i f i c a n t l y influence choice of a t e r r i t o r y by other, l a t e r nesting birds and thereby influence mating success of males. Therefore, i t i s important to establish what determines the pattern of d i s t r i b u t i o n of older experienced females. (a) Experienced females might clump i n the best guality habitat. It has been established that (1) older female redwings start nesting, on average, two weeks before young inexperienced birds (Crawford 1977, Jackson 1971), and (2) the early nesting females are eventually i n large harems (Figs. 8, 11)-Therefore, i f the proposed hypothesis was r i g h t , then the largest harems (which presumably have most experienced females) should be established on the best t e r r i t o r i e s . However, the lack of any s i g n i f i c a n t relationships between harem siz e and various features determining guality of a t e r r i t o r y (except i t s size) that were discussed e a r l i e r does not support t h i s hypothesis. Other evidence against t h i s hypothesis i s the f a c t that changes in t e r r i t o r i a l males that involved yearlings also resulted i n d r a s t i c changes i n the number of females attracted by these males (Table 56). This suggests that female choice of 218 breeding situation i s influenced by the guality of males rather than by habitat guality. (b) Mate f i d e l i t y and the tendency to nest i n the same area i n subseguent years might determine the d i s t r i b u t i o n of experienced females among males' t e r r i t o r i e s . Data on color-banded females from my marsh show that most female redwings renested i n the same t e r r i t o r y within a season (Table 49) and also between seasons, regardless of whether or not the o r i g i n a l male returned in a following year (Table 50). Also other available evidence on redwings suggests that mate f i d e l i t y and t e r r i t o r y tenacity are well developed i n t h i s species (Nero 1956a, Jackson 1971, Searcy 1979). Therefore, i t i s possible that mating success of new t e r r i t o r y holders and conseguently also t h e i r mating success i n subseguent years might be determined by the number of females that were breeding on t h e i r t e r r i t o r i e s i n a previous year and returned. New t e r r i t o r i a l males thus might " i n h e r i t " harems of previous t e r r i t o r y holders which they replaced, This hypothesis could be tested by comparing mating success of o r i g i n a l and new males that were breeding on the same t e r r i t o r i e s . If mating success of a male was i n i t i a l l y determined by the number of females an o r i g i n a l male had acguired, then there should be a positive c o r r e l a t i o n between mating success of new and o r i g i n a l t e r r i t o r y holders. To test t h i s , I selected t e r r i t o r i e s which remained approximately the same i n s i z e and position between subsequent years but whose male owners changed. Because yearling males are f a r l e s s 219 T a b l e 5 6 . Number of f e m a l e s a t t r a c t e d t o t h e same t e r r i t o r i e s i n s u c c e s s i v e y e a r s by d i f f e r e n t m a l e s . T e r r i t o r y Harem s i z e i n Sum o f changes i n number of f e m a l e s when harem number 1976 1977 1978 1979 i n c r e a s e d d e c r e a s e d 1 5 4 * 2 * 1 4 2 6 7 * 1 5 5 6 3 7 7 4 - - 3 4 4 6 - - 2 -5 6 3 - - - 3 6 5 5 ' - - - -7 5 4 4 - - 1 8 5 7 - - 2 -9 - 5 5 - - — 10 - 3 * 2 4 2 1 11 - 8 5 - - 3 12 - 3 3 - - -13 - 3 2 - - 1 14 - - 9 8 - 1 15 - - 6 6 - -T o t a l 11 *** 23 Y e a r l i n g male as a t e r r i t o r y h o l d e r A s i n g l e male d e f e n d i n g t h i s t e r r i t o r y i n 1978 was r e p l a c e d by 3 new males t h a t a c q u i r e d a t o t a l of 8 f e m a l e s Comparison o f t h e t o t a l i n c r e a s e and d e c r e a s e i n number 2 f e m a l e s : X = 4 . 2 4 , 1 - t a i l t e s t , p < 0 . 0 2 5 . 220 a t t r a c t i v e to females than males i n adult plumage (Table 45), I reduced v a r i a t i o n due to this factor by evaluating data on adult males only. There i s a s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n between mating success of new and o r i g i n a l adult males (Fig. 19). However, the great degree of variation in t h i s r e l a t i o n s h i p (Fig. 19), and consequently a small c o e f f i c i e n t of determination (r2=0.34) suggest that mating success of new males can be explained only p a r t i a l l y i n terms of the nesting history on t h e i r t e r r i t o r i e s i n a previous year. I suggest that other factors that might also influence mating success of males are: (1) v a r i a t i o n in overwinter survival of females from d i f f e r e n t harems; and (2) the tendency of some females to renest i n d i f f e r e n t t e r r i t o r i e s (Nero 1956b; Fankhauser 1964; t h i s study, see Tables 49, 50) . The p o s i t i v e correlation between harem s i z e of new and o r i g i n a l males supports the idea that mating success of males i s i n i t i a l l y determined by older experienced females that return to renest i n t h e i r o r i g i n a l t e r r i t o r i e s . This view i s also supported by Searcy's (1979) r e s u l t s on a s i m i l a r number of female redwings nesting in subseguent years i n the same portions of his marsh ("arbitrary t e r r i t o r i e s " ) . However, I suggest that the previous nesting history of t e r r i t o r i e s determines mating success of adult males only. New yearling t e r r i t o r y holders would acguire only one or two females even i f there were as many as 7 females breeding on that t e r r i t o r y with the o r i g i n a l adult male i n a previous year (Table 56). Attractiveness i n terms of age, therefore, also s i g n i f i c a n t l y influences the i n i t i a l mating 221 Fig. 19. Relationship between harem size of previous adult t e r r i t o r y holders and new adult males that replaced them. i _ O E CD C E CD o 8 7 6 5 4 3 2 Y = 0 . 6 0 ( X ) * 1 . 7 3 r = 0 . 5 8 p < 0 . 0 1 — 3 8 H a r e m o f o r i g i n a l m a l e 222 success of males. Changes i n t e r r i t o r y holders resulted, on average, i n s l i g h t l y smaller harems (Fig. 19, Table 56). This suggests that high rates of changes of t e r r i t o r i a l males i n a given population should r e s u l t on average i n smaller harems. On the other hand, high return rates of males and conseguently t h e i r presence for several successive years w i l l r e s u l t i n larger harems (Tables 45, 47). Therefore, the rate of changes of t e r r i t o r i a l male redwings might have a s i g n i f i c a n t influence on the degree of polygyny (as r e f l e c t e d by mean harem size) i n a population. Because the presence of older experienced females probably influences the attractiveness of a t e r r i t o r y to other females, t h i s argument might also be extended to female redwings. Thus i t i s possible that observed differences i n the degree of polygyny among redwing populations (Appendix III) might p a r t i a l l y be explained by di f f e r e n t mortality rates of resident birds. 223 CONCLUSIONS FROM SECTION II Marsh wrens have an important e f f e c t on redwing reproductive success and t h i s negative e f f e c t i s r e f l e c t e d i n redwing reproductive strategy. Contagious nesting by redwing females s i g n i f i c a n t l y reduces impact of wrens. Individuals f a i l i n g to adopt t h i s strategy have lower reproductive success. The hypothesis i s proposed that marsh wrens may have influenced the evolution of a clumped pattern of nesting by redwings. The temporal and s p a t i a l patterns of harem formation, as re f l e c t e d by the dates and places of i n i t i a t i o n of nesting by successive females, vary with harem size of male red-winged blackbirds. Females s e t t l e f i r s t and more synchronously on t e r r i t o r i e s of males attracting large harems, and s e t t l e l a t e and less synchronously on t e r r i t o r i e s acguiring small harems. In addition, females i n large harems s e t t l e closer to each other than females i n small harems. This i s a conseguence of: (1) a stronger clumping tendency of the early breeding females (mostly old experienced b i r d s ) , and (2) a tendency of la t e nesting females (mostly inexperienced birds) to s e t t l e near the early females in larger harems. The stronger clumping tendency by la t e females i n larger harems could be explained only by some kind of stimulation of these females by the early breeding birds because i n small harems with l a t e females only, distances between nests of nearest neighbors were much larger. These findings suggest that increasing reproductive success per female 224 with increasing harem size i s a conseguence of better cooperation i n nest defense against marsh wrens among females i n large harems, presumably because of a greater number of birds involved, greater degree of synchrony i n nesting, and shorter distances between nests. Female red-winged blackbirds are s i g n i f i c a n t l y more l i k e l y to mate with older, more experienced males, and t h i s p r e f e r e n t i a l mating s i g n i f i c a n t l y increases t h e i r f i t n e s s as measured by the number of f l e d g l i n g s . Females appear to assess the guality of males d i r e c t l y , from behavioral features that develop with age of males, and not from the guality of habitat. However, the guality of males i n terms of t h e i r experience cannot explain a l l variation in t h e i r mating success. Another factor influencing mating success of males i s the presence of old experienced females which increase the attractiveness of male t e r r i t o r i e s to other females, thereby increasing the number of mates for the male. The pattern of d i s t r i b u t i o n of these experienced females i s determined by t h e i r tendency to renest i n the same t e r r i t o r i e s within a season as well as i n consecutive years, regardless of whether the o r i g i n a l male returned or was replaced. Conseguently mating success of new males i s i n i t i a l l y determined by the number of females acguired by previous t e r r i t o r y holders. Yearling males, however, always acguired only one or two females even i f a previous t e r r i t o r y holder attracted as many as 7 mates.' Hence the attractiveness of males also s i g n i f i c a n t l y influences t h e i r i n i t i a l mating success. In addition, mating success of males increased s l i g h t l y 225 with the size of their t e r r i t o r i e s , . This finding i s consistent with the evidence on strong s i t e tenacity of females which indicates that by acguiring a larger area (with more returning resident females) males should increase t h e i r mating success. 21St\ S E C T I O N I I I . 226 The evolution of polygyny i n red-winged blackbirds Interference from l o n g - b i l l e d marsh wrens presents an important s e l e c t i v e force acting on redwing reproductive t a c t i c s i n my study marsh. Behavioral interactions between redwings and marsh wrens reduce interference between these birds by s p a t i a l l y segregating t h e i r breeding areas and by keeping marsh wrens from the v i c i n i t y of i n d i v i d u a l redwing nests. In addition, the role of female clumping i n reducing the impact of marsh wrens indicates that the clumped pattern of nesting i s probably another important adaptation evolved by redwings i n response to high nest predation rates caused by marsh wrens. These, and my other findings which I discussed in the previous two sections, have important implications for the theory explaining the evolution of polygyny i n redwings. The purpose of t h i s l a s t section i s to: (1) review the e a r l i e r proposed hypotheses on the evolution of polygyny i n marsh-nesting redwings, and evaluate them i n the l i g h t of my evidence (Chapter 9 ) ; (2) discuss the role of nest predation by marsh wrens as a driving force in the evolution of polygyny i n red-winged blackbirds and to propose a s p e c i f i c model describing the adaptive value of redwing polygyny i n terms of redwing-wren interference (Chapter 10); (3) examine the major assumption of the proposed model on the degree of sympatry between blackbirds and marsh wrens (Chapter 11). 227 CHAPTER 9 The e a r l i e r proposed hypotheses on the evolution of polygyny i n red-winged blackbirds and t h e i r evaluation The adaptive value of polygyny i n marshy-nesting redwings has been intensively studied, p a r t i c u l a r l y i n the l a s t 20 years. However, i n spite of a large amount of data available from di f f e r e n t studies, conclusions reached by various authors are inconsistent and sometimes even appear to be i n d i r e c t contradiction. This can best be seen from the four d i f f e r e n t hypotheses proposed by d i f f e r e n t authors to explain the evolution of polygyny i n redwings (Orians 1969, 1972; Wittenberger 1976; Altmann et a l . 1977; Weatherhead and Robertson 1979) . I w i l l now discuss these and evaluate them i n d i v i d u a l l y . 1 - Heterogeneous d i s t r i b u t i o n of food as a factor selecting for polygyny i n marsh-nesting passerines Verner and Willson (1966) suggested that i n simple, two-dimensional habitats, such as marshes and p r a i r i e s , the productivity of avian food resources should be higher than i n three-dimensional habitats because i t i s concentrated into a narrow v e r t i c a l b elt. Because food productivity i s l i k e l y to be p o s i t i v e l y correlated with variations in i t s d i s t r i b u t i o n , Verner and Willson (op. c i t . ) and l a t e r also Orians (1969) proposed that the presumably great differences i n food 228 a v a i l a b i l i t y between t e r r i t o r i e s i n a marsh are l i k e l y to exceed the "polygyny threshold" and conseguently should select for polygyny in marsh nesting passerines. When applied to redwings, thi s hypothesis i s supported by the fact that polygyny i s more strongly developed i n the more productive marshes i n the semi-a r i d areas of western North America (Orians 1972, 1980). On the other hand, i n most marshes redwings freguently forage outside t h e i r t e r r i t o r i e s (Orians 1961, Case and Hewitt 1963, Voigts 1973) or mainly away from t h e i r t e r r i t o r i e s (Williams 1940; Nero 1956a; Wiens 1965; Snelling 1968; Robertson 1972; Holm 1973; Weatherhead and Robertson 1977a; Pieman et a l . , unpublished data) which suggests that differences i n the d i s t r i b u t i o n of food between t e r r i t o r i e s i s not a major factor, d r i v i n g the evolution of redwing polygyny. Additionally, i n species defending all-purpose t e r r i t o r i e s (Brown 1975), the s i z e of t e r r i t o r i e s i s presumably adjusted to the available food resources (e.g. Stenger 1958, Brown 1964). Therefore, i f the p r o b a b i l i t y of successful breeding i n redwings were influenced primarily by the food resources on t e r r i t o r i e s , then the s i z e of t e r r i t o r i e s should be inversely correlated with food production (Orians 1980). Because the "food hypothesis" of Verner and Willson (1966) and Orians (1969) predicts that the degree of polygyny exhibited by d i f f e r e n t redwing populations should increase with the increasing food productivity of a marsh, the average harem size for di f f e r e n t redwing populations should be inversely correlated with mean t e r r i t o r y size (which r e f l e c t s the productivity of a marsh, according to the theory). To test t h i s p r ediction, I examined data from various marshes (the 229 information on these marshes i s given in Appendix I I I ) . I t appears that, contrary to the prediction made from the "food hypothesis", there i s a weak (but not s i g n i f i c a n t ) p o s i t i v e c o r r e l a t i o n between the average harem si z e and t e r r i t o r y size (Fig. 2 0 ) . Thus i t seems that a high degree of polygyny i n redwings i s more freguently associated with large t e r r i t o r i e s and hence rather with marshes of lower productivity. The interpretation of t h i s analysis i s complicated by the fac t that the amount of foraging within and outside t e r r i t o r i e s varies greatly between populations. I t i s possible that male redwings defend small t e r r i t o r i e s i n habitats where most foraging i s done away from t e r r i t o r i e s , whereas i n marshes where redwings forage mostly within t h e i r t e r r i t o r i e s the size of t e r r i t o r i e s might generally be larger. However, in my study marsh, where redwing t e r r i t o r i e s are, on average, at least roughly four times larger than in other studied marshes (see Apendix I I I ) , female redwings forage almost exclusively outside t h e i r t e r r i t o r i e s (Pieman et a l . , unpublished data). Thus the extremely large t e r r i t o r i e s i n t h i s marsh cannot be explained i n terms of a greater amount of foraging by redwings within t h e i r t e r r i t o r i e s . Therefore, I suggest that the si z e of redwing t e r r i t o r i e s may possibly be determined by a number of f a c t o r s , such as the food productivity of a marsh, the r e l a t i v e amount of foraging by redwings within and outside t e r r i t o r i e s , and probably also by the t o t a l area of suitable habitat available for breeding r e l a t i v e to the area of suitable foraging grounds. The evidence from d i f f e r e n t marshes on redwings foraging F i g . 20. Mean harem s i z e as a function of mean t e r r i t o r y s i z e for d i f f e r e n t redwing populations (data on redwing populations included i n th i s analysis are given i n Appendix I I I ) . 231 outside t h e i r t e r r i t o r i e s , and the analysis of the r e l a t i o n s h i p between harem size and t e r r i t o r y s i z e suggests that variations in the a v a i l a b l i l i t y of blackbird food resources between t e r r i t o r i e s i s not an important factor driving the evolution of polygyny i n redwings. A similar conclusion was also reached by Wittenberger (1976)-. This, however, does not preclude the p o s s i b i l i t y that the abundance of food resources i n marshes and nearby uplands has played an important role i n the evolution of polygyny i n redwings. On the contrary, because the evolution of polygyny i s associated with the reduced r o l e of males i n nesting, and, in redwings, also with much higher density of females breeding on male t e r r i t o r i e s , high productivity of food in and/or near marshes i s a necessary precondition f o r the evolution of polygyny i n redwings and other marsh-nesting birds. 2- Different joredation rates i n marshes and uplands as a factor s e l e c t i n g f o r polygyny i n marsh-nesting redwings Wittenberger (1976) proposed that the evolution of polygyny in redwings may have been driven by di f f e r e n t nest predation rates on marsh and nearby upland redwing t e r r i t o r i e s . The available evidence shows that: (1) marshes provide nesting s i t e s which are safer from predators than those in uplands (Case and Hewitt 1963, Robertson 1972); and (2) i n most marshes female redwings forage frequently or mostly away from t h e i r t e r r i t o r i e s (e.g. Holm 1973). Therefore, the greater success of redwings breeding i n marshes i s a r e s u l t of t h e i r reduced v u l n e r a b i l i t y to predation rather than increased a v a i l a b i l i t y of food 232 resources (Wittenberger 1976). This i s , moreover, supported by the fact that when given a choice between breeding in a marsh of low productivity or i n nearby upland habitat of high productivity, redwings choose marsh (see Holm 1973). Conseguently, Wittenberger (1976) suggested that polygyny i n marsh-nesting blackbirds might have evolved as a r e s u l t of preferences of females for males defending higher guality marsh t e r r i t o r i e s . However, there are several reasons why t h i s hypothesis does not provide an adeguate explanation. F i r s t , i f polygyny i n marsh-nesting redwings has evolved as a r e s u l t of the d i f f e r e n t guality of marsh and upland t e r r i t o r i e s , then i n any given area male redwings which succeeded i n establishing t e r r i t o r i e s i n more suitable marsh habitats should, i n general, attract larger harems. On the other hand, males defending upland t e r r i t o r i e s should freguently be unmated or monogamous. It appears, however, that the mating success of males within i n d i v i d u a l marshes varies greatly, and some t e r r i t o r y holders may even f a i l to acguire any mates at a l l (see Appendix I I I ) . Moreover, males defending upland t e r r i t o r i e s are commonly polygynous. This can be seen from Blakley's (1976) study showing that i n Iowa male redwings breeding i n uplands acguired harems of 1-4 females (mean=2.7 females), and also from Dolbeer's (1976) data from Ohio, where the average harem size of upland redwings was 2.1 females per male. This suggests that the var i a t i o n in mating success between i n d i v i d u a l males i n marshes and uplands i s a conseguence of some features operating within these habitats. 