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UBC Theses and Dissertations

Intrasexual competition, dispersal, territoriality and the mating system of the song sparrow on Mandarte… Arcese, Peter 1988

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INTRASEXUAL COMPETITION, DISPERSAL, TERRITORIALITY AND THE MATING SYSTEM OF THE SONG SPARROW ON MANDARTE ISLAND, B.C. By PETER ARCESE Master of Scie n c e , U n i v e r s i t y of B r i t i s h Columbia, 1984 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA January 1988 (c) Peter Arcese, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Zoology The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 D a t e March 25, 1988 i i ABSTRACT I c o n s i d e r the r e l a t i o n s h i p between n a t a l d i s p e r s a l , the a c q u i s i t i o n of b r e e d i n g r e s o u r c e s and the p r o x i m a t e maintenance of the mating system i n a p o p u l a t i o n of song sparrows ( M e l o s p i z a  m e l o d i a ) r e s i d e n t on Mandarte I s l a n d , B.C.. The g e n e r a l h y p o t h e s i s t e s t e d was t h a t i n t r a s e x u a l c o m p e t i t i o n f o r the r e s o u r c e s t h a t l i m i t r e p r o d u c t i o n i n each sex i s the main f a c t o r a f f e c t i n g p a t t e r n s of n a t a l d i s p e r s a l , t e r r i t o r y t u r n o v e r and m ating system o r g a n i z a t i o n . T h i s study r e v e a l s d i f f e r e n c e s i n t h e c o m p e t i t i v e a b i l i t y of i n d i v i d u a l s and the consequences of t h e s e f o r n a t a l d i s p e r s a l , t e r r i t o r y a c q u i s i t i o n and the a b i l i t y t o m onopolize mates. In t h i s p o p u l a t i o n , n a t a l d i s p e r s a l , t e r r i t o r i a l i t y and the m ating system a r e r e l a t e d t h r o u g h the common mechanism of i n t r a s e x u a l c o m p e t i t i o n f o r b r e e d i n g r e s o u r c e s . T h i s c o m p e t i t i o n a f f e c t e d r e p r o d u c t i o n w i t h i n each sex, and i n the o p p o s i t e sex, r a i s i n g p o t e n t i a l c o n f l i c t s of i n t e r e s t between mated males and f e m a l e s . I i d e n t i f y c o r r e l a t e s of c o m p e t i t i v e a b i l i t y , and c o n s i d e r how i n d i v i d u a l s of each sex maximize the number of young they r a i s e d . A l l s u i t a b l e h a b i t a t was defended even a t low d e n s i t y , r e s u l t i n g i n h i g h l e v e l s of c o m p e t i t i o n f o r space. Song sparrows c o n t e s t e d f o r t e r r i t o r i e s y e a r - r o u n d . Most s e t t l e r s were y e a r l i n g s , but s e v e r a l males were o l d e r and had p r e v i o u s l y owned a t e r r i t o r y . Most f l o a t e r s s e t t l e d by e v i c t i n g owners from a l l or p a r t of t h e i r t e r r i t o r i e s , i n c o n t r a s t t o the assumption of most models of d i s p e r s a l t h a t f l o a t e r s g a i n t e r r i t o r i e s o n l y a f t e r an owner's d e a t h . R e l a t i v e age and dominance w i t h i n c o h o r t s a f f e c t e d r e c r u i t m e n t and t e r r i t o r i a l s t a t u s i n the year f o l l o w i n g h a t c h i n each sex. D i f f e r e n c e s i n age a l s o a f f e c t e d t e r r i t o r i a l and mating b e h a v i o u r , p a r t i c u l a r l y i n males. A l t h o u g h c o m p e t i t i v e a b i l i t y v a r i e d w i t h age, d i f f e r e n c e s i n c o m p e t i t i v e a b i l i t y between males p e r s i s t e d t h r o u g h l i f e . A p p r o x i m a t e l y 9% of male b r e e d i n g a t t e m p t s o c c u r r e d a f t e r a male t e r r i t o r y owner had e v i c t e d a neighbour and t h e r e b y g a i n e d a c c e s s t o an a d d i t i o n a l female. Polygynous males r a i s e d more young than monogamous males. Females v i g o r o u s l y defended t h e i r t e r r i t o r i e s a g a i n s t female i n t r u d e r s , presumably t o p r e v e n t polygyny by t h e i r mates. Females i n polygynous groups o f t e n l o s t the a i d of t h e i r mate, and r a i s e d fewer young, because polygynous males d i d not a i d two females s i m u l t a n e o u s l y . Adding s u p p l e m e n t a l food a l t e r e d female time budgets, i n c r e a s e d t h e i r a b i l i t y t o p r e v e n t s e t t l e m e n t by female f l o a t e r s , and i n c r e a s e d t h e i r r e p r o d u c t i v e s u c c e s s . T h i s s u g g e s t s t h a t females a r e c o n s t r a i n e d i n the amount of time they can spend i n t e r r i t o r y defence by time spent f o r a g i n g . The maintenance of monogamy was a f f e c t e d by d i f f e r e n c e s i n the a b i l i t i e s of i n d i v i d u a l males and females t o defend t h e i r t e r r i t o r i e s a g a i n s t i n t r u d e r s of the same sex. i v D i s p e r s a l d i s t a n c e s were s i m i l a r among se x e s , i n c o n t r a s t t o the u s u a l p a t t e r n of f e m a l e - b i a s e d d i s p e r s a l i n b i r d s . G i v e n the i n t e n s e t e r r i t o r i a l b e h a v i o u r of males and f e m a l e s , t h i s study s u p p o r t s the h y p o t h e s i s t h a t b i a s e s i n d i s p e r s a l a r i s e when t e r r i t o r y e s t a b l i s h m e n t i s s u b s t a n t i a l l y more d i f f i c u l t i n one sex than the o t h e r . D i s p e r s a l on Mandarte was u n r e l a t e d t o i n b r e e d i n g , common p a r e n t a g e , or r e p r o d u c t i v e s u c c e s s . However, e m i g r a n t s from Mandarte were s o c i a l l y s u b o r d i n a t e b i r d s t h a t must have o u t b r e d i f they s e t t l e d s u c c e s s f u l l y . Female e m i g r a n t s from l o c a l p o p u l a t i o n s i n the v i c i n i t y of Mandarte may s e t t l e more o f t e n than males. D i s p e r s a l o c c u r r e d as young b i r d s e s t a b l i s h e d ranges i n the p e r i o d f o l l o w i n g independence, from which they c h a l l e n g e d or r e p l a c e d owners. Adding food d u r i n g the b r e e d i n g p e r i o d reduced d i s p e r s a l i n males and females i n l a t e summer and the f o l l o w i n g s p r i n g . C o m p e t i t i v e a b i l i t y p r o b a b l y a f f e c t s d i s p e r s a l from the n a t a l p o p u l a t i o n , the l i k e l i h o o d of s e t t l e m e n t , and the time of s e t t l e m e n t f o r tho s e b i r d s t h a t remain i n the n a t a l p o p u l a t i o n . V TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES x i i LIST OF FIGURES x i v ACKNOWLEDGEMENTS x v i CHAPTER 1. I n t r a s e x u a l C o m p e t i t i o n , D i s p e r s a l and M a t i n g Systems i n B i r d s 1 INTRODUCTION 1 N a t a l D i s p e r s a l 2 R e p r o d u c t i v e C o m p e t i t i o n and M a t i n g Systems 5 M a t i n g Systems, Demography and D i s p e r s a l 8 Go a l s of the Study ' 9 REFERENCES ..14 CHAPTER 2. Study S p e c i e s , Study S i t e and Methods 21 Study S p e c i e s 21 Mandarte and H a l i b u t I s l a n d s 23 Study P o p u l a t i o n 24 Census Technique 25 D e t e r m i n a t i o n of T e r r i t o r i a l S t a t u s 26 Mapping T e r r i t o r y B o u n d a r i e s 27 Assessment of R e p r o d u c t i o n 27 Age of B i r d s 28 E s t i m a t i n g I n t r u s i o n P r e s s u r e 28 Types of T e r r i t o r y T u r n o v e r s 29 F a t e s of Males 31 Seasonal P a t t e r n s of T e r r i t o r y Turnover 31 Recording Song 32 D e f i n i t i o n s of Mated Status .....32 Food A d d i t i o n Experiment 33 Time Budgets 34 D i s p e r s a l D i s t a n c e 36 E s t i m a t i n g Dominance 36 Relatedness 37 S t a t i s t i c a l Analyses 38 REFERENCES 39 CHAPTER 3. T e r r i t o r y A c q u i s i t i o n and Loss i n Male Song Sparrows 43 INTRODUCTION 43 RESULTS 45 Types of T e r r i t o r y Turnovers 45 The Role of Age 45 Seasonal V a r i a t i o n i n Turnovers 46 T e r r i t o r y Takeover and Polygyny 52 Fates of Deposed T e r r i t o r y Owners 52 Time Spent F l o a t i n g by Deposed Owners 54 Frequency of Regaining a T e r r i t o r y 54 Repeat F l o a t e r s 57 Summer S u r v i v a l of F l o a t e r s and Owners 57 DISCUSSION ...58 F a c t o r s A f f e c t i n g the Mode of T e r r i t o r y A c q u i s i t i o n ..60 P r o b a b i l i t y of Settlement Elsewhere 62 Mode of T e r r i t o r y A c q u s i t i o n and D i s p e r s a l 63 Occurrence of Aggressive Takeovers 63 T e r r i t o r y A c q u i s i t i o n by Neighbours and F l o a t e r s 64 Age and T e r r i t o r y Turnover 65 Routes to T e r r i t o r y Ownership 67 REFERENCES 68 CHAPTER 4. Age, I n t r u s i o n Pressure and T e r r i t o r y Defence Agains t F l o a t e r s i n Male Song Sparrows 72 INTRODUCTION 72 R e l a t i o n s Between Owners and F l o a t e r s on Mandarte I s l a n d 75 RESULTS 76 Temporal P a t t e r n s of I n t r u s i o n 77 F l o a t e r D e nsity, T e r r i t o r y S i z e and Female F e r t i l i t y .77 Male Age and T e r r i t o r y Tenure 78 T e r r i t o r y Q u a l i t y and Male A b i l i t y 81 I n t r u s i o n Pressure and Reproductive Success 83 C ontests over T e r r i t o r y 83 The l e n g t h of c o n t e s t s 84 The number of c h a l l e n g e r s 85 Case h i s t o r i e s 86 Causes of T e r r i t o r y Loss 87 The r o l e of handicaps 87 Handicaps and age 88 The weights of owners, e v i c t e d males and f l o a t e r s .88 Does A b i l i t y to Defend a T e r r i t o r y Change with Age? ..89 DISCUSSION 90 Temporal P a t t e r n s of I n t r u s i o n 91 v i i i Male A b i l i t y , T e r r i t o r y Q u a l i t y and I n t r u s i o n P r e s s u r e 92 I n t r u s i o n P r e s s u r e and R e p r o d u c t i v e Success 93 The Ro l e of F l o a t e r s i n T e r r i t o r y T u r n o v e r s .....94 The T a c t i c s of F l o a t e r s 95 Are F l o a t e r s P u r s u i n g an A l t e r n a t e R e p r o d u c t i v e S t r a t e g y ? 96 Age, Tenure and I n t r u s i o n P r e s s u r e 98 REFERENCES 100 CHAPTER 5. The Form and F u n c t i o n of Song i n Female Song Sparrows 107 INTRODUCTION 107 RESULTS 109 The Form of Female Song 109 The Occurrence of Female Song 110 The C o n t e x t of Female Song 112 DISCUSSION 119 REFERENCES 124 CHAPTER 6. I n t r a s e x u a l C o m p e t i t i o n and M a t i n g System i n a P r i m a r i l y Monogamous Sparrow 127 INTRODUCTION 127 When S h o u l d Females Compete? 129 P r e d i c t i o n s 130 RESULTS 133 O b s e r v a t i o n a l Study 133 Frequency of p o l y g y n y 133 O r i g i n s of polygyny 133 Ages of b i r d s i n polygynous groups 134 Male a i d and female r e p r o d u c t i v e s u c c e s s 137 Male r e p r o d u c t i v e s u c c e s s 139 Use of space by females 141 Sex r a t i o 143 Food A d d i t i o n Study 144 Body mass 144 Male response t o taped song 144 Time budgets 145 S e t t l e m e n t of n o n - t e r r i t o r i a l females ....147 S e t t l e m e n t of n o n - t e r r i t o r i a l males 149 DISCUSSION 151 C o m p e t i t i o n f o r L i m i t i n g Resources 151 Male A i d and R e p r o d u c t i v e Success 153 P o l y g y n y and Male R e p r o d u c t i v e Success 155 Frequency of P o l y g y n y 155 P r o x i m a t e O r i g i n s of P o l y g y n y 156 O p t i o n s of Females i n Polygynous Groups 156 Age and Polygyny 158 H a b i t a t S a t u r a t i o n , Sex R a t i o and Polygyny 158 Use of Space by Females 161 E f f e c t of Food on Time Budgets 161 Food D i s t r i b u t i o n , T e r r i t o r i a l i t y and M a t i n g System .162 REFERENCES 164 CHAPTER 7. I n t r a s e x u a l C o m p e t i t i o n , M a t i n g System and N a t a l D i s p e r s a l i n Song Sparrows 171 INTRODUCTION 171 X Hypotheses About D i s p e r s a l 173 RESULTS 181 D i s p e r s a l W i t h i n Mandarte I s l a n d 181 Y e a r l y v a r i a t i o n i n d i s p e r s a l 181 P o i n t of o r i g i n and d i s t a n c e d i s p e r s e d 184 Sex d i f f e r e n c e s i n d i s p e r s a l 184 Dominance, h a t c h date and d i s t a n c e moved 186 Dominance, h a t c h date and s e t t l e m e n t time on Mandarte 186 S i b l i n g resemblance i n d i s p e r s a l 191 Age of the a c q u i r e d mate 191 I n h e r i t a n c e of n a t a l t e r r i t o r i e s 191 M o r t a l i t y , r e c r u i t m e n t and s e x - b i a s e s i n d i s p e r s a l 193 D i s t a n c e and r e p r o d u c t i v e s u c c e s s 193 I n b r e e d i n g , D i s p e r s a l , D e n s i t y and R e p r o d u c t i o n ..193 E f f e c t of s u p p l e m e n t a l food 195 D i s p e r s a l Between P o p u l a t i o n s 199 E m i g r a t i o n 199 I m m i g r a t i o n 201 H a l i b u t I s l a n d removal experiment .201 S e a s o n a l p a t t e r n s of d i s a p p e a r a n c e 202 DISCUSSION 203 Measures of D i s p e r s a l 205 P o p u l a t i o n D e n s i t y and D i s p e r s a l 208 I n h e r i t a n c e and P o i n t of O r i g i n 210 Sex, Dominance, D i s t a n c e and S e t t l e m e n t Time 211 E f f e c t of Supplemental Food 213 D i s p e r s a l , Mate Q u a l i t y and R e p r o d u c t i v e Success ....213 F a c t o r s A s s o c i a t e d w i t h I n b r e e d i n g 214 D i s p e r s a l Between P o p u l a t i o n s 216 Se a s o n a l P a t t e r n s of D i s a p p e a r a n c e 217 Hypotheses and Pr o x i m a t e Causes of D i s p e r s a l i n Song Sparrows 217 REFERENCES .220 Chapter 8. G e n e r a l D i s c u s s i o n 230 T e r r i t o r i a l B e h a v i o u r and S o c i a l O r g a n i z a t i o n 230 C o m p e t i t i v e A b i l i t y and R e p r o d u c t i v e Success 231 Males 232 Females 234 D i s p e r s a l S t r a t e g i e s and C o m p e t i t i o n f o r Resources ..235 C o n f l i c t s of I n t e r e s t Between Males and Females 237 Male A g g r e s s i o n and Female C h o i c e 239 REFERENCES 240 x i i LIST OF TABLES T a b l e 3.1. Age and t e r r i t o r y t a k e o v e r by n e i g h b o u r s 47 T a b l e 3.2. Type of t u r n o v e r i n r e l a t i o n t o the age of s e t t l e r s 49 T a b l e 3.3. T e r r i t o r y occupancy i n the non-breeding p e r i o d i n r e l a t i o n t o t e r r i t o r y l o c a t i o n 53 T a b l e 3.4. F a t e s of y e a r l i n g males d u r i n g the b r e e d i n g p e r i o d 59 T a b l e 4.1. Mean and maximum r a t e s of i n t r u s i o n p r e s s u r e . ...80 T a b l e 4.2. Changes i n i n t r u s i o n p r e s s u r e f o r 41 males ob s e r v e d i n 2 y e a r s 82 T a b l e 511. The o c c u r r e n c e and c o n t e x t of female song 113 T a b l e 6.1. The number of polygynous a s s o c i a t i o n s r e s u l t i n g from t a k e o v e r s and female s e t t l e m e n t 135 T a b l e 6.2. Number of young r a i s e d by monogamous, a i d e d and unaided females .138 T a b l e 6.3. Number of young r a i s e d by polygynous and monogamous males 140 T a b l e 6.4. Female ne s t placement and male t e r r i t o r y b o u n d a r i e s 142 T a b l e 6.5. Observed and e x p e c t e d s e t t l e m e n t by f l o a t e r s of each sex on f e d and c o n t r o l t e r r i t o r i e s 150 T a b l e 7.1. Hypotheses, p r e d i c t i o n s and assumptions about d i s p e r s a l 174 T a b l e 7.2. T e r r i t o r i a l s t a t u s i n r e l a t i o n t o dominance. ...188 x i i i Table 7.3. S i b l i n g resemblance i n d i s p e r s a l 192 Table 7.4. The numbers and s u r v i v a l of a d u l t s , and recruitment of y e a r l i n g s to the p o p u l a t i o n 194 Table 7.5. The number of t e r r i t o r i e s moved versus the r e l a t e n e s s of the a c q u i r e d mate 196 Table 7.6. Prod u c t i o n of young versus r e l a t e d n e s s of the f i r s t mate 1 98 Table 7.7. A summary of p r e v i o u s s t u d i e s of d i s p e r s a l i n the song sparrow 206 x i v LIST OF FIGURES F i g u r e 1.1. The number of b r e e d i n g females on Mandarte I s l a n d d u r i n g 17 y e a r s 11 F i g u r e 3.1. Male age and the outcome of t e r r i t o r y t u r n o v e r s 48 F i g u r e 3.2. The number of t u r n o v e r s of each type d u r i n g f i v e y e a r s 51 F i g u r e 3.3. Age, s u r v i v a l and the p r o b a b i l i t y of r e g a i n i n g a t e r r i t o r y 56 F i g u r e 4.1. R e l a t i o n s h i p s between age, i n t r u s i o n p r e s s u r e and t e r r i t o r y t e n u r e 79 F i g u r e 5.1. Sonagrams of female sorig sparrows 111 F i g u r e 5.2. The p r o p o r t i o n of females s i n g i n g i n r e l a t i o n t o p o p u l a t i o n d e n s i t y 116 F i g u r e 5.3. D e p i c t i o n of a c o n f l i c t between 3 females f o r the ownership of a b r e e d i n g t e r r i t o r y 118 F i g u r e 6.1. R e l a t i o n s h i p s between age and p o l y g y n y 136 F i g u r e 6.2. Time budgets of f e d and c o n t r o l females 146 F i g u r e 6.3. Time budgets of f e d and c o n t r o l males 148 F i g u r e 7.1. Median d i s t a n c e s moved by males and fe m a l e s . ..182 F i g u r e 7.2. Median number of t e r r i o r i e s moved by males and females 183 F i g u r e 7.3. D i s t a n c e d i s p e r s e d i n r e l a t i o n t o p o i n t of o r i g i n . 185 F i g u r e 7.4. T e r r i t o r i a l s t a t u s i n r e l a t i o n t o h a t c h d a t e . .189 XV F i g u r e 7.5. The e f f e c t of hatch date on settlement time i n male r e c r u i t s 190 F i g u r e 7.6. D i f f e r e n c e between expected and observed frequency of i n b r e e d i n g at d i f f e r e n t d e n s i t i e s 197 F i g u r e 7.7. D i s t a n c e d i s p e r s e d by fed and c o n t r o l young. ..200 F i g u r e 7.8. Seasonal p a t t e r n s of the disappearance of y e a r l i n g s from Mandarte I s l a n d 204 xv i ACKNOWLEDGEMENTS I thank Peter Bets, Anne H e l b i g , Wes Hochachka, Marie-Jose Houde, Doug Reid, Jamie Smith, Anja Tompa and Michaela Waterhouse f o r a s s i s t a n c e i n the f i e l d . P h i l l i p Stoddard and Sara H i e b e r t k i n d l y recorded female song sparrows f o r me on Mandarte I s l a n d and i n S e a t t l e , WA., and Peter M a r l e r and Susan P e t e r s generously allowed me to use sonagrams that they made from c a p t i v e female song sparrows i n New York. S e v e r a l people p r o v i d e d many u s e f u l comments on a l l or p a r t of t h i s t h e s i s i n c l u d i n g : Kim Cheng, John Eadie, Sara H e i b e r t , C h a r l e y Krebs, B i l l Searcy, P h i l l i p Stoddard, Jamie Smith, M i c h a e l Morrison, Robin L i l e y , Don Ludwig, A r t Rodgers, Charles Snowdon, B r i d g e t Stutchbury, Nico Verbeek and Ken Yasukawa. I owe s p e c i a l thanks to my r e s e a r c h committee: Kim Cheng, Don Ludwig, Robin L i l e y , and Nico Verbeek, and i n p a r t i c u l a r to Jamie Smith, my s u p e r v i s o r . My f i e l d w o r k was supported by a grant from N.S.E.R.C. to J.N.M. Smith, and I was supported by a graduate f e l l o w s h i p from the U n i v e r s i t y of B r i t i s h Columbia. The Tsawout and Tseycum Bands of Saanich, B.C. k i n d l y allowed me to work on Mandarte I s l a n d . T h i s t h e s i s i s d e d i c a t e d to f i e l d n a t u r a l i s t s . 1 CHAPTER 1. INTRASEXUAL COMPETITION, DISPERSAL AND MATING SYSTEMS IN BIRDS INTRODUCTION In t h i s t h e s i s , I c o n s i d e r the r e l a t i o n s h i p between n a t a l d i s p e r s a l , the a c q u i s i t i o n of b r e e d i n g r e s o u r c e s and the p r o x i m a t e maintenance of the mating system. I s t u d i e d a p o p u l a t i o n of song sparrows ( M e l o s p i z a m e l o d i a ) r e s i d e n t on Mandarte I s l a n d , B.C., Canada. I f o c u s on the r o l e s of (a) i n t r a s e x u a l c o m p e t i t i o n and (b) i n d i v i d u a l v a r i a t i o n i n c o m p e t i t i v e a b i l i t y on: t e r r i t o r y a c q u i s i t i o n and d e f e n c e , n a t a l d i s p e r s a l , and the mating system. These t o p i c s a r e of c o n s i d e r a b l e c u r r e n t i n t e r e s t because r e c e n t s t u d i e s ( r e v i e w e d below) suggest s t r o n g l i n k s between them. D e s p i t e t h i s , no stu d y has s i m u l t a n e o u s l y i n v e s t i g a t e d these t o p i c s w i t h i n a s i n g l e , well-known v e r t e b r a t e p o p u l a t i o n . E a r l y models of t e r r i t o r i a l i t y , n a t a l d i s p e r s a l and mating systems f o c u s e d p r i m a r i l y on the a g g r e s s i v e b e h a v i o u r of males. The major f a c t o r thought t o a f f e c t female b e h a v i o u r was the c h o i c e of h i g h q u a l i t y mates. I f c o m p e t i t i o n between females a f f e c t s n a t a l d i s p e r s a l , t e r r i t o r y t u r n o v e r and the o r g a n i z a t i o n of m ating systems, we must a s s e s s i t s importance r e l a t i v e t o female c h o i c e of mates or t e r r i t o r i e s . The main g o a l of t h i s t h e s i s i s t h e r e f o r e t o a s s e s s the i n t e n s i t y of c o m p e t i t i o n between i n d i v i d u a l s of each sex, and t o d e t e r m i n e i f i t has a 2 major e f f e c t on the p a t t e r n s of n a t a l d i s p e r s a l , t e r r i t o r y turnover, and mating system observed in the study p o p u l a t i o n of song sparrows. In t h i s chapter, I f i r s t review b r i e f l y the t o p i c of of n a t a l d i s p e r s a l and i t s r e l a t i o n s h i p to t h e o r i e s of the ecology and e v o l u t i o n of s o c i a l o r g a n i z a t i o n . Second, I review c u r r e n t t h e o r i e s of the o r g a n i z a t i o n of mating systems, and I show how competition f o r resources that l i m i t r e p r o d u c t i o n might a f f e c t the spacing behaviour of both males and females. T h i r d , I d e s c r i b e how i n t r a s e x u a l c o m p e t i t i o n , demography and the o r g a n i z a t i o n of mating systems a f f e c t p a t t e r n s of n a t a l d i s p e r s a l . F i n a l l y , I d e s c r i b e the t o p i c s and hypotheses addressed i n subsequent chapters of t h i s t h e s i s . N a t a l D i s p e r s a l N a t a l d i s p e r s a l i s the permanent movement of an organism from the n a t a l s i t e , as d i s t i n c t from breeding d i s p e r s a l , which r e f e r s to movements between breeding s i t e s ( L i d i c k e r 1975; Greenwood 1980). The frequency and extent of n a t a l d i s p e r s a l i s t y p i c a l l y c o r r e l a t e d with other l i f e - h i s t o r y t r a i t s such as f e c u n d i t y , s u r v i v a l and mating system (Hamilton & May 1977; Comins et a l . 1980; Greenwood 1980; Greenwood & Harvey 1982; Dobson 1982; Waser & Jones 1983). P a t t e r n s of d i s p e r s a l are i n t i m a t e l y r e l a t e d to a s p e c i e s ' s o c i a l o r g a n i z a t i o n because they a f f e c t the frequency and type of i n t e r a c t i o n s between 3 i n d i v i d u a l s . The causes of d i s p e r s a l and i t s e v o l u t i o n a r y consequences have t h e r e f o r e been of general and long-standing i n t e r e s t among b i o l o g i s t s (Wright 1940; M i l l e r 1947; Mayr 1963; Wilson 1975; Bradbury & Vehrencamp 1983; Brown 1987). D i s p e r s a l piqued the c u r i o s i t y of p o p u l a t i o n g e n e t i c i s t s s e v e r a l decades ago (e.g. Wright 1940; M i l l e r 1947), because i t i s the means by which genes flow between p o p u l a t i o n s . In. the absence of st r o n g s e l e c t i o n , a r e d u c t i o n i n gene flow was recognized as a p r e r e q u i s i t e to p o p u l a t i o n d i f f e r e n t i a t i o n and s p e c i a t i o n . Recent models of d i s p e r s a l have i n c o r p o r a t e d the i n c l u s i v e f i t n e s s of i n d i v i d u a l s , h a b i t a t s t r u c t u r e and q u a l i t y , and the l e v e l of c o m p e t i t i o n f o r resources w i t h i n p o p u l a t i o n s (e.g. Hamilton & May 1977; Horn 1978,1983; Comins et a l . 1980). P o p u l a t i o n b i o l o g i s t s began e x t e n s i v e r e s e a r c h on d i s p e r s a l i n the l a t e 1950's because of i t s presumed r o l e i n p o p u l a t i o n r e g u l a t i o n ( E r r i n g t o n 1956; L i d i c k e r 1962). At t h i s time, the f a c t o r s causing d i s p e r s a l began to be a c t i v e l y d i s c u s s e d (e.g. Howard 1960; S a d l e i r 1965). Howard (1960) p o s t u l a t e d that a h e r i t a b l e tendency to d i s p e r s e would be favored because d i s p e r s e r s would c o l o n i z e vacant h a b i t a t , extend the s p e c i e s ' range and reduce i n b r e e d i n g i n n a t a l p o p u l a t i o n s . L i d i c k e r (1962) added that d i s p e r s a l c o u l d prevent p o p u l a t i o n s from over-e x p l o i t i n g l o c a l r e s o u r c e s . 4 The view t h a t d i s p e r s a l i s an advantageous p r o p e r t y of p o p u l a t i o n s and s p e c i e s was championed by Wynne-Edwards (1962), who c o n s i d e r e d d i s p e r s a l t o be an a l t r u i s t i c exodus of s u r p l u s i n d i v i d u a l s . In c o n t r a s t , Murray (1967) viewed d i s p e r s a l as a means of m a x i m i z i n g i n d i v i d u a l r e p r o d u c t i v e p o t e n t i a l , w i t h i n the l i m i t s of i n d i v i d u a l a b i l i t y . He suggested t h a t d i s p e r s a l i s s e l e c t e d f o r whenever i n t e n s e l o c a l c o m p e t i t i o n makes s e t t l e m e n t u n l i k e l y , and he o f f e r e d a model based on b e h a v i o u r a l dominance and d i f f e r e n t i a l a c c e s s t o b r e e d i n g r e s o u r c e s t h a t was c o n s i s t e n t w i t h the p a t t e r n s of n a t a l d i s p e r s a l observed i n n a t u r e . These a u t h o r s and o t h e r s s t i m u l a t e d much r e s e a r c h i n t o the r e l a t i o n s h i p between d i s p e r s a l , p o p u l a t i o n dynamics and s o c i a l s t r u c t u r e ( r e v i e w s i n L i d i c k e r 1975; Gaines & McClenaghan 1980; Waser & Jones 1983), but i n d i v i d u a l v a r i a t i o n i n d i s p e r s a l b e h a v i o u r was not g i v e n c l o s e a t t e n t i o n ( e . g . Brown 1975; W i l s o n 1975; but see Myers & Krebs 1971). However, the l a t t e r t o p i c has r e c e n t l y s p a r k e d much i n t e r e s t among r e s e a r c h e r s i n v e s t i g a t i n g r e l a t i o n s h i p s between b e h a v i o u r and the e c o l o g y and e v o l u t i o n of s o c i a l systems ( B e k o f f 1977; Horn 1978, 1983; Greenwood 1980; S h i e l d s 1982, Dobson 1982; Waser & Jones 1983). In s t u d i e s of t h i s s o r t , the main q u e s t i o n s a d d r e s s e d a r e : (1) what a r e the causes and consequences of v a r i a t i o n i n d i s p e r s a l b e h a v i o u r t o the i n d i v i d u a l ? (2) how a r e s o c i a l o r g a n i z a t i o n and d i s p e r s a l r e l a t e d ? (3) what a r e the 5 consequences of such r e l a t i o n s h i p s f o r the e v o l u t i o n of s p e c i e s ? To answer t h e s e q u e s t i o n s w i t h i n a s p e c i e s , one must f i r s t u n d e r s t a n d some of the major f a c t o r s t h a t c o n t r i b u t e t o the s u c c e s s of i n d i v i d u a l s w i t h i n p o p u l a t i o n s . Second, one must a s s e s s the s t r e n g t h of i n t e r a c t i o n s between i n d i v i d u a l b e h a v i o u r and the p h y s i c a l and s o c i a l environment (Bradbury & Vehrencamp 1983; Woolfenden & F i t z p a t r i c k 1984; Brown 1987). I now e x p l o r e some of t h e s e i n t e r a c t i o n s by c o n s i d e r i n g r e l a t i o n s h i p s between i n d i v i d u a l d i f f e r e n c e s i n b e h a v i o u r , the environment, and the mating system. R e p r o d u c t i v e C o m p e t i t i o n and M a t i n g Systems The t h e o r y of mating systems i s founded on the o b s e r v a t i o n t h a t females commit more energy t o each gamete than males ( O r i a n s 1969; T r i v e r s 1972). Thus, female r e p r o d u c t i v e s u c c e s s i s o f t e n l i m i t e d by a c c e s s t o r e s o u r c e s , w h i l e male r e p r o d u c t i v e s u c c e s s i s more l i k e l y t o be l i m i t e d by a c c e s s t o f e r t i l e f e m a l e s . T r i v e r s (1972) argued t h a t s e l e c t i o n t h e r e f o r e f a v o r s c o m p e t i t i v e mate a c q u i s i t i o n by males and c a r e f u l mate c h o i c e by females (see a l s o O r i a n s 1969; S e l a n d e r 1972; H a l l i d a y 1978; W i t t e n b e r g e r 1979; Greenwood 1980). Male r e p r o d u c t i v e c o m p e t i t i o n i s w i d e s p r e a d and w e l l -documented (e . g . i n s e c t s , P a r k e r 1978; f i s h , F a r r 1977; anurans, Arak 1983; mammals, C l u t t o n - B r o c k e t a l . 1982; b i r d s , W i l e y 1974). C u r r e n t t h e o r y s u g g e s t s t h a t , as a consequence of such 6 c o m p e t i t i o n , mature males are o f t e n l a r g e r , more a g g r e s s i v e and b r i g h t l y c o l o r e d , and more endowed with weaponry than females (Selander 1972; Wittenberger 1979; Payne 1984). Females, however, a l s o compete f o r resources c r u c i a l to r e p r o d u c t i o n , (e.g. i n s e c t s , Whitham 1986; f i s h , Schroder 1981; mammals, Hogdon & Larsen 1973, b i r d s , Davies & Houston 1986). Because competition i n females i s u s u a l l y much l e s s overt than i n males, i t remains p o o r l y understood i n general (Wittenberger 1979; Oring 1982; Wasser 1983). Reproductive competition among males i n c l u d e s defence of r e s o u r c e s r e q u i r e d by females, d i r e c t defence of females or advertisement f o r females. In c o n t r a s t , females t y p i c a l l y compete through defence of breeding s i t e s , but r a r e l y by defending males or by advertisement (Wittenberger 1979; Oring 1982; Arak 1983; P e t r i e 1983). E c o l o g i c a l f a c t o r s (e.g. temporal and s p a t i a l d i s t r i b u t i o n of food, breeding s i t e s ) determine the form of most mating systems (Orians 1969; Emlen & Oring 1977; Wittenberger 1979). T h i s i s because the d i s t r i b u t i o n of resources a f f e c t s the a b i l i t y of i n d i v i d u a l s to defend them from c o n s p e c i f i c s (Brown 1964; Brown & Orians 1970). Females respond p r i m a r i l y to resource d i s t r i b u t i o n , while males respond e i t h e r to female d i s t r i b u t i o n or to the d i s t r i b u t i o n of resources that females r e q u i r e (Bradbury & Vehrencamp 1977; Parker 1978; Wittenberger 1979). When resources are s e a s o n a l l y p r e d i c t a b l e and clumped i n 7 space, males can maximize e x c l u s i v e a c c e s s t o females by d e f e n d i n g r e s o u r c e s t h a t females r e q u i r e ( r e s o u r c e d e f e n c e ; Crook 1965; V e r n e r & W i l l s o n 1966). The e x t e n t t o which d e f e n d i n g males e x c l u d e o t h e r males d e t e r m i n e s the l e v e l of p o l y g y n y i n the p o p u l a t i o n ( O r i a n s 1969; Emlen & O r i n g 1977). In c o n t r a s t , when r e s o u r c e s are e i t h e r t h i n l y d i s p e r s e d or s e a s o n a l l y u n p r e d i c t a b l e , males must e i t h e r accompany and defend females d u r i n g f e r t i l e p e r i o d s (female d e f e n c e ; C l u t t o n - B r o c k & Harvey 1976; Bradbury & Vehrencamp 1977), or a d v e r t i s e t h e i r s u i t a b i l i t y ( l e k b r e e d i n g ; W i l e y 1974; Emlen 1976). Females s h o u l d a l s o defend l i m i t i n g r e s o u r c e s a g a i n s t c o n s p e c i f i c s when defence i n c r e a s e s : (1) the q u a l i t y or number of o f f s p r i n g produced, (2) t h e i r r e l a t i v e r e p r o d u c t i v e s u c c e s s (by i n t e r f e r i n g w i t h r e p r o d u c t i o n i n o t h e r f e m a l e s ) , or (3) reduces the p r o b a b i l i t y of h a v i n g t o share male p a r e n t a l c a r e ( W i t t e n b e r g e r 1979; O r i n g 1982; Wasser 1983; Hannon 1984; D a v i e s & Houston 1986). In monogamous s p e c i e s , the r o l e of male c o m p e t i t i o n i s p r o b a b l y r e l a x e d compared t o polygynous and promiscuous s p e c i e s ( S e l a n d e r 1972; W i t t e n b e r g e r & T i l s o n 1980; O r i n g 1982). Males may respond e i t h e r t o the d i s t r i b u t i o n of f e m a l e s , or t o the d i s t r i b u t i o n of the r e s o u r c e s t h a t females r e q u i r e . Females respond t o the d i s t r i b u t i o n of r e s o u r c e s t h a t a f f e c t r e p r o d u c t i v e s u c c e s s . In t h i s c a s e , e i t h e r sex or both might de f e n d r e s o u r c e s (e.g. D a v i e s & Houston 1986). T h i s c o u l d 8 a f f e c t p a t t e r n s of n a t a l d i s p e r s a l i f the c o s t s or b e n e f i t s of resource defence depended on p r o x i m i t y to the n a t a l s i t e (Murray 1967; Greenwood 1980; Dobson 1982). Monogamy i s g e n e r a l l y r a r e among v e r t e b r a t e s , except i n b i r d s where i t i s common (Lack 1968; Kleiman 1977; M o l l e r 1986). Wittenberger & T i l s o n (1980) concluded that most b i r d s were monogamous because males e i t h e r cannot defend enough resources to a t t r a c t a d d i t i o n a l mates or cannot defend more than one female. However, recent work shows that female t e r r i t o r i a l behaviour i n b i r d s reduces polygyny and sometimes e n f o r c e s monogamy on males by p r e v e n t i n g some females from access to t e r r i t o r i e s (Hannon 1983, 1984; Davies 1985; Hurley & Robertson 1985; Stutchbury & Robertson 1985). F u r t h e r , when male p a r e n t a l care i n c r e a s e s r e p r o d u c t i v e success but cannot be shared among females, females can double the number of young they r a i s e by i n s u r i n g e x c l u s i v e access to male a i d (e.g. Smith et a l . 1982; Davies & Houston 1986). T h i s should s e l e c t f o r i n c r e a s e d female a g g r e s s i o n . In p r a c t i c e , however, female t e r r i t o r i a l i t y i s d i f f i c u l t to d e t e c t because i n most monogamous b i r d s defence of a t e r r i t o r y by a male i s e q u i v a l e n t to defence of a female (Smith et a l . 1982). Mating Systems, Demography and D i s p e r s a l If the f a c t o r s that a f f e c t success i n resource defence are s i m i l a r i n males and females, d i s p e r s a l may be more a 9 consequence of s e x - s p e c i f i c demography than of d i f f e r e n c e s i n mating s t r a t e g y (Murray 1967; Waser & Jones 1983). In c o n t r a s t , Greenwood (1980) and Greenwood & Harvey (1982) argued that sex-b i a s e s i n d i s p e r s a l i n monogamous b i r d s are caused by the r e l a t i v e importance of resource defence to males and females, and the c o s t s of i n b r e e d i n g d e p r e s s i o n . They b e l i e v e that males are the main defenders of breeding resources, and that females choose where to s e t t l e r a t h e r than having t h e i r c h o i c e f o r c e d by i n t r a s e x u a l c o m p e t i t i o n . Making the a d d i t i o n a l assumption that males remaining near the n a t a l s i t e are more l i k e l y to a c q u i r e a t e r r i t o r y and breed, they concluded that the c o s t of i n b r e e d i n g d e p r e s s i o n would outweigh the c o s t of d i s p e r s a l i n females, but not i n males. T h i s h y pothesis c l e a r l y depends on the s t r u c t u r e of the mating system and on which sex i s p r i m a r i l y r e s p o n s i b l e f o r resource defence, which are not yet well-understood f o r most s p e c i e s . The advantage of remaining near the n a t a l s i t e i s a l s o p o o r l y known. Few p r e v i o u s s t u d i e s have simultaneously i n v e s t i g a t e d i n t r a s e x u a l c o m p e t i t i o n f o r r e s o u r c e s , n a t a l d i s p e r s a l and the o r g a n i z a t i o n of the mating system. T h i s i s my o v e r a l l goal i n t h i s t h e s i s . Goals of the Study Four c h a r a c t e r i s t i c s make the song sparrow p o p u l a t i o n on Mandarte I s l a n d p a r t i c u l a r l y s u i t a b l e f o r a study of i n t r a s e x u a l 10 c o m p e t i t i o n f o r r e s o u r c e s , n a t a l d i s p e r s a l and the o r g a n i z a t i o n of the mating system: ( 1 ) n e a r l y a l l b i r d s t h a t breed i n the p o p u l a t i o n a r e hatched t h e r e , and t h u s can be o b s e r v e d i n d e t a i l from h a t c h i n g t o the time they s e t t l e on a b r e e d i n g t e r r i t o r y , ( 2 ) the l i f e t i m e p r o d u c t i o n of independent young f o r a l l r e c r u i t s can be d e t e r m i n e d , ( 3 ) the s i z e of the p o p u l a t i o n a l l o w s w e l l - c o n t r o l l e d e x p e r i m e n t s w i t h a s u f f i c i e n t number of e x p e r i m e n t a l s u b j e c t s , and ( 4 ) the n a t u r a l v a r i a t i o n i n p o p u l a t i o n s i z e a l l o w s o b s e r v a t i o n s over a wide range of p d e n s i t i e s ( F i g . 1 . 1 ) . In C hapter 2 , I f i r s t d e s c r i b e the n a t u r a l h i s t o r y of the song sparrow, the study s i t e , and the methods employed i n t h i s s t u d y . F u r t h e r , I have c o n s o l i d a t e d a l l methods r e f e r r e d t o i n subsequent c h a p t e r s t o m i n i m i z e redundancy. Each t e c h n i q u e used i s d e s c r i b e d under a heading t h a t i d e n t i f i e s the t o p i c (e.g. E s t i m a t i n g Dominance, S t a t i s t i c a l A n a l y s e s ) . I l i s t t h e s e i n the t a b l e of c o n t e n t s t o f a c i l i t a t e t h e i r l o c a t i o n . The methods d e s c r i b e d i n c h a p t e r 2 a r e p r e s e n t e d r o u g h l y i n the o r d e r t h a t they are employed i n c o n s e c u t i v e c h a p t e r s of the t h e s i s. In Chapter 3 , I c o n s i d e r p a t t e r n s of t e r r i t o r y a c q u i s i t i o n and l o s s by male song sparrows over the 5 y e a r s of the s t u d y . I a l s o t e s t two assumptions of p o p u l a r models of d i s p e r s a l i n t e r r i t o r i a l a n i m a l s . These a r e : ( 1 ) p r e v i o u s o w n e r ship i s the main f a c t o r a f f e c t i n g the outcome of c o n t e s t s over t e r r i t o r i e s , Y e a r Figure 1.1. The number of females breeding on Mandarte Island during 17 years (Tompa 1964; Smith 1988; t h i s study). 1 2 and (2) t h a t m o r t a l i t y of t e r r i t o r y owners i s the p r i m a r y mechanism by which h a b i t a t becomes a v a i l a b l e f o r s e t t l e m e n t by n o n - t e r r i t o r i a l a n i m a l s ( r e f e r e d t o as ' f l o a t e r s ' , a f t e r Brown 1969 and Smith 1978). I d i s c u s s t h e s e r e s u l t s w i t h r e g a r d t o (a) c o m p e t i t i o n f o r l o c a l t e r r i t o r i e s v e r s u s d i s p e r s a l t o unoccupied s i t e s e l s e w h e r e , (b) the t e m p o r a l p a t t e r n of t e r r i t o r y t u r n o v e r s , and (c) the r o l e of age i n t e r r i t o r y d e f e n c e . In Chapter 4, I t e s t s e v e r a l h y p o t h e s i s about the r o l e of s u r p l u s , n o n - t e r r i t o r i a l f l o a t e r s i n the t e r r i t o r i a l system of the song sparrow. To do t h i s , I d e s c r i b e a g g r e s s i v e i n t e r a c t i o n s between t e r r i t o r i a l and n o n - t e r r i t o r i a l male song sparrows', and I i n v e s t i g a t e s e v e r a l f a c t o r s t h a t might a f f e c t the tendency of f l o a t e r s t o l o c a l i z e on the t e r r i t o r i e s of p a r t i c u l a r males. F i n a l l y , I t e s t the h y p o t h e s i s t h a t the a b i l i t y t o d e f end t e r r i t o r i e s a g a i n s t c h a l l e n g e s by f l o a t i n g male song sparrows i s r e l a t e d t o the age of the t e r r i t o r i a l male. In Chapter 5, I examine the a g g r e s s i v e and t e r r i t o r i a l b e h a v i o u r of female song spa r r o w s , and i t s r e l a t i o n t o female s i n g i n g b e h a v i o u r . U s i n g o b s e r v a t i o n s on the f r e q u e n c y of song by f e m a l e s , and on the s i t u a t i o n s i n which song i s most o f t e n o b s e r v e d , I t e s t the h y p o t h e s i s t h a t song i s an a b e r r a n t b e h a v i o u r of females t h a t i s unimportant i n the c o n t e x t of b r e e d i n g or t e r r i t o r i a l b e h a v i o u r . These r e s u l t s a r e d i s c u s s e d 13 i n l i g h t of r e c e n t h y p o t h e s i s about the r o l e of androgens i n t e r r i t o r i a l b e h a v i o u r . In Chapter 6, I p r e s e n t o b s e r v a t i o n a l d a t a t o t e s t the h y p o t h e s i s t h a t the mated s t a t u s of i n d i v i d u a l males and females depends on t h e i r a b i l i t y t o p a r t i c i p a t e i n t e r r i t o r y d e f e n c e . I then p r e s e n t an e x p e r i m e n t a l t e s t of the e f f e c t s of r e s o u r c e abundance on the time budgets, t e r r i t o r i a l b e h a v i o u r and r e p r o d u c t i v e s u c c e s s of males and f e m a l e s . I f i r s t h y p o t h e s i z e d t h a t female song sparrows f r e e d from f o r a g i n g demands would spend more time i n a c t i v i t i e s d i r e c t l y r e l a t e d t o t e r r i t o r y d e f e n c e . Second, i f female song sparrows defend e x c l u s i v e a c c e s s the t h e i r mate, or t o the r e s o u r c e s c o n t a i n e d w i t h i n t h e i r t e r r i t o r y , I e x p e c t e d t h a t females w i t h e x t r a food would be more a b l e t o p r e v e n t s e t t l e m e n t by s u r p l u s , n o n - t e r r i t o r i a l f e m a l e s . In Chapter 7, I i n v e s t i g a t e p a t t e r n s of n a t a l d i s p e r s a l i n 5 c o h o r t s of young w i t h r e g a r d t o p o p u l a t i o n d e n s i t y , k i n s h i p , r e s o u r c e d i s t r i b u t i o n and subsequent r e p r o d u c t i v e s u c c e s s . In t h i s c h a p t e r I r e v i e w and t e s t s e v e r a l h y p o t h e s i s put f o r w a r d i n the l i t e r a t u r e t o e x p l a i n i n d i v i d u a l v a r i a t i o n i n d i s p e r s a l d i s t a n c e . I a l s o p r e s e n t r e s u l t s on the e f f e c t of food a d d i t i o n on the n a t a l d i s p e r s a l of young r a i s e d on f e d and c o n t r o l t e r r i t o r i e s , and on the l e v e l of subsequent c o m p e t i t i o n f o r e x p e r i m e n t a l t e r r i t o r i e s a f t e r s u p p l e m e n t a l f e e d i n g had ce a s e d . In Chapter 8, I c o n c l u d e the t h e s i s by b r i e f l y d i s c u s s i n g the 14 r o l e s of i n t r a - s e x u a l c o m p e t i t i o n i n d i s p e r s a l , t e r r i t o r i a l i t y and the o r g a n i z a t i o n of mating systems. Cha p t e r s 3-7 of t h i s t h e s i s were w r i t t e n as a s e r i e s of p a p e r s . Because of t h i s , the main development of each t o p i c and the statements of r e l e v a n t hypotheses are d e a l t w i t h i n the i n t r o d u c t i o n s of each of t h e s e c h a p t e r s . REFERENCES Arak, A. 1983. 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Cambridge: Cambridge U n i v e r s i t y P r e s s . S a d l i e r , R.M.F.S. 1965. The r e l a t i o n s h i p between a g o n i s t i c behaviour and p o p u l a t i o n changes i n the deer mouse, Peromyscu maniculatus (Wagner). J . Anim. E c o l . , 34, 331-352. Schroder, S.L. 1981. The r o l e of sexual s e l e c t i o n i n determining the o v e r a l l mating p a t t e r n s and mate c h o i c e of chum salmon. Ph.D. d i s s . , U n i v e r s i t y of Washington, S e a t t l e . Selander, R.K. 1972. Sexual s e l e c t i o n and dimorphism i n b i r d s . In: Sexual S e l e c t i o n and the Decsent of Man. (Ed. by B.G. Campbell), pp. 180-230. Chicago: A l d i n e P r e s s . S h i e l d s , W.J. 1982. P h i l o p a t r y , Inbreeding and the E v o l u t i o n of  Sex. Albany: State U n i v e r s i t y of New York Press. Smith, J.N.M., Yom Tov, Y. & Moses R. 1982. Polygyny, male p a r e n t a l care and sex r a t i o i n song sparrows: an experimental study. Auk, 99, 555-564. Stutchbury, B.J. & Robertson, R.J. 1985. F l o a t i n g p o p u l a t i o n s of female t r e e swallows. Auk, 102, 651-654. 19 T r i v e r s , R.L. 1972. P a r e n t a l investment and sexual s e l e c t i o n . In: Sexual S e l e c t ion and the Descent of Man. (Ed. by B.G. Campbell), pp. 136-179. Chicago: A l d i n e P r e s s . Verner, J . & Wilson, M. F. 1966. the i n f l u e n c e of h a b i t a t s on mating systems of North American p a s s e r i n e b i r d s . Ecology, 47, 143-147. Waser, P.M. & Jones, W.T. 1983. N a t a l p h i l o p a t r y among s o l i t a r y mammals. Quart. Rev. B i o l . , 58, 355-390. Wasser, S.K. 1983. S o c i a l Behaviour of Female V e r t e b r a t e s . New York: Academic Press. Whitham, T.G. 1986. Costs and b e n e f i t s of t e r r i t o r i a l i t y : b e h a v i o r a l and r e p r o d u c t i v e r e l e a s e by competing aphids. Ecology, 67, 139-147. Wiley, R.H. 1974. E v o l u t i o n of s o c i a l o r g a n i z a t i o n and l i f e h i s t o r y p a t t e r n s among grouse. Q. Rev. B i o l . , 49, 201-227. Wilson, E.O. 1975. S o c i o b i o l o g y : the New S y n t h e s i s . Cambridge: Harvard U n i v e r s i t y Press. Wittenberger, J.F. 1979. The e v o l u t i o n of mating systems i n b i r d s and mammals. In: Handbook of Neurobiology: S o c i a l  Behaviour and Communication. (Ed. by P. Ma r l e r & J . Vandenbergh), pp. 271-349. New York: Plenum P r e s s . Wittenberger, J.F. & T i l s o n , R.L. 1980. The e v o l u t i o n of monogamy: hypotheses and evidence. Ann. Rev. S y s t . , 11, 197-232. Woolfenden, G.E. & F i t z p a t r i c k , J.W. 1984. The F l o r i d a Scrub  Jay: Demography of a C o o p e r a t i v e l y Breeding B i r d . P r i n c e t o n : P r i n c e t o n U n i v e r s i t y Press. Wright S. 1940. Breeding s t r u c t u r e of p o p u l a t i o n s i n r e l a t i o n to s p e c i a t i o n . Am. Nat., 74, 232-248. 20 Wynne-Edwards, V.C. 1962. Animal D i s p e r s i o n in R e l a t i o n to S o c i a l Behaviour. Edinburough: O l i v e r & Boyd. 21 CHAPTER 2. STUDY SPECIES, STUDY SITE AND METHODS Study Species The song sparrow i s a t i m i d , s e c r e t i v e i n h a b i t a n t of shrublands that border watercourses, damp meadows and c o a s t l i n e (Nice 1937, M a r s h a l l 1948a,b). I t breeds throughout North America from c e n t r a l Mexico n o r t h to the t r e e l i n e (Marshall 1948a). Song sparrows are mainly ground-feeding g r a n i v o r e s i n winter, and t h e r e f o r e e i t h e r migrants, p a r t i a l migrants or year-round r e s i d e n t s depending on snow cover (Nice 1937). In s p r i n g and summer, song sparrows feed p r i m a r i l y on i n s e c t s and l e a f and flower buds, and feed mainly i n s e c t s to t h e i r young (Nice 1937; Johnston 1956b, Tompa 1964). The song sparrow i s a p r i m a r i l y monogamous p a s s e r i n e that o f t e n mates f o r l i f e , and l i v e s mostly w i t h i n a small (ca. 0.05-1.0 ha) multi-purpose t e r r i t o r y (Nice 1937; Johnston 1956a; Tompa 1964). The low frequency of n a t u r a l polygyny (ca. 3%) can be i n c r e a s e d by removing males d u r i n g the breeding p e r i o d (Smith et a l . 1982), or i n migratory p o p u l a t i o n s by e x p e r i m e n t a l l y i n c r e a s i n g t e s t o s t e r o n e l e v e l s i n males ( w i n g f i e l d 1984). Song sparrows mature at 1 year of age, o f t e n f l e d g e more than one brood per year (max=4) of up to f i v e young and t y p i c a l l y breed f o r from 1-3 seasons (max=8, Nice 1937; Smith 1988). 22 J u v e n i l e s gain independence at 25-30 days of age and o f t e n j o i n other young in loose f l o c k s . T e r r i t o r i a l behaviour among young b i r d s appears a f t e r the p o s t - j u v e n i l e molt (Tompa 1964). Dominance h i e r a r c h i e s form among young b i r d s d u r i n g the l a t e summer and f a l l , and s o c i a l s t a t u s i n these f l o c k s determines which young remain i n the p o p u l a t i o n and e v e n t u a l l y s e t t l e (Arcese & Smith 1985). Among j u v e n i l e s , males o f t e n dominate females at feeders, and young hatched e a r l y i n the s p r i n g are g e n e r a l l y dominant to those hatched l a t e r (Arcese & Smith 1985). O f f s p r i n g do not resemble parents i n dominance s t a t u s (P.Bets un p u b l i s h e d ) . Some j u v e n i l e s move w i t h i n r e s t r i c t e d ranges from e a r l y f a l l to the f o l l o w i n g s p r i n g . The degree of l o c a l i z a t i o n among j u v e n i l e s v a r i e s , but many gain t e r r i t o r i e s i n these areas from f a l l through s p r i n g (Tompa 1964; own o b s e r v a t i o n s ) . J u v e n i l e s that f a i l to s e t t l e by the time breeding commences become h i g h l y s e c r e t i v e and l i v e on the t e r r i t o r i e s of breeders (Smith et a l . 1982). Females, however, r a r e l y remain u n s e t t l e d a f t e r breeding commences, although many u n s e t t l e d females sometimes remain i n the p o p u l a t i o n u n t i l egg-l a y i n g begins, but disappear t h e r e a f t e r (own o b s e r v a t i o n s ) . In most years, a small number of male t e r r i t o r y owners remains unmated (Smith et a l . 