233 Second, because predators operating within marshes and uplands d i f f e r greatly, redwings breeding i n these two types of habitats should be exposed to d i f f e r e n t s e l e c t i v e pressures. In upland habitats, redwing nests are usually e a s i l y accessible to a variety of land predators which are not present i n marshes (Robertson 1972, Howard 1977). Also, i n addition to taking contents of redwing nests, these upland predators may freguently k i l l the breeding females (Blakley 1976), which suggests that redwings breeding i n uplands cannot e f f i c i e n t l y reduce nest predation through cooperation i n nest defense,. Therefore, female redwings breeding i n uplands should presumably be selected for c r y p t i c nesting which could be achieved through: (1) the dispersed pattern of d i s t r i b u t i o n of the breeding females (this should select against a high degree of polygyny); and (2) selecting better concealed nesting sites,. This view i s supported by data on a more dispersed d i s t r i b u t i o n of redwing nests i n an upland habitat than i n a nearby marsh studied by Howard (1977). Conversely, females breeding i n marshes are rarely k i l l e d by predators. Despite the much larger number of studies on marsh nesting redwings, only Weatherhead and Robertson (1977a) reported two suspected cases where a predator may have k i l l e d the breeding female. This i s consistent with the view that much of the redwing nesting mortality i s probably caused by marsh wrens which do not present any danger to adult redwings or to older young. This idea i s further supported by my observations on the clumping tendency of breeding females, which increases 234 t h e i r chances of success (see Chapter 6 ) , and the fact that redwings are more successful i n sparse c a t t a i l , presumably because here they can more e f f i c i e n t l y defend th e i r nests against wrens than i n dense c a t t a i l (see Chapter 5). Therefore, marsh and upland redwing populations might present l o c a l l y adapted demes with l i t t l e or no exchange of genes (Robertson 1972, Howard 1977), This view i s supported by r e s u l t s of 3 studies which showed that, despite the d i f f e r e n t types of predators, redwings from marsh and nearby upland populations had s i m i l a r reproductive success (Howard 1977; Goddard and Board 1967; Robertson 1972, 1973a,b). Hence, i f upland and marsh redwing populations were at l e a s t p a r t i a l l y i s o l a t e d , then the polygynous pattern of nesting exhibited by birds from these habitats must have been driven and maintained by forces operating within these habitats. The test of t h i s hypothesis w i l l reguire data on pre-reproductive dispersal and movement rates of redwings both within and between habitats- Such evidence i s not available at present. Third, i t i s known that widespread redwing nesting i n upland habitats i s of recent o r i g i n (Bent 1958, Dykstra 1960, Orians 1980). The expansion of redwing breeding range from marshes into uplands has probably resulted from: (1) a large scale destruction of marsh habitats (Orians 1961, Auclair 1976); (2) increasing s u i t a b i l i t y of uplands to redwings through a g r i c u l t u r a l a c t i v i t i e s ; and (3) probably associated with that, a recent population increase (Dykstra 1960, Weins and Dyer 1975). Thus polygyny i n redwings most l i k e l y evolved before 235 t h e i r recent spread into uplands, presumably i n response to se l e c t i v e forces operating within marsh habitats. Therefore, I suggest that Wittenberger*s (1976) hypothesis invoking differences between marshes and uplands as a possible force driving the evolution of polygyny i n marsh-nesting redwings i s unlikely . 3 . The ^sexy son" hypothesis Weatherhead and Robertson (1977a) examined redwing reproductive t a c t i c s i n terms of Orians' (1969) model which proposes that polygyny evolved as a r e s u l t of large differences in the guality of i n d i v i d u a l t e r r i t o r i e s . They obtained two kinds of evidence which contradict Orians' model: (1) they found no correlation between harem si z e and various features of the habitat which determine t e r r i t o r y guality (also this study, chapter 8 ) ; and (2) i n high density situations ( c h a r a c t e r i s t i c of large harems) females fledged fewer young than i n low density s i t u a t i o n s . Therefore, Weatherhead and Robertson (1977a) concluded that some females must have made poor choices of habitat for th e i r breeding. In order to explain t h i s contradiction, these authors proposed that females were choosing a t t r a c t i v e ("sexy") males defending poor guality t e r r i t o r i e s to increase their ultimate f i t n e s s rather than t h e i r immediate reproductive success. This i s because half of the offspring of such females w i l l be male, which w i l l i n h e r i t some of t h e i r fathers' genes, and af t e r reaching maturity w i l l presumably also a t t r a c t large harems to their t e r r i t o r i e s . Thus 236 females which made poor choice of a t e r r i t o r y might s t i l l increase t h e i r f i t n e s s ultimately through t h e i r "sexy sons". To evaluate the chances of evolving the "sexy son" strategy by females, we have to examine the i n t e n s i t y of selection for t h i s and a l t e r n a t i v e reproductive strategies. The following discussion of t h i s problem i s based on an assumption that reproductive strategies of animals are subject to natural selection which, i n turn, drives the evolution of various adaptations, such as the mating pattern. On t h e o r e t i c a l grounds, i t i s obvious that the i n t e n s i t y of selection for any p a r t i c u l a r adaptation should be highest when the return (in terms of the effects on f i t n e s s of females) i s immediate. This i s because adaptations which increase female f i t n e s s immediately should be at a competitive advantage over those whose e f f e c t s on f i t n e s s are delayed to a l a t e r time and, therefore, are l e s s predictable. Individuals possessing genes which control t r a i t s with immediate e f f e c t s on t h e i r f i t n e s s should, therefore, reproduce at a faster rate and t h e i r genes should conseguently spread faster than those c o n t r o l l i n g behavioral t r a i t s with e f f e c t s on f i t n e s s which are, for example, delayed to the future generations. This view i s consistent with t h e o r e t i c a l considerations on the variance in offspring numbers. G i l l e s p i e (1979) demonstrated mathematically that the addition of a random element to the number of offspring of a given genotype, and hence increasing the variance in offspring numbers, w i l l lower the f i t n e s s of that genotype as measured by i t s mean frequency i n the next generation. 237 Since the "sexy son" strategy implies t h i s element of randomness i n the number of progeny ( i . e . . the f i t n e s s of females i s not increased immediately through the number of their o f f spring but i t might p o t e n t i a l l y be raised through the number of offspring of their highly a t t r a c t i v e sons), i t becomes obvious that i t could evolve through natural s e l e c t i o n only i n the absence of selection for a more e f f i c i e n t strategy with more immediate e f f e c t s on female f i t n e s s . Therefore, the "sexy son" strategy might evolve only under those circumstances when the genetic guality of males (their genetically controlled attractiveness to females) i s the most important feature which females assess when choosing a breeding s i t u a t i o n . Other q u a l i t i e s of males (e.g. t h e i r t e r r i t o r y defense, nest defense behavior, and parental care), therefore, should not influence female choice of a breeding s i t u a t i o n . Conseguently, the test of the "sexy son" hypothesis reguires that we demonstrate that: (1) male attractiveness to females i s genetically controlled and hence heritable; and (2) features other than the genetically controlled attractiveness of males should play a r e l a t i v e l y smaller role i n female choice of breeding sit u a t i o n s . There are no data available on redwings to establish to what extent male attractiveness to females i s genetically controlled. But there are two kinds of information which strongly suggest that females do not choose breeding s i t u a t i o n s on the basis of a genetically controlled attractiveness of males. F i r s t , females p r e f e r e n t i a l l y join old experienced males and harems with experienced females (see Chapter 8). Older 238 birds are experienced breeders which w i l l increase t h e i r reproductive success through more e f f i c i e n t nest defense against predators (see Chapter 8)- Second, because a majority of older females generally return to t h e i r o r i g i n a l nesting areas regardless of whether theic o r i g i n a l male returned (Table 50), the number of females acguired by newly established males i s , i n general, determined by the mating success of the previous t e r r i t o r y holders which they replaced (see Chapter 8). This evidence suggests that female choice i s not influenced by the genetically controlled attractiveness of males, but i t i s influenced by their' strong nest s i t e tenacity, and by the presence of experienced birds of both sexes. Both factors increase t h e i r reproductive success and hence t h e i r immediate f i t n e s s . Therefore, i t i s not necessary to invoke the "sexy son" hypothesis (Weatherhead and Robertson 1979) to explain the adaptive value of polygyny in red-winged blackbirds. 4. Cooperation between females as a force selecting for polygyny i n redwings Altmann et a l . (1977) noticed that fledging success per nest increased with harem size i n four studies of blackbirds. They suggested that t h i s s i t u a t i o n could r e s u l t either from great differences i n the guality of breeding s i t u a t i o n s i n those marshes and an appropriate d i s t r i b u t i o n of females among t e r r i t o r i e s of different quality (i..e. Orians' (1969) hypothesis), or, a l t e r n a t i v e l y , from some form of cooperation between females. This second possible explanation led Altmann 239 et a l . (1977) to formulate the "cooperative female choice model" as an alternative to the "competitive female choice model" (this i s i d e n t i c a l with the model proposed by Orians) . The most important assumption of the "cooperative female choice model" i s that the addition of females to a harem w i l l improve the f i t n e s s of a l l females i n that harem. Altmann et a l . (1977) did not examine the v a l i d i t y of the cooperative female choice model as applied to redwings nor did they attempt to i d e n t i f y the type of cooperation between females and s e l e c t i v e forces possibly driving the evolution of redwing polygyny. The p o s s i b i l i t y of cooperation between female redwings was examined by Lenington (1980). She collected data on the influence of various t e r r i t o r y guality and nest s i t e guality parameters on reproductive success of females and mating success of males in order to distinguish between four models for mate se l e c t i o n : the cooperative female choice model, the competitive female choice model, the long term strategy model, and the nest s i t e guality model (since the l a s t two models present s p e c i a l cases of the competitive female choice model, I w i l l not examine them here). Lenington (1980) did not obtain any evidence supporting an assumption of the cooperative female choice model that, within t e r r i t o r i e s of given g u a l i t y , female's reproductive success should increase with increasing harem size. On the other hand, she found that mating success of males was correlated with some t e r r i t o r y guality parameters ( t e r r i t o r y size and vegetation density). From this evidence Lenington concluded that the competitive female choice model best f i t s her 240 data. I suggest, however, that the data on the basis of which Lenington (1980) rejected the cooperative female choice model are inconclusive f o r the following reasons. F i r s t , Lenington did not examine redwing nesting t a c t i c s in r e l a t i o n to a factor which might have driven the evolution of cooperation between females. Her index of t e r r i t o r y guality thus might not be relevant. Since predation was an important source of nesting mortality i n her marshes, i t i s necessary to evaluate nesting success of females from harems of various sizes i n terms of actual nest predation pressures within i n d i v i d u a l t e r r i t o r i e s . These data should then be used for examining the r o l e of cooperation i n nest defense between females from harems of various s i z e s . Second, Lenington did not study the influence of temporal and s p a t i a l organization of nesting by females from harems of various s i z e s . As indicated by my results (Chapter 7), such information i s important and might lead to d i f f e r e n t conclusions from Lenington's study. Hence I conclude that Lenington's (1980) study does not provide a v a l i d test of the cooperative female choice model as applied to red-winged blackbirds. The hypothesis that redwing polygyny may have evolved as a result of selection for some form of cooperation between females i s supported by my results which suggest that high nest predation rates caused by marsh wrens may have driven the evolution of cooperation between females in nest defense. The 241 possible role of marsh wrens, as a s p e c i f i c s e l e c t i v e force dr i v i n g the evolution of female clumping tendency, and the role of other factors l i k e l y to influence female f i t n e s s i n the evolution of polygyny i n redwings w i l l be discussed i n the following Chapter. 242 CHAPTER 10 An alternative model on the evolution of polygyny i n red-winged blackbirds 1. Nest destruction by marsh wrens as a s e l e c t i v e force Red-winged blackbirds and other marsh nesting passerines suffer from the highest nest mortality rates, mainly through predation, of a l l Temperate Zone passerines (Ricklefs 1969). Evidence that (1) the success of redwing nests improves with increasing distance from the nearest marsh wren nest (Table 20) , and (2) i n successful nests fledging at least one young, the proportion of eggs surviving to fledge decreases with proximity to marsh wrens (Table 22) suggests that, in the marsh I studied, marsh wrens are responsible f o r most redwing nesting mortality. This view i s also supported by the fa c t that using the redwing nest with eggs as a bait i n a trap s p e c i f i c a l l y designed for marsh wrens (see Appendix I ) , I was able to capture almost a l l t e r r i t o r i a l males. In f i v e years of trapping marsh wrens i n th i s way, I have not, however, captured any other predators. On only two occasions, female brown-headed cowbirds, Molothrus <__.££, triggered ray traps while presumably t r y i n g to remove the redwing eggs. Since marsh wrens and redwings greatly overlap i n t h e i r breeding range (Bent 1948, 1958; t h i s study, see Chapter 243 11), the destruction of redwing nests by wrens should present an important s e l e c t i v e force acting on redwings.. 2• Marsh wrens as a f a c t o r s e l e c t i n g for cooperation bs£ws§.fi female redwings Some form of cooperation between females as an agent possibly driving the evolution of harem polygyny i n any given species should normally be indicated by: (1) the formation of generally large harems and conseguently s h i f t e d sex r a t i o towards females i n breeding populations; (2) clustered d i s t r i b u t i o n of females on male t e r r i t o r i e s ; and (3) increasing average reproductive success per female with increasing harem s i z e . Such evidence i s available f o r the red-winged blackbird. The average sex r a t i o within various redwing populations i s almost always highly shifted towards females (see Appendix I I I ) ; marsh-nesting redwings breed i n a clumped pattern (Mayr 1941; Caccamise 1977; t h i s study, see Chapter 6); and i n three out of six redwing populations the average reproductive success per female increased with harem siz e (Goddard and Board 1967, data organized by Orians 1972; Holm 1973; t h i s study, see Fig. 16). In addition, I have shown that by breeding close to other conspecifics, females which have marsh wrens nearby s i g n i f i c a n t l y increase their chances of rearing any young (see Chapter 6). Theoretical consequences of these findings are shown in F i g . 21. Evidence of the adaptive value of the clumped nesting pattern, i n terms of reduced nest predation, thus suggests that female redwings cooperate in defense of their 244 F i g . 21. Relationship between success of redwings and t h e i r distance from marsh wrens and other c o n s p e c i f i c s . 245 nests against marsh wrens. The idea of mutual nest defense i s supported by two kinds of evidence: F i r s t , on many occasions I observed group attacks of p o t e n t i a l predators such as marsh hawks, r e d - t a i l e d hawks, sparrow hawks, short-eared owls, great blue herons, and crows by redwings of both sexes throughout the breeding season- Similar observations have also been made in other marshes (Bent 1958, Nero 1956b, Robertson 1972). In May 1976 I observed a group of excited redwings (7 females and 2 males) gathered near a redwing nest in which eggs were recently l a i d . These birds were direc t i n g t h e i r attacks at a male marsh wren which was low i n the vegetation.. When I checked the redwing nest a f t e r t h i s incident, the eggs were already detroyed. Small holes i n the she l l s indicated marsh wren egg destruction, Similar group attack of two marsh wrens, involving 4 female and 2 male redwings, which took place near a redwing nest destroyed most l i k e l y by these wrens was observed by my f i e l d assistant i n May 1979. Second, mutual nest defense by redwings i s also evident from freguent attacks upon f i e l d observers when approaching active redwing nests. These attacks were made not only by the female from the closest nest but freguently also by other females from the same harem, the resident male, and sometimes even by a neighbor and his females (Pieman, unpublished data). The above observations, however, do not provide s u f f i c i e n t evidence for cooperation between females as a force dr i v i n g the 246 evolution of polygyny i n redwings or any other species because they might be explained a l t e r n a t i v e l y by the concentration of females i n the most suitable parts of a habitat. Therefore, i t i s necessary to develop methods which would allow us to discriminate between these two alt e r n a t i v e explanations. By examining the competitive and cooperative female choice models (Altmann et a l . 1977), i t becomes obvious that the most important difference between the resource defense polygyny and harem defense polygyny (see Emlen and Oring 1977) i s the assumption of the influence of new females s e t t l i n g in a harem on the f i t n e s s of e a r l i e r s e t t l e d i n d i v i d u a l s (Fig. 22) . Therefore, examining the influence of new females on the f i t n e s s of the e a r l i e r settled i n d i v i d u a l s should, t h e o r e t i c a l l y , provide the most appropriate t e s t . P r a c t i c a l l y , however, t h i s test would be extremely d i f f i c u l t because the f i t n e s s of e a r l i e r s e t t l e d birds i s immediately altered as other females s e t t l e i n a t e r r i t o r y . Since f i t n e s s of animals can usually be measured only i n terms of the f i n a l reproductive output, which i s also a f i n a l product of a l l interactions between females from a harem, i t i s impossible to estimate the influence of the addition of females to any p a r t i c u l a r harem on the average female f i t n e s s . Therefore, the influence of additional new females on the f i t n e s s of e a r l i e r s e t t l e d i n d i v i d u a l s could probably be studied only by removing late s e t t l i n g birds and examining the e f f e c t s of t h i s removal on the f i t n e s s of the remaining female(s) compared to that of females from control (unmanipulated) harems. However, because in redwings there appears to be no rela t i o n s h i p 22. Influence of addition of females to a harem on the average fitness per female as predicted by the competitive female choice model ( f u l l l i ne) and cooperative female choice model (dashed l i n e ) . ' Harem size 248 between the various features determining t e r r i t o r y guality and the number of females attracted to a t e r r i t o r y (Chapter 8; see also Weatherhead and Robertson 1977a), one would have to consider such features when evaluating results of the proposed removal experiment. Because of t h i s and other problems associated with the various possible side e f f e c t s of the removal experiment, this test may not be f e a s i b l e . A l t e r n a t i v e l y , cooperation could be studied more d i r e c t l y by examining the degree of cooperation between females within harems of d i f f e r e n t sizes with regard to the agent which has been driving the evolution of a given cooperative strategy. This assumes, however, that we have discovered that driving force. Since my data indicate that i n redwings the evolution of cooperation between females has probably been driven by high nest predation rates, i t should be possible to test the hypothesis on the role of cooperation between females i n nest defense in the evolution of the clumped pattern of nesting i n redwings by examining the role of cooperation i n nest defense within i n d i v i d u a l harems. Because, i n general, the effectiveness of redwing nest defense i s higher when female redwings exhibit a greater degree of clumping (Table 36), I predict that within harems of any given size the degree of cooperation, as indicated by the distance from the nearest conspecific neighbor, should be r e f l e c t e d i n reproductive success of female redwings. But, i f there i s no cooperation between females involved, and large harems are simply attracted to the most suitable t e r r i t o r i e s (females concentrate in areas 249 where lim i t e d resources are abundant), then a l l females from harems of any given size should have approximately s i m i l a r chances of success, regardless of t h e i r distance from conspecifics. To test these ideas, I examined nesting success of redwing females from harems of various s i z e s , which had the nearest conspecific neighbor nearby (within 30m) or farther away. Within harems of any p a r t i c u l a r s i z e the presence of a conspecific neighbor within 30m resulted i n a higher success (Table 57). However, the difference between success of redwing females which had a conspecific near and farther away i s s i g n i f i c a n t only i n 3 out of 8 cases (Table 57). But because the r e l a t i o n s h i p between nesting success and the distance from the nearest conspecific i s consistent for harems of a l l s i z e s , i t seems reasonable to conclude that i t i s the degree of female clumping within t e r r i t o r i e s of males a t t r a c t i n g harems of various s i z e s , and thereby cooperation between females i n nest defense, rather than the guality of male t e r r i t o r i e s , which increases female chances of success. Therefore, I conclude that the benefits of female clumping have been selecting f o r the clumping tendency i n female red-winged blackbirds. This conclusion i s also supported by the fact that i n my study marsh mating success of males was not correlated with various features determining t e r r i t o r y guality (see Chapter 8). This does not, however, preclude the p o s s i b i l i t y that female choice of a breeding s i t u a t i o n might be influenced by the guality of a male and female (s) i n his harem. This problem w i l l be discussed 250 Table 57. Nesting success of female red-winged blackbirds as re l a t e d to the mating status of t h e i r male and the distance from the nearest co n s p e c i f i c neighbor. The mating status of a male was determined from the highest number of simultaneously active nests on hi s t e r r i t o r y . Data on successful nests and nests destroyed by predators from 1976-1979 were included. Harem Percent nests successful when distance from the nearest female neighbor was Sample si z e P (1-tailed) s i z e 1 - 30m 30 ,5m - more 2 37.5 • 22.2 17 * >0.4 3 64.0 33.3 58 4.21 <0.01 4 55.6 34.8 68 1.86 >0.05 5 51.9 28.2 147 5.54 «0.01 6 53.9 30.0 85 2.59 >0.05 7 38.2 30.4 91 0.18 >0.3 8 8.3 0.0 32 * >0.3 9 82.4 37.5 25 * <0.05 Tested by Fisher exact p r o b a b i l i t y t e s t . 251 l a t e r . 3- The nature of cooperation between female redwings Females from a harem might increase t h e i r success by defending a roughly c i r c u l a r area around t h e i r nests whose radius i s larger than the distance between nests of the nearest neighbors (see also Horn 1968). Each female from a clump thus might defend, in addition to her nest, a nest of one or more neighbors. This would imply a true active cooperation between females i n defense of their nests against predators (marsh wrens) . The benefits of the clumped nesting pattern to females, however, do not have to reguire active cooperation between females i n nest defense. Because marsh wrens tend to avoid high redwing density areas (see Chapter 4), an a l t e r n a t i v e explanation could be that the increased success of females exhibiting a greater degree of clumping might be a conseguence of t h e i r more e f f e c t i v e exclusion of wrens from the v i c i n i t y of t h e i r own nests. I suggest, however, that t h i s rather "passive mutual protection" r e s u l t i n g from the marsh wren tendency to avoid redwings, could be p a r t i c u l a r l y important in habitats where the density of marsh wrens i s low and hence wrens have enough space available to avoid redwings through the s p a t i a l segregation of t h e i r a c t i v i t y centers. Moreover, since the defense of nests by redwings against marsh wrens appears to be influenced by learning ( i . e . redwings learn faster under high 252 wren density conditions when the rate of redwing-wren int e r a c t i o n s i s high; see Chapters 2 , 3 ), redwing nest defense against marsh wrens and hence also the active cooperation between females should become more e f f e c t i v e under high wren density conditions. On the other hand, when the density of marsh wrens i s low, active defense should be poor, whereas the e f f e c t s of the "passive mutual protection" should become more pronounced. These two hypotheses are supported by my data. In 1976, the density of marsh wrens was low and the degree of s p a t i a l segregation between redwing and marsh wren nesting s i t e s was generally high (Chapter 4). However, as the distance between the nearest redwing nests decreased, the redwing-wren internest distance on average increased, thereby indicating the increasing e f f i c i e n c y of redwings i n the exclusion of marsh wrens from the v i c i n i t y of t h e i r nests. Since the rates of redwing-wren interactions are low when the density of marsh wrens i s low (see Chapter 2 ), the greater success of female redwings which had conspecifics nearby could be explained by the "passive mutual protection" presumably re s u l t i n g from the tendency of marsh wrens to avoid areas of high redwing density. This i s also supported by the f a c t that the impact of marsh wrens on redwing nesting decreases with increasing redwing-wren distance (Table 20). In addition, i n spite of a greater degree of s p a t i a l segregation between redwing and marsh wren nests i n 1976, marsh wrens had a r e l a t i v e l y greater impact on redwing nesting i n t h i s year than in 1977 when the density of wrens was much higher (see 253 Chapter 4). Presumably t h i s i s because the "passive mutual protection" r e s u l t i n g from the e f f e c t s of redwing clumping on marsh wren d i s t r i b u t i o n i s probably less e f f i c i e n t when compared with the active nest defense by females through t h e i r agonistic behavior s p e c i f i c a l l y directed towards marsh wrens. In 1977 the density of marsh wrens was high and the degree of s p a t i a l segregation between redwing and marsh wren nests was generally low (Table 21). Like i n 1976, also i n 1977, female redwings which exhibited a greater degree of clumping had higher success (Tables 34, 36). However, i n 1977, because redwings had no s i g n i f i c a n t e f f e c t on the d i s t r i b u t i o n of marsh wrens through their clumping, presumably because of extremely high wren densities, a higher success of females with a conspecific neighbor nearby could be explained by a more e f f i c i e n t nest defense against marsh wrens and hence better active mutual protection. This i s also supported by the fact that the rate of redwing-wren interactions i s high when the density of marsh wrens i s high (see Chapter 2). Under these conditions with generally small redwingswren internest distances, the passive mutual protection presumably does not contribute s i g n i f i c a n t l y to the higher success of female redwings which clump more. 4. The influence of male nest defense behavior on female clumping, tendency As male redwings become older, they have a greater influence on the reproductive success of t h e i r females (see Chapter 8 ) . Because the males do not a s s i s t t h e i r mates with 254 most of the nesting duties (females alone build the nest, incubate, and feed nestlings; Bent 1958), I conclude that t h e i r contribution to nesting must be through the defense of nests against predators and probably also through the exclusion of conspecific intruders which i n t e r f e r e with the breeding females (e.g. Nero 1956b). This i s supported by observations of aggression by male redwings towards marsh wrens (Allen 1914, Nero 1956a, Orians and Willson 1964, Burt 1970, Verner 1975, t h i s study), other types of predators (Nero 1956b, Robertson 1972, t h i s study), and conspecific intruders (Nero 1956b, t h i s study). But i f females were di s t r i b u t e d uniformly or randomly on male t e r r i t o r i e s , t h i s contribution by males to the defense of each nest should become smaller with increasing harem s i z e because males can, presumably, defend only a small area e f f e c t i v e l y . On the other hand, through t h e i r concentration on a small area females should increase t h e i r success by increasing the e f f i c i e n c y of male nest defense.. Therefore, male nest defence behavior probably presents another factor favoring the evolution of female clumping tendency. 5. Negative effects of clumping as a force setting an upper l i m i t to harem size As harem size increases, competition between females f o r l i m i t e d resources such as food, space, suitable nesting s i t e s , and male contribution to nesting should become more severe. Therefore, as the number of females on a t e r r i t o r y increases, the negative e f f e c t s of clumping on the average female f i t n e s s 255 should become more pronounced. There are two kinds of evidence which support t h i s view. F i r s t , i n two studies the average fledging success per female increased with harem size but, after i t reached i t s maximum for harems of 4-6 females, i t started decreasing (Goddard and Board 1967, as organized by Orians 1972; thi s study, see F i g . 16). Second, Weatherhead and Robertson (1977a) and Orians (1980) observed that when the density of female redwings was highest, redwings fledged fewer young. These negative e f f e c t s on the reproductive output of female redwings should, therefore, select for some mechanism through which females could set an upper l i m i t to the degree of clumping and hence harem s i z e , thereby maximizing t h e i r reproductive output. In the following part I w i l l discuss two possible ways how redwings might achieve t h i s . A . T e r r i t o r i a l behavior of females Females from a harem might prevent additional birds from s e t t l i n g in t h e i r t e r r i t o r i e s through agonistic behavior i f the addition of such birds should reduce their f i t n e s s . This hypothesis i s based on an assumption that the breeding space i s a l i m i t i n g factor. In other words, as the number of females s e t t l e d on a t e r r i t o r y of a male increases, competition for space should become more severe u n t i l a l l space within a t e r r i t o r y i s defended, and conseguently additional i n d i v i d u a l s are prevented from s e t t l i n g by the t e r r i t o r i a l behavior of resident females. Therefore, the hypothesis that the resistance of s e t t l e d birds might prevent new females from s e t t l i n g 256 requires that I f i r s t examine t h i s assumption. Female redwings have well developed t e r r i t o r i a l behavior (Nero 1956b). Females establish r e l a t i v e l y small t e r r i t o r i e s by subdividing the t e r r i t o r y of a male, from which they exclude other females by means of song-spread, b i l l - t i l t i n g , and attack (Nero, 1956b). In addition, i t i s known that females breed i n a clumped pattern (see Chapter 6). Because the clumped pattern of nesting by females i s evident even on t e r r i t o r i e s which a t t r a c t the largest harems (see Chapter 7), this would appear to indicate that the breeding space cannot be a l i m i t i n g factor and, therefore, the t e r r i t o r i a l behavior of e a r l i e r s e t t l e d i n d i v i d u a l s should not play a role i n determining the pattern of settlement of late a r r i v i n g females. However, the fact that female redwings increase their reproductive success through cooperation i n nest defense offers an alternative explanation. Because e f f i c i e n t cooperation i n nest defense reguires that the distances between neighboring females are reduced (see Chapter 6), females should be selected to choose nesting s i t e s near e a r l i e r s e t t l e d birds. Therefore, the addition of new females to a harem should r e s u l t i n t h e i r concentration i n a clump. This i s supported by data on distances of consecutively s e t t l i n g females i n medium size and large harems from the f i r s t s e t t l e d female on t e r r i t o r i e s of males, which indicate that consecutive females tend to s e t t l e , on the average, at si m i l a r distances from the f i r s t female, thereby increasing the degree of clumping in a harem (see Table 41). However, as a clump (harem) reaches i t s optimum size and density, the resistance of s e t t l e d females 257 to new birds might become strong enough to prevent new indi v i d u a l s from s e t t l i n g within or near the clump- Under such circumstances a new female faces a choice of either (1) s e t t l i n g elsewhere within the same t e r r i t o r y of a resident male, or (2) leaving that t e r r i t o r y and finding another one with fewer females which w i l l not prevent her from s e t t l i n g near them. With regard to the advantages of the female clumping strategy i n terms of increased mutual protection against nest predators i t i s obvious that a female should choose the second option because thi s w i l l increase her f i t n e s s . The benefits of cooperative inte r a c t i o n s between females i n a harem should select for the clumping tendency and, therefore, the space suitable for nesting (within and near the clumps of the breeding birds) i s probably lim i t e d . The view that the agonistic interactions between females might play an important r o l e i n the d i s t r i b u t i o n of females throughout the breeding population i s thus reasonable. (a) The theory of habitat d i s t r i b u t i o n , and a test of the role of female t e r r i t o r i a l i t y To test the hypothesis that the resident females might prevent new birds from s e t t l i n g through t h e i r agonistic behavior, I w i l l examine the available data on the pattern of settlement of females i n terms of the theory of habitat d i s t r i b u t i o n in birds. But before that, I w i l l outline t h i s theory as i t was put forward by Fretwell and Lucas (1969), discuss a new hypothetical type of habitat d i s t r i b u t i o n i n some birds, and then examine the implications of t h i s theory with 258 regard to the redwing data. The theory of habitat d i s t r i b u t i o n i n birds i s based on an assumption that individuals can assess habitat guality and conseguently p r e f e r e n t i a l l y s e t t l e i n the highest guality habitat available. This i s a reasonable assumption because in d i v i d u a l s which s e t t l e i n r e l a t i v e l y poor habitats are selected against (Fretwell and Lucas 1969), However, as new birds continue to s e t t l e i n a habitat of any p a r t i c u l a r g u a l i t y , they may influence i t s res u l t i n g s u i t a b i l i t y to additional birds i n two ways. F i r s t , in some species the addition of birds to a given habitat w i l l always reduce the s u i t a b i l i t y of that habitat to new individ u a l s and to the residents, presumably because of increasing competition f o r limited resources* Such species w i l l be referred to as non-Allee type of birds. In other species, however, the addition of new i n d i v i d u a l s may i n i t i a l l y increase the r e s u l t i n g s u i t a b i l i t y of a habitat to new birds* This s i t u a t i o n , which was f i r s t discussed i n d e t a i l by Allee et a l . (1949) , and has been since referred to as " A l l e e 1 s principle",, may be explained i n terms of certa i n advantages of group l i f e in animals. Birds to which the Allee's p r i n c i p l e applies w i l l be further referred to as Allee type species. In addition to t h e i r influence on the r e s u l t i n g s u i t a b i l i t y of a habitat, either through increased competition or through some form of cooperative in t e r a c t i o n s , s e t t l e d birds may influence the pattern of d i s t r i b u t i o n of unsettled i n d i v i d u a l s through t h e i r behavior towards them. Fretwell and Lucas (1969) discussed two possible situations i n t h i s regard. F i r s t , i f a l l 259 i n d i v i d u a l s always chose those sit u a t i o n s where t h e i r f i t n e s s i s highest, and thereby made the " i d e a l free choice^ of habitat (this i s not d i r e c t l y influenced by e a r l i e r s e t t l e d birds; only through t h e i r effects on resources), t h e i r r e s u l t i n g d i s t r i b u t i o n i n a habitat would be y. ideal f reey. Under these circumstances a l l individuals should have the same f i t n e s s , regardless of the i n i t i a l guality of d i f f e r e n t habitats. Second, i f the unsettled i n d i v i d u a l s were prevented from entering the best guality habitats by the t e r r i t o r i a l behavior of s e t t l e d birds, and therefore were forced to s e t t l e in lower guality habitats where their f i t n e s s i s lower, the d i s t r i b u t i o n of birds would be " i d e a l despotic^. This implies that the f i t n e s s of individuals settled in better habitats which support higher densities of birds, should be higher than that of birds in poorer habitats. In order to outline the implications of the " i d e a l free" and " i d e a l despotic" habitat d i s t r i b u t i o n s for the non-Allee type and Allee type species, I w i l l discuss each case separately. (1) Non-Allee type ideal free d i s t r i b u t i o n (Fig- 23); The r e l a t i o n s h i p between the f i t n e s s of i n d i v i d u a l s s e t t l i n g i n high (S1), medium (S2) , and low (S3) guality habitats as a function of population density i s determined by competition between i n d i v i d u a l s . For t h i s reason the f i t n e s s of individuals i n a l l habitats decreases with increasing population density. Because in d i v i d u a l s make an "ideal free choice" of the best available habitat, they s e t t l e f i r s t i n S1, where the maximum achievable 260 F i g . 23. Non-Allee type i d e a l free d i s t r i b u t i o n . As a consequence of the " i d e a l free choice" assumption, the r e s u l t i n g f i t n e s s of i n d i v i d u a l s i n d i f f e r e n t q u a l i t y habitats i s s i m i l a r when the density of a population i s intermediate (o) or high (•). See text f o r further explanation. (Modified from Fretwell and Lucas 1969.) Population density 261 f i t n e s s i s B1. But after t h e i r density increased to a point when t h e i r f i t n e s s was reduced to B2 (the maximum achievable f i t n e s s i n S2), additional birds should be s e t t l i n g also i n S2. As densities of birds i n S1 and S2 increase further and t h e i r f i t n e s s i s reduced to B3, new birds should s t a r t also s e t t l i n g in S3. Because individuals are assumed to make an " i d e a l free choice" of habitat, the fi t n e s s of birds under intermediate and high population densities should be the same i n d i f f e r e n t guality habitats. (2) Non-Allee type ideal despotic d i s t r i b u t i o n (Fig. 24); The addition of new individ u a l s to any guality habitat i s assumed always to have negative e f f e c t s on f i t n e s s of a l l birds. Birds should s t a r t s e t t l i n g f i r s t i n a high guality habitat S i . i However, as new individ u a l s are added and t h e i r f i t n e s s i s reduced to the l e v e l A1, the resistance of s e t t l e d birds towards new unsettled i n d i v i d u a l s becomes so high that these i n d i v i d u a l s are forced into the medium guality habitat S2. Si m i l a r l y , when the f i t n e s s of birds i n habitat S2 decreases to the l e v e l A2, and t h e i r resistance makes s e t t l i n g here uneconomical, unsettled birds should s t a r t entering the poor guality habitat S3. As a consequence, the average f i t n e s s of se t t l e d birds should decrease from high to low guality habitats. (3) Allee type ideal free d i s t r i b u t i o n (Fig. 25); Since i t i s assumed that the addition of individuals to a population should i n i t i a l l y increase f i t n e s s of a l l settled birds, the f i t n e s s curves for habitats of d i f f e r e n t guality f i r s t increase and then, as a conseguence of increasing negative e f f e c t s of 24. Non-Allee type i d e a l despotic d i s t r i b u t i o n . Thick parts of the f i t n e s s curves S^-S^ i n d i c a t e the regions of settlement of i n d i v i d u a l s i n a habitat as determined by t e r r i t o r i a l behavior of established b i r d s . See text f o r explanation. (Modified from Fretwell and Lucas 1969.) Population density 263 competition, decrease. If there are two habitats available of di f f e r e n t guality (S1, S2), birds should s e t t l e f i r s t in a high guality habitat St. After reaching the maximum achievable f i t n e s s (B1), new birds should continue s e t t l i n g i n S1 u n t i l t h e i r f i t n e s s i s reduced to the guality of S2 which i s i d e n t i c a l with i t s i n i t i a l s u i t a b i l i t y (this i s indicated by x on the f i t n e s s curve for S1). However, the addition of new birds to a population should result i n some birds s e t t l i n g i n S2. But as new i n d i v i d u a l s s e t t l e in S2, they increase i t s s u i t a b i l i t y for other b i r d s , whereas the addition of birds to S1 would reduce f i t n e s s of birds settled here. Because of t h i s sudden difference between resulting s u i t a b i l i t i e s of S1 and S2 (SKS2) , Fretwell and Lucas (1969) proposed that, i n order to increase t h e i r f i t n e s s , the birds which s e t t l e d e a r l i e r i n S I should move into S2, whose s u i t a b i l i t y i s now increasing as a consequence of some benefits of group l i f e (this movement of birds from S1 to S2 i s indicated on Fig. 25 by a dashed arrow). This transfer of birds from S1 to S2 should continue u n t i l the r e s u l t i n g s u i t a b i l i t i e s of these habitats are egual (reach the l e v e l B2). Additional birds should then s e t t l e i n both S1 and S2 in such a way that t h e i r resulting s u i t a b i l i t i e s (as measured by the f i t n e s s of individuals) are egual. Thus t h i s pattern of settlement should r e s u l t i n a similar f i t n e s s of in d i v i d u a l s s e t t l e d i n S1 and S2. An important implication of t h i s hypothesis i s that species exhibiting this type of habitat d i s t r i b u t i o n may show large changes i n d i s t r i b u t i o n with r e l a t i v e l y small increases i n population s i z e (Fretwell and Lucas, 1969). 