1980). The p a i r r e s i d e s on the t e r r i t o r y year-round, except where the s p e c i e s i s migratory, but t e r r i t o r i a l behaviour peaks d u r i n g 23 the autumn and s p r i n g (Nice 1937; Johnston 1956b; Tompa 1964). Males are more v i s i b l y t e r r i t o r i a l than females, but females a l s o f i g h t , e s p e c i a l l y i n l a t e winter and s p r i n g (Nice 1937; Tompa 1964; Smith et a l . 1980; own o b s e r v a t i o n s ) . Few b i r d s change t e r r i t o r i e s w i t h i n or between seasons, though females may do so more than males (Nice 1937; but see Weatherhead & Boak 1986). Mandarte and H a l i b u t I s l a n d s Mandarte I s l a n d i s l o c a t e d i n the Haro S t r a i t , approximately 25 km ENE of V i c t o r i a , B.C., Canada. T h i s small (6 ha), long and narrow i s l a n d (ca. 700 by 90 m) i s covered by a t h i n l a y e r of s o i l that supports a dense cover of grasses and herbs that remain green throughout most of the year. A d e p r e s s i o n along the a x i s of the i s l a n d supports a dense growth of shrubs that cover approximately 1.5 ha of the i s l a n d ' s land a r e a . T h i s shrub area, along with the surrounding meadows and i n t e r t i d a l areas, are the h a b i t a t s used f o r f e e d i n g and breeding by song sparrows. A few l a r g e c o n i f e r s , and s e v e r a l s m a l l e r b r o a d - l e a f e d t r e e s (e.g. Prunus spp., Quercus garryana) grow i n c l u s t e r s throughout the i s l a n d . Canopy height v a r i e s from 1-5 m. H a l i b u t I s l a n d , s i t u a t e d about 1.3 km S of Mandarte, i s the c l o s e s t i s l a n d with a r e s i d e n t song sparrow p o p u l a t i o n . H a l i b u t supports a small p o p u l a t i o n of song sparrows (ca. 10 24 p a i r s ; Tompa 1963) and i s about h a l f the s i z e of Mandarte in land area. H a l i b u t i s l e s s s u i t a b l e as a h a b i t a t f o r song sparrows than i s Mandarte because i t i s covered p r i m a r i l y by c o n i f e r o u s f o r e s t . However, patches of dense shrub that border the f o r e s t p rovide song sparrow h a b i t a t which c l o s e l y resembles that found on Mandarte. Study P o p u l a t i o n The song sparrow p o p u l a t i o n on Mandarte I s l a n d has been i n t e n s i v e l y s t u d i e d s i n c e 1974. Tompa (1964) a l s o s t u d i e d t h i s p o p u l a t i o n from 1960 to 1964, and he p r o v i d e s a d e t a i l e d d e s c r i p t i o n of the h a b i t a t and the b i o l o g y of the song sparrows that are r e s i d e n t there year-round. The general f i e l d methods used s i n c e 1974 are d e s c r i b e d i n Smith & Zach (1979), Smith (1981,1988) and Arcese & Smith (1985). Some data presented here were c o l l e c t e d i n these p r e v i o u s s t u d i e s . In b r i e f , most a d u l t s are t e r r i t o r i a l year-round. Y e a r l i n g s begin to a c q u i r e t e r r i t o r i e s i n t h e i r f i r s t summer, but many remain n o n - t e r r i t o r i a l u n t i l the beginning of t h e i r f i r s t breeding season (Chap. 3,4,6,7). The defended h a b i t a t i s composed p r i m a r i l y of dense shrub (e.g. Symphoricarpos  a l b u s , Rosa nutkana), and i s used f o r n e s t i n g and f e e d i n g . Observations of i n d i v i d u a l b i r d s can be made r e l a t i v e l y e a s i l y . As a r e s u l t , v i r t u a l l y a l l song sparrow nests are found d u r i n g i n c u b a t i o n , and a l l young are banded as n e s t l i n g s with a unique 25 combination of c o l o r e d , p l a s t i c r i n g s and a numbered aluminum r i n g . Census Technique I v i s i t e d the i s l a n d at l e a s t monthly f o r 2-5 days from September to February, and l i v e d semi-permanently on the i s l a n d from March through August, from September 1981 through 1986. During t h i s time, the l o c a t i o n s of a l l b i r d s s i g h t e d were recorded on maps drawn from an a e r i a l photograph, and s p e c i a l a t t e n t i o n was p a i d to c o n f l i c t s over t e r r i t o r i e s . From 1 January 1982 to 10 March 1987, i n t e n s i v e censuses of the p o p u l a t i o n were made by myself and up to 2 h e l p e r s . During a census, the e n t i r e i s l a n d was searched by walking on a l l t r a i l s , around a l l shrub patches and along the e n t i r e s h o r e l i n e . I r o u t i n e l y crawled through p a r t i c u l a r l y dense shrub patches. While s e a r c h i n g , a l l b i r d s s i g h t e d were f o l l o w e d u n t i l i d e n t i f i e d or l o s t . When no b i r d s were evident i n an area, I sat motionless f o r up to s e v e r a l minutes l i s t e n i n g f o r song sparrow sounds and watching f o r motion. Many extremely t i m i d b i r d s (e.g. female f l o a t e r s ) were i d e n t i f i e d and observed i n t h i s way. Playbacks of taped song were somtimes used to l o c a t e b i r d s i n wi n t e r . Because shrub patches cover l e s s than one-t h i r d of the 6 ha i s l a n d , and are w e l l - d i v i d e d by t r a i l s , the e n t i r e i s l a n d was r e l a t i v e l y e a s i l y censused. 26 Determination of T e r r i t o r i a l Status Song sparrows on Mandarte I s l a n d are r e s i d e n t year-round. T e r r i t o r i e s of shrubs and adjacent g r a s s l a n d or i n t e r t i d a l areas are defended by song, t h r e a t p o s t u r e s , p u r s u i t s and f i g h t s throughout the year by most a d u l t s and a few y e a r l i n g s . Non-t e r r i t o r i a l f l o a t e r s l i v e s e c r e t i v e l y on the t e r r i t o r i e s of other males, but are r e l a t i v e l y e a s i l y observed o u t s i d e of the breeding p e r i o d . T e r r i t o r i a l behaviour i s most pronounced i n autumn, l a t e winter and s p r i n g , but song and t e r r i t o r i a l a g g r e s s i o n occur throughout the year. I used the presence or absence of s e v e r a l behaviours to determine the t e r r i t o r i a l s t a t u s of males and females. I co n s i d e r e d b i r d s to be t e r r i t o r i a l i f they sang, perched prominently and unchallenged, and/or c o n s i s t e n t l y e v i c t e d other b i r d s from an area of shrubland f o r at l e a s t a day. T e r r i t o r y h o l d e r s r a r e l y allowed f l o a t e r s of the same sex to s i n g or perch prominently i n the shrub f o r more than b r i e f p e r i o d s . However, f l o a t e r s o c c a s i o n a l l y remained f o r longer p e r i o d s when owners were o f f the t e r r i t o r y f e e d i n g or b a t h i n g . The postures and v o c a l i z a t i o n s of f l o a t e r s and t e r r i t o r y h o l d e r s d i f f e r e d s t r i k i n g l y : f l o a t e r s f r e q u e n t l y gave ' t s i p ' c a l l s (Nice 1943:274), always h e l d t h e i r wings f l a t upon t h e i r backs and u s u a l l y r a i s e d t h e i r c r e s t f e a t h e r s when s i t t i n g prominently i n a shrub or when approached by a t e r r i t o r y h o l d e r . 27 T e r r i t o r y owners r a r e l y gave t s i p c a l l s , but f r e q u e n t l y gave 'tchunks', 'caterwauls' and 'threat notes' (Nice 1943:274). Male owners f r e q u e n t l y sang, but females r a r e l y d i d so (Chap. 5). Owners r a i s e d t h e i r c r e s t s only when alarmed (e.g. i n presence of a hawk) and t y p i c a l l y h e l d t h e i r wings e r e c t along the l e n g t h of t h e i r body. The only exceptions to t h i s occurred when f l o a t e r s t r i e d to e v i c t t e r r i t o r y owners by p e r c h i n g prominently and s i n g i n g i n t h e i r t e r r i t o r y . F u r t h e r d e s c r i p t i o n s of the behaviours of f l o a t e r s and of t r a n s i t i o n s i n t e r r i t o r i a l s t a t u s are given i n Chapters 3-6. Mapping T e r r i t o r y Boundaries T e r r i t o r y s i z e was measured d u r i n g the l a s t 2 weeks of A p r i l by p l o t t i n g on maps t r a c e d from a l o w - l e v e l a e r i a l photograph the s i n g i n g p o s t s , movements and p o i n t s of boundary d i s p u t e s between a l l b i r d s . T e r r i t o r y boundaries f o r males and females were drawn by connecting these p o i n t s . Boundaries c o u l d be most a c c u r a t e l y estimated from March to August, when ob s e r v a t i o n s were made n e a r l y c o n t i n u o u s l y . Home-ranges of f l o a t e r s were determined by p l o t t i n g s i g h t i n g s on maps and connecting the p o i n t s to form minimum convex polygons. Assessment of Reproduction T e r r i t o r i e s were v i s i t e d at l e a s t every 5 days throughout the breeding p e r i o d each year. At each v i s i t , males and 28 females were observed f o r 20-60 min, or u n t i l the stage in the n e s t i n g c y c l e was determined. Regular nest checks revealed when nest f a i l u r e o ccurred or the number of young that hatched. Most nests were found d u r i n g i n c u b a t i o n or e a r l i e r . A f t e r h a t c h i n g , n e s t l i n g s were weighed and color-banded at about day 6. The number of f l e d g l i n g s was determined by nest checks and by observing parents feeding newly-fledged young (Smith 1978; Smith & Merkt 1980). The number of young reachi n g independence was determined from r e s i g h t i n g s and captures i n mist nets d u r i n g i n t e n s i v e censuses of the sparrow p o p u l a t i o n on the i s l a n d throughout the summer and f a l l . Age of B i r d s As there i s l i t t l e immigration to the p o p u l a t i o n (Tompa 1964, Chap. 7), n e a r l y a l l i n d i v i d u a l s observed i n t h i s study were color-marked and of known age. Because few i n d i v i d u a l s of e i t h e r sex s u r v i v e beyond 4 years, a l l b i r d s 4 or o l d e r were pooled i n analyses i n v o l v i n g age. E s t i m a t i n g I n t r u s i o n Pressure Observations on r a t e s of i n t r u s i o n were made between 1 March and 31 August i n 1984 and 1985. I recorded i n t r u s i o n p ressure on the t e r r i t o r i e s of 57 males from 1-6 years of age dur i n g 80 h of o b s e r v a t i o n i n 1984. In 1985, I recorded i n t r u s i o n p r e s s u r e on the t e r r i t o r i e s of 75 males of the same 29 range of ages duri n g 214 h of o b s e r v a t i o n . An i n t r u s i o n was recorded whenever a f l o a t e r was observed t r e s p a s s i n g on the t e r r i t o r y of the f o c a l male. Forty-one t e r r i t o r i a l males were observed in both y e a r s . A l l o b s e r v a t i o n s were made du r i n g 578 p e r i o d s (15 to 80 min) from 0600 to 2100 hours. I n i t i a l r e s u l t s showed that the number of i n t r u s i o n s , but not the i n t r u s i o n r a t e , v a r i e d s i g n i f i c a n t l y with the l e n g t h of the o b s e r v a t i o n p e r i o d . I t h e r e f o r e s t a n d a r d i z e d s t a t i s t i c a l l y f o r each b i r d ' s o b s e r v a t i o n p e r i o d l e n g t h by r e g r e s s i n g the number of i n t r u s i o n s on p e r i o d l e n g t h . I then used the l o g -transformed r e s i d u a l s ( h e r e a f t e r c a l l e d standard i n t r u s i o n p r e s s u r e , SIP) as the y - v a r i a t e i n l a t e r a n a l y s e s . T h i s method proved s u p e r i o r to using r a t e s because the transformed r e s i d u a l s were normally d i s t r i b u t e d , while r a t e s were not normally d i s t r i b u t e d even a f t e r t r a n s f o r m a t i o n ( G i l b e r t 1973; Sokal & Rohlf 1981). The s l o p e s and y - i n t e r c e p t s of the y e a r l y r e g r e s s i o n s were s i m i l a r ( a l l P>0.10). T e r r i t o r y tenure was measured as the number of days that a male defended a t e r r i t o r y d u r i n g 1 March to 31 August. Types of T e r r i t o r y Turnovers I d e f i n e d 5 types of turnover i n t e r r i t o r y ownership (males o n l y ) : (1) I n s e r t i o n s o c c u r r e d when f l o a t e r s s e t t l e d on the border between two t e r r i t o r i e s or when f l o a t e r s s e t t l e d p r i m a r i l y 30 w i t h i n one t e r r i t o r y . (2) Take-overs by f l o a t e r s o c c u r r e d when f l o a t e r s a c q u i r e d the e n t i r e t e r r i t o r y of an owner. (3) Replacements by f l o a t e r s o c c u r r e d when f l o a t e r s began defending a t e r r i t o r y that was known to have been unoccupied f o r at l e a s t one day. (4) Take-overs by neighbours o c c u r r e d when a t e r r i t o r y owner expanded i t s t e r r i t o r y to i n c l u d e most (>75%) of a neighbour's t e r r i t o r y , and e i t h e r e v i c t e d the owner or f o r c e d i t to remain i n a small p o r t i o n of i t s former t e r r i t o r y . (5) Replacements by neighbours, as f o r f l o a t e r s , o c c u r r e d when a neighbour annexed an undefended p o r t i o n of shrub a f t e r the disappearance of an owner. In a l l cases, turnovers were scored as take-overs only i f a c o n f l i c t had been observed e i t h e r p r e v i o u s to or d u r i n g the o b s e r v a t i o n of a change in ownership. In a few cases, no c o n f l i c t was observed and the t e r r i t o r y was known to have been occupied when l a s t watched: these cases were scored as replacements. These took p l a c e i n winter when v i s i t s to the i s l a n d were l e s s frequent and the o v e r a l l number of turnovers that o c c u r r e d was s m a l l . The a c t u a l number of take-overs i s t h e r e f o r e s l i g h t l y underestimated. The dates of turnovers were known by o b s e r v a t i o n or estimated as e i t h e r the date that the owner was l a s t seen (take-overs) or the date when a replacement was f i r s t observed (replacements). 31 Fates of Males I determined the f a t e s of former male owners for each turnover recorded. Owners e i t h e r : (1) remained t e r r i t o r i a l on a p o r t i o n of t h e i r p r e v i o u s t e r r i t o r y ; (2) d i s p e r s e d to a nearby t e r r i t o r y ; (3) became a n o n - t e r r i t o r i a l f l o a t e r ; or (4) disappeared from the p o p u l a t i o n . Because f l o a t e r s are s e c r e t i v e and d i f f i c u l t to observe, some males that a p p a r e n t l y disappeared from the p o p u l a t i o n probably s u r v i v e d undetected as f l o a t e r s . Thus, the number of males that became f l o a t e r s a f t e r being e v i c t e d from t h e i r t e r r i t o r i e s i s probably underestimated. Seasonal P a t t e r n s of T e r r i t o r y Turnover To examine the seasonal p a t t e r n of t e r r i t o r y t u r nover, data were pooled i n t o 6 2-month p e r i o d s . These a r e : Jan.-Feb., a p e r i o d of i n c r e a s i n g pre-breeding t e r r i t o r i a l a c t i v i t y ; Mar.-Apr., the f i r s t two months of breeding; May-June, peak of the breeding p e r i o d ; July-Aug., end of breeding and the p e r i o d of p o s t - n u p t i a l molt; Sept.-Oct., autumn p e r i o d of t e r r i t o r i a l behaviour; and Nov.-Dec, mid-winter p e r i o d with only low l e v e l s of t e r r i t o r i a l behaviour. P o o l i n g o b s e r v a t i o n s i n t h i s way minimized e r r o r because I was very c o n f i d e n t of the 2-month p e r i o d w i t h i n which tur n o v e r s o c c u r r e d . 32 Recording Song During the s p r i n g s of 1984 and 1985, P.K. Stoddard and S.M. H i e b e r t recorded songs from three females d u r i n g a study of song r e p e r t o i r e f u n c t i o n i n males. One of these females was recorded while s i n g i n g spontaneously. The others sang i n response to a playback of songs recorded from a male on a non-a d j o i n i n g t e r r i t o r y . The playback speaker was p l a c e d i n the cen t e r of each female's t e r r i t o r y . Songs were recorded on a Uher 4000 tape recorder with a 41 cm p a r a b o l i c microphone. A f t e r r e c o r d i n g s were band-pass f i l t e r e d to reduce ambient n o i s e , sonagrams were made on a Kay 6061B Sona-graph with a 150 Hz p l u g - i n f i l t e r . D e f i n i t i o n s of Mated Status I r e f e r t o females as 'monogamous' i f they: (1) occupied a t e r r i t o r y that was overlapped by a s i n g l e male, and (2) r e c e i v e d a i d r a i s i n g t h e i r young from a male whose t e r r i t o r y d i d not o v e r l a p completely that of an a d d i t i o n a l female. During the i n c u b a t i o n stage, male a i d was d e f i n e d as alarm c a l l i n g when a predator or human entered the t e r r i t o r y or approached the n e s t . A f t e r h a t c h i n g , only females whose t e r r i t o r i a l male fed her young were d e f i n e d as monogamous. When a male's t e r r i t o r y overlapped the m a j o r i t y of the areas u t i l i z e d by two or more females, males were d e f i n e d as 33 polygynous. The r e s i d e n t females were d e f i n e d as e i t h e r 'aided' or 'unaided' using the same c r i t e r i a as f o r monogamous females. S e v e r a l females were l e f t unaided a f t e r t h e i r o r i g i n a l mate was r e p l a c e d by a f l o a t e r . These females were pooled with unaided females i n polygynous groups f o r analyses of the e f f e c t of male a i d on r e p r o d u c t i v e success. Food A d d i t i o n Experiment In 1985, I p l a c e d a s i n g l e small feeder i n the c e n t e r s of 15 t e r r i t o r i e s l o c a t e d i n 2 b l o c k s of 7 and 8 contiguous t e r r i t o r i e s . I chose t h i s arrangement to minimize the number of neighbouring p a i r s with p o t e n t i a l access to adjacent feeders, and to maximize the s i m i l a r i t y of fed and c o n t r o l t e r r i t o r i e s i n h a b i t a t s t r u c t u r e . T e r r i t o r y owners used the feeders immediately a f t e r I i n s t a l l e d them, and continued to use them throughout the experiment to feed themselves, and t h e i r n e s t l i n g s and f l e d g l i n g s . A few neighbours and f l o a t e r s a l s o used the f e e d e r s , but t h e i r v i s i t s probably o f t e n went unseen. A d e t a i l e d account of the e f f e c t s of t h i s experiment on r e p r o d u c t i o n i s given i n Arcese & Smith (1988). Supplemental food c o n s i s t e d of moistened, p e l l e t e d dog chow (Ra l s t o n - P u r i n a Inc., 21% crude p r o t e i n ) , v i t a m i n s o l u t i o n , mealworms (Tenebrio sp.) and m i l l e t seed. Food was provided twice d a i l y from 28 February, 5 weeks before the onset of l a y i n g , to 24 J u l y , a f t e r the l a s t young had l e f t t h e i r n e s t s . 34 My h e l p e r s and I were absent from Mandarte duri n g 9 of 35 days before l a y i n g , 7 of 87 days d u r i n g the l a y i n g and r e a r i n g p e r i o d and 5 of 24 days a f t e r the l a s t c l u t c h was i n i t i a t e d . Before l e a v i n g , a l l feeders were heaped with food. Time Budgets I recorded the a c t i v i t i e s of 13 fed and 14 c o n t r o l females, and 14 males of each type, d u r i n g 2 h a l f - h o u r p e r i o d s on d i f f e r e n t days (55 h t o t a l ) . C o n t r o l t e r r i t o r i e s were chosen to match c l o s e l y i n h a b i t a t s t r u c t u r e the t e r r i t o r i e s with added food. Observations were made between 7 March and 15 A p r i l , 1985, from 1-6 weeks p r i o r to commencement of i n c u b a t i o n by the f o c a l females. To reduce v a r i a t i o n due to time of day and weather, b i r d s were observed only between 700 and 1100 h, when temperatures exceeded 4 C, and when wind was l i g h t or absent. Observations were made while standing on a 3 m ladder p l a c e d so that t e r r i t o r i e s c o u l d be observed with l i t t l e d i s t u r b a n c e to the b i r d s . I waited 5 min a f t e r mounting the ladder before beginning my o b s e r v a t i o n s . B i r d s ignored me soon a f t e r my a r r i v a l because song sparrows on Mandarte I s l a n d are accustomed to o b servers. I recorded the behavior of the f o c a l b i r d every 7.5 s, when prompted by an e l e c t r o n i c metronome. Seven types of behavior were d e f i n e d : (1) f e e d i n g ( s e a r c h i n g f o r , c a p t u r i n g and consuming food), 35 (2) p r e e n / l o a f ( s i t t i n g low i n shrubs or on the ground, u s u a l l y with f e a t h e r s f l u f f e d and wings tucked onto the back, or sunning and preening on a prominent perch "e.g. Nice 1 943:48-J ) , (3) a l e r t ( s i t t i n g prominently, u s u a l l y above the shrub canopy, with f e a t h e r s sleeked and wings h e l d e r e c t along the le n g t h of the body), (4) defense/chase ( t h r e a t e n i n g and/or e v i c t i n g non-t e r r i t o r i a l i n t r u d e r s , a g g r e s s i v e p o s t u r i n g opposite neighbour on or near a shared b o r d e r ) , (5) d i s p l a y ( f o r females: c o p u l a t i o n s o l i c i t a t i o n , p u f f -wave d i s p l a y , song, c h a t t e r and/or caterwaul; f o r males: song, puff-sing-wave, pounce, c h a t t e r and/or caterwaul "Nice 1943:274; Chap. 5 J ) , (6) nest b u i l d i n g (females only; grass p u l l i n g and c a r r y i n g ) , (7) out of s i g h t ( e i t h e r on or o f f the t e r r i t o r y ) . The p r o p o r t i o n of time spent i n each a c t i v i t y was estimated as the number of p e r i o d s f o r which the behavior was recorded, d i v i d e d by the t o t a l number of p e r i o d s (480). Most behaviors that I recorded l a s t e d s e v e r a l seconds or long e r . Thus, my method was s u f f i c i e n t f o r the goals of t h i s study (Tacha et a l . 1985). I a l s o recorded a l l t e r r i t o r i a l i n t r u s i o n s t hat e l i c i t e d chases from one or both of the f o c a l p a i r . 3 6 D i s p e r s a l D i s t a n c e N a t a l d i s p e r s a l d i s t a n c e was estimated as the s t r a i g h t - l i n e d i s t a n c e between the approximate c e n t e r s of the n a t a l t e r r i t o r y and the t e r r i t o r y of f i r s t s e t t l e m e n t . To estimate the number of t e r r i t o r i e s t r a v e r s e d , I d i v i d e d the l e n g t h of the u t i l i z e d p o r t i o n of the i s l a n d (650 m) by the number of t e r r i t o r i a l females that produced the c o h o r t . I then converted d i s t a n c e moved in meters i n t o t e r r i t o r y e q u i v a l e n t s by d i v i s i o n . T h i s method g i v e s only a rough, minimum estimate of the number of t e r r i t o r i e s t r a v e r s e d by n a t a l d i s p e r s e r s on Mandarte because some young that s e t t l e d c l o s e to t h e i r n a t a l s i t e were observed to have v i s i t e d l a r g e p o r t i o n s of the i s l a n d before s e t t l i n g . I estimated the d i s t a n c e t r a v e l e d by fed and c o n t r o l young by l a t e August, 1985, as the f u r t h e s t d i s t a n c e from the n a t a l t e r r i t o r y at which young were observed. S e v e r a l young b i r d s were i n v o l v e d i n feeding or s i t e dominance experiments from 1983 to 1985. These b i r d s are i n c l u d e d i n analyses presented here because removing them had no e f f e c t on the r e s u l t s . E s t i m a t i n g Dominance Dominance was estimated by observing a g g r e s s i v e i n t e r a c t i o n s at 4 evenly spaced fe e d e r s , and c a l c u l a t i n g the p r o p o r t i o n of t o t a l i n t e r a c t i o n s won by each i n d i v i d u a l (Ketterson 1979; Kikkawa 1981; Arcese & Smith 1985). 37 Observations were made p r i m a r i l y from mid-July to l a t e September, from 1982-84. Winners and l o s e r s of encounters were recorded only when the outcome was c l e a r . For a l l 3 c o h o r t s , 8378 a g g r e s s i v e encounters were recorded. T h i s method of e s t i m a t i n g dominance p r e d i c t s the p r o b a b i l i t y of winning encounters most a c c u r a t e l y when the number of p r e v i o u s encounters observed i s l a r g e (Arcese & Ludwig 1986). I t h e r e f o r e conducted c o r r e l a t i o n analyses using both weighted and unweighted dominance estimates, and excluded b i r d s observed i n fewer than 5 encounters (Arcese & Ludwig 1986). B i r d s observed i n fewer than 5 encounters were i n c l u d e d as subordinates i n frequency analyses (Kikkawa 1981; Arcese & Smith 1985). Relatedness Parentage was assumed from the behaviour of t e r r i t o r i a l p a i r s . However, 3 e x t r a - p a i r c o p u l a t i o n s were observed i n a t o t a l of about 100, suggesting that the a s s o c i a t i o n of males and females on the same t e r r i t o r y may not always p r e d i c t parentage c o r r e c t l y . A pedigree of the p o p u l a t i o n based on assumed matings was c o n s t r u c t e d beginning with the 1981 cohort, f o r which r e l a t e d n e s s was p a r t l y known. The pedigrees of a l l young that s e t t l e d with a mate were t r a c e d , and those that shared common 38 r e l a t i v e s were noted. P a i r s with complete pedigrees and no common r e l a t i v e s among parents or grand-parents were c o n s i d e r e d to be not c l o s e l y r e l a t e d . Matings were c l a s s e d as m i l d l y i n b r e d when they took p l a c e with grandparents, u n c l e s , aunts or c o u s i n s , and c l o s e l y inbred when between s i b s or parents and t h e i r o f f s p r i n g ( S h i e l d s 1982). S t a t i s t i c a l Analyses Data were analyzed using standard techniques ( S i e g e l 1956; G i l b e r t 1973; Sokal & Rolhf 1981). I transformed data (e.g. by the angular or l o g a r i t h m i c t r a n s f o r m a t i o n ) i f g r a p h i c a l a n a l y s e s showed that they were not approximately normally d i s t r i b u t e d (Sokal & Rolhf 1981). In a l l analyses of v a r i a b l e s i n which I employed parametric techniques, I a l s o c a r r i e d out non-parametric t e s t s f o r comparison. In these cases, only the r e s u l t s of the parametric t e s t s are given, unless the two techinques gave c o n t r a s t i n g r e s u l t s . When t r a n s f o r m a t i o n d i d not improve the d i s t r i b u t i o n of the data (e.g. as i n the case of number of young produced), only non-parametric t e s t s were employed. However, I sometimes used c o r r e l a t i o n c o e f f i c i e n t s as d e s c r i p t i v e s t a t i s t i c s of n o n - s i g n i f i c a n t r e l a t i o n s h i p s . In some cases, the u n i t of data was the n e s t i n g attempt r a t h e r than the i n d i v i d u a l because males and females f r e q u e n t l y p a r t i c i p a t e d i n more than one type of mating a s s o c i a t i o n w i t h i n seasons and between y e a r s . B i r d s that were known or presumed to have had access to the feeders i n 1985 were excluded from comparisons of 39 r e p r o d u c t i v e success. I used G - t e s t s with the W i l l i a m s c o r r e c t i o n and F i s h e r ' s exact p r o b a b i l i t y t e s t s to analyze most frequency data (Sokal & Rolhf 1981). In a few cases, c h i - s q u a r e d s t a t i s t i c s were used when expected values were small but adjacent c e l l s c o u l d not be combined because of the hypothesis being t e s t e d . In these cases, I ad j u s t e d c h i - s q u a r e by the c o n t i n u i t y c o r r e c t i o n to give a more c o n s e r v a t i v e t e s t (Sokal & Rolhf 1981). S t a t i s t i c a l comparisons are 2 - t a i l e d except where I made p r i o r p r e d i c t i o n s about the outcome of t e s t s . In an a l y s e s where I made repeated comparisons of r e l a t e d data, I r e v i s e d the s i g n i f i c a n c e l e v e l a c c o r d i n g t o the B o n f e r r o n i i n e q u a l i t y . T h i s s t a t e s that only p r o b a b i l i t i e s l e s s than or equal to the i n i t i a l l e v e l of s i g n i f i c a n c e ( i . e . P=0.05) d i v i d e d by the number of t e s t s conducted should be co n s i d e r e d s t a t i s t i c a l l y s i g n i f i c a n t ( M i l l e r 1981). REFERENCES Arcese, P. & Smith, J.N.M. 1988. E f f e c t s of p o p u l a t i o n d e n s i t y and supplemental food on r e p r o d u c t i o n i n the song sparrow. J . Anim. E c o l . , 57, i n p r e s s . Arcese, P. & Ludwig, D. 1986. Improving estimates of dominance based on r a t i o s . Condor, 88, 106-107. Arcese, P. & Smith, J.N.M. 1985. Phenotypic c o r r e l a t e s and e c o l o g i c a l consequences of dominance i n song sparrows. J . Anim. E c o l . , 54, 817-830. 40 G i l b e r t , N. 1973. B i o m e t r i c a l I n t e r p r e t a t i o n . Oxford: Clarendon Press. Johnston, R.F. 1956a. P o p u l a t i o n s t r u c t u r e i n s a l t marsh song sparrows p a r t I. Environment and annual c y c l e . Condor, 58, 24-44. Johnston, R.F. 1956b. P o p u l a t i o n s t r u c t u r e i n s a l t marsh song sparrows p a r t I I . D e n s i t y , age s t r u c t u r e and maintenance. Condor, 58, 254-274. Ke t t e r s o n , E.D..1979. Aggressive behavior i n w i n t e r i n g dark-eyed juncos: determinants of dominance and t h e i r p o s s i b l e r e l a t i o n to geographic v a r i a t i o n . Wilson B u l l . , 91, 371— 383. Kikkawa, J . 1981. Winter s u r v i v a l i n r e l a t i o n to dominance c l a s s e s among s i l v e r e y e s , Zosterops l a t e r a l i s  c h l o r o c h e p h a l a , of Heron I s l a n d , Great B a r r i e r Reef. I b i s , 122, 437-446. M a r s h a l l , J.T. 1948a. E c o l o g i c a l races of song sparrows' i n the San F r a n c i s c o Bay r e g i o n : Part I. H a b i t a t and abundance. Condor, 50, 193-215. M a r s h a l l , J.T. 1948b. E c o l o g i c a l races of song sparrows i n the San F r a n c i s c o Bay r e g i o n : Part I I . Geographic v a r i a t i o n . Condor, 50, 233-256. M i l l e r , R.G. J r . 1981. Simultaneous S t a t i s t i c a l I n f e r e n c e . 2nd edn. New York: S p r i n g e r - V e r l a g . N i c e , M.M. 1937. S t u d i e s i n the l i f e h i s t o r y of the song sparrow. Part I. Trans. L i n n . Soc., N.Y., 4, 1-247. Nice, M.M. 1943. S t u d i e s i n the l i f e h i s t o r y of the song sparrow. I I . The behavior of the song sparrow and other p a s s e r i n e s . Trans. L i n n . Soc., N.Y., 6, 1-329. S h i e l d s , W.J. 1982. P h i l o p a t r y , Inbreeding and the E v o l u t i o n of  Sex. Albany: State U n i v e r s i t y of New York P r e s s . 41 S i e g e l , S. 1956. Non-parametric S t a t i s t i c s f o r the B e h a v i o u r a l and S c i e n c e s . New York: McGraw-Hill. Smith, J.N.M. 1978. D i v i s i o n of labour by song sparrows fe e d i n g f l e d g e d young. Can. J . Zoo l . , 56, 187-191. Smith, J.N.M. 1981. Cowbird p a r a s i t i s m , host f i t n e s s and the age of the host female i n an i s l a n d song sparrow p o p u l a t i o n . Condor, 83, 152-161 . Smith, J.N.M. 1988. Determinants of l i f e t i m e r e p r o d u c t i v e success i n the song sparrow. In: Reproductive Success (Ed. by T.H. C l u t t o n - B r o c k ) . Chicago: Chicago U n i v e r s i t y P r e s s . Smith, J.N.M. & Zach, R. 1979. H e r i t a b i l i t y of some morphological c h a r a c t e r s i n a song sparrow p o p u l a t i o n . E v o l u t i o n , 33, 460-467. Smith, J.N.M. & Merkt, J.R. 1980. Development and s t a b i l i t y of s i n g l e - p a r e n t f a m i l y u n i t s i n the song sparrow. Can. J . Zool . , 58, 1869-1875. Smith, J.N.M., Yom Tov, Y. & Moses R. 1982. Polygyny, male p a r e n t a l care and sex r a t i o i n song sparrows: an experimental study. Auk, 99, 555-564. Smith, J.N.M., Montgomerie, R.D., T a i t t , M.J. & Yom-Tov, Y. 1980. A winter f e e d i n g experiment on an i s l a n d song sparrow p o p u l a t i o n . O e c o l o g i a , 47, 164-170. Soka l , R.R. & Rohlf, J.F. 1981. Biometry. San F r a n c i s c o : W.H. Freeman. Tacha, T.C., Vohs, P.A. & Iverson, G.C. 1985. A comparison of i n t e r v a l and continuous sampling methods f o r b e h a v i o r a l o b s e r v a t i o n s . J . F i e l d O r n i t h . , 56, 258-264. Tompa, F.S. 1963. F a c t o r s determining the numbers of song sparrows Melospiza melodia (Wilson) on Mandarte I s l a n d . Ph.D. d i s s . , U n i v e r s i t y of B r i t i s h Columbia, Vancouver, B.C. Tompa, F.S. 1964. F a c t o r s determining the numbers of song sparrows Melospiza melodia (Wilson) on Mandarte I s l a n d , B.C., Canada. Acta Z o o l . Fenn., 109, 1-73. Weatherhead, P.J. & Boak, K.A. 1986. S i t e i n f i d e l i t y i n song sparrows. Anim. Behav., 34, 1311-1318. W i n g f i e l d , J.C. 1984. Androgens and mating systems: t e s t o s t e r o n e - i n d u c e d polygyny i n normally monogamous b i r d s . Auk, 101, 665-671. 43 CHAPTER 3. TERRITORY ACQUISITION AND LOSS IN MALE SONG SPARROWS INTRODUCTION In many s p e c i e s , r e p r o d u c t i v e success depends on the s u c c e s s f u l a c q u i s i t i o n of a t e r r i t o r y (e.g. Wiley 1973; Clutton-Brock et a l . 1982; Smith 1988). T h i s f a c t has spawned s e v e r a l s t u d i e s of the m o r p h o l o g i c a l (e.g. Searcy 1979; Hannon & Roland 1984) and b e h a v i o u r a l c h a r a c t e r s a s s o c i a t e d with t e r r i t o r y a c q u i s i t i o n (e.g. Knapton & Krebs 1974; Arcese & Smith 1985). However, few s t u d i e s have c o n s i d e r e d how i n d i v i d u a l s a c q u i r e t h e i r f i r s t t e r r i t o r y , expand a c u r r e n t one or r e g a i n l o s t t e r r i t o r i e s (e.g. Smith 1978; Yasukawa 1979; Pieman 1987). Models of d i s p e r s a l and t e r r i t o r y a c q u i s i t i o n u s u a l l y assume that i n d i v i d u a l s gain t e r r i t o r i e s only a f t e r the death of owners or by c o l o n i z i n g vacant h a b i t a t s (e.g. Murray 1967; F r e t w e l l & Lucas 1970; Waser 1985). In s e v e r a l b i r d p o p u l a t i o n s , however, i n d i v i d u a l s do not always wait u n t i l owners d i e before attempting to become t e r r i t o r i a l . Instead, n o n - t e r r i t o r i a l i n d i v i d u a l s ( f l o a t e r s , Brown 1969; Smith 1978) sometimes c h a l l e n g e and e v i c t t e r r i t o r y h o l d e r s t h a t are not able to v i g o r o u s l y defend themselves (e.g. Crook & S h i e l d s 1985; Freed 1987; Chap. 4). F u r t h e r , i n d i v i d u a l s that l o s e t h e i r t e r r i t o r i e s may not simply become pa r t of a 'doomed 44 s u r p l u s ' of non-breeders ( E r r i n g t o n 1956), but c o u l d j o i n the f l o a t i n g p o p u l a t i o n , c h a l l e n g e t e r r i t o r y owners and p o s s i b l y regain a t e r r i t o r y and reproduce. Because t h i s view of t e r r i t o r i a l dynamics i s r e l a t i v e l y new, and may c o n t r a d i c t some assumptions of c u r r e n t models, i t i s important to know what c o n d i t i o n s favour t h i s type of t e r r i t o r i a l system and how o f t e n i t occurs i n the w i l d . T h i s r e q u i r e s d e t a i l e d d e s c r i p t i o n s of the t e r r i t o r i a l dynamics i n a po p u l a t i o n over s e v e r a l y e a r s . In t h i s chapter, I present the r e s u l t s of a 5-year study of the timing and mode of t e r r i t o r y a c q u i s i t i o n and l o s s by male song sparrows, and the f a t e s of deposed t e r r i t o r y owners. In the song sparrow p o p u l a t i o n r e s i d e n t on Mandarte I s l a n d , B.C., most a d u l t s are t e r r i t o r i a l year-round, and c o n t e s t s f o r t e r r i t o r y ownership occur throughout the year between f l o a t e r s and owners, and between s e t t l e d neighbours. Three q u e s t i o n s are addressed here. (1) How and when are t e r r i t o r i e s acquired? (2) Who occupies vacant t e r r i t o r i e s a f t e r an owner d i e s ? And (3) what are the f a t e s of males that l o s e t h e i r t e r r i t o r i e s ? I a l s o s p e c u l a t e on f a c t o r s that a f f e c t the l e v e l of competition f o r t e r r i t o r i e s , the resources that are defended by males and the extent to which t h i s p o p u l a t i o n may be r e p r e s e n t a t i v e of ot h e r s . 45 RESULTS Types of T e r r i t o r y Turnovers The l e a d i n g cause of turn o v e r s was the i n s e r t i o n of f l o a t e r s among e s t a b l i s h e d t e r r i t o r y owners (107 of 292 t o t a l t u rnovers or 37%). In 33 of these 107 i n s e r t i o n s , the f l o a t e r took some area from 2 or more t e r r i t o r y owners, while i n the remaining 74 a p o r t i o n of a t e r r i t o r y was taken from only one owner. O v e r a l l , f l o a t e r s a c q u i r e d t e r r i t o r i e s i n 71% (206 of 292) of a l l t u r n o v e r s , of which at l e a s t 76% (156 of 206) i n v o l v e d the a g g r e s s i v e a c q u i s i t i o n of a t e r r i t o r y . C l e a r l y , many f l o a t i n g male song sparrows a c t i v e l y sought t e r r i t o r i e s , r a t h e r than w a i t i n g f o r owners t o d i e . In 86 of 292 turnovers recorded (30%), neighbours a c q u i r e d an adjacent t e r r i t o r y . Forty-two of these 86 turnovers were the outcome of a g g r e s s i v e encounters. The remaining 44 oc c u r r e d as replacements. Thus, t e r r i t o r y owners a l s o f r e q u e n t l y a c q u i r e d a d d i t i o n a l shrubland by e v i c t i n g other males from a l l or p a r t of t h e i r t e r r i t o r y . The Role of Age The outcomes of c o n t e s t s f o r ownership between e s t a b l i s h e d neighbours showed that the ages of the owners i n v o l v e d p l a y e d a r o l e i n determining which owner won (Table 3.1). For example, 46 males 4 or more years o l d l o s t most or a l l of t h e i r t e r r i t o r i e s i n a l l 13 c o n t e s t s i n v o l v i n g t h i s a g e - c l a s s (P<0.001, binomial t e s t ) . F i g u r e 3.1a shows that the p r o b a b i l i t y of wresting a t e r r i t o r y from a neighbour d e c l i n e d s h a r p l y a f t e r a male reached 2 years of age (G =23.63, df=3, P<0.001). There was a s i m i l a r , but n o n - s i g n i f i c a n t , r e l a t i o n s h i p between the ages of new versus o l d owners of t e r r i t o r i e s a f t e r replacements of neighbours, when no c o n f l i c t s i n v o l v i n g the missing male had been observed (G =3.60, df = 3, P>0.10; F i g . 3.1b). P o o l i n g these data showed that 2- and 3-year-old males were s i g n i f i c a n t l y more l i k e l y to a c q u i r e adjacent t e r r i t o r i e s from neighbours than were 1- and 4-year-old males r e g a r d l e s s of the t a c t i c used to g a i n the t e r r i t o r y (G =15.24, df=3, P<0.005). Y e a r l i n g and o l d e r f l o a t e r s were e q u a l l y l i k e l y to a c q u i r e t e r r i t o r i e s by i n s e r t i o n , takeover or replacement (G =0.30, df=2, P>0.98, Table 3.2). O v e r a l l , however, y e a r l i n g s accounted f o r the m a j o r i t y of turnovers that i n v o l v e d f l o a t e r s (161 of 206 or 78%) because few b i r d s remained n o n - t e r r i t o r i a l a f t e r t h e i r f i r s t s p r i n g . Seasonal V a r i a t i o n i n Turnovers The number of turnovers per 2-month p e r i o d v a r i e d s e a s o n a l l y , and the p a t t e r n of v a r i a t i o n was s i m i l a r over 5 years ( F i g . 3.2). E i g h t y - t h r e e percent of turnovers o c c u r r e d from January through June, the months p r i o r to and d u r i n g the 4* Table 3.1. The ages of owners that won and l o s t t e r r i t o r i e s as a r e s u l t of takeovers by nei ghbours. Winner Age (years) Loser Age (years) 1 2 3 4+ 1 9 4 2 0 2 2 6 0 0 3 4 5 0 0 4 5 4 0 A) Take-overs B) Replacements 1 2 3 4* 1 2 3 4+ Male Age ( y e a r s ) Figure 3.1. The relationship between male age and the outcome of n a t u r a l l y occurring t e r r i t o r y turnovers between neighours. Boxes depict the t o t a l number of males of each age that gained t e r r i t o r i e s , divided by the t o t a l number of each age (indicated) that were involved i n takeovers (A) or replacements (B). Table 3.2. Type of turnover i n r e l a t i o n to the age of the s e t t l i n g f l o a t e r . Type of turnover F l o a t e r age I n s e r t i o n Take-over Replacement Y e a r l i n g 2+ years 85 22 37 12 39 1 1 50 breeding p e r i o d . In 2 y e a r s , (1983,1985) turnovers were a l s o common du r i n g the p e r i o d of autumn t e r r i t o r i a l i t y from September through October (Tompa 1964). B i r d s l e a s t o f t e n a c q u i r e d t e r r i t o r i e s from November through December. There was, however, a s i g n i f i c a n t d i f f e r e n c e i n the seasonal frequency of t u r n o v e r s i n v o l v i n g neighbours or f l o a t e r s when the data f o r a l l 5 years were pooled i n t o 6 2-month p e r i o d s (G =14.19, df=5, P<0.02). F l o a t e r s a c q u i r e d t e r r i t o r i e s most f r e q u e n t l y d u r i n g the 4-month p e r i o d from January through A p r i l , but neighbours r a r e l y expanded t h e i r t e r r i t o r i e s o u t s i d e of the breeding p e r i o d (March-June; F i g . 3.2). For a l l three types of turnovers i n v o l v i n g f l o a t e r s , the median p r o p o r t i o n of y e a r l y turnovers peaked p r i o r to or at the beginning of breeding. Takeovers and replacements by neighbours peaked d u r i n g breeding. These peaks c o n t r i b u t e d to s i g n i f i c a n t d i f f e r e n c e s i n the seasonal occurrence of each type of turnover ( a l l P<0.01, K r u s k a l - W a l l i s ANOVA's, N=5 years f o r each of 6 2-month p e r i o d s ) . Turnovers were r a r e i n winter even though many song sparrows sometimes l e f t t h e i r t e r r i t o r i e s f o r a p o r t i o n of the day to feed on seeds (e.g. Sorbus and Rubus spp.) d e p o s i t e d at a s t a r l i n g (Sturnus v u l g a r i s ) roost on the i s l a n d . To t e s t i f the d i s t r i b u t i o n of t h i s food source on the i s l a n d i n f l u e n c e d t e r r i t o r y occupancy, I compared the frequency that I observed owners o f f t h e i r t e r r i t o r y at l e a s t once d u r i n g a census f o r 1982 1983 1984 1985 1986 Figure 3.2. The number of turnovers of each type recorded during 6 bi-monthly periods from 1982-6 (see Methods) . Shaded regions represent the main breeding period from March through (Jl June each year. 52 b i r d s that owned t e r r i t o r i e s w i t h i n the s t a r l i n g r o o s t to those b i r d s that owned t e r r i t o r i e s elsewhere on the i s l a n d . As expected i f food d i s t r i b u t i o n were important to the a b i l i t y of owners to remain on t h e i r t e r r i t o r i e s , v i r t u a l l y no 'roost owners' were observed o f f - t e r r i t o r y even dur i n g mid-winter, when 20-78% of a l l other t e r r i t o r y h o l d e r s l e f t t h e i r t e r r i t o r i e s f o r some p o r t i o n of the day (Table 3.3). F u r t h e r , n e a r l y a l l b i r d s observed o f f - t e r r i t o r y were seen i n the s t a r l i n g r o o s t f e e d i n g on seeds and s p r o u t i n g c o t e l y d o n s . However, I found no evidence that the r a t e of t e r r i t o r y turnover d i f f e r e d between these two groups. T e r r i t o r y Takeover and Polygyny Male t e r r i t o r y owners f r e q u e n t l y gained p o t e n t i a l access to adjacent t e r r i t o r i a l females when they annexed t h e i r nieghbour's t e r r i t o r y (63 of 86 cases or 73%). In 24 of 63 such cases, the encroaching male was p r e v i o u s l y unmated, but i n the remaining 39 cases (45% of a l l neighbour turnovers) males became polygynous. No f l o a t e r s s i m u l t aneously gained ownership of areas that encompassed more than one female's t e r r i t o r y . Fates of Deposed T e r r i t o r y Owners In 39% (113 of 292) of a l l t u r n o v e r s the o r i g i n a l owners .remained t e r r i t o r i a l on p o r t i o n s of t h e i r former t e r r i t o r i e s . Ten males (3%) moved to a nearby t e r r i t o r y j u s t p r i o r t o Table 3.3. The number of t e r r i t o r i a l males observed o f f t h e i r t e r r i t o r y f o r a p o r t i o n of the day at l e a s t once w i t h i n the census p e r i o d i n r e l a t i o n to t e r r i t o r y l o c a t i o n from September to February, 1985-6. Month L o c a t i o n On/Off Sept. Oct. Nov. Dec. Jan. Feb Roost T e r r i t o r i e s Other T e r r i t o r i e s On Off On Off 5 0 26 0 6 4 0 0 33 16 1 4 8 0 5 18 9 7 0 0 24 41 19 3 54 replacement or a f t e r an i n s e r t i o n , and i n 72 cases (25%) males became n o n - t e r r i t o r i a l f l o a t e r s (8 males l o s t t e r r i t o r i e s and subsequently f l o a t e d twice and 2 males d i d so 3 t i m e s ) . Males disappeared a f t e r 98 of 292 turnovers (34%). E i g h t y - t h r e e of these were replacements and 15 were takeovers. In some cases, former t e r r i t o r y owners that became f l o a t e r s disappeared from the p o p u l a t i o n w i t h i n a few days of t e r r i t o r y l o s s , and some were known to be i n poor p h y s i c a l c o n d i t i o n or have s e r i o u s i n j u r i e s (e.g. broken l e g , u s e l e s s or m i s s i n g f o o t ; see Chap. 4). Time Spent F l o a t i n g by Deposed Owners The l e n g t h of time spent f l o a t i n g by males that l o s t t h e i r t e r r i t o r i e s v a r i e d from 1 day to over 2 y e a r s , although only 2 cases exceeded a year. The d i s t r i b u t i o n of days spent f l o a t i n g f o r b i r d s t h a t regained a t e r r i t o r y (range=1-784 days, median=l60, N=36) d i d not d i f f e r s t a t i s t i c a l l y from that f o r b i r d s that disappeared from the p o p u l a t i o n without r e g a i n i n g a t e r r i t o r y (range=1-517 days, median=75, N=32; Dmax=0.24, P>0.10, Kolmogrov-Smirnov t e s t ) . Frequency of Regaining a T e r r i t o r y T h i r t y - s i x of 68 males (53%) t h a t l o s t t h e i r t e r r i t o r y and were known to have become a f l o a t e r l a t e r regained one, but 69% of these 36 males gained t e r r i t o r i e s without females. However, 55 s e v e r a l of these males subsequently gained mates, and o v e r a l l , 27 of the 36 males (40%) that regained a t e r r i t o r y subsequently bred. Ten of 36 males (28%) regained t h e i r former t e r r i t o r y , 15 of 36 (42%) s e t t l e d on an adjacent t e r r i t o r y and the 11 remaining males (30%) s e t t l e d 2 or more t e r r i t o r i e s d i s t a n t . The frequency with which males regained a t e r r i t o r y a f t e r having l o s t one and f l o a t e d appeared to depend on age ( F i g . 3.3). However, because of the small number of b i r d s i n v o l v e d , t h i s p a t t e r n was not s t a t i s t i c a l l y s i g n i f i c a n t (G =4.55, df=3, P>0.1). The frequency with which males regained a t e r r i t o r y might be expected to depend of age i f a d u l t s u r v i v a l a l s o depended on age. I t h e r e f o r e r e - c a l c u l a t e d the expected p r o b a b i l i t y of r e g a i n i n g a t e r r i t o r y based on the observed s u r v i v a l r a t e s of males of d i f f e r e n t age (Nol & Smith 1987). Although 1- and 4-year-old males each d i d more p o o r l y , and 2-and 3-year-old males each d i d b e t t e r than expected by t h e i r p r o b a b i l i t i e s of s u r v i v a l ( F i g . 3.3), these d i f f e r e n c e s were a l s o not s t a t i s t i c a l l y s i g n i f i c a n t (G =3.11, df=3, P>0.1). Two- and 3-year-old males spent on average 2 months l e s s as f l o a t e r s a f t e r l o s i n g a t e r r i t o r y than d i d 1- or 4-year-old males, but t h i s d i f f e r e n c e was a l s o not s t a t i s t i c a l l y s i g n i f i c a n t (H=4.07, df=3, P>0.1, K r u s k a l - W a l l i s ANOVA). cn c - iH c -rH tu cn I CD OC - P c CD CJ c CD Q_ o rj) o CO o o to o in o O m A* Male Age (years) Figure 3.3. The percent of males that regained a t e r r i t o r y a f t e r l o s i n g one i n r e l a t i o n to t h e i r age at the time of loss ( s o l i d boxes), and the expected s u r v i v a l rates to the following breeding season for males the same age (open boxes; Nol & Smith 1987). 