264 F i g . 25. A l l e e type i d e a l free d i s t r i b u t i o n . Birds s e t t l e f i r s t i n high q u a l i t y habitat (sp but a f t e r reaching the i n i t i a l s u i t a b i l i t y of a low q u a l i t y habitat (S^) , some i n d i v i d u a l s from S^ move into where t h e i r f i t n e s s i n i t i a l l y increases u n t i l f i t n e s s of birds i n both habitats i s the same (o). Further i n d i v i d u a l s s e t t l e i n both habitats so that the r e s u l t i n g f i t n e s s e s i n these habitats are s i m i l a r (•). See text f o r further explanation. (Modified from Fretwell and Lucas 1969.) Population density 265 (4) Allee type i d e a l despotic d i s t r i b u t i o n (Fig. 26); This type of d i s t r i b u t i o n , though not described by Fretwell and Lucas (1969), i s the fourth hypothetical case of habitat d i s t r i b u t i o n , analogous to the non-Allee type i d e a l despotic d i s t r i b u t i o n . Benefits of s o c i a l l i f e are r e f l e c t e d i n i n i t i a l increases in f i t n e s s with increasing population density i n both high guality (S1) and low guality (S2) habitats. At low d e n s i t i e s , i n d i v i d u a l s should s e t t l e only i n S1, but as t h e i r density increases, the resistance of the resident birds towards new unsettled i n d i v i d u a l s should increase u n t i l most of these in d i v i d u a l s are forced to s e t t l e i n S2. Further addition of new i n d i v i d u a l s should r e s u l t i n their settlement i n S2, with a small proportion of the most persistent birds probably succeeding i n entering S i . Because the maximum f i t n e s s achievable through the cooperative interactions i s assumed to decrease with decreasing guality of a habitat (B2<B1), the r e s u l t i n g f i t n e s s of settled i n d i v i d u a l s should always decrease with decreasing habitat guality. Determining the role of t e r r i t o r i a l behavior i n the d i s t r i b u t i o n of a given species reguires that we examine the r e s u l t i n g f i t n e s s in habitats of d i f f e r e n t guality. If the process of d i s t r i b u t i o n of i n d i v i d u a l s in a population i s " i d e a l free", i n d i v i d u a l s s e t t l e d i n d i f f e r e n t guality habitats, where th e i r densities are adjusted to compensate f o r differences i n the i n i t i a l guality of these habitats, should have a s i m i l a r f i t n e s s , regardless of whether a given species belongs to the Allee type or non-Allee type of animals^ On the other hand, i f 266 F i g . 26. A l l e e type i d e a l despotic d i s t r i b u t i o n . Birds s e t t l e f i r s t i n high q u a l i t y habitat (x). But as the density of birds i n S^ increases to a point when established birds prevent new i n d i v i d u a l s from s e t t l i n g i n through t e r r i t o r i a l behavior (o), these i n d i v i d u a l s enter low q u a l i t y habitat S2 (o). I f density increases further, almost a l l new birds are forced to s e t t l e i n S2 (•). See text f o r further explanation. Population density 267 the d i s t r i b u t i o n of birds in a given population i s "despotic", i n d i v i d u a l s s e t t l e d i n high guality habitats should a t t a i n a higher f i t n e s s than those in low guality habitats (Fretwell and Lucas 1969). This implication also applies to both Allee and non-Allee types of birds. In redwings, t e r r i t o r i e s which attra c t the f i r s t breeding females also acguire the largest harems (Figs- 8, 11; see also Orians 1980). Because the theory of habitat d i s t r i b u t i o n predicts that: (1) the f i r s t i n d i v i d u a l s should s e t t l e i n the best habitat available; and (2) the f i n a l density of s e t t l e d birds should be highest in habitats of the best i n i t i a l g u a l i t y , these redwing t e r r i t o r i e s should present the highest guality s i t u a t i o n s . The fact that i n my study population the average female f i t n e s s , as r e f l e c t e d by mean reproductive success, increased with harem size u n t i l i t reached the maximum for harems of 4-6 females (Fig. 16) suggests that the d i s t r i b u t i o n of females i n t h i s population i s despotic.. Conseguently, because some females could not have made the " i d e a l free choice" (this would be indicated by a s i m i l a r reproductive success per female i n harems of a l l s i z e s ) , the resistance from s e t t l e d birds must have prevented new females from s e t t l i n g i n the highest guality situations. This supports the hypothesis that female t e r r i t o r i a l behavior plays an important role i n l i m i t i n g the density of breeding females on a male t e r r i t o r y and hence i n setting an upper l i m i t to harem s i z e . Because evidence from my marsh strongly suggests that redwings cooperate i n nest defense against marsh wrens, the d i s t r i b u t i o n of the breeding females 268 must, therefore, approach the "Allee type i d e a l despotic d i s t r i b u t i o n " (see Fig. 26). The problem of the role of agonistic interactions among females i n determining the pattern of d i s t r i b u t i o n of females i n a habitat has also been examined by Orians (1980), who suggested that i t might be costly for females which are f u l l y involved i n breeding to prevent new i n d i v i d u a l s from s e t t l i n g , near them (leaving the nest increases the r i s k s of loosing the ent i r e clutch due to c h i l l i n g and predators). The fact that female intolerance of other females decreases with the progress of their nesting (Nero 1956b) i s consistent with t h i s view. To test the role of agonistic behavior of s e t t l e d females on the pattern of settlement of new females, Orians (1980, pp. 60-62) examined the seguence of nesting by successive females. He hypothesized that i f females are more agonistic prior to egg-laying, and t h e i r agonistic behavior prevents new females from s e t t l i n g , the seguence of nesting by successive females should be more uniform than random i n time (since female intolerance of other females i s probably highest during the f i r s t 7 days after a female s e t t l e d , successive females should be separated from one another by more than a week). But a more random seguence of settlement of females should indicate that agonistic behavior of e a r l i e r s e t t l e d females has l i t t l e or no e f f e c t on new females. Because i n most cases the i n i t i a t i o n of nesting by successively s e t t l i n g females was, on the average, shorter than 7 days (Orians 1980, Table 3.1), Orians concluded that agonistic i n t e r a c t i o n s between females are not important i n determining 2 6 9 the pattern of female settlement on male t e r r i t o r i e s . The evidence discussed by Orians (1.980) , however, does not provide an adequate test of the r o l e of female agonistic interactions f o r two reasons. F i r s t , agonistic interactions between females should play an increasingly important r o l e as harem siz e increases and the breeding space becomes l i m i t e d . Therefore, as a harem becomes larger, new females should experience stronger resistance from the e a r l i e r s e t t l e d i n d i v i d u a l s . Conseguently, i f female t e r r i t o r i a l i t y was important i n preventing new females from s e t t l i n g i n large harems, the l a s t s e t t l i n g females, which should experience most resistance from already established birds, should be, on the average, more asynchronous i n nesting (the time i n t e r v a l between the i n i t i a t i o n of nesting by consecutively s e t t l i n g females should be longer)i On the other hand, the f i r s t females might s e t t l e r e l a t i v e l y synchronously because under low density conditions space i s not presumably limited and consequently the resistance of s e t t l e d individuals should not be important, The data presented by Orians were on the second and t h i r d nests only from harems of a l l sizes (Orians 1980, Table 3.1). The short time i n t e r v a l s between s e t t l i n g for these females reported by Orians are consistent with the view that females s e t t l i n g early should not experience much resistance from already established i n d i v i d u a l ( s ) , But Orians did not examine time i n t e r v a l s between females which s e t t l e l a s t on t e r r i t o r i e s acguiring large harems. Therefore, his evidence i s not conclusive. Moreover, i f females from a harem can e f f i c i e n t l y prevent 270 new i n d i v i d u a l s from s e t t l i n g throughout th e i r nesting, the argument developed by Orians (1980) would be inadequate. In addition to t h i s , there are two other reasons which indicate that Orians' speculation on the e f f e c t s of changes i n female resistance towards new a r r i v a l s as related to the stage of their nesting probably does not provide an adeguate test. F i r s t , because inexperienced (one year old) and experienced (at least two years old) females i n i t i a t e nesting, on the average, two weeks apart (Crawford 1977), the pattern of female settlement i s l i k e l y to be influenced by t h i s factor and possibly also by age-related differences i n behavior of females;. Second, I predict that nest predation, which presumably selects for cooperation among females i n nest defense through th e i r clumping tendency, should also select for nesting synchrony between neighbors. This i s because the cooperation between female neighbors which started nesting at a s i m i l a r time should presumably be more e f f i c i e n t than that between females which started nesting, for example, two weeks apart. This view i s supported by the fact that i n my marsh a majority of a l l females selected neighbors which had i n i t i a t e d nesting recently (Fig,. 12) . These suggestions indicate that the test of the r o l e of female t e r r i t o r i a l behavior i s d i f f i c u l t * However, the fact that i n my marsh females from small harems set t l e d l a s t and selected l e s s suitable situations for their nesting (see Chapter 7) strongly suggests that they must have been prevented by resident females from choosing higher guality nesting s i t u a t i o n s on t e r r i t o r i e s of males acguiring large harems and thereby from making an " i d e a l free choice". 271 In addition, evidence i s available which shows that the t e r r i t o r i a l behavior of settled females probably plays a ro l e i n preventing new females from s e t t l i n g even in Orians' marshes. Holm (1973), who studied redwings i n the same marshes, reported that females from the larger harems had, on the average, the highest reproductive success. Therefore, the d i s t r i b u t i o n of females in these marshes should, t h e o r e t i c a l l y , be rather "despotic" and not " i d e a l free" as suggested by Orians (1980). Second, i n spi t e of Orians' suggestion, i t i s l i k e l y that females might be e f f i c i e n t i n preventing new individ u a l s from s e t t l i n g nearby even later when they are f u l l y involved i n nesting. This view i s supported by two fa c t s : (1) Females are intol e r a n t of intruding females throughout th e i r nesting. This can be seen from the fact that I was able to capture incubating females or even females feeding nestlings throughout the breeding season (between early A p r i l and la t e June) using a trap baited with a female intruder (see Appendix I I ) . In addition, O'Connor (1976) found no s i g n i f i c a n t differences i n responses of female redwings during the nest-building, egg-laying, incubation, and nestling periods towards a caged conspecific female intruder, that she experimentally offered at three d i f f e r e n t distances from nests of examined females. This evidence thus indicates that the e a r l i e r observations of i n i t i a l l y higher aggression among female redwings (Nero 1956b) might r e f l e c t higher rates of interactions early i n a season, when there are many unsettled females searching for suit a b l e o breeding situations, 272 (2) Since females breed i n a clumped pattern t h e i r exclusion of new individuals from the clump i s probably not energetically expensive^ This hypothesis i s supported by the following evidence. In order to capture and band females, I used a trap baited with a female intruder (see Appendix I I ) . I set t h i s trap approximately 2 m from the redwing nests^ In most cases I captured resident females shortly a f t e r setting t h i s trap but, on 8 occasions, I captured neighboring females whose nests were 8-46 m far from the nest where I set the trap (the evidence that those 8 females were not from the nests closest to where I trapped them i s based on d i r e c t observations of these color-banded birds on their nests). These data indicate that, in addition to excluding conspecific; female intruders from the immediate v i c i n i t y of their own nests, the breeding females w i l l also attack intruders as f a r as 46 m away from their nests and w i l l attack them even i f these intruders are close to the nests of t h e i r neighbors. An important implication of these observations i s that since the resident females apparently attack strange intruders even i f these are close to the nests of t h e i r neighbors, the resident females must have developed mutual tolerance towards each other which allowed them to breed close to one another, but the i r l e v e l of aggression towards strange intruders must have remained high,. This view i s also supported by the f a c t that females from harems of 2-11 did not d i f f e r s i g n i f i c a n t l y i n the l e v e l of aggression towards a conspecific intruder during experiments conducted by O'Connor (1976).- Thus i n d i v i d u a l 273 females breeding i n a clump probably defend, i n addition to the i r nest, also the nest (s) of the nearest neighbor(s) against intruding conspecifics. Therefore, females from a harem probably cooperate i n excluding conspecific intruders from t h e i r breeding area. On the basis of these considerations I conclude that the t e r r i t o r i a l behavior of female redwings i s probably an important factor influencing the d i s t r i b u t i o n of females i n a breeding population,. B. Males as a possible factor setting an upper l i m i t to harem s i z e ; There should be selection for an optimum harem siz e that w i l l maximize the reproductive output of males,. The increase i n harem above t h i s optimum should have negative effects on male f i t n e s s because of increasing competition among females for lim i t e d resources, which should reduce not only the f i t n e s s of females, but eventually also the f i t n e s s of a male. In addition, i t has been suggested that male redwings may be able to e f f i c i e n t l y keep track of and defend from intruding males only a certain number of females (Weatherhead and Robertson 1977a). Further increase of the harem above t h i s s i z e , therefore, might res u l t i n reduced e f f i c i e n c y of excluding male intruders and conseguently an increasing rate of stolen copulations. Hence, male redwings should also be selected to prevent new females from joining t h e i r harem i f these birds have adverse effects on male f i t n e s s . However, two kinds of evidence indicate that males are probably not important i n se t t i n g an 274 upper l i m i t to harem size;. F i r s t , i t appears that the optimum harem size which has been favored by natural s e l e c t i o n i s that which i s most productive i n terms of the female reproductive strategy (see Figs. 16, 18)- Since males produce more young i f they acquire harems larger than i s optimum from the female point of view (Figs,. 16, 18) , an upper l i m i t to harem s i z e i s presumably set by female behavior and not by a male.. Second, the fact that females from a harem breed i n a clumped pattern suggests that the e f f i c i e n c y of males in keeping track of t h e i r females, excluding male intruders, and thereby preventing stolen copulations should stay, within certain l i m i t s , r e l a t i v e l y constant with increasing harem s i z e . In addition, the fact that subseguent females from the largest harems (6-9 females) s t a r t nesting, on the average, at roughly three-day i n t e r v a l s (Fig. 11, Table 39) indicates that males are usually engaged i n courting not more than two or three females simultaneouslyw This indicates that even males which acguired the largest harems might be r e l a t i v e l y e f f i c i e n t i n preventing stolen copulations. Therefore, male redwings probably do not play an important role i n s e t t i n g an upper l i m i t to harem s i z e . 6. The r o l e of nest predation and competition for l i m i t e d resources in the evolution of polygyny i n redwings The influence of increasing harem size on the average f i t n e s s per female i n terms of. (1) cooperation among females i n nest defense, and (2) limited resources such as food, space suitable for nesting, and male contribution to nest defense can 275 be graphically described by two kinds of f i t n e s s curves (Fig. 27). As harem size on any p a r t i c u l a r t e r r i t o r y increases, the e f f i c i e n c y of nest defense by females i n a harem should also increase u n t i l i t approaches a c e r t a i n maximum. Conseguently, the average female fi t n e s s increases with harem size u n t i l i t reaches the hypothetical maximum f i t n e s s (Fig 27, curves 1-3). But also, as the number of females i n a harem increases, i n t r a s p e c i f i c competition for l i m i t e d resources should become more intense. Hence, the average f i t n e s s per female, as related to the li m i t e d resources, decreases with increasing harem size (Fig. 27, curves 1'-3'). The intercept between the two f i t n e s s curves indicates the optimum harem size which should be selected for under that p a r t i c u l a r wren density because here females achieve, under given conditions, the highest r e a l i z a b l e f i t n e s s . Therefore, polygyny i n red-winged blackbirds may have evolved as a compromise between two driving forces: ( 1 ) . nest predation which s e l e c t s for cooperation among females i n nest defense; and (2) competition for limited resources or other negative e f f e c t s of clumping, which set an upper l i m i t to harem size presumably through the t e r r i t o r i a l behavior of females. In the following part I w i l l survey the most important factors which are l i k e l y to play a role i n determining the optimum harem siz e and hence the degree of polygyny in various marshes, and discuss their possible s i g n i f i c a n c e under various environmental conditions* Fig. 27. The average fitness per female as a function of harem size as related to marsh wren nest destruction (curves 1-3) and competition for limited food (curves l ' - 3 ' ) . Conditions i n a habitat with low density of marsh wrens are described by curves 1 and 1', whereas curves 3 and 3' i l l u s t r a t e the predicted relationships i n a habitat with high wren density. The intercept between the two types of fitness curves indicates the highest (realizable) fitness (RF), which females could achieve under the given conditions by breeding i n harems of optimum size (OH). See text for further explanation. Hypothetical maximum fitness OH^ 0 H 2 0 H 3 >• to ON Harem size 277 A. The i n t e n s i t y of nest predation In marshes with generally high nest predation rates due to marsh wrens chances of success of single, monogamously mated females should be low. But, the addition of other females to a harem should substantially reduce nest predation through improved nest defense, thereby selecting for large harems (Fig. 27, curve 3). On the other hand, when marsh wrens are less abundant and have small impact on redwing nesting, f i t n e s s of monogamously mated females should be r e l a t i v e l y higher and could be increased only s l i g h t l y by the addition of another one or two females. Under these circumstances small harems should be selected for (Fig. 27, curve 1),. This view i s supported by data from my marsh which show that i n situations with marsh wrens nearby (these simulate habitats with high wren density) redwing success increased with increasing number of females i n a clump (Table 36). But when marsh wrens were farther away (this s i t u a t i o n resembles habitat with low density of marsh wrens), female redwings were always r e l a t i v e l y more successful, regardless of the clump size (Table 36). Therefore, I predict that as the average intensity of nest predation in various marshes increases, redwings should be selected for more e f f i c i e n t cooperation in nest defense, which they can presumably achieve by increasing the degree of clumping and hence harem siz e . As the i n t e n s i t y of nest predation i n various marshes increases, redwings might be able to completely compensate for 278 i t by establishing larger, and, with cooperation i n nest defense, more e f f i c i e n t harems. This i s a reasonable assumption because by increasing t h e i r density redwings might be able to e n t i r e l y exclude marsh wrens from the v i c i n i t y of t h e i r nesting s i t e s . This s i t u a t i o n i s shown in F i g . 