57 Repeat F l o a t e r s A l l males on Mandarte spend the e a r l y p a r t of t h e i r l i f e as f l o a t e r s . However, many males a l s o do not a c q u i r e t e r r i t o r i e s before breeding begins and remain unmated f o r part or a l l of the f o l l o w i n g breeding p e r i o d . These males might remain as f l o a t e r s because they are i n h e r e n t l y l e s s a b l e to defend t e r r i t o r i e s than are males that have a c q u i r e d a t e r r i t o r y . I f so, such males might a l s o become f l o a t e r s l a t e r i n l i f e more o f t e n than males that had a c q u i r e d a t e r r i t o r y by the b e g i n i n g of breeding i n t h e i r f i r s t year of l i f e . To t e s t t h i s , I c o n s i d e r e d males from 4 c o h o r t s (1982-5) that owned a t e r r i t o r y at some time i n t h e i r l i v e s , but had d i e d by August 1987. These males were c l a s s e d as f l o a t e r s or as t e r r i t o r y owners in l a t e March of t h e i r f i r s t breeding season, and by whether or not they f l o a t e d a f t e r owning a t e r r i t o r y . Only 7 of 39 (18%) t e r r i t o r y owners i n e a r l y l i f e were observed as f l o a t e r s l a t e r . In c o n t r a s t , 18 of the 39 (46%) y e a r l i n g f l o a t e r s t h a t e v e n t u a l l y a c q u i r e d a t e r r i t o r y , a l s o became f l o a t e r s l a t e r i n l i f e (G =7.15, df=1, P<0.01). Summer S u r v i v a l of F l o a t e r s and Owners The s u r v i v a l of y e a r l i n g s by the end of breeding was u n r e l a t e d to t h e i r t e r r i t o r i a l s t a t u s on 1 March, as t h e i r f i r s t breeding season commenced, except in 1985 when y e a r l i n g s that 58 began the breeding season as f l o a t e r s s u r v i v e d b e t t e r than y e a r l i n g t e r r i t o r y owners (Table 3.4). In 3 of the 4 cohorts f o r which I was had such data, t e r r i t o r y owners s u r v i v e d from 6-22% b e t t e r than d i d f l o a t e r s , but when pooled over a l l cohorts there was no d i f f e r e n c e i n s u r v i v a l (Table 3.4). In c o n t r a s t , both the annual s u r v i v a l and l i f e t i m e r e p r o d u c t i v e success of y e a r l i n g t e r r i t o r y owners was s i g n i f i c a n t l y higher than that of y e a r l i n g f l o a t e r s (Smith & Arcese ms). DISCUSSION During 5 years, n o n - t e r r i t o r i a l male song sparrows most f r e q u e n t l y gained breeding t e r r i t o r i e s by e v i c t i n g r e s i d e n t owners from a l l or p a r t of t h e i r t e r r i t o r i e s . T h i s r e s u l t c o n t r a s t s with the assumptions of most c u r r e n t models of d i s p e r s a l and h a b i t a t settlement i n t e r r i t o r i a l animals. (e.g. Murray 1967; F r e t w e l l & Lucas 1970; Waser 1985; but see Buechner 1987). These assumptions are t h a t : (1) p r e v i o u s ownership i s the main f a c t o r that determines the outcome of c o n t e s t s over t e r r i t o r i e s and (2) m o r t a l i t y of t e r r i t o r y owners i s the primary mechanism by which h a b i t a t becomes a v a i l a b l e f o r set t l e m e n t . I now c o n s i d e r f a c t o r s t h a t a f f e c t the l e v e l of co m p e t i t i o n f o r l o c a l t e r r i t o r i e s versus d i s p e r s a l to unoccupied s i t e s , the temporal p a t t e r n of t e r r i t o r y t urnovers and the r o l e of age i n t e r r i t o r y defence. Table 3.4. The f a t e s of y e a r l i n g males i n 4 cohorts at the end of breeding (31 J u l y ) i n r e l a t i o n to t e r r i t o r i a l s t a t u s at the commencement of t h e i r f i r s t breeding season (1 March). T e r r i t o r i a l s t a t u s Beginning of season End of season Percent Cohort Owner/Floater Owner F l o a t e r Absent S u r v i v a l 1982 Owner 22 0 4 84.6 F l o a t e r 5 1 2 62.5 1983 Owner 1 4 0 3 82.4 F l o a t e r 10 1 6 8 76.5 1 984 Owner 10 1 2 84.6 F l o a t e r 7 1 1 5 78.3 1985 Owner 1 1 4 33.3* F l o a t e r 9 1 1 90.9 T o t a l Owner 47 2 1 3 75.8 F l o a t e r 31 29 16 78.9 * G w=6.45, df=2, P<0.05 60 F a c t o r s A f f e c t i n g the Mode of T e r r i t o r y A c q u i s i t i o n Three f a c t o r s that c o u l d extend l e n g t h of the p e r i o d of co m p e t i t i o n f o r a l o c a l l y l i m i t i n g supply of t e r r i t o r i e s or other breeding resources a r e : (1) year-round r e s i d e n c e and t e r r i t o r y defence, (2) an extended breeding p e r i o d , and (3) a s u r p l u s of p o t e n t i a l breeders. In r e s i d e n t , s e a s o n a l l y breeding p o p u l a t i o n s the d i s t r i b u t i o n and abundance of resources w i l l determine i f t e r r i t o r y defence i s economical (Brown & Orians 1970). Resource defence o u t s i d e the breeding p e r i o d c o u l d improve s u r v i v a l by ensuring access to food and cover, and enhance f u t u r e r e p r o d u c t i v e success by promoting access to h i g h - q u a l i t y breeding s i t e s . On Mandarte, male song sparrows sometimes leave t h e i r t e r r i t o r i e s to feed i n winter (Table 3.3), but they remain v i g o r o u s l y t e r r i t o r i a l when c h a l l e n g e d on t h e i r t e r r i t o r i e s year-round. I suggest that because co m p e t i t i o n f o r t e r r i t o r i e s i s strong on Mandarte I s l a n d , that males gain t e r r i t o r i e s o p p o r t u n i s t i c a l l y , and that once s e t t l e d , defend them throughout the non-breeding p e r i o d to ensure access to a breeding s i t e i n upcoming seasons. When year-round defence i s the norm and h a b i t a t i s l i m i t e d , queues f o r ownership based on co m p e t i t i v e a b i l i t y may form and r e s u l t i n the most able i n d i v i d u a l s g a i n i n g the best s i t e s , s e t t l i n g f i r s t or both (Brown 1969; Popp & De Vore 1979; Chap. 61 7). Smith (1978) d e s c r i b e d such a system i n the rufo u s -c o l l a r e d sparrow, Z o n o t r i c h i a c a p e n s i s , and a s i m i l a r s i t u a t i o n e x i s t s in song sparrows on Mandarte I s l a n d (Arcese & Smith 1985; Chap. 4 & 7). When unoccupied s i t e s are very s c a r c e , the most able f l o a t e r s should seek out and c h a l l e n g e t e r r i t o r y h o l d e r s whose a b i l i t y i s p o o r l y matched to the q u a l i t y of the t e r r i t o r y that they defend. T h i s c o u l d occur when vigo u r d e c l i n e s because of i n j u r y or o l d age, or i f l e s s v i g o r o u s b i r d s sometimes a c q u i r e s u p e r i o r s i t e s by chance. On Mandarte, f l o a t e r s do c h a l l e n g e and overthrow those t e r r i t o r y h o l d e r s that are l e a s t able to defend themselves: these are t y p i c a l l y the youngest and o l d e s t males i n the p o p u l a t i o n and those with i n j u r i e s or handicaps (Table 3.1; Chap. 4). When breeding seasons are long, f l o a t e r s that continue to co n t e s t f o r t e r r i t o r i e s may produce more young than those that wait f o r owners to disappear, e s p e c i a l l y when the s u r v i v a l of owners i n the absence of c h a l l e n g e s by f l o a t e r s i s high and the p r o b a b i l i t y of s u r v i v i n g to the next breeding season f o r f l o a t e r s i s low. These c o n d i t i o n s are met on Mandarte: few t e r r i t o r i e s became a v a i l a b l e a f t e r owners disappeared ( F i g . 3.2) and the annual s u r v i v a l r a t e of f l o a t e r s was lower than f o r t e r r i t o r y h o l d e r s (Smith & Arcese ms) even though d i f f e r e n c e s i n summer s u r v i v a l were smal l (Table 3.4). Thus, as expected, most f l o a t e r s gained a t e r r i t o r y by e v i c t i n g owners from a l l or 62 p a r t of t h e i r former t e r r i t o r y , and i n s e r t i o n s and takeovers by f l o a t e r s o c c u r r e d throughout the breeding p e r i o d ( F i g . 3.2). The degree of h a b i t a t s t a t u r a t i o n should f u r t h e r i n f l u e n c e the t a c t i c s of f l o a t e r s by a f f e c t i n g the p r o b a b i l i t y of s e t t l i n g without engaging i n c o n f l i c t s . During t h i s study, p o p u l a t i o n s i z e more than doubled (36-78 t e r r i t o r i a l males i n A p r i l ) . Because the p r o p o r t i o n of males that remain as f l o a t e r s a f t e r breeding commences i n c r e a s e s with p o p u l a t i o n s i z e (Smith & Arcese ms), I expected and found that the p r o p o r t i o n of f l o a t e r s t h a t gained t e r r i t o r i e s by takeovers and i n s e r t i o n s i n c r e a s e d with p o p u l a t i o n s i z e (R =0.90, N=5, P=0.05, Spearman rank c o r r e l a t i o n , 1 - t a i l e d ) . P r o b a b i l i t y of Settlement Elsewhere F a c t o r s that promote l o c a l c o m p e t i t i o n f o r t e r r i t o r i e s would appear to i n c r e a s e the b e n e f i t s of d i s p e r s i n g i n search of unoccupied h a b i t a t elsewhere. However, my l i m i t e d data suggest that breeding o p p o r t u n i t i e s f o r males elsewhere are few. A l l 4 males that I observed o f f Mandarte were f l o a t e r s . F u r t h e r , s e v e r a l y e a r l i n g male immigrants were captured on Mandarte each year, but only 2 immigrant males have s e t t l e d and bred i n 12 years of study (Smith 1988; Chap. 7). T h i s suggests that male song sparrows hatched i n the v i c i n i t y of Mandarte are u n l i k e l y to s e t t l e o u t s i d e of t h e i r n a t a l p o p u l a t i o n . T h i s c o u l d e x p l a i n why males c o n t e s t f o r l o c a l t e r r i t o r i e s even when 63 the p r o b a b i l i t y of settlement i s low. Mode of T e r r i t o r y A c q u s i t i o n and P i s p e r s a l In s p e c i e s where the r i s k s of e v i c t i n g t e r r i t o r y owners outweigh the b e n e f i t s of t e r r i t o r i a l s t a t u s , c u r r e n t models of d i s p e r s a l and h a b i t a t settlement may s a t i s f a c t o r i l y c h a r a c t e r i z e settlement p a t t e r n s (e.g. Murray 1967; F r e t w e l l & Lucas 1970; Waser 1985). However, i n p o p u l a t i o n s l i k e the one d e s c r i b e d here, the assumptions of these models are not met (Buechner 1987). When f l o a t e r s form s o c i a l h i e r a r c h i e s and c o n t e s t f o r access to l o c a l t e r r i t o r i e s i n s t e a d of d i s p e r s i n g i n search of vacant h a b i t a t , time from h a t c h i n g to s e t t l e m e n t , r a t h e r than d i s t a n c e moved, may be the most a p p r o p r i a t e v a r i a b l e with which to c h a r a c t e r i z e d i f f e r e n t i n d i v i d u a l s . F u r t h e r , f l o a t e r s probably a l t e r t h e i r t a c t i c s depending on the l e v e l of h a b i t a t s a t u r a t i o n , the b e n e f i t s d e r i v e d by owning a t e r r i t o r y , the r i s k s of e v i c t i n g owners and t h e i r own c o m p e t i t i v e a b i l i t y . Recent advances i n d i s p e r s a l m o d e l l i n g may a l l o w such v a r i a b l e s to be i n c o r p o r a t e d i n the f u t u r e (e.g. Parker & S u t h e r l a n d 1986; Buechner 1987). Occurrence of A g g r e s s i v e Takeovers P a t t e r n s of t e r r i t o r y a c q u i s i t i o n and l o s s i n male song sparrows are s i m i l a r to those d e s c r i b e d i n l e s s d e t a i l f o r r u f o u s - c o l l a r e d sparrows (Smith 1978), red grouse and willow 64 ptarmigan (Lagopus lagopus, Hannon 1983; Ma r t i n 1984; Watson 1985), blue grouse (Dendragapus obscurus, Lewis & Zwickel 1980), European k e s t r e l s ( F a l c o t i n n u n c u l u s , V i l l a g e 1983), t r o p i c a l house wrens (T r o g l o d y t e s aedon, Freed 1987), magpies ( P i c a p i c a , Birkhead et a l . 1986) and f a n - t a i l e d warblers ( C i s t i c o l a  j u n c i d i s , Ueda 1986) and o t h e r s . However, a g g r e s s i v e t e r r i t o r y a c q u i s i t i o n by f l o a t e r s of other s p e c i e s g e n e r a l l y appears to be l e s s common than i n song sparrows (e.g. Pieman 1987; Birkhead & Clarkson 1985; but see Ueda 1986). There are at l e a s t two ex p l a n a t i o n s f o r why my r e s u l t s d i f f e r from o t h e r s . F i r s t , when s p e c i e s can not be monitored as completely as the song sparrows i n t h i s study, many takeovers w i l l be mistaken as replacements, as indeed some were here. Second, there i s l i k e l y to be c o n s i d e r a b l e v a r i a t i o n among s p e c i e s and po p u l a t i o n s with regard to the modes of t e r r i t o r y a c q u i s i t i o n . Some reasons f o r t h i s were summarized above. T e r r i t o r y A c q u i s i t i o n by Neighbours and F l o a t e r s F l o a t e r s u s u a l l y s e t t l e d d u r i n g the 4-month p e r i o d from J a n u a r y - A p r i l ( F i g . 3.2), most of which precedes the peak of c l u t c h i n i t i a t i o n by females (Smith et a l . 1980; Arcese & Smith 1988). In c o n t r a s t , neighbours r a r e l y expanded t h e i r t e r r i t o r i e s o u t s i d e of the breeding p e r i o d (March-June; F i g . 3.2). T h i s i s expected i f neighbours u s u a l l y expand only a f t e r most f l o a t e r s have s e t t l e d , or i f neighbours expand p r i m a r i l y to gain access to females. 65 The f i r s t e x p l a n a t i o n i s u n l i k e l y . I n t r u s i o n pressure d e c l i n e d only s l i g h t l y from s p r i n g to summer (Chap. 4) and f l o a t e r s s e t t l e d throughout t h i s p e r i o d , when owners a l s o annexed neighbouring t e r r i t o r i e s ( F i g . 3.2). F u r t h e r , I o f t e n observed neighbours t r y i n g to prevent f l o a t e r s from s e t t l i n g i n vacant neighbouring t e r r i t o r i e s . In support of the second e x p l a n a t i o n , however, 45% of males that annexed a neighbouring t e r r i t o r y became polygynous at l e a s t t e m p o r a r i l y . Freed (1987) d e s c r i b e d a s i m i l a r s i t u a t i o n i n t r o p i c a l house wrens. Male song sparrows that defended more females a l s o produced more young than those that defended a s i n g l e female (Chap. 6). In c o n t r a s t , f l o a t e r s o f t e n gained t e r r i t o r i e s o u t s i d e of the breeding p e r i o d , when doing so d i d not ensure reproduct i o n . Age and T e r r i t o r y Turnover The r e p r o d u c t i v e advantages gained by a c q u i r i n g a d d i t i o n a l t e r r i t o r i e s and mates suggests that t e r r i t o r y h o l d e r s should compete to r e p l a c e m i s s i n g neighbours, and t h a t c o m p e t i t i v e l y s u p e r i o r t e r r i t o r y owners should e v i c t t h e i r l e s s a b l e neighbours. Three s t u d i e s suggest that c o m p e t i t i v e a b i l i t y i n song sparrows peaks around age 2-3 on average. Annual s u r v i v a l d e c l i n e s by about 20% a f t e r age 2 (Nol & Smith 1987), 1- and 4-y e a r - o l d males are l e s s a b l e to defend a g a i n s t f l o a t e r s than are 2- and 3-year-old males (Chap. 4) and 2-year-old males become 66 polygynous much more o f t e n than younger or o l d e r males (Chap. 6). As expected by these p a t t e r n s , middle-aged males were more l i k e l y to gain a d d i t i o n a l t e r r i t o r i e s than 1- or 4-year-old males ( F i g . 3.1). Age i s c l e a r l y a key f a c t o r t hat a f f e c t s behaviour and r e p r o d u c t i v e success i n male song sparrows. However, the tendency f o r males that f l o a t e d when young to a l s o f l o a t a f t e r h o l d i n g a t e r r i t o r y , suggests that i n a d d i t i o n to age-dependent v a r i a t i o n i n a b i l i t y , some males are i n h e r e n t l y l e s s able to defend t e r r i t o r i e s than o t h e r s . In a separate a n a l y s i s (Chap. 4), males were found to experience c o n s i s t e n t l y higher or lower i n t r u s i o n p r essure than the mean f o r t h e i r a g e - c l a s s a c r o s s years, even though these means v a r i e d s u b s t a n t i a l l y . Because any e f f e c t of t e r r i t o r y q u a l i t y c o u l d be r u l e d out, t h i s a n a l y s i s a l s o suggested that inherent d i f f e r e n c e s i n a b i l i t y e x i s t between i n d i v i d u a l s . As an i n t e r e s t i n g c o n t r a s t , h e r i t a b l e f a c t o r s , such as morphology (Smith & Dhondt 1980), have so f a r been found to have no measurable e f f e c t on behaviour or r e p r o d u c t i o n i n male song sparrows (Arcese & Smith 1985, and P. Bets, unpublished ms c i t e d t h e r e i n ; S c h l u t e r & Smith 1986; Smith 1988). I suggest that inherent d i f f e r e n c e s i n - the a b i l i t y to defend t e r r i t o r i e s may o r i g i n a t e d u r i n g development. 67 Routes to T e r r i t o r y Ownership For male song sparrows on Mandarte I s l a n d , the routes to t e r r i t o r y ownership can be complex. In song sparrow p o p u l a t i o n s elsewhere, the r a t e of t e r r i t o r y turnover can be h i g h i n migratory (Weatherhead & Boak 1986) and non-migratory p o p u l a t i o n s (P.K. Stoddard & M. Beecher p e r s o n a l communication). The e x i s t e n c e of f l o a t e r s i n e a r l y s p r i n g has been e x p e r i m e n t a l l y confirmed i n a p r i m a r i l y migratory song sparrow p o p u l a t i o n i n New York ( W i n g f i e l d 1985), and i n a mainland, r e s i d e n t p o p u l a t i o n near Mandarte I s l a n d (Knapton & Krebs 1974). I t remains u n c l e a r , however, how o f t e n f l o a t e r s e x i s t throughout the breeding p e r i o d , or what r o l e f l o a t e r s might play i n the t e r r i t o r i a l system i n other p o p u l a t i o n s . To date, the l i t e r a t u r e p r o v i d e s few s i m i l a r examples from temperate-zone s p e c i e s that defend t e r r i t o r i e s year-round (e.g. Birkhead et a l . 1986; Eden 1987). However, a recent study of t r o p i c a l house wrens (Freed 1987) p r o v i d e s some s t r i k i n g p a r a l l e l s to my study. These s t u d i e s show that year-round t e r r i t o r i a l i t y and extended breeding p e r i o d s are important e c o l o g i c a l f a c t o r s a s s o c i a t e d with the development of complex, n o n - t e r r i t o r i a l 'underworld' systems (Smith 1978). Comparative s t u d i e s of the song sparrow may i l l u s t r a t e the extent and v a r i a t i o n to which underworld systems develop i n temperate environments. 68 REFERENCES Arcese, P. & Smith, J.N.M. 1988. The e f f e c t s p o p u l a t i o n d e n s i t y and supplemental food on r e p r o d u c t i o n i n song sparrows. J . Anim. E c o l • , 57, i n p r e s s . Arcese, P. & Smith, J.N.M. 1985. Phenotypic c o r r e l a t e s and e c o l o g i c a l consequences of dominance i n song sparrows. J . Anim. E c o l . , 54, 817-830. Birkhead, T.R. & Cla r k s o n , K. 1985. 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E c o l . , 52, 807-820. Hannon, S.J. & Roland, J . 1984. Morphology and t e r r i t o r y a c q u i s i t i o n i n willow ptarmigan. Can. J . Z o o l . , 62, 1502-1506. Knapton, R.W. & Krebs, J.R. 1974. Settlement p a t t e r n s , t e r r i t o r y s i z e and breeding d e n s i t y i n the song sparrow, Melospiza  melodia• Can. J . Z o o l . , 52, 1413-1420. Lewis, R.A. & Zwickel, F.C. 1980. Removal arid replacement of male blue grouse on p e r s i s t e n t and t r a n s i e n t s i t e s . Can. J . Z o o l . , 58, 1417-1423. M a r t i n , K. 1984. Reproductive defence p r i o r i t i e s of male willow ptarmigan (Lagopus l a g o p u s ) : enhancing mate s u r v i v a l or extending p a t e r n i t y o p t i o n s . Behav. E c o l . S o c i o b i o l • , 16, 57-63. Murray, B.G. 1967. D i s p e r s a l i n v e r t e b r a t e s . Ecology, 48, 975-978. Nice, M.M. 1943. Studi e s i n the l i f e h i s t o r y of the song sparrow. I I . The behavior of the song sparrow and other p a s s e r i n e s . Trans. L i n n . Soc. N.Y., 6, 1-329. Nol, E. & Smith, J.N.M. 1987. E f f e c t s of age and breeding experience on seasonal r e p r o d u c t i v e success i n the song sparrow. J . Anim. E c o l . , 56, 301-313. 70 Pieman, J . 1987. T e r r i t o r y e s t a b l i s h m e n t , s i z e and t e n a c i t y by male red-winged b l a c k b i r d s ( A g e l a i u s phoeniceus). Auk, 104, 405-412. Parker, G.A. & Sutherland, W.J. 1986. I d e a l f r e e d i s t r i b u t i o n s when i n d i v i d u a l s d i f f e r i n c o m p e t i t i v e a b i l i t y : phenotype-l i m i t e d i d e a l f r e e models. Anim. Behav., 34, 1222-1242. Popp, J.L. & DeVore, I. 1979. Agg r e s s i v e c o m p e t i t i o n and s o c i a l dominance theory, s y n o p s i s . In: The Great Apes, P e r s p e c t i v e s on Human E v o l u t i o n , V o l 5 (Ed. by D.A. Hamburg & E.R. McCown),pp. 317-338. Menlo Park, C a l i f o r n i a : Benjamin/Cummings. S c h l u t e r , D. & Smith, J.N.M. 1986. N a t u r a l s e l e c t i o n on beak and body s i z e i n the song sparrow. E v o l u t i o n , 40, 221-231. Searcy, W.A. 1979. Male c h a r a c t e r i s t i c s and p a i r i n g success i n red-winged b l a c k b i r d s . Auk, 96, 353-363. S i e g e l , S. 1956. Nonparametric S t a t i s t i c s f o r the B e h a v i o r a l S c i e n c e s . New York: McGraw-Hill.-Smith, J.N.M. 1981. Cowbird p a r a s i t i s m , host f i t n e s s , and age of the host female i n an i s l a n d song sparrow p o p u l a t i o n . Condor, 83, 152-161. Smith, J.N.M. 1988. Determinants of l i f e t i m e r e p r o d u c t i v e success i n the song sparrow. In: Reproductive Success (Ed. by T.H. C l u t t o n - b r o c k ) , Chicago: U n i v e r s i t y of Chicago Press. Smith, J.N.M. & Arcese, P. The f i t n e s s of f l o a t e r s : Consequences of a l t e r n a t e t e r r i t o r i a l b ehaviors i n a non-migratory sparrow. Submitted ms. Smith, J.N.M. & Dhondt, A.A. 1980. Experimental c o n f i r m a t i o n of h e r i t a b l e morphological v a r i a t i o n i n a n a t u r a l p o p u l a t i o n of song sparrows. E v o l u t i o n , 34, 1155-1158. Smith, J.N.M., Montgomerie, R.D., T a i t t , M.J. & Yom-Tov, Y. 1980. A winter f e e d i n g experiment on an i s l a n d song sparrow p o p u l a t i o n . O e c o l o g i a , 47, 164-170. Smith, S.M. 1978. The underworld i n a t e r r i t o r i a l sparrow: a d a p t i v e s t r a t e g y f o r f l o a t e r s . Am. Nat., 112, 571-582. Soka l , R.R. & Rohlf, J.F. 1981. Biometry. San F r a n c i s c o : W.H. Freeman. Tompa, F.S. 1964. F a c t o r s determining the numbers of song sparrows Melospiza melodia (Wilson) on Mandarte I s l a n d , B.C., Canada. Acta Z o o l . Fenn. , 109, 3-73. Ueda, 1986. A polygamous s o c i a l system of the f a n - t a i l e d warbler C i s t i c o l a j u n c i d i s . Ethology, 73, 43-55. V i l l a g e , A. 1983. The r o l e of n e s t - s i t e a v a i l a b i l i t y and t e r r i t o r i a l behaviour i n l i m i t i n g the breeding d e n s i t y of k e s t r e l s . J . Anim. E c o l . , 52, 635-645. Waser, P.M. 1985. Does co m p e t i t i o n d r i v e d i s p e r s a l ? Ecology, 66, 1 170-1175. Watson, A. 1985. S o c i a l c l a s s , s o c i a l l y - i n d u c e d l o s s , r e cruitment and breeding of red grouse. O e c o l o g i a , 67, 493-498. Weatherhead, P.J. & Boak, K.A. 1986. S i t e i n f i d e l i t y i n song sparrows. Anim. Behav., 34, 1311-1318. Wiley, R.H. 1973. T e r r i t o r i a l i t y and non-random mating i n sage grouse, Centrocercus urophasianus. Anim. Behav. Monogr., 6, 87-161. W i n g f i e l d , J.C. 1985. Short-term changes i n plasma l e v e l s of hormones duri n g establishment and defence of a breeding t e r r i t o r y i n male song sparrows, Melospiza melodia. Horm. Behav., 19, 174-187. Yasukawa, R.C. 1979. T e r r i t o r y establishment i n red-winged b l a c k b i r d s : the importance of a g g r e s s i v e behavior and ex p e r i e n c e . Condor, 81, 258-264. 72 CHAPTER 4. AGE, INTRUSION PRESSURE AND TERRITORY DEFENCE AGAINST FLOATERS IN MALE SONG SPARROWS INTRODUCTION The a b i l i t y to gain and defend t e r r i t o r i e s , mates and other c r u c i a l r esources has a profound i n f l u e n c e on an animal's f i t n e s s (Clutton-Brock et a l . 1982; Howard 1983; Smith 1988). However, the a g g r e s s i v e behaviour of dominant animals sometimes prevents subordinates from access to such resources (reviews i n Brown 1969; Watson & Moss 1970; P a t t e r s o n 1980; Smith & Arcese 1986). In t e r r i t o r i a l s p e c i e s , these subordinates o f t e n become ' f l o a t e r s ' ( a f t e r Brown 1969; Smith 1978) that s u r v i v e i n groups o u t s i d e the main breeding h a b i t a t (e.g. C a r r i c k 1963; Birkhead & Clarkson 1985) or r e s i d e s i n g l y and i n c o n s p i c u o u s l y on the t e r r i t o r i e s of breeders (e.g. Tompa 1964; Smith 1978). F l o a t e r s a r i s e i n breeding p o p u l a t i o n s when s u i t a b l e h a b i t a t s become s a t u r a t e d with dominant t e r r i t o r y owners (Brown 1969; Watson & Moss 1970). When f l o a t e r s are a c o n s i s t e n t f e a t u r e of the s o c i a l environment of a s p e c i e s , i t may be u n p r o f i t a b l e f o r owners to t r y to e v i c t them from t h e i r t e r r i t o r i e s (Brown 1964,1969,1982; Davies & Houston 1981,1983a,b). In such cases, owners might t o l e r a t e f l o a t e r s or even r e c r u i t them onto t h e i r t e r r i t o r i e s as ' s a t e l l i t e s ' (e.g. Van R h i j n 1973; Davies & Houston 1981) or 'helpers' (e.g. Emlen & Vehrencamp 1983; Woolfenden & 73 F i t z p a t r i c k 1984). Recent s t u d i e s have shown that subordinate s a t e l l i t e s may b e n e f i t t e r r i t o r y owners by a t t r a c t i n g mates (e.g. Van R h i j n 1973; L i l l 1976) or by defending the t e r r i t o r y (e.g. Davies & Houston 1981,1983b; Wirtz 1981). Helpers may a i d breeding p a i r s i n v a r i o u s ways (see Emlen & Vehrencamp 1983; Wolfenden & F i t z p a t r i c k 1984). In c o n t r a s t , f l o a t e r s that do not become h e l p e r s or s a t e l l i t e s do not a s s i s t breeders i n r a i s i n g young or defending the breeding area. These f l o a t e r s are, however, o f t e n capable of breeding when the o p p o r t u n i t y to gain a t e r r i t o r y a r i s e s (e.g. Smith 1978; Hannon 1983; V i l l a g e 1983). S e v e r a l s t u d i e s suggest that f l o a t e r s might hinder t e r r i t o r y owners (e.g. Tompa 1964; Birkhead & Clarkson 1985). For example, t e r r i t o r y owners o f t e n spend time and energy chasing f l o a t e r s from t h e i r t e r r i t o r i e s (Myers et a l . 1979,1981; Hixon 1980 and r e f e r e n c e s t h e r e i n ) , and owners may reduce the s i z e of t h e i r t e r r i t o r i e s at high l e v e l s of i n t r u s i o n p ressure (Norton et a l . 1981). F l o a t e r s can impose a f u r t h e r r e p r o d u c t i v e c o s t on owners i f they f e r t i l i z e females that r e s i d e on or v i s i t the owner's t e r r i t o r y (Howard 1978; Wirtz 1981). In b i r d s , f l o a t i n g females c o u l d p a r a s i t i z e the nests of t e r r i t o r y owners (Gowaty 1983). Moreover, f l o a t e r s sometimes a c q u i r e t e r r i t o r i e s by e v i c t i n g t e r r i t o r y owners or by usurping p a r t s of t h e i r t e r r i t o r i e s (e.g. Tompa 1964; Birkhead & Clarkson 1985). In such cases, f l o a t e r s c o u l d be a s e r i o u s t h r e a t to t e r r i t o r y owners, and t h e i r r e l a t i o n s h i p with owners 74 c o u l d t h e r e f o r e be important i n a f f e c t i n g s o c i a l o r g a n i z a t i o n . S e v e r a l f a c t o r s might a f f e c t both i n t r u s i o n pressure by f l o a t e r s and an owner's a b i l i t y t o defend a g a i n s t them. For example, f l o a t e r s might c o n c e n t r a t e i n high q u a l i t y h a b i t a t s to await the deaths of breeders (Smith 1978), or they might i n t r u d e s e l e c t i v e l y on t e r r i t o r i e s with f e r t i l e females to seek c o p u l a t i o n s (Ford 1983). A l t e r n a t e l y , f l o a t e r s might t r y to gain t e r r i t o r i e s by c h a l l e n g i n g owners that seem p a r t i c u l a r l y v u l n e r a b l e to takeover. T e r r i t o r y owners probably a l s o vary i n t h e i r a b i l i t y to e v i c t f l o a t e r s . In the polygynous red deer (Cervus elaphus) f o r example, middle-aged males defended t h e i r harems a g a i n s t c h a l l e n g e r s most s u c c e s s f u l l y (Clutton-Brock et a l . 1982). Some evidence suggests that i n b i r d s , t e r r i t o r y defence a b i l i t y i n c r e a s e s with age and experience (Yasukawa 1979; Ewald & Rohwer 1980; DeVos 1983). Robinson (1986), however, found that the a b i l i t y of polygynous male caciques (Cacicus c e l a ) , to defend f e r t i l e females d e c l i n e d with age. Dhondt (1985) suggested that t e r r i t o r i a l defence a b i l i t y a l s o d e c l i n e d among o l d male great t i t s (Parus major), but there have been no r e p o r t s on the f a c t o r s that a f f e c t an owner's a b i l i t y to defend a t e r r i t o r y a g a i n s t f l o a t e r s . Because of t h e i r g e n e r a l l y inconspicuous behaviour, f l o a t e r s are d i f f i c u l t to study, and t h e i r r e l a t i o n s with t e r r i t o r y owners remain p o o r l y understood. I s t u d i e d the behaviour of t e r r i t o r i a l and f l o a t i n g male song sparrows on 75 Mandarte I s l a n d , where the a c t i v i t i e s of f l o a t e r s can be observed i n u n u s u a l l y complete d e t a i l . In t h i s chapter, I s p e c i f i c a l l y address three q u e s t i o n s about the e f f e c t of i n t r u s i o n s by f l o a t e r s on t e r r i t o r i a l males. F i r s t , d i d the d e n s i t y of f l o a t e r s i n p a r t i c u l a r areas a f f e c t i n t r u s i o n p ressure? Second, d i d f l o a t e r s s e l e c t i v e l y i n t r u d e on the t e r r i t o r i e s of males: (1) with f e r t i l e females, (2) with l a r g e t e r r i t o r i e s , or (3) i n a p a r t i c u l a r a g e - c l a s s ? T h i r d , d i d i n t r u s i o n s by f l o a t e r s a f f e c t the r e p r o d u c t i v e success of t e r r i t o r i a l males or the number of days that they defended a t e r r i t o r y ? F u r t h e r , because i n t r u s i o n p r essure was recorded over s u c c e s s i v e years, I was able to separate the e f f e c t s of males and t h e i r t e r r i t o r i e s i n determining i n t r u s i o n p r e s s u r e . A d d i t i o n a l o b s e r v a t i o n s on the s o c i a l o r g a n i z a t i o n and l i f e t i m e r e p r o d u c t i v e success of f l o a t e r s are given elsewhere (Chap 3; Smith & Arcese ms). I now g i v e a b r i e f d e s c r i p t i o n of the r e l a t i o n s between f l o a t e r s and t e r r i t o r y owners i n the study p o p u l a t i o n . R e l a t i o n s Between Owners and F l o a t e r s on Mandarte I s l a n d On Mandarte, song sparrows are p r i m a r i l y monogamous, u s u a l l y l i v e from 1-4 y e a r s , and are s e x u a l l y mature at 1 year of age (Smith 1988). Possession of a t e r r i t o r y i n the year a f t e r h a t c h i n g has an important e f f e c t on l i f e t i m e r e p r o d u c t i v e success (Smith 1988; Smith & Arcese ms). Some males f a i l to a c q u i r e t e r r i t o r i e s before the onset of breeding each year. 76 These f l o a t e r s l i v e s i n g l y and s e c r e t i v e l y on the t e r r i t o r i e s of breeders. There they await the deaths of t e r r i t o r y h o l d e r s , or p r e c i p i t a t e a takeover by d i r e c t l y c h a l l e n g i n g them. F l o a t e r s are e v i c t e d by t e r r i t o r i a l males on d i s c o v e r y , but f l o a t e r s t h at do not s i n g or perch prominently i n t e r r i t o r i e s may go undetected (personal o b s e r v a t i o n ) . Competition between males f o r t e r r i t o r i e s i s e a s i l y observed; i t occurs when they f i g h t over vacant areas, when they t r y to s e t t l e between e s t a b l i s h e d t e r r i t o r i e s , or when f l o a t e r s t r y to overthrow owners. The number of f l o a t e r s i n c r e a s e s with p o p u l a t i o n d e n s i t y , and f l o a t e r s sometimes comprise over 25% of the male p o p u l a t i o n (Smith & Arcese 1987, ms). RESULTS Only r a r e l y were f l o a t e r s seen by observers when they d i d not appear to have been d e t e c t e d by t e r r i t o r y owners. F l o a t e r s t y p i c a l l y were extremely s e c r e t i v e and e i t h e r flew or remained m o t i o n l e s s when owners were near. T h i s response appeared to depend upon being d e t e c t e d . T e r r i t o r y owners that d e t e c t e d an i n t r u d e r u s u a l l y cocked t h e i r heads, craned t h e i r necks, v o c a l i z e d (song, tchunk, puff-sing-wave; Nice 1943) and then chased the i n t r u d e r . Owners t r i e d t o e v i c t f l o a t e r s whenever they were d e t e c t e d . I n t r u s i o n p r e s s u r e , as measured here, probably r e p r e s e n t s the t e r r i t o r y owners p e r c e i v e d r a t e of i n t r u s i o n by f l o a t e r s , r a t h e r than the a c t u a l r a t e , s i n c e f l o a t e r s on Mandarte r e s i d e on defended t e r r i t o r i e s , but are 77 only d e t e c t e d o c c a s i o n a l l y . Temporal P a t t e r n s of I n t r u s i o n At the beginning of the study p e r i o d i n 1984, f l o a t e r s comprised 24 of 86 (28%) males present on Mandarte. There were s l i g h t l y fewer f l o a t e r s i n 1985 (16 of 94, 17%). Standard i n t r u s i o n p r e s s u r e (SIP) was not s i g n i f i c a n t l y c o r r e l a t e d with the time of day i n e i t h e r year (1984: r=-0.085, N=158, P>0.10; and 1985: r=-0.095, N=420, P>0.05). SIP d e c l i n e d s l i g h t l y as the season progressed i n 1984 (r=-0.130, N=158, P<0.10), and i n 1985 (r=-0.l97, N=420, P<0.01), probably because many f l o a t e r s gained t e r r i t o r i e s from A p r i l - A u g u s t (Chap. 3 ) . I found no evidence of n o n - l i n e a r i t y i n these r e l a t i o n s h i p s (but see Peek 1971). F l o a t e r D e n s i t y , T e r r i t o r y S i z e and Female F e r t i l i t y Male t e r r i t o r i e s were overlapped by from 0-10 f l o a t e r home ranges (mean=5.4), but there was no c o r r e l a t i o n between the number of f l o a t e r ranges that overlapped a t e r r i t o r y and e i t h e r SIP or tenure i n e i t h e r year ( a l l P>0.50). Thus, the d e n s i t y of f l o a t e r s i n an area d i d not a f f e c t the i n t r u s i o n p r essure recorded on p a r t i c u l a r t e r r i t o r i e s . S i m i l a r l y , n e i t h e r SIP nor tenure were r e l a t e d to t e r r i t o r y s i z e , nor to whether males had mates ( a l l P>0.10). F u r t h e r , SIP on t e r r i t o r i e s where females were f e r t i l e (the n e s t - b u i l d i n g and l a y i n g p e r i o d s ) was not 78 higher than on t e r r i t o r i e s where females were not f e r t i l e (P>0.40, 1985 d a t a ) , even though f e r t i l e females use conspicuous v o c a l i z a t i o n s and postures to s o l i c i t c o p u l a t i o n s (Nice 1943, own o b s e r v a t i o n s ) . Male Age and T e r r i t o r y Tenure Male age was r e l a t e d to standard i n t r u s i o n pressure and to tenure ( F i g . 4.1), and the form of the r e l a t i o n s h i p s was c o n s i s t e n t a c r o s s y e a r s . These r e l a t i o n s h i p s were n o n - l i n e a r ( F i g . 4.1); 1-year-old and 4-year-old males s u f f e r e d higher SIP, and h e l d t e r r i t o r i e s f o r s h o r t e r p e r i o d s , than d i d 2- and 3-year-olds. Note, however, that while 4-year-old b i r d s were i n t r u d e d on most i n 1984, 1-year-olds were i n t r u d e d on most i n 1985. As expected from these r e l a t i o n s h i p s , tenure and SIP were i n v e r s e l y r e l a t e d i n 1984 and i n 1985 (r=-0.43, N=57, and r=-0.46, N=75, r e s p e c t i v e l y , both P<0.005). Table 4.1 g i v e s the mean and maximum r a t e s of i n t r u s i o n observed f o r each age-c l a s s and year. If the p a t t e r n s of i n t r u s i o n and l o s s of t e r r i t o r y r e s u l t e d p r i m a r i l y from d i f f e r e n c e s i n age, then males should experience lower i n t r u s i o n pressure as 2-year-olds than as 1-year-olds. They should a l s o experience higher i n t r u s i o n p r essure a f t e r age 3 ( F i g . 4.1). Table 4.2 shows that f o r 41 b i r d s observed i n both years, t h i s p a t t e r n was observed, but the d i f f e r e n c e was s t a t i s t i c a l l y s i g n i f i c a n t only f o r the comparison of 1- and 2-1 2 3 4+ AGE OF OWNER Figure 4.1. The relationships between age of the t e r r i t o r i a l male and standard intrusion pressure ( F 2 5 4=10.1, P«.0.001 and F 2 7 2=9.5, P<0.001; 1984 and 1985), and between age and t e r r i t o r y tenure (F„ 7,.=3.2, P<0.05 and F, 77=10.2, P<0.001; 1984 and 1985). Table 4.1. Mean (SE) and maximum r a t e s of i n t r u s i o n per hour recorded on t e r r i t o r i e s of d i f f e r e n t aged males i n two y e a r s . Age C l a s s (years) Year 1 2 3 4+ 1 984 Mean(SE) 2.5(0.6) 0.9(0.2) 2.3(0.6) 11.7(7.7) Maximum 8.6 3.9 6.0 42.0 N 22 20 10 5 1985 Mean(SE) 3.6(0.8) 1.3(0.4) 1.9(0.8) 2.1(0.4) Maximum 14.4 8.0 15.0 5.14 N 18 26 18 13 81 y e a r - o l d s . The f a i l u r e to observe the p r e d i c t e d i n c r e a s e beyond age 3 i s p a r t l y e x p l a i n e d by the lower r a t e of i n t r u s i o n a g a i n s t 4-year-olds i n 1985 ( F i g . 4.1). T e r r i t o r y Q u a l i t y and Male A b i l i t y V a r i a t i o n i n i n t r u s i o n pressure c o u l d a l s o r e s u l t from d i f f e r e n c e s among t e r r i t o r i e s i n h a b i t a t s t r u c t u r e or i n t h e i r q u a l i t y as breeding s i t e s . F l o a t e r s might be more or l e s s d e t e c t a b l e i n p a r t i c u l a r h a b i t a t s , or they might c o n c e n t r a t e i n high q u a l i t y t e r r i t o r i e s . I d i d not measure these v a r i a b l e s ; however, I c o u l d t e s t i f SIP v a r i e d p r e d i c t a b l y f o r e i t h e r the same t e r r i t o r y a c r o s s years, or w i t h i n years f o r d i f f e r e n t male owners. To do t h i s , I compared SIP on 49 t e r r i t o r i e s that remained s i m i l a r i n s i z e and shape a c r o s s y e a r s . I c o r r e c t e d s t a t i s t i c a l l y f o r d i f f e r e n c e s i n SIP due to owner age, and these c o r r e c t e d measures were normally d i s t r i b u t e d . I p r e d i c t e d that SIP on these t e r r i t o r i e s should be p o s i t i v e l y c o r r e l a t e d a c r o s s y e a r s , e s p e c i a l l y i n those t h a t changed owners. However, n e i t h e r p r e d i c t i o n was supported by the data ( a l l t e r r i t o r i e s : r=0.20, N=49, P>0.1; those changing ownership: r=-0.44, N=41, P<0.05). I then compared SIP f o r 41 males that were observed in each year, r e g a r d l e s s of the resemblance of t h e i r t e r r i t o r i e s a c r o s s Table 4.2. Changes i n standard i n t r u s i o n p r e ssure f o r 41 males observed i n 2 years. Age C l a s s (years) 1-2 2-3 3+ 0.32* 0.13 0.03 0.09 0.16 0.09 13 17 11 Mean change SE * p<0.01, Wilcoxon matched-pairs signed-ranks t e s t . 83 years, a f t e r a d j u s t i n g f o r t h e i r a g e - c l a s s means. These r e s i d u a l s were normally d i s t r i b u t e d and p o s i t i v e l y c o r r e l a t e d (r=0.37, N=41, P<0.03, 1 - t a i l e d ) , showing that males v a r i e d p r e d i c t a b l y i n SIP a c r o s s years with r e s p e c t to the mean of t h e i r a g e - c l a s s . In c o n t r a s t , when I compared SIP on t e r r i t o r i e s with d i f f e r e n t owners i n the same season I found no c o r r e l a t i o n (Spearman rank=-0.08, N=9, P>0.50). These r e s u l t s s t r o n g l y suggest that i n t r u s i o n pressure i s a c o n s i s t e n t a t t r i b u t e of p a r t i c u l a r males, but not of t e r r i t o r i e s . I n t r u s i o n Pressure and Reproductive Success Males that h e l d t e r r i t o r i e s f o r longer p e r i o d s produced more independent young each season (1984: r=0.26, N=57, P=0.05 and 1985: r=0.35, N=75, P<0.01). Because SIP was n e g a t i v e l y r e l a t e d to tenure i n both years, I expected negative c o r r e l a t i o n s between i n t r u s i o n p r essure and r e p r o d u c t i v e success. These c o r r e l a t i o n s were nega t i v e , but they were not s t a t i s t i c a l l y s i g n i f i c a n t (1984: r=-0.15, N=57, P>0.05 and 1985: r=-0.l9, N=75, P>0.05). The average e f f e c t of f l o a t e r s on the seasonal r e p r o d u c t i v e success of t e r r i t o r y owners was t h e r e f o r e s m a l l . Contests over T e r r i t o r y I observed a l l or part of 89 c o n t e s t s between f l o a t e r s and owners over t e r r i t o r y d u r i n g both study p e r i o d s . C o n t e s t s , as 84 I d e f i n e d them, began with a f l o a t e r s i n g i n g on the owner's t e r r i t o r y . The l a r g e m a j o r i t y of i n t r u s i o n s d i d not i n v o l v e s i n g i n g and were t h e r e f o r e not i n c l u d e d by d e f i n i t i o n . I c l a s s i f i e d c o n t e s t s broadly i n t o s h o r t c o n t e s t s that l a s t e d f o r l e s s than 1 h and long c o n t e s t s that l a s t e d more than 1 h. There were 29 short and 60 long c o n t e s t s and at l e a s t 3 of the long c o n t e s t s l a s t e d up to 2 days. In a l l long and i n s e v e r a l short c o n t e s t s f l o a t e r s returned from chases r e p e a t e d l y to s i n g or perch i n the owner's t e r r i t o r y . Contests ended when the f l o a t e r no longer r e t u r n e d to the t e r r i t o r y a f t e r a chase or when the owner no longer chased the f l o a t e r . The l e n g t h of c o n t e s t s Of the longer c o n t e s t s ( i n c l u d i n g the 3 l a s t i n g longer than a day), 63% (38 of 60) r e s u l t e d i n settlement by a f l o a t e r . No short c o n t e s t s (0 of 29) r e s u l t e d i n se t t l e m e n t . Thus, f l o a t e r s t h a t were able to a v o i d e v i c t i o n f o r longer p e r i o d s were much more l i k e l y to be s u c c e s s f u l (G =32.14, df=1, P<0.001). Of the s u c c e s s f u l settlement attempts, 53% (20 of 38) r e s u l t e d i n p a r t i a l a c q u i s i t i o n of the owner's t e r r i t o r y , and 47% (18 of 38) i n t o t a l e v i c t i o n of the owner. O v e r a l l , 43% (38 of 89) of c o n t e s t s r e s u l t e d i n settlement by a f l o a t e r , but t h i s estimate was probably i n f l a t e d by the f a c t t h a t long c o n t e s t s were more l i k e l y to have been observed. 85 The number of c h a l l e n g e r s I observed from 2-4 f l o a t e r s on the t e r r i t o r y d u r i n g 9 u n u s u a l l y i n t e n s e c o n t e s t s . More than one f l o a t e r o f t e n sang d u r i n g these c o n t e s t s , and defending males seemed to spend much more time c h a s i n g than each f l o a t e r spent evading. In a l l 9 of these c o n t e s t s the owner l o s t a l l or a major part of the t e r r i t o r y , compared to 29 l o s s e s i n 80 (36%) c o n t e s t s when only one f l o a t e r was observed (P=0.0005, F i s h e r ' s exact t e s t ) . The p r o b a b i l i t y of s u c c e s s f u l settlement thus i n c r e a s e d when more than one f l o a t e r was p r e s e n t . In one case, however, the usurper was not one of the f l o a t e r s i n i t i a l l y observed i n t r u d i n g . Neighbours p a r t i c i p a t e d i n c o n t e s t s when f l o a t e r s perched i n or flew through t h e i r t e r r i t o r i e s , otherwise they d i d not i n t e r f e r e (but see Tompa 1964). The replacement of owners by f l o a t e r s o f t e n seemed to take p l a c e o v e r n i g h t , with the p r e v i o u s owner simply not appearing on the t e r r i t o r y i n the morning. In the 18 e v i c t i o n s that I observed p a r t o f , only 4 former owners were ever trapped or r e s i g h t e d . Two of these b i r d s a c q u i r e d a t e r r i t o r y the f o l l o w i n g year; one adjacent and the other 3 t e r r i t o r i e s d i s t a n t from those l o s t . I t h e r e f o r e suspect that many former owners d i e d of exposure, s t a r v a t i o n , or i n j u r y , soon a f t e r e v i c t i o n , r a t h e r than e n t e r i n g the f l o a t e r p o p u l a t i o n (see a l s o Watson 1985). High m o r t a l i t y , r e s u l t i n g from i n j u r i e s a c q u i r e d while l o s i n g c o n t e s t s , i s suspected i n s e v e r a l ungulate s p e c i e s 86 (Clutton-Brook et a l . 1982 and r e f e r e n c e s t h e r e i n ) . Case h i s t o r i e s On 4 oc c a s i o n s from 1982-5, I observed the a c t u a l t r a n s i t i o n of ownership, as i n d i c a t e d by the p r e v i o u s owner f a i l i n g to chase a s i n g i n g i n t r u d e r . In one of these cases, the owner s u f f e r e d a compound f r a c t u r e of i t s t i b i o t a r s u s when i t s foot was caught i n a shrub while pursuing a f l o a t e r . T h i s owner con t i n u e d to s i n g from low i n a shrub, but i t no longer chased the f l o a t e r which t h e r e a f t e r sang throughout the t e r r i t o r y . The former owner was not observed a g a i n . In a s i m i l a r case, I observed an owner u n s u c c e s s f u l l y attempting to e v i c t a p e r s i s t e n t i n t r u d e r . T h i s b i r d gained -control of the t e r r i t o r y soon a f t e r my a r r i v a l . T h i s deposed owner had a l s o r e c e n t l y s u f f e r e d a s i m i l a r compound f r a c t u r e , though I d i d not witness i t s occurrence. In t h i s case, the former owner s u r v i v e d to the f o l l o w i n g breeding season as a f l o a t e r and s u c c e s s f u l l y regained a t e r r i t o r y adjacent to i t s p r e v i o u s one. In a t h i r d case, I witnessed a male with a swollen and u s e l e s s foot being e v i c t e d from h i s t e r r i t o r y by a f l o a t e r that had been r e p e a t e d l y observed attempting to s i n g on the t e r r i t o r y on previous days. The usurper had taken c o n t r o l of most of the t e r r i t o r y by the time of my a r r i v a l on the day of takeover. The deposed male was e v e n t u a l l y chased from the t e r r i t o r y and was not seen a g a i n . In the f i n a l case, the takeover was 87 l a r g e l y completed by the time of my a r r i v a l . I observed the former owner s i n g i n g q u i e t l y low i n a shrub, while two f l o a t e r s c o n t e s t e d the t e r r i t o r y . The former owner remained at h i s perch with h i s f e a t h e r s e r e c t u n t i l n i g h t f a l l and he was not seen t h e r e a f t e r . Causes of T e r r i t o r y Loss The r o l e of handicaps The case h i s t o r i e s suggest that handicaps ( i . e . a m i s s i n g or u s e l e s s f o o t , l e g or eye) p l a y a r o l e i n t e r r i t o r y l o s s . However, many of the turnovers summarized above o c c u r r e d i n the absence of an obvious handicap. F u r t h e r , males o c c a s i o n a l l y r e t a i n t e r r i t o r i e s even when they possess apparent handicaps (personal o b s e r v a t i o n ) . My o b s e r v a t i o n s on the occurrence of handicaps d u r i n g t h i s study show that only 12.5% (1 of 8) of the males with handicaps h e l d t h e i r t e r r i t o r y f o r the remainder of the breeding season, while 79% (109 of 138) of the v i s i b l y h e a l t h y males d i d so (P=0.0005, F i s h e r ' s exact t e s t ) . Thus, t e r r i t o r y l o s s i s sometimes a s s o c i a t e d with a v i s i b l e handicap, and these handicaps hinder t e r r i t o r i a l males. Of a l l owners that disappeared d u r i n g the study p e r i o d s , however, most (29 of 39; 74.4%) appeared to be h e a l t h y p r i o r to the l o s s of t h e i r t e r r i t o r y . I do not know what p r o p o r t i o n of handicaps r e s u l t from 88 c o n f l i c t with f l o a t e r s , but my r e s u l t s do show that 46% (18 of 39) of disappearances of t e r r i t o r y owners d u r i n g the study p e r i o d s were a s s o c i a t e d with c o n t e s t s with f l o a t e r s (see a l s o Chap. 3). F l o a t e r s may t h e r e f o r e account f o r a l a r g e p r o p o r t i o n of a l l disappearances of t e r r i t o r i a l males. Handicaps and age Of the 8 handicapped males, one was 2-years, 5 were 3-ye a r s , and 2 were 4- or more y e a r s - o l d . No t e r r i t o r i a l 1-year-o l d s were observed with handicaps. P o o l i n g adjacent age-c l a s s e s , I found that o l d e r b i r d s were more l i k e l y to possess handicaps (1 of 90 1- and 2-year-olds versus 7 of 64 3- and 4-y e a r - o l d s ; P=0.019, F i s h e r ' s exact t e s t ) . The weights of owners, e v i c t e d males and f l o a t e r s Many males that l o s t t h e i r t e r r i t o r i e s to f l o a t e r s d i d not possess v i s i b l e handicaps. However, these a p p a r e n t l y healthy males might have been i n poor c o n d i t i o n . I captured and weighed 6 t e r r i t o r i a l males before and from 1-6 months a f t e r they l o s t t h e i r t e r r i t o r i e s to f l o a t e r s . These males were s i g n i f i c a n t l y h e a v i e r before t e r r i t o r y l o s s than a f t e r (mean(se): 25.73(0.30)g versus 24 .10(0.59)g; t = - 3 . l 6 , df = 5, P=0.025). They were a l s o s i g n i f i c a n t l y l i g h t e r a f t e r t e r r i t o r y l o s s than 50 males that r e t a i n e d t h e i r t e r r i t o r i e s (24.10(0.59)g versus 