27 by the f i t n e s s curves 1-3, describing the average f i t n e s s per female related to marsh wrens as a function of harem s i z e , which ultimately reach (as a conseguence of " i d e a l cooperation" in nest defense among a certain number of females) the hypothetical maximum f i t n e s s under a l l , low, medium, and high wren density conditions. A l t e r n a t i v e l y , i t i s possible that the highest e f f i c i e n c y with which female redwings could reduce the impact of marsh wrens might decrease with increasing harem size (Fig- 28).. This could be a conseguence of the fact that, as the density of marsh wrens increases, the degree of s p a t i a l segregation between redwing and wren nest s i t e s decreases (see Chapter 4), which might in turn r e s u l t i n the generally reduced success of females from a harem. I f t h i s were true, then under extremely high marsh wren density conditions, redwing cooperation i n nest defense should become highly inefficient,. . Under these circumstances redwings should rather avoid breeding i n such habitat. This view i s supported by observations from a s a l t -water marsh i n Georgia with an extremely dense population of marsh wrens (Kale 1965),. T e r r i t o r y s i z e of male marsh wrens i n t h i s habitat was, on the average, around 100 sguare meters (Kale 1965), as compared with 450 sguare meters for marsh wrens studied by Verner (1965), and approximately 1200 sguare meters 279 Fig. 28. Average fitness per female redwing as a function of harem size as related to marsh wrens (density of marsh wrens i n a habitat increases from 1 to 4). I t i s assumed that females cannot f u l l y compensate for increasing density of marsh wrens by increasing harem siz e . See text for further explanation. U) c a> o •o _w ui v> a> c Harem size 280 for marsh wrens in my study marsh (Pieman, unpublished data). Red-winged blackbirds were nesting along the upland edge of t h i s marsh where marsh wrens were absent (Kale, personal communication). Redwings i n t h i s marsh thus apparently avoid high marsh wren density areas. However, since t h i s marsh supports almost pure stands of marsh grass, Spartina a l t e r n i f l o r a , which might not provide suitable nesting s i t e s f or large blackbirds, the s p a t i a l segregation of redwings and marsh wrens in t h i s and other spartina marshes i n Georgia (Kale, personal communication) might have resulted also from t h i s f a c t . The f i n a l test would reguire either reducing the density of marsh wrens experimentally or introducing new type of vegetation which would provide suitable nesting substrate f o r redwings and examining e f f e c t s of these experiments on the choice of breeding habitat by redwings. However, the test of the hypotheses on the maximum f i t n e s s which female redwings could achieve by breeding i n harems most e f f i c i e n t i n nest defense i s l i k e l y to be extremely complicated because i t reguires us to be able to discriminate between the ef f e c t s of: (1) reduced e f f i c i e n c y i n nest defense as a possible conseguence of the decreasing degree of s p a t i a l segregation between redwings and marsh wrens; and (2) other factors which might set an upper l i m i t to harem size even before the cooperation i n nest defense could reach i t s maximum possible e f f i c i e n c y . I do not have data to examine these hypotheses further. 281 B. The r o l e of food resources The a v a i l a b i l i t y of a s u f f i c i e n t amount of food i n a habitat i s a prerequisite for successful reproduction of any species,. The a v a i l a b i l i t y of food resources, both in time and space, i s therefore one of the most important factors which should influence the evolution of various reproductive strategies in di f f e r e n t animals (e.g. Crook 1965). The productivity of avian food resources i n marshes i s probably higher than in three-dimensional habitats because i t i s concentrated into a narrow v e r t i c a l belt (Verner and Willson 1966). In addition, because marshes are simple habitats with a low d i v e r s i t y of avian prey species, birds which c o l l e c t most of their food there should be more e f f i c i e n t in u t i l i z i n g these resources. Therefore, a high abundance of food i n marshes should allow the evolution of harem polygyny i n blackbirds which i s accompanied with freeing the males from most of the nesting duties and increased density of females on male t e r r i t o r i e s -Red-winged blackbirds forage mostly outside t h e i r t e r r i t o r i e s , either within the same marsh (Nero 1956a, Orians 1973, Pieman unpublished data), i n another nearby marsh (Holm 1973, Orians 1980), or on adjacent uplands (Snelling 1968, Holm 1972, Orians 1980). The location of redwing foraging grounds r e l a t i v e to t h e i r marsh nesting area i s determined by the productivity of redwing food resources i n these habitats (Orians 1980) . The fact that when given a choice between breeding i n a 2 8 2 marsh of low productivity or i n an adjacent upland habitat with high productivity of food, redwings w i l l select the marsh (see Holm 1973) indicates that marshes must provide safer nesting s i t e s than uplands (Wittenberger 1976). This and the fact that redwings tend to forage mostly away from their t e r r i t o r i e s indicate that t h e i r selection of nesting grounds i s determined by t h e i r safety against predators rather than the abundance of food resources. However, t h i s does not imply that food i s not an important l i m i t i n g factor. On the contrary, i t seems l i k e l y that food should play an important role i n setting an upper l i m i t to the density of birds i n a population and hence harem size because, as the number of females on a. t e r r i t o r y of a male increases, competition for food among them should also increase. Therefore, redwings should be selected to prevent a d d i t i o n a l females from s e t t l i n g on t h e i r t e r r i t o r i e s i f these new i n d i v i d u a l s increase competition f o r food enough to e f f e c t a net decrease in the reproductive output of established birds. In habitats where redwings forage mostly within t h e i r t e r r i t o r i e s (Linford 1935), the resident birds should be able to estimate food resources d i r e c t l y from th e i r foraging e f f i c i e n c y , as determined by the density and a v a i l a b l i l i t y of suitable prey species, and consequently respond to the reduced a v a i l a b i l i t y of food by changes in th e i r behavior towards new females. On the other hand, i n habitats where most of the foraging i s done outside t e r r i t o r i e s , females might evaluate food a v a i l a b i l i t y from the time reguired for foraging. I assume that as the number of females i n a harem increases, the depletion of 283 resources closest to t h e i r t e r r i t o r y (these should be u t i l i z e d f i r s t ) w i l l be faster and consequently females w i l l be forced to make energetically more expensive foraging t r i p s to more distant places. These longer t r i p s should i n turn have e f f e c t s on the time and energy budgets of females and also, through the length of time t h e i r nests are l e f t unguarded, on the nest predation rates. Therefore, i n such habitats, behavior of the resident females towards new individuals attempting to s e t t l e near them might be controlled by their foraging e f f i c i e n c y as measured by the length of t r i p s to the foraging grounds and the abundance of food i n these areas. The role of food (and probably also other l i m i t i n g factors) in s e t t i n g an upper l i m i t to harem size i n redwings could be viewed as a two-stage process. F i r s t , on the basis of the available evidence i t appears that females themselves play the most important role i n the evolution of the optimum harem s i z e in various habitats by adjusting the degree of clumping and conseguently cooperation i n nest defense to the i n t e n s i t y of nest predation i n a marsh. However, since the competition for limited food among females presumably increases with harem s i z e , females should be also selected to prevent new birds from s e t t l i n g in their harems when costs associated with the addition of new birds, due to t h i s increasing competition for food, exceed benefits of cooperation among them (see F i g . 27). In t h i s way li m i t e d food resources should play an important role i n s e t t i n g an upper l i m i t to harem siz e . Second, at the same time as natural s e l e c t i o n presumably 284 favors females which exhibit optimal clumping, i t should also select for those males which adjusted the size of th e i r t e r r i t o r i e s to the- average a v a i l a b i l i t y of food resources and the degree of female clumping selected for, so that they could accomodate, from the female viewpoint, harems of the optimum size- However, since males should, hypothetically,. fledge more young from harems which are larger than optimum from the female viewpoint (Fig. 29), i t i s also possible that males might esta b l i s h t e r r i t o r i e s which are adjusted to their own best reproductive strategy. But, there are three hypothetical reasons to believe that t h i s i s unlikely; F i r s t , the most common harems are those which produce most young per female (Figs. 16, 18). Therefore, selection for males establishing large t e r r i t o r i e s which could accomodate harems maximizing the male reproductive output should be weak or perhaps non-existent. Second, since costs to the male associated with the defense of a t e r r i t o r y presumably increase with (1) the size of an area defended, and (2) the in t e n s i t y of competition among males for the l i m i t e d breeding space, males might be selected against establishing t e r r i t o r i e s which are larger than necessary for accomodating harems of the optimum s i z e , as determined by the female strategy, Third, the defense of a t e r r i t o r y by a male against conspecifics should also have negative e f f e c t s on female f i t n e s s through i t s influence on the time and energy a male can spend defending nests of his females against predators. Hence, males should also be selected to conserve their time and energy for nest defense against predators by establishing a t e r r i t o r y which i s adjusted, i n terms of the female strategy, to the F i g . 29. Number of f l e d g l i n g s per female ( f u l l l i n e ) and male (dashed l i n e ) as a function of harem s i z e . F l e d g l i n g p r o d u c t i v i t y curves are i d e a l i z e d but approximate r e a l data from my redwing population. Notice that females fledge most young from harems that are smaller than the most productive harems of males. Because the f l e d g l i n g productivity of males i s a function of female p r o d u c t i v i t y , difference between the most productive harems of males and females i s , t h e o r e t i c a l l y , determined by the steepness of the descending part of the female p r o d u c t i v i t y curve and hence by negative e f f e c t s of female clumping. cu O E CD CD CL cn c o >-d CD E CD CL cn c 3 o T 1 1 r 7 8 9 10 11 12 Harem size to CO l_n 286 optimum harem size* The suggestion that the size of male t e r r i t o r i e s should be adjusted to the average food conditions in a habitat to accomodate the optimum harem size implies that males should not be responsive to short-time changes i n food productivity.. On the other hand, female redwings should be able to assess the a v a i l a b i l i t y of food resources i n a habitat and respond by adjusting t h e i r behavior towards other females. Therefore, i n a given habitat harem size should be adjusted to food a v a i l a b i l i t y . These predictions are supported by data from a long-term study of one redwing population which showed that, i n spite of great changes i n the productivity of blackbird food (insects) i n that marsh between years, the number of t e r r i t o r i a l males remained s i m i l a r . However, the decrease i n food productivity resulted i n the decrease of the breeding female population (Brenner 1966, 1968; see also appendix I I I ) . Also i n the following four years fluctuations in the size of t h i s population were associated only with changes i n the number of females breeding i n the area (Davis and Peek 1972). My r e s u l t s on the s t a b i l i t y of the male redwing population and changes i n the female population i n f i v e consecutive years (Pieman, unpublished data) are also consistent with t h i s view. I therefore suggest that food may have played a two-fold r o l e i n the evolution of the optimum harem size within i n d i v i d u a l marshes by: (1) setting an upper l i m i t to the number of females in a harem which can s t i l l e f f i c i e n t l y u t i l i z e the food resources within or near t h e i r t e r r i t o r i e s ; and (2) acting on 287 males to adjust the size of a t e r r i t o r y such that i t could accomodate, under the average food a v a i l a b i l i t y conditions, a certa i n optimum number of females res u l t i n g i n the most e f f e c t i v e achievable cooperation* These presumed rol e s of lim i t e d food resources are graphically i l l u s t r a t e d i n F i g . 27. C. I n t r a s p e c i f i c competition for limited breeding space I assume that as the average harem size selected for i n various marshes increases, the defense of a t e r r i t o r y and females breeding on i t should become more costly f o r two reasons. F i r s t , on a t e r r i t o r y l e v e l , as the number of females in a harem increases, the e f f i c i e n c y of males' i n keeping track of t h e i r mates and defending them against conspecific male intruders should decrease because males can e f f i c i e n t l y defend only a l i m i t e d area and a certain maximum number of females. This view i s supported by evidence on promiscuous matings by female redwings (Bray et a l * 1975, Roberts and Kennelly 1980). The f a c t that female redwings breed i n a clumped pattern, however, indicates that, within certain l i m i t s , males should be able to e f f i c i e n t l y defend several females simultaneously. Therefore, I suggest that males should become s i g n i f i c a n t l y l e s s e f f i c i e n t in the defense of their females only i f they acquired extremely large harems, where females are more synchronous i n nesting and their nests are b u i l t over a larger area. Second, on a population l e v e l , as the optimum harem selected for in various marshes increases, and hence the sex 288 r a t i o o f b r e e d i n g a n i m a l s b e c o m e s m o r e s h i f t e d t o w a r d s f e m a l e s , t h e n u m b e r o f n o n - b r e e d i n g s u r p l u s m a l e s , w h i c h a r e p r e v e n t e d b y t e r r i t o r i a l b i r d s f r o m e s t a b l i s h i n g t e r r i t o r i e s ( B e e r a n d T i b b i t s 1 9 5 0 , O r i a n s 1 9 6 1 , P e e k 1 9 7 1 ) , s h o u l d i n c r e a s e , T h e s e s u r p l u s m a l e s f r e q u e n t l y i n t r u d e i n t o t e r r i t o r i e s o f t h e r e s i d e n t m a l e s , d i s t u r b the b r e e d i n g f e m a l e s , a n d e n g a g e t h e r e s i d e n t m a l e s i n a g o n i s t i c i n t e r a c t i o n s ( N e r o 1 9 5 6 b , P i e m a n , u n p u b l i s h e d d a t a ) , I n t e r f e r e n c e f r o m i n t r u d i n g m a l e s s h o u l d h a v e f o u r k i n d s o f n e g a t i v e e f f e c t s o n r e d w i n g r e p r o d u c t i v e r a t e s . F i r s t , t h e y s h o u l d a f f e c t t h e t i m e a n d e n e r g y b u d g e t s o f r e d w i n g s w h i c h a r e e n g a g e d i n t h e s e i n t e r a c t i o n s . S e c o n d , i n t e r a c t i o n s b e t w e e n r e s i d e n t m a l e s a n d i n t r u d e r s s h o u l d r e d u c e t h e c o n t r i b u t i o n o f a m a l e t o n e s t d e f e n s e a g a i n s t p r e d a t o r s . T h i r d , d i s t u r b a n c e o f t h e b r e e d i n g f e m a l e s b y i n t r u d e r s s h o u l d i n c r e a s e t h e v u l n e r a b i l i t y o f u n g u a r d e d r e d w i n g n e s t s t o m a r s h w r e n s . F o u r t h , a s t h e e f f i c i e n c y o f m a l e s i n d e f e n d i n g t h e i r m a t e s d e c r e a s e s , e i t h e r a s a c o n s e g u e n c e o f i n c r e a s i n g h a r e m s i z e o n a t e r r i t o r y l e v e l , o r a s a r e s u l t o f i n c r e a s i n g i n t e n s i t y o f c o m p e t i t i o n b e t w e e n m a l e s f o r l i m i t e d b r e e d i n g s p a c e o n a p o p u l a t i o n l e v e l , s t o l e n c o p u l a t i o n s s h o u l d i n c r e a s e . B e c a u s e t h e s e n e g a t i v e e f f e c t s o f b e h a v i o r a l i n t e r a c t i o n s b e t w e e n r e d w i n g s o n t h e i r f i t n e s s p r e s u m a b l y i n c r e a s e w i t h i n c r e a s i n g h a r e m s i z e , t h e y s h o u l d s e t a n u p p e r l i m i t t o h a r e m s i z e . 289 D. Contribution of males to nest defense Hale redwings take an active part i n defense of nests of t h e i r mates against various predators, including marsh wrens (Nero 1956b; this study, see Chapter 2). However, as the number of females breeding on a t e r r i t o r y increases, male contribution to the defense of each nest should decrease. But females from a harem should be able, to a certain degree, to compensate for these costs of increasing harem size by clumping in a small area, thereby presumably increasing male contribution to defense of each nest. In addition, females also p a r t i a l l y compensate for these losses by increasing the e f f i c i e n c y of cooperation i n mutual protection of nests among themselves. In spite of these facts, i t i s l i k e l y that as the number of females i n a harem exceeds a certain optimum, and t h e i r nests become di s t r i b u t e d over a larger area, male contribution to defense of each nest should decrease more s i g n i f i c a n t l y . This i s especially l i k e l y to be true i f nest predators are small, inconspicuous animals, such as marsh wrens, which can ea s i l y approach the poorly guarded redwing nests without being noticed. Under such circumstances a male could simultaneously defend e f f i c i e n t l y only a small number of highly clumped nests. E- I n t e r s p e c i f i c interactions As the density of marsh wrens in various habitats increases, the rate of interactions between redwings and marsh wrens should also increase as a conseguence of decreasing degree of s p a t i a l segregation between these species. This hypothesis 290 i s supported by data from my study marsh on: (1) higher rate of inter a c t i o n s between redwings and marsh wrens i n situations when marsh wrens were abundant than i n situ a t i o n s with low wren densities i n the same season (see Chapter 2); and (2) higher rate of interactions i n a year with high density of marsh wrens than i n a year with low wren density (Pieman, unpublished data),. These behavioral interactions presumably reduce interference between redwings and marsh wrens through the s p a t i a l segregation of their a c t i v i t y centres (e. g. Orians and Willson 1964). But, i n addition to t h i s benefit, these agonistic i n t e r a c t i o n s with marsh wrens should also have negative conseguences on female f i t n e s s through t h e i r e f f e c t s on female time and energy budgets (see Chapter 2, 4) and increasing the r i s k of los i n g the whole unattended clutches due to c h i l l i n g or predators (Orians 1980). Because such costs should increase with increasing density of marsh wrens i n various marshes, they should play an important role i n setting an upper l i m i t to the highest f i t n e s s female redwings could potentially attain and hence also to the harem size selected for in marshes with d i f f e r e n t densities of marsh wrens,. In other words, as the density of marsh wrens and hence the intensity of nest predation increases, the costs of i n t e r s p e c i f i c agonistic interactions to the breeding females should increase, thereby reducing the maximum r e a l i z a b l e f i t n e s s which females could, on the average, achieve by breeding i n optimum s i z e harems (see Fig. 27). Various negative e f f e c t s of female clumping which I discussed above might a l l play a r o l e i n setting an upper l i m i t 291 to the highest f i t n e s s females could achieve and hence to harem size selected f o r . I t i s probable, however, that the r e l a t i v e s i g n i f i c a n