25.53(0.17)g; t=3.11, df=54, P<0.005). In c o n t r a s t , 10 89 f l o a t e r s that had not yet h e l d a t e r r i t o r y were i d e n t i c a l i n weight (25.47(0.53)g) to the 50 t e r r i t o r y h o l d e r s (25.53(0.17)g). Thus, weight l o s s was p e c u l i a r to males that l o s t t h e i r t e r r i t o r i e s , r a ther than to t e r r i t o r y ownership per se. The weight l o s s of deposed males i s not l i k e l y to have been a d i r e c t r e s u l t of the c o n t e s t f o r the t e r r i t o r y , as a l l 6 males were weighed at l e a s t 1 month a f t e r being e v i c t e d . F u r t h e r , only one of these males showed s i g n s of a handicap (a w e l l -healed l e g f r a c t u r e ) that c o u l d have reduced f o r a g i n g e f f i c i e n c y . L a s t l y , n e a r l y a l l c a p t u r e s were made i n the l a t e summer and were c o r r e c t e d f o r time of day (Dhondt & Smith 1980). I t i s t h e r e f o r e u n l i k e l y that seasonal or d a i l y v a r i a t i o n s i n weight a f f e c t e d the r e s u l t s . I suggest that weight l o s s r e f l e c t s the poor c o n d i t i o n of males that have been e v i c t e d by f l o a t e r s . Does A b i l i t y to Defend a T e r r i t o r y Change with Age? One hypothesis to account f o r the r e l a t i o n s h i p s that I observed between i n t r u s i o n p r e s s u r e , age and t e r r i t o r y tenure i s that the a b i l i t y to defend t e r r i t o r i e s depends p a r t l y on age, and that f l o a t e r s d e t e c t and t r y to overthrow the males that are l e a s t a b l e to defend themselves (the a b i l i t y h y p o t h e s i s ) . A c r i t i c a l p r e d i c t i o n of the a b i l i t y hypothesis i s that the p r o b a b i l i t y of t e r r i t o r y l o s s per i n t r u s i o n i s r e l a t e d to age i n 90 a n o n - l i n e a r way. Thus, 1- and 4-year-old males should have l o s t t h e i r t e r r i t o r i e s more o f t e n than expected by d i f f e r e n c e s i n i n t r u s i o n pressure alone. To t e s t t h i s p r e d i c t i o n , I compared observed values of tenure with those p r e d i c t e d f o r each a g e - c l a s s by the r e g r e s s i o n of tenure on SIP. In both y e a r s , 1- and 4-year-olds h e l d t e r r i t o r i e s f o r s h o r t e r , and 2- and 3-y e a r - o l d s h e l d t e r r i t o r i e s f o r longer, than expected by d i f f e r e n c e s i n i n t r u s i o n pressure (G =6.70, df=3, P<0.05; and G =7.32, df=3, P<0.05, 1 - t a i l e d t e s t s , 1984 and 1985, r e s p e c t i v e l y ) . The pooled r e s u l t was a l s o s t a t i s t i c a l l y s i g n i f i c a n t {G =9.71, df=3, P<0.025). DISCUSSION My r e s u l t s suggest a d r a m a t i c a l l y d i f f e r e n t r e l a t i o n s h i p between f l o a t e r s and t e r r i t o r y owners than has yet been d e s c r i b e d (e.g. Orians 1961; C a r r i c k 1963; D e l i u s 1965; Smith 1978; Watson 1985). F l o a t e r s most o f t e n i n t r u d e d c o n s p i c u o u s l y on the t e r r i t o r i e s of the males that were the l e a s t a b l e to defend a g a i n s t them. These were c o n s i s t e n t l y the young, o l d and handicapped males i n the p o p u l a t i o n . In c o n t r a s t , i n t r u s i o n p ressure was u n r e l a t e d to t e r r i t o r y s i z e , the d e n s i t y of f l o a t e r s i n a p a r t i c u l a r area, to the f e r t i l i t y of t e r r i t o r i a l females or even to whether a female was present on a t e r r i t o r y . Contrary to the idea that f l o a t e r s sometimes a i d t e r r i t o r y owners (e.g. W i r t z 1981; Davies & Houston 1983b), on Mandarte I s l a n d at l e a s t some f l o a t e r s are a t h r e a t to t e r r i t o r y 91 owners that are not able to v i g o r o u s l y defend t h e i r t e r r i t o r i e s . In the d i s c u s s i o n f o l l o w i n g , I c o n t r a s t my r e s u l t s with those of other s t u d i e s and I c o n s i d e r f u r t h e r the r e l a t i o n s h i p between age and the a b i l i t y to defend t e r r i t o r i e s . Temporal P a t t e r n s of I n t r u s i o n I n t r u s i o n s by f l o a t i n g male song sparrows d i d not vary p r e d i c t a b l y with the time of day. In c o n t r a s t , K a c e l n i c k & Krebs (1983) found that the c o l o n i z a t i o n r a t e of empty t e r r i t o r i e s by p a i r s of great t i t s was h i g h e s t i n the morning. They suggested that the dawn chorus of t e r r i t o r i a l t i t s might be a response to d a i l y v a r i a t i o n i n i n t r u s i o n p r e s s u r e . These c o l o n i z i n g b i r d s , however, were probably r e s i d e n t s from adjacent, p o o r - q u a l i t y h a b i t a t s r a t h e r than being f l o a t e r s (Krebs 1971; K a c e l n i c k & Krebs 1983). T h i s study c o n s i d e r e d only i n t r u s i o n s by n o n - t e r r i t o r i a l males that r e s i d e d on the t e r r i t o r i e s of breeders, but were only o c c a s i o n a l l y d e t e c t e d by them. D a i l y v a r i a t i o n i n the song r a t e of song sparrows (Nice 1943) i s t h e r e f o r e probably not a response to i n t r u s i o n s by f l o a t e r s . Peek (1971) noted the h i g h e s t r a t e of i n t r u s i o n by f l o a t i n g male red-winged b l a c k b i r d s ( A g e l a i u s p h o e n i c i u s ) d u r i n g A p r i l . I n t r u s i o n r a t e decreased i n May, and t e r r i t o r i a l males that were removed a f t e r the peak of breeding were not r e p l a c e d by f l o a t e r s (see a l s o O r i a n s 1961). On Mandarte, i n t r u s i o n pressure 92 d e c l i n e d s l i g h t l y from March to J u l y , probably because some f l o a t e r s s e t t l e d during t h i s p e r i o d . However, the replacement of owners by f l o a t e r s continued throughout the year on Mandarte (Chap. 3, F i g . 3.2), as i t d i d d u r i n g a study of a r e s i d e n t p o p u l a t i o n of the r u f o u s - c o l l a r e d sparrow ( Z o n o t r i c h i a c a p e n s i s ; Smith 1978). Seasonal v a r i a t i o n i n i n t r u s i o n pressure i s probably l e s s i n s p e c i e s that are t e r r i t o r i a l year-round versus those that are only s e a s o n a l l y t e r r i t o r i a l . In s p e c i e s that are t e r r i t o r i a l year-round, f l o a t e r s may have to c h a l l e n g e owners o p p o r t u n i s t i c a l l y to assure a c q u i s i t i o n of a t e r r i t o r y . In s e a s o n a l l y t e r r i t o r i a l s p e c i e s there may be a s m a l l e r advantage to a c q u i r i n g a t e r r i t o r y once the p r o b a b i l i t y of breeding i s low (Yasukawa 1979; Chap. 3). Male A b i l i t y , T e r r i t o r y Q u a l i t y and I n t r u s i o n Pressure Males experienced s i m i l a r i n t r u s i o n pressure a c r o s s years with respect to the means f o r t h e i r a g e - c l a s s e s . In c o n t r a s t , i n t r u s i o n p r e s s u r e was not c o n s i s t e n t on the same t e r r i t o r y between years or f o r d i f f e r e n t owners of the same t e r r i t o r y w i t h i n y e a r s . V a r i a t i o n i n i n t r u s i o n pressure was t h e r e f o r e u n l i k e l y to have r e s u l t e d from d i f f e r e n c e s i n h a b i t a t s t r u c t u r e or t e r r i t o r y q u a l i t y , but i t may r e f l e c t the d e f e n s i v e a b i l i t y of i n d i v i d u a l males. I n t r u s i o n pressure was a l s o u n r e l a t e d to the number of f l o a t e r home ranges that overlapped a t e r r i t o r y . T h i s shows that f l o a t e r d e n s i t y d i d not a f f e c t i n t r u s i o n pressure and i t suggests that f l o a t e r s s e l e c t and c h a l l e n g e 93 p a r t i c u l a r males w i t h i n t h e i r home range. F l o a t e r s c o u l d do t h i s by c o n c e n t r a t i n g t h e i r time i n a t e r r i t o r y and by making themselves conspicuous t h e r e . Conspicuous i n t r u s i o n , f o l l o wed by the i n e v i t a b l e chase, might enable f l o a t e r s to assess and monitor the vigour of t e r r i t o r i a l males. A s t r a t e g y of t h i s s o r t suggests an 'honest' system of assessment (Zahavi 1977), s i n c e i t would be d i f f i c u l t f o r owners to conceal weaknesses in t h e i r d e f e n s i v e a b i l i t y . Males that were e v i c t e d from t h e i r t e r r i t o r i e s by f l o a t e r s o f t e n disappeared, and they o c c a s i o n a l l y s u f f e r e d from v i s i b l e handicaps that probably c o n t r i b u t e d to t h e i r e v i c t i o n . Males f r e q u e n t l y disappeared or were e v i c t e d , however, when they possessed no v i s i b l e handicap. The l o s s i n weight of males that became f l o a t e r s a f t e r being e v i c t e d suggests that these a p p a r e n t l y h e a l t h y males were a l s o i n poor p h y s i c a l c o n d i t i o n . Robinson (1986) a l s o found t h a t , among male c a c i q u e s , weight l o s s was a s s o c i a t e d with decreased dominance and a b i l i t y to defend f e r t i l e females. I n t r u s i o n Pressure and Reproductive Success I n t r u s i o n p ressure was n e g a t i v e l y c o r r e l a t e d to tenure, and tenure was p o s i t i v e l y c o r r e l a t e d with the number of young r a i s e d to independence i n both y e a r s . As expected, the r e l a t i o n s h i p s between i n t r u s i o n p r essure and seasonal r e p r o d u c t i v e success were negative i n each year, but they were not s t a t i s t i c a l l y 94 s i g n i f i c a n t . Because few b i r d s breed s u c c e s s f u l l y a f t e r being e v i c t e d by f l o a t e r s , however, f l o a t e r s have a p o t e n t i a l l y d e t r i m e n t a l e f f e c t on the l i f e t i m e r e p r o d u c t i v e success of i n d i v i d u a l males. The Role of F l o a t e r s i n T e r r i t o r y Turnovers Smith (1978; p e r s o n a l communication) found that f l o a t e r s i n a p o p u l a t i o n of r u f o u s - c o l l a r e d sparrows d i d not c h a l l e n g e t e r r i t o r y owners, and i n s t e a d r e p l a c e d owners only a f t e r t h e i r presumed death. While some replacements were undoubtably of t h i s s o r t on Mandarte, many c l e a r l y were not. At l e a s t 46% of a l l disappearances recorded d u r i n g the study p e r i o d s were a s s o c i a t e d with a p r o t r a c t e d c h a l l e n g e by one or more f l o a t e r s . Birkhead & C l a r k s o n (1985) a l s o r e p o r t e d that n o n - t e r r i t o r i a l magpies ( P i c a p i c a ) c h a l l e n g e d t e r r i t o r y owners by i n t r u d i n g c o n s p i c u o u s l y on t h e i r t e r r i t o r i e s , and that i n t r u d e r s o c c a s i o n a l l y (4% of 'gatherings') succeeded i n a c q u i r i n g the t e r r i t o r y or a p o r t i o n of i t . However, many of the ga t h e r i n g s t h a t they observed (38%) l a s t e d l e s s than 5 minutes. In the 3 c o n t e s t s that i n v o l v e d s e r i o u s f i g h t s between owners and i n t r u d e r s , the i n t r u d e r s e v i c t e d the owners (Birkhead & C l a r k s o n 1985). These r e s u l t s suggest that f l o a t e r s may account f o r a s u b s t a n t i a l p r o p o r t i o n of turn o v e r s i n t e r r i t o r y ownership i n some s p e c i e s , e s p e c i a l l y those where f l o a t e r s are not t o l e r a t e d on t e r r i t o r i e s as s a t e l l i t e s or h e l p e r s . T h i s c o u l d a l s o be tru e even i n p o p u l a t i o n s where the a c t i v i t i e s of f l o a t e r s have 95 yet to be s t u d i e d . The T a c t i c s of F l o a t e r s Subordinate i n d i v i d u a l s might pursue one of v a r i o u s s t r a t e g i e s once they have been prevented from s e t t l i n g i n s u i t a b l e breeding h a b i t a t s by t e r r i t o r y owners (see I n t r o d u c t i o n ) . On Mandarte, u n s e t t l e d b i r d s remained s e c r e t i v e l y i n the main breeding h a b i t a t as f l o a t e r s , and many f l o a t e r s gained t e r r i t o r i e s by c h a l l e n g i n g owners t h a t were s u s c e p t i b l e to e v i c t i o n or by r e p l a c i n g owners that disappeared. The e v i c t i o n of owners was most l i k e l y when more than one f l o a t e r was observed d u r i n g a c o n t e s t . In one case, however, replacement was by a f l o a t e r other than one of the o r i g i n a l i n t r u d e r s . F u r t h e r , f l o a t e r s that maintained t h e i r c h a l l e n g e s to owners longer by re p e a t e d l y r e t u r n i n g to s i n g on c o n t e s t e d t e r r i t o r i e s were more l i k e l y to s e t t l e . Two other s t u d i e s of b i r d s have a l s o i d e n t i f i e d f a c t o r s that may a f f e c t the s e v e r i t y of c o n t e s t s over t e r r i t o r i e s . Ewald (1985) found that staged c o n t e s t s over feeders between non-breeding hummingbirds ( T r o c h i l i d a e ) l a s t e d longer when the c o n t e s t a n t s were most s i m i l a r i n age and i n the l e v e l of reward they had p r e v i o u s l y r e c e i v e d . He suggested that hummingbirds adj u s t the energy expenditure i n agg r e s s i o n to t h e i r expected energy gain from winning a c o n t e s t . Krebs (1982) showed that male great t i t s were more l i k e l y to l o s e t h e i r t e r r i t o r y the 96 longer they were h e l d i n c a p t i v i t y , and that replacement males fought these r e l e a s e d owners harder as t h e i r own r e s i d e n c e time i n c r e a s e d . He proposed that the c o s t of t e r r i t o r y establishment to a replacement decreased with r e s i d e n c e time, and that t h i s c o s t was a key f a c t o r i n a replacement's d e c i s i o n to e s c a l a t e or r e t r e a t from the c o n t e s t . The t a c t i c s employed by f l o a t e r s i n c o n t e s t s with owners on Mandarte are probably shaped by two f a c t o r s . F i r s t , f l o a t e r s r e s i d e on only a few t e r r i t o r i e s (ca. 5-10). I t i s t h e r e f o r e l i k e l y that f l o a t e r s are n e a r l y as f a m i l i a r with t e r r i t o r i e s as are owners, and have a s i m i l a r p o t e n t i a l a b i l i t y to e x p l o i t them fo r food. Second, owners r a r e l y r e g a i n a t e r r i t o r y a f t e r l o s i n g a c o n t e s t , and should t h e r e f o r e ' c o n t i n u e to f i g h t to the l i m i t of t h e i r a b i l i t y . My r e s u l t s suggest some p o t e n t i a l l y u s e f u l t a c t i c s : (1) t e s t male vigour by conspicuous i n t r u s i o n , (2) c o n t e s t the t e r r i t o r y i f a weakness i s d e t e c t e d , or (3) j o i n c o n t e s t s i n v o l v i n g other f l o a t e r s . The l e n g t h of c o n t e s t s may be determined by continuous assessment by the f l o a t e r of the owner's a b i l i t y to defend the t e r r i t o r y , and by the l i m i t s of the f l o a t e r ' s own a b i l i t y and a v a i l a b l e energy. Are F l o a t e r s Pursuing an A l t e r n a t e Reproductive Strategy? Ford (1983), Flood (1985) and others have suggested that i n b i r d s i n t r u s i o n s by males are f r e q u e n t l y d i r e c t e d towards f e r t i l e females. Males that i n t r u d e p r i m a r i l y to c o p u l a t e with 97 r e c e p t i v e females c o u l d be pursuing an a l t e r n a t e r e p r o d u c t i v e s t r a t e g y to t e r r i t o r y defence (Howard 1978; Dunbar 1982). I found no evidence to support t h i s f o r n o n - t e r r i t o r i a l male song sparrows. I n t r u s i o n s were as common on t e r r i t o r i e s without f e r t i l e ( l a y i n g ) females as they were when females were f e r t i l e . I n t r u s i o n p r essure was a l s o s i m i l a r on t e r r i t o r i e s with mated or unmated males. B u i t r o n (1983) found that 'monogamous' t e r r i t o r i a l female magpies were v i s i t e d by e x t r a - p a i r males more o f t e n when they were f e r t i l e . These v i s i t i n g males, however, were n e a r l y always experienced t e r r i t o r y h o l d e r s from up to 2.5 km away, rat h e r than being n o n - t e r r i t o r i a l males. The 3 e x t r a - p a i r c o p u l a t i o n s that I have observed a l s o i n v o l v e d neighbour t e r r i t o r y owners. I have never observed female song sparrows perform s o l i c i t a t i o n d i s p l a y s i n the presence of f l o a t e r s , nor have I observed f l o a t e r s to approach t e r r i t o r i a l females. I t i s p o s s i b l e t h a t ' f l o a t e r males i n t r u d e s e c r e t i v e l y when seeking c o p u l a t i o n s , but do so c o n s p i c u o u s l y only to c h a l l e n g e t e r r i t o r i a l males. However, t e r r i t o r i a l males o f t e n f o l l o w t h e i r mates c l o s e l y while they are l a y i n g and i t seems u n l i k e l y that f l o a t e r s would go undetected. These o b s e r v a t i o n s suggest that f l o a t e r male song sparrows do not i n t r u d e p r i m a r i l y to c o p u l a t e with t e r r i t o r i a l females. 98 Age, Tenure and I n t r u s i o n Pressure I found s t r i k i n g l y c o n s i s t e n t r e l a t i o n s h i p s between age, i n t r u s i o n p r e s s u r e and tenure d u r i n g 2 years. Tenure and i n t r u s i o n p r e s s u r e c o u l d be n o n - l i n e a r l y r e l a t e d to age f o r two reasons. F i r s t , young b i r d s are i n e x p e r i e n c e d i n s k i l l s r e l a t e d to resource defence, such as f o r a g i n g and f i g h t i n g . Second, o l d b i r d s may be l e s s v i g o r o u s because of p h y s i o l o g i c a l changes a s s o c i a t e d with age, or because of i n j u r i e s and a i l m e n t s that compound with age to cause a handicap. I f i r s t c o n s i d e r young b i r d s . Song sparrows on Mandarte forage e x t e n s i v e l y on l e a f - r o l l e r s ( T o r t r i c i d a e , Lepidoptera) and s l o w - f l y i n g i n s e c t s ( D i p t e r a and Hymenoptera) e a r l y i n s p r i n g , but these prey are uncommon in summer by the time most young are f o r a g i n g f o r themselves. Thus, y e a r l i n g sparrows have had l i t t l e p r e v i o u s experience with the main food items a v a i l a b l e i n s p r i n g , the most inte n s e p e r i o d of t e r r i t o r i a l a c t i v i t y (Tompa 1964), and t h e r e f o r e probably have l e s s energy a v a i l a b l e f o r t e r r i t o r y defence than more experienced b i r d s . T e r r i t o r i a l b i r d s , i n c l u d i n g song sparrows (Chap. 6), i n c r e a s e t h e i r time spent i n t e r r i t o r i a l defence when p r o v i d e d with supplemental food (Searcy 1979; Ydenberg 1984; Tamm 1985). Foraging e f f i c i e n c y i s s i m i l a r l y important to t e r r i t o r y defence in unsupplemented p o p u l a t i o n s (Davies & Houston I983a,b), and f o r a g i n g e f f i c i e n c y i n b i r d s f i r s t i n c r e a s e s with age (Orians 1969; Dunn 1972; Verbeek 1977; Groves 1978; Quinney & Smith 99 1980), and then reaches a p l a t e a u (Pugesek 1983). F i g h t i n g a b i l i t y a f f e c t s access to t e r r i t o r y and mates i n many s p e c i e s (LeBeouf & Peterson 1969; G e i s t 1971; Wiley 1973; Kikkawa 1981; Clutton-Brock et a l . 1982; Howard 1983; Arcese & Smith 1985). F i g h t i n g success c l e a r l y i n c r e a s e s as j u v e n i l e b i r d s and mammals mature ( G e i s t 1971; Ewald & Rohwer 1980 Kikkawa 1981; Clutton-Brock et a l . 1982). A l a c k of f i g h t i n g experience among 1-year-old song sparrows c o u l d a l s o e x p l a i n t h e i r lower a b i l i t y to defend t e r r i t o r i e s . Long-term data on f i g h t i n g success are sparse. The data presented here suggest that the f i g h t i n g success of male song sparrows d e c l i n e s a f t e r the age of 2 or 3 y e a r s . I found that v i s i b l e handicaps were more common i n b i r d s o l d e r than 2 y e a r s , and these handicaps were a s s o c i a t e d with t e r r i t o r y l o s s . Such handicaps c o u l d reduce f o r a g i n g e f f i c i e n c y as w e l l as f i g h t i n g a b i l i t y . S i m i l a r o b s e r v a t i o n s have been made on ungulates and primates, where p o s t - r e p r o d u c t i v e males showed poor muscle-tone, poor c o n d i t i o n , and p o t - b e l l i e s ( G e i s t 1971; Jarman 1979; Clutton-Brock et a l . 1982, Dunbar 1984). F u r t h e r evidence f o r other b i r d s i s i n d i r e c t , but s u r v i v a l (Coulson & Wooller 1976) and r e p r o d u c t i v e success (Burley 1980; Haymes & Blokpoel 1980; C o l l i a s et a l . 1986, Robinson 1986) d e c l i n e beyond middle-age i n some s p e c i e s . While the mechanisms f o r such d e c l i n e s remain u n c e r t a i n , t h i s study demonstrates that the e f f e c t of age on the a b i l i t y to defend resources i s n o n - l i n e a r 100 even i n a r e l a t i v e l y s h o r t - l i v e d monogamous s p e c i e s . REFERENCES Arcese, P. & Smith, J.N.M. 1985. 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F a c t o r s determining the numbers of song sparrows Melospiza melodia (Wilson) on Mandarte I s l a n d , B.C., Canada. Acta Z o o l . Fenn., 109,1-73. van R h i j n , J.G. 1973. Be h a v i o u r a l dimorphism i n male r u f f s , Philomachus puqnax ( L . ) . Behaviour, 47,153-229. Verbeek, N.A.M. 1977. Age d i f f e r e n c e s i n d i g g i n g frequency of h e r r i n g g u l l s on a dump. Condor, 79,123-125. 106 Watson, A. 1985. S o c i a l c l a s s , s o c i a l l y - i n d u c e d l o s s , r ecruitment and breeding of red grouse. O e c o l o g i a , 67,493-498. Watson, A. & Moss, R. 1970. Dominance, spacing behaviour and ag g r e s s i o n i n r e l a t i o n to p o p u l a t i o n l i m i t a t i o n i n v e r t e b r a t e s . In: Animal P o p u l a t i o n s i n R e l a t i o n to t h e i r  Food Resources (Ed. by Watson, A . ) , pp.167-218. Oxford: B l a c k w e l l S c i e n t i f i c P u b l i c a t i o n s . Wiley, R.H. 1973. T e r r i t o r i a l i t y and non-random mating i n sage grouse, Centrocercus urophasianus. Anim. Behav. Monogr., 6,87-161. W i r t z , P. 1981. T e r r i t o r i a l defence and t e r r i t o r y take-over by s a t e l l i t e males i n the waterbuck Kobus e l l i p s i p r y m n u s . Behav. E c o l . S o c i o b i o l . , 8,161-162. Woolfenden, G.E. & F i t z p a t r i c k , J.W. 1984. The F l o r i d a scrub j a y . Demography of a c o o p e r a t i v e - b r e e d i n g b i r d . P r i n c e t o n : P r i n c e t o n U n i v e r s i t y P r e s s . V i l l a g e , A. 1983. The r o l e of n e s t - s i t e a v a i l a b i l i t y and t e r r i t o r i a l behaviour i n l i m i t i n g the breeding d e n s i t y of k e s t r e l s . J . Anim. E c o l . , 52,621-634. Yasukawa, K. 1979. T e r r i t o r y establishment i n red-winged b l a c k b i r d s : the importance of a g g r e s s i v e behavior and expe r i e n c e . Condor, 81,258-264. Ydenberg, R.C. 1984. The c o n f l i c t between fee d i n g and t e r r i t o r y defence i n the great t i t . Behav. E c o l . S o c i o b i o l . , 15,103-108. Zahavi, A. 1977. R e l i a b i l i t y i n communication systems and the e v o l u t i o n of a l t r u s i m . In: E v o l u t i o n a r y Ecology (Ed. by B. Stonehouse & CM. P e r r i n s ) , pp.253-259. 