c e of these agents i n setting t h i s upper l i m i t may change with the number of females i n a harem, the abundance of food resources, and the in t e n s i t y of nest predation,. Therefore, the r e s u l t i n g f i t n e s s curve describing the negative e f f e c t s of various degrees of female clumping under d i f f e r e n t sets of conditions should probably present a combination of these factors,* 7. A model on the r o l e of predation i n the evolution of polygyny, and i t s implications Considerations on the role of marsh wrens, which presumably select f o r redwing clumping tendency, and the negative e f f e c t s of increasing harem size on female f i t n e s s are summarized i n a three-dimensional model presented i n Figure 30. This model describes the average f i t n e s s per female as a function of harem size i n habitats with various i n t e n s i t i e s of nest predation caused by marsh wrens. The ascending part of each f i t n e s s curve (small harems) r e f l e c t s the benefits of cooperation among females i n nest defense achieved through clumping, whereas the descending parts of these f i t n e s s curves r e f l e c t the combined negative e f f e c t s of increasing harem s i z e . The highest f i t n e s s which females could achieve under the given marsh wren density conditions or the highest r e a l i z a b l e f i t n e s s i s c a l l e d the maximum f i t n e s s (MF), and the number of females on a male's t e r r i t o r y which a t t a i n the highest (optimum) fit n e s s as a 292 Fig. 30. Model of the role of marsh wrens and limited resources i n the evolution of polygyny i n marsh-nesting red-winged blackbirds. I t i s assumed that as the intensity of nest predation i n various marshes increases (from 1 to 5), as a consequence of increasing density of marsh wrens, redwings are selected for establishing larger harems more e f f i c i e n t i n nest defense. See text for further explanation. Fitness 293 consequence of the most e f f i c i e n t achievable cooperation i s c a l l e d the optimum harem (OH) . o I assume that i n a marsh without marsh wrens (nest predation rates equal zero) the addition of females to a harem w i l l always have negative effects on the average f i t n e s s per female because: (1) clumping has no positive e f f e c t s on female f i t n e s s ; and (2) the addition of new females to a harem w i l l increase i n t r a s p e c i f i c competition for l i m i t e d resources. Therefore, under such circumstances monogamy should, t h e o r e t i c a l l y , be selected f o r (Fig. 30, curve 1). On the other hand, when the density of marsh wrens and hence the i n t e n s i t y of nest predation i s high, redwings should be selected for establishing large harems which could more e f f i c i e n t l y reduce impact of marsh wrens through the cooperation i n nest defense (Fig. 30, curve 5). From t h i s model three major predictions on the influence of marsh wren nest predation on the degree of polygyny i n redwings can be made. F i r s t , the degree of polygyny selected for i n various marshes should be p o s i t i v e l y correlated with the average i n t e n s i t y of nest predation. In other words, as the average nest predation rates in d i f f e r e n t marshes increase, redwings should be selected to e s t a b l i s h larger harems, more e f f i c i e n t i n nest defense (see F i g . 30). This prediction i s supported by a positive c o r r e l a t i o n between the average harem size and nest predation rates i n various marshes obtained by M. McVay (personal communication). I also examined t h i s prediction using a l l a v a i l a b l e data on nesting success of redwings from various 294 marshes throughout North America. Since nest predation i s the major cause of redwing nesting mortality (e.g. Ricklefs 1969), the proportion of nests fledging at least one young can be used as a r e l i a b l e index of predation pressures i n a given habitat. As the average harem size of redwings increases, their nesting success, on the average, decreases (Fig* 31). This r e s u l t i s thus also consistent with the prediction from my model. Second, because the costs of clumping presumably increase with harem s i z e , there should be a negative c o r r e l a t i o n between harem s i z e selected for i n various redwing populations and the highest f i t n e s s which females could achieve by breeding i n optimum siz e harems (see Fig. 30) . Test of t h i s prediction reguires data on the average reproductive success of females from optimum size harems from marshes with various densities of marsh wrens. However, I do not have data to test t h i s prediction. Third, as the i n t e n s i t y of nest predation and hence presumably harem siz e selected for i n various marshes increases, the difference between the average f i t n e s s of monogamously mated females and f i t n e s s of females from harems of the optimum size should increase (in Fig. 30 (MF5-F5)>(MF4-F4)>(MF3-F3) e t c . ) . Therefore, the i n t e n s i t y of selection for polygyny should be highest i n habitats with high nest predation rates* The test of t h i s prediction i s complicated by the f a c t that i n redwings the guality of male t e r r i t o r i e s and the number of females attracted to them are not correlated (see Chapter 8; Weatherhead and Robertson 1977a). Because of t h i s , the analysis of female 295 Fig. 31. Nesting success of various redwing populations as a function of the i r degree of polygyny. Data on redwing populations included i n this analysis are given i n Appendix I I I . Mean harem size 2 9 6 f i t n e s s as a function of harem siz e does not provide an adequate test. Therefore, a v a l i d test of t h i s prediction requires the evaluation of the influence of the addition of new females to i n d i v i d u a l harems on f i t n e s s of the e a r l i e r s e t t l e d i n d i v i d u a l s . However, such data are not available. * The available evidence from various marshes supports the view that polygyny in marsh nesting redwings has probably been driven by high nest predation rates perhaps caused by marsh wrens, which select for cooperation among females i n nest defense and hence increased density of females within male t e r r i t o r i e s . Additional evidence supporting t h i s view comes from two studies which investigated relationships between predation pressure on redwing nests and redwing nesting densities within a marsh (Robertson 1973a, Caccamise 1976). A common conclusion of both authors was that high predation pressures on nests under high nest density conditions are reduced with further increase i n nest densities. Robertson (1973a) proposed that t h i s could r e s u l t from s a t i a t i o n of the predator population.* However, because in my marsh marsh wrens did not eat the contents of the attacked nests, predation for food i s an unlikely explanation (see Chapter 1). Hence, i f marsh wrens are the most important selective force acting on redwings i n most marshes, Robertson's suggestion that decreasing nest predation pressure with increasing density of redwing nests could r e s u l t from s a t i a t i o n of the predators would have to be rejected* Caccamise (1977), on the other hand, proposed that reduced nest predation pressures under high redwing nest density 297 conditions might r e s u l t from possible mutual protection from predators. This view i s consistent with my findings on the adaptive value of redwing harem polygyny with regard to marsh wrens. A. Evidence from the comparative method The evolution of a given behavior may sometimes be explained by comparing c l o s e l y related species which exhibit a spectrum of behaviors of varying degrees of s i m i l a r i t y to the one i n guestion (Tinbergen 1959). This comparative method i s based on three major assumptions (Alcock 1975): (1) Behavior has a genetic component and i s shaped by natural selection; (2) Closely related species are l i k e l y to share some behavior patterns; (3) The more widespread a behavior pattern i s i n a group of related species, the higher the pr o b a b i l i t y that i t i s s i m i l a r to an ancient behavior pattern. One way of using the comparative method i n the reconstruction of the evolution of a given adaptation i s to compare cl o s e l y related species which have been exposed to the same se l e c t i v e forces f o r various lengths of time. By comparing such species we should be able to detect differences i n the degree with which they have adapted to the same selective forces. Such a comparison should, conseguently, allow us to es t a b l i s h the probable direction of the evolution of a given adaptation towards a more e f f i c i e n t one> 298 Selective forces driving the evolution of polygyny i n North American marsh-dwelling blackbirds can be examined by comparing two e c o l o g i c a l l y very similar species, the red-winged blackbird and yellow-headed blackbird (from the following t h e o r e t i c a l considerations I excluded the t r i c o l o r e d blackbird because of i t s highly c o l o n i a l nesting presumably associated with the u t i l i z a t i o n of highly unpredictable food resources located far away from the breeding colonies; see Orians 1961). Both species build t h e i r nests mostly i n emergent vegetation and usually forage within marshes or on nearby uplands (Orians 1980). Therefore, i t i s reasonable to conclude that these species have been exposed to the same s e l e c t i v e forces operating within North American marshes, This i s supported by the fac t that both redwings and yellowheads exhibit s i m i l a r reproductive strategies; they are strongly t e r r i t o r i a l and highly polygynous (Orians 1980). I t has been suggested, however^ that the red-winged blackbird i s a more recent member of the marsh avifauna (Allen 1914). The yellowhead, on the other hand, has probably been associated with marshes for a longer period of time (Burt 1970). This view i s supported by the following evidence: (1) The yellowhead i s a food s p e c i a l i s t r e s t r i c t e d to the more productive lakes, whereas the redwing i s more v e r s a t i l e (Willson and Orians 1963); (2) In addition to nesting in t y p i c a l marsh vegetation such as c a t t a i l , bulrush, and sedge, redwings freguently u t i l i z e shrubs or low trees as a nesting substrate, 299 frequently even far from water (e.g. Allen 1914, Nero 1956a# Orians 1961, M i l l e r 1968, Burt 1970). Yellowheads, on the other hand, normally build their nests over water i n emergent vegetation and only ra r e l y u t i l i z e shrubs (Linsdale 1938, Fautin 1941, Weller and Spatcher 1965, Orians 1980). (3) Marsh nesting redwings respond more vigorously to t e r r e s t r i a l predators than do yellowheads ( S i g l i n and Weller 1963); (4) The juvenile plumage of redwings i s dark, d u l l brown, which i s the feature c h a r a c t e r i s t i c of the more arboreal North American blackbirds, whereas the juvenile plumage of yellowheads i s tan, which i s c h a r a c t e r i s t i c of true marsh birds (Linsdale 1938). This evidence indicates that yellowheads are better adapted to marshes than redwings, which s t i l l exhibit many features showing a closer relationship to t e r r e s t r i a l breeding environments. Therefore, i f the evolution of polygyny i n these two blackbirds has been driven by high nest predation rates i n marsh habitats, then yellowheads should have evolved more e f f i c i e n t methods of reducing nest predation than redwings. This should be r e f l e c t e d p a r t i c u l a r l y i n the e f f i c i e n c y of the nest defense behavior, the degree of polygyny, and the s p a t i a l organization of nesting which should r e f l e c t the e f f i c i e n c y of cooperation among females. 300 (a) Nest defense behavior The available evidence shows that attacks by the yellow-headed blackbird on l o n g - b i l l e d marsh wrens are much more intense than those by redwings (Burt 1970, Verner 1975).. Burt (1970) observed that male yellowheads not only chased marsh wrens more consistently, but that they also captured them and pecked them vigorously. He has not, however, detected wren mortality as a resu l t of these attacks. In addition, during, many hours of observations of interactions between redwings and marsh wrens I saw no captures of wrens by redwings, and I know of no reports from other studies. Moreover, the fact that yellowheads exhibit a higher degree of intolerance towards marsh wrens i s evident from Burt's (1970) data on hatching success of yellowhead and redwing nests which had marsh wren courtship centers nearby or farther away (Table 58). Because yellowheads had greater success than redwings i n si t u a t i o n s with marsh wrens nearby, they must have been more e f f e c t i v e i n nest defense against marsh wrens. But i n si t u a t i o n s with marsh wrens farther away both species had s i m i l a r success. However, the guestion a r i s i n g from Burt's data i s : why did yellowheads have lower success when marsh wrens were farther away? There are two possible explanations: F i r s t , in the l i g h t of the e a r l i e r considerations, the cooperation in nest defense between yellowheads which had marsh wrens farther away may have been passive and hence less e f f i c i e n t than the active mutual nest protection i n situations with marsh wrens nearby, Second, yellowheads with nests farther away from marsh wrens may have 301 Table 58. Hatching success of red-winged blackbirds and yellow-headed blackbirds related to t h e i r distance from marsh wren courting centers (from Burt 1970). Percent nests successful (N) Species within 100 feet outside 100 feet Total Yellow-headed blackbird 79.5 (21) 54.8 (31) 61.5 (52) Red-winged blackbird 33.3 (24) 53.1 (32) 44.6 (56) 302 exhibited a lower degree of clumping and hence less e f f i c i e n t cooperation. These suggestions, however, remain speculative because there are no data available for examining them further. (b) The degree of polygyny As the length of exposure of a population to the marsh conditions increases, the degree of polygyny exhibited by i n d i v i d u a l s from that population should, on the average, approach a certain optimum. In addition, as blackbirds gradually evolve, in terms of the cooperation i n nest defense and l i m i t e d resources, towards the most e f f i c i e n t degree of polygyny, the degree of v a r i a t i o n i n harem size within a population should become smaller because a l l birds ought to behave more i d e a l l y ; In other words, we could, expect more "mistakes" from birds which are i n the e a r l i e r stages of evolution towards the most e f f i c i e n t strategy* Data on sex r a t i o s of yellowheads and redwings breeding in the same marsh show that these species exhibit, on the average, a s i m i l a r degree of polygyny (Orians 1980)* This indicates that both blackbird species have probably been exposed to s e l e c t i v e pressures operating i n marshes long enough to approach, on the average, the optimum degree of polygyny selected for. A test of the second prediction ( i . e . the degree of v a r i a t i o n in harem size should decrease as a function of the length of exposure to marsh conditions) reguires data on the freguency of occurrence of harems of various sizes within i n d i v i d u a l populations of redwings and yellowheads* This information was given by Orians 303 (1980), who showed that within the same marshes both blackbird species exhibit also a s i m i l a r degree of va r i a t i o n i n harem si z e . Thus redwings and yellowheads do not seem to d i f f e r i n t h e i r degree of polygyny. (c) S p a t i a l organization of nests Yellowheads exhibit a stronger clumping tendency than redwings because, within the same marsh, nests of simultaneously breeding female yellowheads appear to be located closer to each other than nests of female redwings (Table 59)- Similar observations have been also made in central Washington (J- F. Wittenberger, personal communication) and southern B r i t i s h Columbia ( E . Guindon, personal communication).. The smaller distances between yellowhead nests may have also contributed to the higher hatching success of yellowheads near marsh wren courtship s i t e s observed by Burt (Table 58), and the o v e r a l l higher nesting success of yellowheads compared to redwings breeding i n the same marsh (Table 58). The above evidence thus supports the view that yellow-headed blackbirds, which have probably been associated with marshes for a longer period of time, have evolved, i n terms of in t e r a c t i o n s with l o n g - b i l l e d marsh wrens, a more e f f i c i e n t reproductive strategy to cope with marsh wren predation than red-winged blackbirds. Hence, I suggest that the reproductive strategy of marsh-nesting redwings should be evolving towards that exhibited at present by yellowheads. 304 Table 59. Comparison of distances between the nearest simultaneously active nests of red-winged blackbirds and yellow-headed blackbirds. Data were obtained by measuring distances between the nearest blackbird nests with a r u l e r from maps of d i s t r i b u t i o n of nests given by Burt (1970). Mean distance (m) ±SE (N) •-• between the nearest simultaneously Species active nests T-value d.f. p Red-winged blackbird 15.3±3.3 (16) Yellow-headed blackbird 4.9±1.6 (15) 2.84 21 <0.01 \ 305 B, Problems associated with the model In some areas of their breeding range redwings do not coexist with l o n g - b i l l e d marsh wrens. What factors could have driven the evolution of polygyny in redwings i n those areas? What features have been driving the evolution of polygyny i n redwings as opposed to c o l o n i a l i t y ? Could redwing populations nesting i n upland habitats have any impact on marsh nesting redwings through the exchange of genes? These guestions present several major d i f f i c u l t i e s with the view that nest predation by marsh wrens may have driven the evolution of polygyny i n redwings. I w i l l now discuss these problems i n d i v i d u a l l y . (a) Are l o n g - b i l l e d marsh wrens the only agent s e l e c t i n g f o r cooperation between female redwings? Two major assumptions of the proposed model (Fig. 30) are that; (1) marsh nesting redwings occur sympatrically with long-b i l l e d marsh wrens throughout most of their breeding range, and (2) behavioral interactions between these species are s i m i l a r throughout t h e i r range of sympatry. These assumptions are supported by available data (see Chapter 11).. However* i n some areas of the redwing breeding range (e.g. Central America and the most northern areas of the redwing breeding range i n Canada) l o n g - b i l l e d marsh wrens are not present (e.g. Bent 1948, 1958; Godfrey 1966). I propose two explanations on the evolution of polygyny i n redwings breeding i n such areas. F i r s t , gene flow between populations with and without marsh wrens might be maintaining polygynous matings i n areas without marsh wrens. 306 Second, other nest predators e c o l o g i c a l l y eguivalent to long-b i l l e d marsh wrens may select for contagious nesting by female redwings in areas without l o n g - b i l l e d marsh wrens. For example, s h o r t - b i l l e d marsh wrens, Cistothorus platensis, are also known to destroy nests of other passerines as well as conspecifics (Pieman and Pieman 1980). Hence they should present an important s e l e c t i v e force acting upon redwings i n marshes of eastern North America and Central America where the two species are sympatric (e.g. Bent 1948, 1958). But, i n addition to various species of wrens, a number of other small predators, whose impact redwings could reduce through the cooperation i n nest defense, might influence redwing reproductive t a c t i c s i n a way s i m i l a r to l o n g - b i l l e d marsh wrens. In s p i t e of the f a c t that the model (Fig. 30) was derived from data on i n t e r a c t i o n s between redwings and l o n g - b i l l e d marsh wrens, i t also applies to any other predators whose impact on redwing reproductive strategy i s s i m i l a r and who could be i n h i b i t e d by redwings i n a s i m i l a r way. Support for t h i s p o s s i b i l i t y comes from Orians' (1973) study of redwings i n Costa Rica, outside the breeding range of the l o n g - b i l l e d marsh wren. Important features which characterize the nesting of t r o p i c a l blackbirds are high nest predation rates (see Appendix III) and freguent p a r t i a l nestling losses. The predators responsible for nesting losses are unknown, however, based on the appearance of depredated nests, Orians (1973) suggested that predators must be small animals. Also the f a c t that nestling growth rates in Orians' study are 307 independent of the number of young i n a nest suggests that small predators rather than starvation are responsible for p a r t i a l n estling losses. The s i m i l a r i t y of Orians' (1973) findings on predation of redwing nests in Costa Rica with those made i n other studies of redwings i n the Temperate Zone thus supports the view that polygyny i n t r o p i c a l nesting redwings may also be an adaptation to strong nest predation pressures. (b) why have redwings evolved harem polygyny as opposed to c o l o n i a l i t y ? I t i s not known what features select for polygyny as opposed to c o l o n i a l i t y (Orians 1972). This guestion i s esp e c i a l l y important i n the case of highly polygynous red-winged blackbirds because of several potential benefits of c o l o n i a l i t y . Colonial nesting i s generally associated with group defense against predators (Kruuk 1964, Horn 1968, Brown and Orians 1970, Hoogland and Sherman 1976), s p a t i a l l y and temporally variable food resources (Fisher 1954, Horn 1968), and some other functions l i s t e d by Brown and Orians (1970) which do not apply to redwings. I assume that nest predation by marsh wrens should select for c o l o n i a l nesting in blackbirds. This reproductive strategy should presumably improve the f i t n e s s of redwings through t h e i r selection of optimal nest s i t e s ( a l l or most birds could breed r e l a t i v e l y far from marsh wrens), and improved nest defense. Furthermore, a monogamous c o l o n i a l system should reduce -freguently reported nestling starvation (e.g. Holm 1973, Robertson 1973b) because both parents could feed young. 308 However, the evolution of true c o l o n i a l monogamous nesting i n redwings must have been prevented by some counterselective forces. To resolve the problem of the evolution of polygyny as opposed to true c o l o n i a l i t y , i t i s necessary to examine a l l possible effects of predation and the d i s t r i b u t i o n of food resources which present probably the most important factors influencing the evolution of reproductive strategies (e.g-Crook 196 5). To avoid complications associated with the evolution of polyandry in a small number of nidifugous species (see Emlen and Oring 1977), I w i l l confine the following discussion of the roles of predation and food resources i n the evolution of various spacing patterns, and conseguently also mating systems, to nidicolous birds only. High predation rates on breeding birds or t h e i r nests might presumably drive the evolution of two alternative strategies by the affected species* F i r s t , they might select for grouping of some birds because of advantages such as improved predator detection, cooperation in defense against predators, and predator confusion (Brown 1975). Second, those species of birds which cannot reduce predation rates through some form of predator-related cooperation should be selected to avoid predators either through camouflage and concealment (presumably most of the smaller and less conspicuous species) or by choosing for t h e i r breeding inaccessible places such as holes, high trees, c l i f f s , or islands (mostly larger bi r d s ; e. g. Lack 1968). These avoidance strategies should conseguently favor a 309 dispersed (less conspicuous) or uniform d i s t r i b u t i o n of breeding i n d i v i d u a l s . In the case of species nesting at places inaccessible to predators, i t could also lead to c o l o n i a l nesting i f these places were i n short supply (see Fig. 32). The d i s t r i b u t i o n of food resources may also have two kinds of e f f e c t s on the reproductive strategies of birds, If food i s abundant, uniformly dis t r i b u t e d , seasonally stable, and hence economically defendable, t e r r i t o r i a l i t y should be selected for (Brown 1964). On the other hand, i f food resources are unpredictable i n time and space, c o l o n i a l nesting should be favored, presumably, because of the increased e f f i c i e n c y of u t i l i z i n g such food resources through cooperation between indi v i d u a l s from a colony (Bcown 1964, 1975; Horn 1968). Combining predation and food i n various possible ways we generate four reproductive strategies which should, t h e o r e t i c a l l y , present the most e f f i c i e n t adaptations under the given circumstances (Fig* 32). Thus, hypothetically, true c o l o n i a l nesting may evolve only when food resources are highly unpredictable and conseguently se l e c t f o r cooperation i n foraging and thereby for the tendency to group. Since the highly unpredictable nature of the d i s t r i b u t i o n of food resources in time and space reguires the cooperation of both parents i n feeding young, monogamy should be selected for (e.g. Orians 1961). I f predation also drives the evolution of grouping either through some form of cooperation in antipredator defense or through the avoidance of predators by breeding at scarce and inaccessible places, then this should present an F i g . 32. Presumed roles of d i f f e r e n t patterns of d i s t r i b u t i o n of food resources and predation pressures i n the evolution of various reproductive strategies i n passerines. See text f o r further explanation. Cooperation through grouping / Food resources Territoriality Coloniality, monogamy Territoriality, harem polygyny Territoriality, monogamy Territoriality, resource defense polygyny Breeding at inaccessible places Camouflage, dispersion Cooperation through grouping Predation Predator avoidance Co i—• o 311 addi t i o n a l factor promoting the evolution of true monogamous c o l o n i a l system (Fig. 32). On the other hand, i f food resources are economically defendable, and hence, presumably, drive the evolution of t e r r i t o r i a l i t y , the resu l t i n g reproductive strategy of a given species should be determined by the nature of predation. I assume that i n situations when predation selects for cooperation through grouping, true harem polygyny (definded as "harem defense polygyny" by Emlen and Oring 1977) should evolve as a compromise between two driving forces (one selecting for t e r r i t o r i a l i t y and the other f o r grouping; see F i g . 32). This strategy should, t h e o r e t i c a l l y , r e s u l t i n the most e f f i c i e n t u t i l i z a t i o n of r e l a t i v e l y evenly d i s t r i b u t e d food by i n d i v i d u a l s defending t e r r i t o r i e s which are adjusted to the available food resources, and, i n addition, i t should also reduce predation rates through improved cooperation between females nesting c o l o n i a l l y on male t e r r i t o r i e s . But i f predation selects for one of the two most important predator avoidance strategies, and food resources drive the evolution of t e r r i t o r i a l i t y , then t e r r i t o r i a l i t y and monogamy should r e s u l t (Fig. 32). However, i f the a v a i l a b i l i t y of resources important for female nesting such as food, suitable nesting s i t e s , and the inte n s i t y of predation varies greatly between t e r r i t o r i e s and the differences between high and low guality t e r r i t o r i e s exceed the "polygyny threshold" (see Verner and S i l l s o n 1966, Orians 1969), polygynous matings on the highest quality t e r r i t o r i e s should occur. This s i t u a t i o n would, therefore, lead to resource 312 defense polygyny (see Emlen and Oring 1977). Therefore, I conclude that the most important factor determining the evolution of true c o l o n i a l i t y as opposed to polygyny i s the nature of the d i s t r i b u t i o n of food resources i n a habitat. The view that economically defendable food resources se l e c t i n g for t e r r i t o r i a l i t y should present an important force sel e c t i n g for harem polygyny i n redwings as opposed to true c o l o n i a l i t y i s supported by the following facts : (1) marshes are highly productive habitats where food resources are predictable and seasonally stable (Orians 1969, Verner and Willson 1966) ; (2) redwings of both sexes have well developed t e r r i t o r i a l behavior (e.g. Nero 1956b); (3) the s i z e of a redwing population i s correlated with food resources i n a marsh (Brenner 1966, 1968). Thus i t i s reasonable to conclude that the t e r r i t o r i a l behavior, that has most l i k e l y evolved as a mechanism of i n t r a s p e c i f i c competition (e.g. Brown 1975, Wilson 1975), probably presents the most important force counterselecting the evolution of a true c o l o n i a l system in redwings. The present redwing reproductive strategy, therefore, most l i k e l y represents a compromise between two opposite s e l e c t i v e pressures (redwing females breed c o l o n i a l l y within male t e r r i t o r i e s ) . This view i s also supported by comparative evidence on three c l o s e l y related blackbird species breeding i n North American marshes: the red-winged blackbird, yellow-headed blackbird, and t r i c o l o r e d blackbirds. Both redwings and 313 yellowheads generally forage near t h e i r nests, either within a marsh or on adjacent uplands (e.g.. Orians 1961, Willson 1966). Therefore, these species should, t h e o r e t i c a l l y , be selected for establishing and defending a t e r r i t o r y whose size ought to be adjusted to the available food resources. In f a c t , both redwings and yellowheads establish r e l a t i v e l y large t e r r i t o r i e s (e.g. Orians 1961, 1980; Willson 1966), and the size of redwing t e r r i t o r i e s in various marshes appears to be generally adjusted to food resources available i n a habitat (Orians 1972, 1980). On the other hand, t r i c o l o r e d blackbirds generally u t i l i z e more distant foraging grounds where food resources are highly unpredictable both i n time and space (Orians 1961). This condition should favor the evolution of a high degree of c o l o n i a l i t y i n t r i c o l o r s , shown by extremely small t e r r i t o r i e s the size of which does not vary much between colonies, that probably f a c i l i t a t e s the e f f i c i e n t u t i l i z a t i o n of unpredictable food (Orians 1961, 1980). Presumably as a conseguence of these differences i n foraging strategies redwings and yellowheads are highly polygynous, whereas t r i c o l o r s exhibit only a low degree of polygyny (see Orians 1961). This evidence thus supports the view that foraging within marshes and nearby uplands, where food resources are abundant and predictable but also limited, may have selected for t e r r i t o r i a l i t y , thereby counteracting the evolution of true c o l o n i a l i t y i n redwings and yellowheads. 314 (c) What i s the influence of upland-nesting blackbirds? Another factor that may have played an important r o l e i n the evolution of polygyny i n marsh-nesting redwings i s the opposite s e l e c t i v e forces acting upon redwing reproductive strategies i n marsh and upland habitats. Marshes generally support high density populations, whereas uplands maintain populations of low densities (Case and Hewitt 1963, Robertson 1972, Blakley 1976, Howard 1977, Orians 1980). Robertson (1973 a) suggested that, because of the presence of many d i f f e r e n t predators in uplands, there should be s e l e c t i o n for more dispersed or s o l i t a r y nesting which should suffer less predation. In f a c t , more dispersed nesting has been observed i n an upland redwing population studied by Howard (1977). ' This view on d i f f e r e n t s e l e c t i v e pressures i n uplands i s supported by addi t i o n a l evidence on high mortality rates of breeding female redwings (26% of a l l females were k i l l e d by predators on or near the i r nests) i n an upland habitat studied by Blakley (1976). I can f i n d no similar information i n more numerous studies of redwings breeding in marshes. Conseguently, i t i s possible that the lower degree of polygyny i n upland nesting redwings as compared with nearby located marsh nesting populations (Case and Hewitt 1963, Howard 1977) might be a conseguence of the selection for more dispersed and c r y p t i c nesting i n uplands. In marshes, on the other hand, there appears to be selection for c o l o n i a l nesting. Hence, i f these two kinds of reproductive strategies are genetically controlled and there i s exchange of genes between habitats, the opposite s e l e c t i v e forces might 315 hinder the development of more e f f i c i e n t nesting systems i n these habitats (monogamy i n uplands; polygyny with highly c o l o n i a l l y nesting females in male t e r r i t o r i e s within marshes). Several studies have examined the p o s s i b i l i t y that marsh and upland redwing populations might be e c o l o g i c a l l y i s o l a t e d and hence possibly genetically d i f f e r e n t . These studies compared upland and marsh-nesting redwing populations with regard to various morphological (Dyer 1964), physiological (Dyer 1968) , and behavioral features (Holcomb and Twiest 1968, Stone 1969) i Because there are some differences in the morphology (Dyer 1964) and physiology (Dyer 1968) between the two kinds of populations, marsh and upland-nesting redwings might present l o c a l l y adapted populations with l i t t l e or no exchange of genes. On the other hand, the evidence on nesting by a single female redwing i n an upland habitat and then in a marsh (Fankhauser 1964), and the i n f l u x of new females i n a marsh l a t e r i n a breeding season (Fankhauser 1964, Jackson 1971, Holm 1973, Dolbeer 1976) indicate that genetic separation between marsh and upland populations i s unlikely. These contradictory data indicate the need for more research on the rates of redwing movements between habitats, and on o r i g i n of differences between these two types of populations. 316 8- The role of previous breeding experience in the evolution of polygyny i n redwings In addition to i t s probable r o l e in the evolution of clumping by females, nest predation by marsh wrens also appears to influence the mating pattern within a breeding redwing population through the guality of males and females i n terms of t h e i r previous breeding experience. This i s supported by the increasing mating success of males with t h e i r age, and a positive c o r r e l a t i o n between harem size and the presence of old, experienced females on t e r r i t o r i e s of either inexperienced or experienced males (see Chapter 8). The r o l e of age-related experience of redwings i n determining the mating pattern i n a breeding population could be explained in terms of the e f f i c i e n c y of nest defense against predators as follows. F i r s t , since male redwings do not p a r t i c i p a t e in most of the' nesting duties (e. g. Bent 1958), but play an important role i n the defense of nests against marsh wrens (see Chapter 2), the influence of th e i r previous breeding experience on female choice of the breeding s i t u a t i o n could be explained by t h i s , presumably age (experience)-related contribution to nesting., This view i s supported by the higher reproductive success of females mated to experienced males (Table 48). Second, most old experienced females generally come back to their o r i g i n a l nesting areas i n consecutive years, regardless of whether the o r i g i n a l male returned to his t e r r i t o r y (Table 50). This indicates that mating success of new males i s i n i t i a l l y determined by the number of females which were breeding i n the 317 area during a previous year and returned. This s i t e tenacity of females may increase their f i t n e s s i n terms of the interference i n t e r a c t i o n s with marsh wrens in two ways. F i r s t , a returning male i s l i k e l y to more e f f i c i e n t l y defend nests of his females because of his previous breeding experience* Therefore, by returning to the same t e r r i t o r y , and remating with the same male i n consecutive years, females should increase their chances of nesting success. Second, since females can also increase t h e i r nesting success through cooperation i n nest defense with other females from a harem, returning to the same area and renesting with the same f a m i l i a r neighbors should also increase female f i t n e s s . This i s probably because the e f f i c i e n c y of cooperation between females i s highest when distances between neighbors are small and neighbors are more synchronous i n nesting. This would most l i k e l y be achieved in harems where females are f a m i l i a r with each other and have developed a mutual tolerance i n a previous year (s). This view i s supported by a greater degree of nesting synchrony and smaller average distances between female neighbors i n the large harems containing most old experienced females (see Chapter 7). Another support comes from my trapping breeding females at their nests. This indicates that breeding neighbors have developed mutual tolerance but exhibit a high l e v e l of aggression towards strange female intruders. In addition, returning to the l a s t year's t e r r i t o r y should speed up the establishment of t e r r i t o r i e s by females, the formation of pair bonds between the f a m i l i a r mates, and hence also the i n i t i a t i o n of nesting by females early i n a season and t h e i r renesting l a t e r in that season. 318 Third, because old, experienced females are more successful i n nesting, they could influence ' nest s i t e choice by inexperienced females, which generally s t a r t nesting l a t e r (Crawford 1977, Jackson 1971). This i s supported by a po s i t i v e c o r r e l a t i o n between harem siz e and the presence of old experienced females on t e r r i t o r i e s of inexperienced and experienced males, and the f a c t that, on the average, there are more l a t e nesting (presumably inexperienced) females joining harems with early nesting birds (Fig. 11). Hence, mating success of males i s determined by: (1) mating success of the previous t e r r i t o r y holders and the overwinter s u r v i v a l rates of th e i r females; (2) the attractiveness of an area to new females i n terms of the number of ol d , experienced females breeding i n i t ; and (3) the attractiveness of males to females in terms of th e i r age (experience). These factors, therefore, probably determine the variation in the mating pattern within a breeding redwing population. The f a c t that female choice of nesting si t u a t i o n s i s influenced by the guality of males i n terms of their age (experience), and by the presence of old experienced females on th e i r t e r r i t o r i e s , has important implications on the theory of the evolution of polygyny i n red-winged blackbirds. The previous breeding experience of redwings of both sexes has probably, along with the selection for female clumping tendency and hence cooperation i n nest defense, driven the evolution of polygyny i n redwings. Differences in the guality of i n d i v i d u a l redwing t e r r i t o r i e s i n terms of the number of experienced birds 319 present on them, however, presents a force selecting for resource defense polygyny as opposed to true harem polygyny (or harem defense polygyny; see Emlen and Oring 1977). Hence, in marsh-nesting redwings, true harem polygyny which has presumably evolved as a s p e c i f i c reproductive strategy i n response to se l e c t i o n for the clumping tendency of females, occurs i n combination with male resource defense polygyny (females p r e f e r e n t i a l l y join older experienced males and harems with experienced females). 9. Model combining competitive and cooperative interactions between females To adeguately explain the adaptive value of the polygynous pattern of nesting i n redwings, i t i s necessary to consider both the competitive and cooperative interactions between females as forces which have driven i t s evolution. The model which incorporates these two kinds of interactions i s presented i n Fig. 33. The role of cooperative interactions between females i n nest defense and the negative e f f e c t s of clumping i s shown by the r e l a t i o n s h i p between the average female f i t n e s s and harem s i z e , which produce the e a r l i e r discussed bell-shaped f i t n e s s curves. However, in order to graphically i l l u s t r a t e the influence of guality of the breeding s i t u a t i o n ( i . e. guality as r e f l e c t e d by the previous breeding experience of birds on a t e r r i t o r y ) on the mating pattern i n a population, i t i s necessary to consider the female preferences for the best guality s i t u a t i o n s i n terms of the i n t e n s i t y of competition 320 F i g . 33. Model of the adaptive value of redwing polygyny combining the cooperative and competitive i n t e r a c t i o n s between females. Fitness curves 1-4 present breeding s i t u a t i o n s of high to low q u a l i t y , as measured by the presence of old experienced male and females. See text f or explanation. Fitness 1> No. set t led females in populat ion 321 between females for t h i s resource. This i s achieved i n Fi g . 33 by introducing the number of females s e t t l e d in a population as a variable. Female redwings continue to s e t t l e in a breeding area over a longer period of time, as indicated by a gradual increase i n the number of simultaneously active nests early i n a season (Fig. 34). Therefore, the number of females s e t t l e d i n a population, as described by the X axis in Fig. 33, also r e f l e c t s time i n the early part of the breeding season. In order to discuss the re l a t i o n s h i p s between the average f i t n e s s per female as a function of harem s i z e , and guality of the breeding s i t u a t i o n , I chose four si t u a t i o n s from high (fitness curve 1) to low (fitness curve 4) qu a l i t y . I assumed that the highest guality s i t u a t i o n occurs when there are several experienced females and an experienced male present on a t e r r i t o r y , whereas the lowest guality s i t u a t i o n would be c h a r a c t e r i s t i c of t e r r i t o r i e s with inexperienced birds only. The number of situ a t i o n s of d i f f e r e n t g u a l i t i e s i n a given population and hence the number of f i t n e s s curves describing these si t u a t i o n s would, however, be determined by a number of combinations with males of d i f f e r e n t experience and a varying number of experienced females s e t t l i n g on t h e i r t e r r i t o r i e s . Differences i n guality of breeding situations should have three major e f f e c t s on the temporal organization of nesting within a breeding population, the average f i t n e s s per female as a function of harem s i z e , and hence also on the mating pattern. F i r s t , females should s e t t l e f i r s t i n the best guality s i t u a t i o n s and l a s t i n poor guality situations (see also Altmann Fig. 34. Number of active redwing nests as a function of time in the f i r s t part of the breeding season. Notice that the number of breeding females increases gradually over approximately 30 days. 323 et a l . . 1977, Fretwell and Lucas 1969, Orians 1972, 1980). This i s true for marsh-nesting redwings where there i s a c o r r e l a t i o n between harem size and the date of the i n i t i a t i o n of nesting by the f i r s t female from a harem (Orians 1980; t h i s study, see Chapter 7). Therefore, as the guality of breeding si t u a t i o n s decreases (in Fig. 33 from the f i t n e s s curve 1 to 4), the f i r s t females s t a r t s e t t l i n g progressively l a t e r . This i s because, as the number of birds settled i n a habitat increases, the density of females i n better quality s i t u a t i o n s becomes too high and these s e t t l e d females probably prevent most of the new birds from s e t t l i n g near them, thereby forcing the add i t i o n a l i n d i v i d u a l s into lower guality s i t u a t i o n s . Second, the i n i t i a l f i t n e s s of the f i r s t female s e t t l e d i n a high guality s i t u a t i o n should be higher than that of the f i r s t females i n lower guality s i t u a t i o n s (in Fig. 33; F1 > F2 > F3 > F4; the predicted relationship i s indicated by a dashed l i n e X). This i s because the f i r s t females which s e t t l e i n a marsh are old experienced birds (e.g. Crawford 1977, Jackson 1971), which are more successful in nesting (Crawford 1977). In addition, since both male and female redwings generally return to t h e i r o r i g i n a l t e r r i t o r i e s in subseguent years, i n the best guality s i t u a t i o n s old experienced females should also benefit from more e f f i c i e n t nest defense by experienced males. On the other hand, the f i r s t (inexperienced) females entering low guality situations ( t e r r i t o r i e s without experienced females and with inexperienced males) should have, on the average, lower f i t n e s s presumably because of the lack of previous experience as re l a t e d 324 p a r t i c u l a r l y to t h e i r nest defense behavior. Third, the highest achievable f i t n e s s measured i n terms of the reproductive success of females breeding i n harems of the optimum siz e should be, on the average, attained by females entering the best guality breeding situations ( t e r r i t o r i e s with experienced females and males), but i t should decrease with the decreasing quality of the breeding si t u a t i o n (therefore, i n F i g . 33, MF1 > MF2 > MF3 > MF4). This should be a conseguence of the presence or absence of old experienced birds which appear to be more e f f i c i e n t i n nest defense both i n d i v i d u a l l y and through the cooperation with t h e i r neighbors.. In addition, since old experienced females are probably also more e f f i c i e n t in foraging, t h i s should further increase t h e i r reproductive success. This prediction i s supported by the f i n d i n g that females breeding i n the most productive harems (from the female viewpoint) of experienced males fledged, on the average, more young than females from the most productive harems of inexperienced males (these present a lower guality breeding s i t u a t i o n ; see Table 48). A s i m i l a r kind of information that would allow us to evaluate the influence of the presence of old experienced females on the average female f i t n e s s i s not available. However, there are two kinds of evidence which indicate that, l i k e old males, also experienced females should increase the average f i t n e s s of females which join them. F i r s t , experienced females are more successful breeders (Crawford 1977), probably because they are more e f f i c i e n t i n nest defense 325 against marsh wrens,. This i s supported by the stronger responses by experienced than inexperienced females towards experimentally offered marsh wren nests (see Chapter 3). Second, large harems including the early nesting females (Orians 1980, t h i s study), which are generally old experienced birds (Crawford 1977, Jackson 1971), produce more young per female than small harems (Fig. 16; see also Holm 1973),. This i s probably the r e s u l t of better nest defense by females i n these harems as indicated by smaller distances between neighbors and a greater degree of synchrony in nesting (see Chapter 7). However, i t i s impossible to separate these effects from improved nest defense simply through the increased number of females i n a harem. Fourth, the optimum harem s i z e , i n terms of the maximum female productivity of fl e d g l i n g s , should decrease with decreasing guality of a breeding s i t u a t i o n (in Fig. 33, 0H1 > OH2 > 0H3 > 0H4; the predicted trend i s also indicated by a dashed l i n e Y). This prediction i s based on the assumption that redwings lacking the previous breeding experience are not as e f f i c i e n t i n nest defense as older experienced birds. Moreover, the f a c t that young birds are less experienced and hence probably l e s s e f f i c i e n t in foraging should influence both the e f f i c i e n c y of t h e i r nest defense against predators and the number of young they could adeguately feed. This prediction i s supported by the evidence that the most productive harems of inexperienced males are smaller than those of experienced males (Table 48) . , 326 The role of the previous breeding experience of redwings of both sexes i n determining the variation i n mating success of i n d i v i d u a l males within a breeding population can be interpreted only i n terms of resource defense polygyny,. Thus the pattern of d i s t r i b u t i o n of polygynous matings i s presumably determined by differences i n the guality of i n d i v i d u a l t e r r i t o r i e s i n terms of the presence or absence of old experienced males and females. The observations concerning t h i s type of polygyny i n redwings which I made are generally consistent with predictions of the competitive female choice model (e.g. Altmann et a l . 1977, Orians 1969). However, there i s one major difference between the implications of the presented model on the adaptive value of polygyny i n redwings i n terms of resource defense polygyny (see Fig. 33) and the competitive female choice model (Altmann et a l . 1977). Because the s e l e c t i o n for female preferences for high guality breeding s i t u a t i o n s has presumably been driven by the guality of cooperation between indivi d u a l s involved, the addition of females to a harem i n situations of any p a r t i c u l a r guality should i n i t i a l l y increase the average female f i t n e s s (Fig. 33). In contrast to t h i s * the competitive female choice model predicts that the addition of females to a harem should always have negative e f f e c t s on the average female f i t n e s s becase of increasing competition for limited resources (Orians 1969, Altmann et a l , . 1977, see F i g . 22),. Hence, t h i s prediction i s not v a l i d for redwings and, t h e o r e t i c a l l y , f o r any other species in which resource defense polygyny occurs i n combination with true harem polygyny, and in which the agent driving the evolution of these two forms of polygyny i s the 327 same. Therefore, i n order to determine whether such birds exhibit resource defense polygyny i n combination with true harem polygyny, we have to examine t h e i r mating pattern i n r e l a t i o n to the agent which has driven the evolution of true harem polygyny. 328 CHAPTER 11 Overlap i n breeding d i s t r i b u t i o n of red-winged blackbirds # i I_llow-headed blackbirds, and l o n g - b i l l e d marsh wrensi test of the major assumption of the proposed theory Introduction The evidence I c o l l e c t e d during t h i s study suggests that l o n g - b i l l e d marsh wrens present an important force selecting for polygyny i n red-winged blackbirds i n a marsh I studied. But the p l a u s i b i l i t y of the theory of the evolution of polygyny i n redwings, which I discussed i n the previous Chapter, depends c r i t i c a l l y on the assumption of broad geographic sympatry of the breeding populations of blackbirds and marsh wrens. In addition, i t also depends on the assumption that ecological and behavioral r e l a t i o n s h i p s , observed between blackbirds and marsh wrens i n t h i s study, are similar throughout their range of sympatry. There i s information available which shows that, on a broad geographical scale, the breeding ranges of redwings and marsh wrens greatly overlap i n most areas of North America (e.g. Bent 1948, 1958). But the proposed theory reguires more accurate information which would allow us to evaluate the degree of sympatry between these species within i n d i v i d u a l marshes throughout North America. 329 In t h i s Chapter I w i l l examine the extent of sympatry between redwing and marsh wren breeding populations in d i f f e r e n t parts of t h e i r breeding range. In addition, because the proposed theory on the role of marsh wrens in the evolution of redwing polygyny might apply to yellow-headed blackbirds, which are also highly polygynous (e.g, Willson 1966), I w i l l examine the degree of sympatry between yellowheads and marsh wrens. To determine the degree of overlap between these three marsh-nesting passerines in North American marshes, I conducted a guestionnaire study which was designed to answer the following guestions: (1) How freguently are breeding redwings, yellowheads, and marsh wrens sympatric? (2) Is the freguency of sympatry geographically variable? (3) Does redwing population size influence the probability of sympatry with marsh wrens? (4) How frequent are redwing populations of various sizes? (5) What i s the r e l a t i v e abundance of marsh wrens and redwings i n i n d i v i d u a l marshes? Are abundances of the two species related? (6) Do the two species nest i n i n d i v i d u a l marshes every year? How i s t h i s influenced by t h e i r abundances? (7) Do s i m i l a r behavioral interactions between blackbirds and marsh wrens occur in di f f e r e n t portions of their geographic ranges? 330 Methods To evaluate the degree of sympatry in d i s t r i b u t i o n of red-winged blackbirds, yellow-headed blackbirds, and l o n g - b i l l e d marsh wrens, I prepared a guestionaire that asked f o r the following information: (1) presence or absence of the three species i n i n d i v i d u a l marshes; (2) the size of male redwing population (size categories were 1-5, 6-10, 11-20, and more than 20 males)- (3) Abundance of marsh wrens compared to redwings (less common, as common, more common than redwings). And (4) presence of breeding marsh wrens each year or i n some years only. A t o t a l of 1000 guestionaires were mailed i n winter and spring 1978 to ornithologists or zoologi c a l i n s t i t u t i o n s throughout the United States and Canada, Sixty-one completed guestionaires were returned, giving information on 170 marshes. Results 1. Overlap in breeding d i s t r i b u t i o n of redwings and marsh wrens Redwings and marsh wrens have a s i g n i f i c a n t tendency to nest i n the same marshes, based on information from 170 marshes (Table 60). According to my informants, both species bred i n 85% of marshes, only redwings bred i n 14%, and only marsh wrens occurred i n just one marsh. 3 3 1 T a b l e 6 0 . Summary o f d a t a o n d i s t r i b u t i o n o f b r e e d i n g p o p u l a t i o n s o f r e d - w i n g e d b l a c k b i r d s a n d l o n g - b i l l e d m a r s h w r e n s i n d i f f e r e n t m a r s h e s . N u m b e r (%) o f m a r s h e s M a r s h w r e n s R e d - w i n g e d b l a c k b i r d s T o t a l P r e s e n t A b s e n t P r e s e n t 1 4 4 ( 8 5 ) 1 . ( 1 ) 1 45 A b s e n t 25 ( 1 4 ) 0 ( 0 ) 25 T o t a l 169 1 170 N o t e : F a g e r ' s i n d e x o f a f f i n i t y = 0 . 8 8 1 ( s i g n i f i c a n t i f g r e a t e r t h a n 0 . 5 ) . 332 To evaluate the frequency of sympatry between redwing and marsh wren populations i n d i f f e r e n t areas of their breeding ranges, I compared marshes i n western, c e n t r a l , and eastern North America. I considered western North America to be that area west of (not including) Saskatchewan, North and South Dakotas, Nebraska, Kansas, Oklahoma, and Texas. Areas east of (not including) Ontario, Ohio, Kentucky, Tennessee, and Alabama are eastern North America. The results summarized i n Table 61 show that the degree of sympatry between redwings and marsh wrens i s s i m i l a r i n marshes i n these three regions. Data on the siz e of male redwing populations and the presence or absence of marsh wrens are available on 128 marshes (Table 62). Of these, most (52%) supported large redwing populations with more than 20 t e r r i t o r i a l redwing males. The rest of the marshes were di s t r i b u t e d roughly egually among the 3 smaller redwing population s i z e categories. The pro b a b i l i t y that marsh wrens and redwings nest i n the same marsh increases with si z e of the redwing population (Table 62). Whereas only 55% of 22 marshes with only 1-5 male redwings had marsh wrens, 92% of 66 marshes with more than 20 male redwings had wrens. Because marsh wrens are more l i k e l y to nest sympatrically with large redwing populations, and because most redwing populations i n my sample were large ones, I can conclude that most redwings i n my sample (and perhaps i n North America) nested sympatrically with marsh wrens. Marsh wrens were less abundant than redwings i n 74% of 3 3 3 T a b l e 6 1 . O v e r l a p i n d i s t r i b u t i o n o f r e d - w i n g e d b l a c k b i r d s a n d l o n g - b i l l e d m a r s h w r e n s i n w e s t e r n , c e n t r a l , a n d e a s t e r n N o r t h A m e r i c a . , R a n g e o f d i s t r i b u t i o n N u m b e r (%) m a r s h e s w i t h i n N o r t h A m e r i c a b o t h s p e c i e s o n e s p e c i e s T o t a l W e s t e r n 2 8 ( 8 8 ) 4 ( 1 2 ) 32 C e n t r a l 59 ( 7 5 ) 20 ( 2 5 ) 79 E a s t e r n 5 1 ( 8 6 ) 8 ( 1 4 ) 59 T o t a l 1 3 8 32 170 N o t e : X 2 = 4 . 0 9 ; d . f . = 2 ; p > 0 . 1 ( 2 - t a i l e d t e s t ) . 334 Table 62. Degree of overlap between red-winged blackbirds and l o n g - b i l l e d marsh wrens, as rel a t e d to the s i z e of redwing male populations. Number of marshes Number of redwing males i n population Marsh wrens 1 - 5 6 - 10 1 1 - 2 0 >20 Tot a l Present 12 10 21 61 104 Absent 10 6 3 5 24 Total 22 16 24 ' 66 128 % marshes with wrens present 55 63 87 92 % marshes with wrens of t o t a l with wrens 11 10 20 59 100 % marshes of a l l marshes 17 12 19 52 100 Note: Differences i n the degree of sympatry between redwings and marsh wrens as rel a t e d to the s i z e of male redwing populations were tested by chi-square (Fisher exact p r o b a b i l i t y test used where indicated with an a s t e r i s k ) ; S=significant; NS=not s i g n i f i c a n t (<*=0.05). A, B, C, D stands f o r male redwing populations of 1-5, 6-10, 11-20, and more than 20, res p e c t i v e l y . 335 marshes i n which both occured; they were more abundant i n only 9% of marshes (Table 63). However, there i s no clear r e l a t i o n s h i p between the abundance of marsh wrens as related to redwings and the size of redwing male populations (Table 63). Both species bred every year i n 82% of marshes i n which both occurred (Table 64)- Furthermore, both species are more l i k e l y to breed every year i n marshes that support large redwing populations (Table 64). 2. Sympatry among yellowheads^ redwings, and marsh wrens Dis t r i b u t i o n of yellowheads, redwings, and marsh wrens i s summarized in Table 65. Both redwings and marsh wrens were present i n 95% of marshes that supported yellowhead populations,. Only marsh wrens and yellowheads were present in only one marsh, and only redwings and yellowheads were present in only two. Thus yellowheads almost always breed sympatrically with the other two species. 3. Do marsh wrens and blackbirds i n t e r f e r e throughout their range of sympatry? Direct observations of marsh wrens pecking eggs of blackbirds or observations of high nesting f a i l u r e of blackbirds when marsh wrens are nearby have been made in 6 widely separated areas in North America (Table 66). In addition, both aggression by blackbirds towards marsh wrens and compression of marsh wren t e r r i t o r i e s after blackbirds' a r r i v a l in marshes have been 336 Table 63. Summary of data on the abundance of marsh wrens as related to redwings and the size of redwing male populations. Number (%) of marshes Abundance of " " ~~~ " marsh wrens Number of redwing males in population related to redwings 1 - 5 6 - 10 11 - 20 >20 Total (%) More common 1 (11) 1 (20) 1 (9) 3 (7) 6 (9) As common 1 (11) 1 (20) 1 (9) 9 (20) 12 (17) Less common 7 (78) 3 (60) 9 (82) 33 (73) 52 (74) Total 9 (100) 5 (100) 11 (100) 45 (100) 70 (100) 337 Table 64. Temporal s t a b i l i t y of marsh wren populations as r e l a t e d to the s i z e of redwing male populations. Number (%) of marshes Number of redwing males i n population Marsh wrens present 1 - 5 6 - 1 0 1 1 - 2 0 >20 To t a l (%) Each year 2 (25) 4 (80) 9 (82) 38 (93) 53 (82) In some years 6 (75) 1 (20) 2 (18) 3 (7) 12 (18) To t a l 8 (100) 5 (100) 11 (100) 41 (100) 65 (100) Note: Difference i n temporal s t a b i l i t y of marsh wren populations as re l a t e d to the siz e of male redwing populations were tested by Fisher exact test (of=0.05); S=significant; NS=not s i g n i f i c a n t . A, B, C , D stands f o r redwing populations of 1-5, 6-10, 11-20, and more than 20 males, res p e c t i v e l y . A B C D NS ^ NS ^ \ NS ^ / 3 3 8 T a b l e 6 5 . D e g r e e o f s y m p a t r y o f b r e e d i n g p o p u l a t i o n s o f y e l l o w -h e a d e d b l a c k b i r d s , r e d - w i n g e d b l a c k b i r d s , a n d l o n g - b i l l e d m a r s h w r e n s . N o . (%) o f m a r s h e s w i t h y e l l o w - h e a d e d b l a c k b i r d s R e d - w i n g e d b l a c k b i r d s M a r s h w r e n s P r e s e n t A b s e n t T o t a l P r e s e n t 5 8 ( 9 5 ) 1 ( 2 ) 59 A b s e n t 2 ( 3 ) 0 ( 0 ) 2 T o t a l 60 1 61 N o t e : F a g e r ' s i n d e x o f a f f i n i t y = 0 . 9 1 ( s i g n i f i c a n t i f g r e a t e r t h a n 0 . 5 ) . Table 66. Summary of information on d i f f e r e n t types of interactions between marsh wrens and redwings (R) or yellowheads (Y) i n d i f f e r e n t areas of North America. Type of marsh wren - blackbird interactions Destruction of Aggression by Compression of m. State b l a c k b i r d eggs blackbirds wren t e r r i t o r i e s (Province) by marsh wrens towards m. wrens owing to blackbirds Source New York R - - A l l e n (1914) Iowa R R,Y - Burt (1970) Wisconsin - R R Nero (1956b) Manitoba R R - Pieman (unpublished data) Utah - - Y Fautin (1940) Alber t a R - - Pieman (unpublished data) Washington R,Y R>,Y R,Y Orians and Willson (1964), Verner (1975) B r i t i s h Columbia R,Y R,Y R Pieman (t h i s study), Runyan (1979), E. Guindon (pers. comm.) 340 reported from several geographically distant areas (Table 66 ) , . Both of these can be considered mechanisms or consequences of interference competition. This information, therefore, indicates that i n t e r s p e c i f i c interference competition i s general over the geographic ranges of these three species. Discussion This study v e r i f i e s a major assumption of the hypothesis that polygyny i n marsh nesting passerines evolves i n response to i n t e r s p e c i f i c interference (see Chapter 10). This assumption, that blackbirds nest sympatrically with marsh wrens, i s supported by two kinds of evidence. F i r s t , the two species nest sympatrically i n most of 170 marshes surveyed. Second, the freguency of sympatry between redwings and marsh wrens does not vary geographically. This finding i s even more s i g n i f i c a n t than the above s t a t i s t i c s suggest. Not only do most redwing populations breed sympatrically with marsh wrens, but because large redwing populations are more l i k e l y to be sympatric with marsh wrens, most redwing individuals nest near marsh wrens. Therefore i t seems reasonable to conclude that most marsh-nesting redwings i n North America are exposed to marsh wren interference. In most marshes redwings appear, from my questionnaire returns, to be more abundant than marsh wrens. However, i t i s possible that estimates of r e l a t i v e abundance may be inaccurate 341 because redwings of both sexes are more conspicuous than marsh wrens. Therefore, wren abundance i s probably underestimated. However, even i f t h i s i s true, impact of marsh' wrens on redwing nesting success i n marshes maintaining sparse marsh wren populations i s not necessarily i n s i g n i f i c a n t , In my study marsh densities of marsh wrens were usually greater than redwing densities. But, I also found that i n d i v i d u a l redwing nests suffered r e l a t i v e l y more from marsh wren interference i n a year when marsh wrens were less abundant. This result seems counterintuitive at f i r