107 CHAPTER 5. THE FORM AND FUNCTION OF SONG IN FEMALE SONG SPARROWS INTRODUCTION Nice (1943) reviewed the occurrence of female b i r d song and spec u l a t e d on i t s e v o l u t i o n . A f t e r a c o n s i d e r a b l e gap, renewed i n t e r e s t i n the song of female b i r d s has concerned i t s r o l e i n c o u r t s h i p and maintenance of the p a i r bond, f a m i l y u n i t and t e r r i t o r y ( B e l e t s k y 1982,1983a,b; R i c h i s o n 1983,1986). In t h i s study, I focused on s i n g i n g female song sparrows (Melospiza  melodia) i n a color-marked i s l a n d p o p u l a t i o n . I d i s c u s s the context of female song, present sonagrams from 3 females, and I compare my o b s e r v a t i o n s to those of Nice (1943). F i n a l l y , I spec u l a t e on the f u n c t i o n of female song i n song sparrows. Except i n a few s p e c i e s , s i n g i n g by female p a s s e r i n e s i s rar e (reviews i n Nice 1943; Nottebohm 1975; R i c h i s o n 1983). Nice (1943) suggested that female p a s s e r i n e s f e l l i n t o two groups depending on whether song was a common or e x c e p t i o n a l aspect of t h e i r behaviour. She p l a c e d song sparrows i n the l a t t e r group. Nice (1943) s p e c u l a t e d f u r t h e r that the female song sparrows she observed s i n g i n g were u n u s u a l l y a g g r e s s i v e i n d i v i d u a l s , and had abnormally high l e v e l s of androgens. Her suggestion i s reasonable because, i n some s p e c i e s i n which females r a r e l y or never s i n g , s i n g i n g can be induced by implanting t e s t o s t e r o n e ( r e f e r e n c e s i n Nice 1943; Nottebohm 108 1975). However, W i n g f i e l d (1984a,b) has shown that i n w i l d song sparrows, c i r c u l a t i n g l e v e l s of t e s t o s t e r o n e i n females at the time of t e r r i t o r y establishment and j u s t p r i o r to nest b u i l d i n g are almost as high as those recorded i n males at the same time. T h i s i s the p e r i o d d u r i n g which Nice (1943) heard females s i n g i n g . If s i n g i n g by female song s p a r r o w s . r e s u l t s from unusually high l e v e l s of androgens i n a few b i r d s , such behaviour may be aberrant and t h e r e f o r e unimportant. A l t e r n a t i v e l y , s i n g i n g females may be r a r e l y but r e g u l a r l y observed under p a r t i c u l a r and i n t e r e s t i n g circumstances, thus suggesting a f u n c t i o n f o r female song. For example, R i c h i s o n (1983) concluded that s i n g i n g by female black-headed grosbeaks (Pheucticus  melanocephalus) f u n c t i o n e d i n f a m i l y group maintenance: females sang most f r e q u e n t l y d u r i n g the f l e d g l i n g p e r i o d and young b i r d s o r i e n t e d themselves towards t h e i r parents song a f t e r becoming separated from them. B e l e t s k y (I983a,b) showed that female red-winged b l a c k b i r d s ( A q e l a i u s phoeniceus) sang d i f f e r e n t song types i n the context of i n t r a s e x u a l a g gression or pair-bond maintenance, and that these songs a l s o d i f f e r e d from the f a m i l i a r t e r r i t o r i a l song of males. In t h i s paper, I show that s i n g i n g by female song sparrows occurred p r i m a r i l y in the context of t e r r i t o r y defence a g a i n s t female i n t r u d e r s . I suggest that s i n g i n g by female song sparrows i n d i c a t e s high l e v e l s of androgens, and that such high 109 l e v e l s are a normal consequence of a g g r e s s i v e i n t e r a c t i o n s . Androgens may be s e c r e t e d i n response to repeated or prolonged c o n f l i c t s between females f o r t e r r i t o r y ownership, as demonstrated f o r male song sparrows by W i n g f i e l d (1984b). RESULTS The Form of Female Song Female 60194 was f i r s t observed to s i n g on 20 February 1984. The song was composed of a s e r i e s of c l e a r , melodic w h i s t l e s , l a s t i n g about 2 seconds, sung 1-6 times from a perch above the shrub canopy. The song was loud ( a u d i b l e from 3 t e r r i t o r i e s d i s t a n t , about 80 meters) and resembled that of a subadult p u r p l e f i n c h (Carpodacus purpureus). The songs of 11 other females heard subsequently v a r i e d i n volume, l e n g t h and complexity, but most resembled the songs of 60194. The female songs recorded were s i m i l a r to male songs i n frequency range, timing and o v e r a l l s t r u c t u r e ( F i g . 5.1, compare F1-4 with M1-2). P. Marler and S. Peters ( p e r s o n a l communication) have a l s o recorded songs from l i v e - t u t o r e d , c a p t i v e females implanted with t e s t o s t e r o n e ( F i g . 5.1, T1-2). Sonagrams of r e c o r d i n g s from 2 of these females show a simple (T1) and more complex song (T2) that n e a r l y span the range of song v a r i a b i l i t y recorded i n the w i l d ( F i g . 5.1). O v e r a l l , males and females sang s i m i l a r s y l l a b l e types i n c l u d i n g pure 110 tones, buzzes, frequency modulated notes and r a p i d sweeps ( F i g . 5.1). On average, however, females tended to s i n g s h o r t e r songs that i n c l u d e d fewer d i f f e r e n t note types i n each song than d i d males. N e v e r t h e l e s s , the longer songs c o u l d e a s i l y pass fo r male songs to a human l i s t e n e r and might have done so i n unbanded p o p u l a t i o n s . Male song sparrows on Mandarte s i n g 4-12 song types and 30 or more v a r i a t i o n s on each type ( H i e b e r t , Stoddard, & Arcese ms). These males s i n g 5-40 songs of one type, a l t e r n a t i n g among v a r i a t i o n s , before s w i t c h i n g to another type. The females that were recorded sang v a r i a t i o n s the way males do, and 1 female was recorded long enough (60 songs) to d i s c o v e r that she had at l e a s t two d i s c r e t e song types. The Occurrence of Female Song S i n g i n g females were r a r e l y observed over the study p e r i o d (Table 5.1). Only 12 of approximately 140 females were heard s i n g i n g d u r i n g 267 female-years of o b s e r v a t i o n . Of these 12, 10 sang on only 1 day or 2 c o n s e c u t i v e days. Two females were heard s i n g i n g i n more than 1 year. In c o n t r a s t , t e r r i t o r i a l males on Mandarte I s l a n d sang more or l e s s throughout the year, with peaks d u r i n g l a t e winter, s p r i n g and f a l l . Table 5.1 shows that female song sparrows sang mainly between 20 February and the beginning of e g g - l a y i n g , which J l l kHz 6f F1 kHz 6 F2 I ' 1 1 1 yuuvW " f r r kHz 6r F3 kHz 6r F4 kHz T1 kHz T2 kHz 4r «• M1 kHz 6f U t t t r t i f f M2 1 tec Figure 5.1. Sonagrams from 3 females recorded on Mandarte Island (Fl-3), 1 from Discovery Park, Seattle (F4), and 2 recorded i n New York by P Marler and S. Peters from captive, live-tutored females implanted with testosterone (Tl-2). Two male songs (Ml-2) from Mandarte Island are presented for comparison. 1 12 o c c u r r e d between l a t e March and m i d - A p r i l . Only 1 female ( 6 0 3 0 0 ) was heard s i n g i n g a f t e r she had begun to l a y eggs. T h i s female sang r e g u l a r l y from 7 A p r i l to 18 June, 1985, u s u a l l y when she came up from her nest and before she returned to incubate eggs or brood n e s t l i n g s . S i n g i n g by females, though always uncommon, has been heard more o f t e n i n t h i s p o p u l a t i o n when p o p u l a t i o n d e n s i t y was h i g h . F.S. Tompa (personal communication) heard a female s i n g i n t h i s p o p u l a t i o n only d u r i n g the year of h i g h e s t p o p u l a t i o n d e n s i t y d u r i n g h i s 4-year study. T h i s was a l s o the o n l y year i n which he observed t e r r i t o r i a l c o n f l i c t s between females (Tompa 1964, p e r s o n a l communication). J.N.M. Smith ( p e r s o n a l communication) a l s o heard a female s i n g i n t h i s p o p u l a t i o n i n the year of h i g h e s t d e n s i t y d u r i n g the p e r i o d from 1974-79. No female was heard s i n g i n g i n t h i s study p r i o r to 1984, d e s p i t e i n t e n s i v e o b s e r v a t i o n . Three females were observed s i n g i n g i n 1984, 9 i n 1985 and 2 i n 1986 (Table 5.1). F i g u r e 5.2 shows that fewer than the o v e r a l l average number of females were observed s i n g i n g when p o p u l a t i o n d e n s i t y was low, while more than expected were observed i n high d e n s i t y y e a r s . The Context of Female Song Female song was n e a r l y always a s s o c i a t e d with t e r r i t o r y defence and i t r e s u l t e d most f r e q u e n t l y when females d i s p u t e d t e r r i t o r y ownership. T h i s can be seen by c o n s i d e r i n g each 113 Table 5.1. The occurrence and context of female song i n the song sparrow. No females were observed s i n g i n g i n 1982 or 1983. Year Female Number 1984 1985 1986 60194 20 Feb: E v i c t i n g a female f l o a t e r . 22 Feb: E v i c t i n g same and s e t t l i n g new boundary. 3 Apr: In response to male playback. 15 Apr: E v i c t i n g a female f l o a t e r . Not heard s i n g i n g , 60279 20 Feb: Taking Not heard s i n g i n g . Not heard s i n g i n g , over new t e r r i t o r y by e v i c t i n g former female owner. 60300 10 Mar: Taking 3 Apr: In response Not heard s i n g i n g . over new t e r r i t o r y to male playback, by e v i c t i n g former 7 Apr: E v i c t i n g a female owner. female f l o a t e r , then r e g u l a r l y to at l e a s t 18 June. 60204 Not heard s i n g i n g . 8 Apr: E v i c t i n g a (Deceased) female f l o a t e r . Table 5.1. Cont. 60108 Not h e a r d s i n g i n g . 25 Apr: On border as a d j a c e n t female i s e v i c t e d by a f l o a t e r female. (Deceased) 60254 Not heard s i n g i n g . 17 Apr: No apparent Not heard s i n g i n g . c o n f l i c t . 60126 Not heard s i n g i n g . 3 Apr: Response t o Not heard s i n g i n g . male playback. 59239 (Year of hatch) 19 Apr: Attempting to s e t t l e between e s t a b l i s h e d t e r r i t o r i e s . Not heard s i n g i n g . 59114 (Year of hatch) 18 Apr: Taking over Not heard s i n g i n g , a d j a c e n t t e r r i t o r y by e v i c t i n g owner. 115 Table 5.1. Cont. 58909 (Year of hatch) 29-30 Apr: Near female (Deceased) f l o a t e r that s e t t l e s on adjacent t e r r i t o r y . 59133 (Year of hatch) Not heard s i n g i n g . • 16 Mar: No apparent c o n f l i c t , but s e t t l e d w i t h i n the pre v i o u s month. 59150 (Year of hatch) Not heard s i n g i n g , s i n g . 3-4 Apr: During i n t e n s e f i g h t s with female f l o a t e r . D) C 'Bi CO c o o a o •15« 10-•05 • oH .82 85 84 .86 83 i . I I I I I 20 30 40 50 60 70 Number of Territorial Females Figure 5.2. The proportion of females (transformed axis) heard singing i n r e l a t i o n to the t o t a l number of females breeding. The dashed l i n e i n d i c a t e s the average proportion of females heard singing over the study. The data are heterogeneous across years (C^ , =12.8, df=4, P<0.025) . 1 1 7 o b s e r v a t i o n of a s i n g l e day or s e r i e s of 2 days of s i n g i n g by a female as an event. Of 17 such events, 12 (70.5%) occ u r r e d d u r i n g vigorous c o n f l i c t s between females f o r t e r r i t o r y ownership (Table 5.1). In these 12 cases, songs were heard from 8 t e r r i t o r i a l females and 4 i n t r u d e r s (3 neighbor t e r r i t o r y h o l d e r s , 1 n o n - t e r r i t o r i a l f l o a t e r ) . F i g u r e 5.3 shows a t y p i c a l t e r r i t o r i a l c o n f l i c t between 3 females that took p l a c e over at l e a s t 3 days. Two of these females (60194 and 60279) sang r e p e a t e d l y as the t h i r d (60182) was chased and u l t i m a t e l y e v i c t e d . The e v i c t e d female l a t e r s e t t l e d 2 t e r r i t o r i e s d i s t a n t with an unmated y e a r l i n g . Two a d d i t i o n a l females sang when no c o n f l i c t was apparent. In both cases, however, home ranges of n o n - t e r r i t o r i a l females encompassed pa r t of the s i n g e r ' s t e r r i t o r y . Three a d d i t i o n a l o b s e r v a t i o n s of female song were made a f t e r p l a y i n g a tape r e c o r d i n g of male song. One of these o b s e r v a t i o n s was made u n i n t e n t i o n a l l y while t r y i n g to s t i m u l a t e a t e r r i t o r i a l male to s i n g . T h i s i s the only female (60126) that was not a l s o observed s i n g i n g under n a t u r a l circumstances. Two other females that responded to playbacks with song were 60300 and 60194, the most a c t i v e s i n g e r s observed d u r i n g the study (Table 5.1). No other playbacks were conducted d u r i n g s p r i n g or summer, but playbacks were f r e q u e n t l y used as an a i d du r i n g l a t e winter I 50m Figure 5.3. Depiction of a c o n f l i c t between 3 females for the ownership of a breeding t e r r i t o r y (band numbers refer to those in Table 5.1). Birds defend areas of shrub (outlined) surrounded by grassland or rocky i n t e r -t i d a l . Female 60279 intruded from her previous te r r i t o r y (solid arrow), sang (black dots) and chased female 60182 from her former t e r r i t o r y . Female 60194 chased female 60182 whenever she entered her t e r r i t o r y and she sang repeatedly (open dots). Female 60182 settled several days later with an unmated male (broken arrow). 119 censuses. Females never responded to these with song, though " t i k s " , "tchunks" and "caterwauls" (Nice 1943:274) were common. Females were never observed responding to i n t r u d i n g or n e i g h b o r i n g males with song, though they d i d sometimes respond to these b i r d s a g g r e s s i v e l y ( i . e . supplant, chase and puff-wave posture "as d e s c r i b e d f o r males as "puff-sing-wave" by Nice 1943:274). DISCUSSION Recent experimental s t u d i e s i n d i c a t e that female song i n p a s s e r i n e s p l a y s a r o l e i n c o u r t s h i p , pair-bond and f a m i l y group maintenance, and i n t e r r i t o r y defence ( B e l e t s k y 1982, l983a,b; R i c h i s o n 1983,1986). However, these s t u d i e s have d e a l t only with s p e c i e s ' i n which females commonly s i n g . No f u n c t i o n has been suggested f o r female song i n s p e c i e s where such behaviour i s r a r e (e.g. Rufous-sided towhee ( P i p i l o e rythropthalmus), i n d i g o bunting ( P a s s e r i n a cyanea), Nolan 1958 song sparrow, Nice 1943; t h i s s t u d y ) . Nice (1943) b e l i e v e d female song served no f u n c t i o n i n the song sparrow, and that i t r e s u l t e d from u n u s u a l l y h i g h l e v e l s of androgens i n a few abnormal i n d i v i d u a l s . In the f o l l o w i n g d i s c u s s i o n , I compare my r e s u l t s with those of Nice (1943) and s p e c u l a t e about the i m p l i c a t i o n s of female song i n my study p o p u l a t i o n . Nice (1943:127) d e s c r i b e d the song of females that she observed as " s h o r t , simple and e n t i r e l y unmusical", with the 1 20 exception of 1 b i r d (K135). On A p r i l 2, 1934, Nice (1943:127) noted that female K135 "has a s p e c i a l p l a c e on the l o c u s t where she s i n g s ; her song i s s h r i l l and loud; i t reminds me a l i t t l e of a w h i t e - t h r o a t ( ( Z o n o t r i c h i a a l b i c o l l i s ) ) . There are at l e a s t two v e r s i o n s ; they are not as unmusical as most female Song sparrows." The s i n g i n g behaviour of most females that I have heard most c l o s e l y resembled that d e s c r i b e d f o r K135 by N i c e . However, not a l l females that sang on Mandarte produced m u s i c a l , m a l e - l i k e songs. Sonagram F1 i n F i g u r e 5.1 r e p r e s e n t s one such song that more c l o s e l y resembled the songs that Nice r e p o r t e d . There i s a l s o c o n s i d e r a b l e v a r i a t i o n i n the s i m i l a r i t y of song types of males and females a c r o s s s p e c i e s . For example, i n some s p e c i e s i n which females commonly s i n g , such as the r e d -winged b l a c k b i r d , female songs do not resemble those of males (Bele t s k y 1983a,b). In o t h e r s , however, females s i n g songs that are e i t h e r n e a r l y as well-developed as males ( i . e . b l a c k -headed grosbeak, R i c h i s o n 1983) or e q u a l l y well-developed (reviewed i n Farabaugh 1982). In s p e c i e s i n which females r a r e l y s i n g , female songs are u s u a l l y l e s s w ell-developed than those of males, and are o f t e n d e s c r i b e d as resembling those of immature males (e.g. Nolan 1958; Nice 1943; t h i s s t u d y ) . T h i s suggests that there may be a reduced c a p a c i t y f o r female song l e a r n i n g and/or performance i n s p e c i e s i n which males commonly s i n g but females r a r e l y do so. 121 Nice (1943) d e f i n e d f i v e stages and M u l l i g a n (1966) four i n the song development of male song sparrows. These began with a s o f t w a rbling stage and culminated i n f u l l c r y s t a l l i z e d song. The females we recorded appeared to be n e a r i n g the f i n a l stage. More of t h e i r songs were fragments than would be expected of a mature male, but they d i d s i n g f u l l , complex songs. F u r t h e r , in a l l of the female songs recorded, i n t e r n o t e i n t e r v a l s were s i m i l a r to those of c r y s t a l l i z e d male song. T h i s suggests e i t h e r t h a t : 1) p r a c t i c e s i n g i n g by females does not have as important i n a r o l e i n song development as i n males, 2) that some females p r a c t i c e s i n g i n g as y e a r l i n g s , or 3) that the song l e a r n i n g process i s somewhat d i f f e r e n t f o r male and female song sparrows. Nice (1943:127) observed young, unhanded but suspected females s i n g i n g i n f a l l ; a female that she hand-raised a l s o produced j u v e n i l e m a l e - l i k e warbles. However, I have never made such o b s e r v a t i o n s on known j u v e n i l e females. I t h e r e f o r e suggest that p r a c t i c e - s i n g i n g i s l e s s important f o r female song sparrows than i t i s f o r males, perhaps because females s i n g fewer and simpler song types. F u r t h e r , the song l e a r n i n g process f o r males and females might d i f f e r i f female song l e a r n i n g f u n c t i o n e d i n s p e c i e s r e c o g n i t i o n , mate choice or i n b r e e d i n g avoidance (review i n Payne 1983) as w e l l as i n t e r r i t o r y e stablishment and defence. In t h i s case, song l e a r n i n g i n females might be more h i g h l y developed f o r r e c o g n i t i o n than f o r performance. 122 I observed s i n g i n g females almost e x c l u s i v e l y during the p e r i o d j u s t p r i o r to n e s t - b u i l d i n g ( l a t e February to e a r l y A p r i l ) . T h i s p e r i o d matches c l o s e l y the dates given by Nice (1943:127) of 12 February f o r the e a r l i e s t song and 19 A p r i l f o r the l a t e s t . Nice (1943) a l s o notes that Wetherbee ( c i t e d i n Nice) r e p o r t e d a color-banded female song sparrow that sang f r e q u e n t l y from A p r i l through at l e a s t 17 June, matching my re c o r d f o r female 60300 (Table 5.1). Nice (1943) observed at l e a s t 2 females to s i n g when they p a r t i c i p a t e d with t h e i r mates i n e v i c t i n g i n t r u d i n g males (one unbanded). For example, "On Feb. 26, 1932, K56 was s i n g i n g almost c o n s t a n t l y , whenever she was r e s t i n g from chasing 120M (a male), a new a r r i v a l that morning. At times she chased while her mate sang. She perched on top of e l d e r s and sang very l o u d l y , o f t e n answering her mate's songs" (Nice 1943:127). Nice (1943) mentioned no t e r r i t o r i a l c o n f l i c t s when she observed other females to s i n g . My o b s e r v a t i o n s are c o n s i s t e n t with N i c e ' s , with the notable e x c e p t i o n that 83% (10 of 12) of the females that I heard s i n g i n g were i n v o l v e d i n c o n f l i c t s with other females over t e r r i t o r y ownership. In c o n t r a s t , I never observed females to s i n g when n o n - t e r r i t o r i a l male f l o a t e r s attempted to e v i c t t e r r i t o r i a l males or to s e t t l e between t e r r i t o r i a l neighbors, even though such attempts were common du r i n g the study (Chap. 1 23 3). T h i s c l e a r l y shows that on Mandarte I s l a n d , s i n g i n g by female song sparrows i s r e l a t e d to competition with other females f o r breeding space. In t h i s regard, my o b s e r v a t i o n of an i n c r e a s e i n the i n c i d e n c e of female song with breeding d e n s i t y i s c o n s i s t e n t with two e f f e c t s of d e n s i t y on breeding performance: average r e p r o d u c t i v e success i s s t r o n g l y depressed (Arcese & Smith 1988), but v a r i a t i o n i n r e p r o d u c t i v e success i s i n c r e a s e d among females at high p o p u l a t i o n d e n s i t y (Smith & Arcese 1986). I suggest t h a t , as p o p u l a t i o n d e n s i t y and v a r i a t i o n i n r e p r o d u c t i v e success i n c r e a s e , c o m p e t i t i o n among females f o r high q u a l i t y t e r r i t o r i e s or males becomes more i n t e n s e . Such com p e t i t i o n commonly leads to t e r r i t o r i a l c o n f l i c t s between females at high p o p u l a t i o n d e n s i t i e s (personal o b s e r v a t i o n s ) . Androgen l e v e l s may i n c r e a s e i n females i n v o l v e d i n prolonged c o n f l i c t s , as has been demonstrated i n w i l d male song sparrows ( W i n g f i e l d 1985b). P. Marler and S. P e t e r s (personal communication) have re p e a t e d l y found that s i n g i n g can be e a s i l y induced i n female song sparrows with t e s t o s t e r o n e implants. I t i s t h e r e f o r e reasonable to conclude that c o n f l i c t - i n d u c e d e l e v a t i o n s i n androgen would have a s i m i l a r e f f e c t i n h i g h l y s t i m u l a t e d females. Androgen l e v e l s i n w i l d females are known to normally approach the l e v e l s observed in males d u r i n g the p e r i o d s i n which Nice (1943) and I observed females s i n g i n g ( W i n g f i e l d 1984a,b). 124 I t h e r e f o r e suggest that s i n g i n g by female song sparrows i s a rare but normal aspect of female behaviour that can be e x p l a i n e d i n two ways: (1) m e c h a n i s t i c a l l y , i n terms of the r e l a t i o n s h i p between c i r c u l a t i n g l e v e l s of androgens and the frequency and i n t e n s i t y of a g g r e s s i v e i n t e r a c t i o n s , and (2) e v o l u t i o n a r i l y , i n terms of the r o l e of female v o c a l i z a t i o n s i n a g g r e s s i v e c o n t e s t s over resources that a f f e c t r e p r o d u c t i v e success. These c o n c l u s i o n s remain t e n t a t i v e , however, because the s p e c i f i c r e l a t i o n of androgen l e v e l s to t e r r i t o r i a l c o n f l i c t i n females has not been s t u d i e d . T h e r e f o r e , experimental s t u d i e s are now needed to t e s t i f female songs or perhaps other v o c a l i z a t i o n s (e.g. caterwauls, c h i t t e r s , t h r e a t notes; Nice 1943) i n the song sparrow, serve a t e r r i t o r i a l f u n c t i o n as i n s p e c i e s such as the blue grouse (Dendragapus obscurus, Hannon 1980). REFERENCES Arcese, P. & Smith, J.N.M. 1988. E f f e c t s of p o p u l a t i o n d e n s i t y and supplemental food on r e p r o d u c t i o n i n song sparrows. J . Anim. E c o l , i n p r e s s . B e l e t s k y , L.D. 1982. V o c a l i z a t i o n s of female northern o r i o l e s . Condor, 84, 445-447. B e l e t s k y , L.D. 1983a. Aggressive and pair-bond maintenance songs of female red-winged b l a c k b i r d s ( A g e l a i u s phoeniceus). Z. T i e r p s y c h o l . , 63, 47-54. B e l e t s k y , L.D. 1983b. Aggressive response to " s e l f " songs by female red-winged b l a c k b i r d s , A g e l a i u s phoeniceus. Can. J . Zo o l . , 61, 462-465. 125 Farabaugh, S.M. 1982. The e c o l o g i c a l and s o c i a l s i g n i f i c a n c e of d u e t t i n g , pp. 85-124. In: A c o u s t i c Communication i n B i r d s , Vol.2 (Ed. by D.E. Kroodsma & E.H. M i l l e r ) . Academic Pr e s s : New York. Hannon, S.J. 1980. The c a c k l e c a l l of female blue grouse: does i t have a mating or a g g r e s s i v e f u n c t i o n ? Auk, 97, 404-407. M u l l i g a n , J.A. 1966. S i n g i n g behavior and i t s development i n the song sparrow, Melospiza melodia. Univ. C a l i f . P u b l . Zool., 81, 1-76. Nice, M.M. 1943. St u d i e s i n the l i f e h i s t o r y of the song sparrow. I I . Trans. L i n n . Soc. N.Y., 6, 1-328. Nolan, V. 1958. Si n g i n g by female i n d i g o buntings and r u f o u s -s i d e d towhee. Wilson B u l l . , 70, 287-288. Nottebohm, F. 1975. Voc a l behavior i n b i r d s , pp. 278-332. In: Avian B i o l o g y , V o l . 5 , (Ed. by D.S. Farner & J.R. K i n g ) . Academic Press, New York. Payne, R.B. 1983. B i r d song, sexual s e l e c t i o n and female mating s t r a t e g i e s , pp. 55-90, In: S o c i a l Behaviour of Female V e r t e b r a t e s (Ed. by S. K. Wasser), Academic Press:New York. R i c h i s o n , G. 1983. The f u n c t i o n of s i n g i n g i n female b l a c k -headed grosbeaks (Pheucticus melanocephalus). Auk, 100, 105-116. R i c h i s o n , G. 1986. The s i n g i n g behavior of female northern c a r d i n a l s . Condor, 88, 156-159. Smith, J.N.M. 1981. Cowbird p a r a s i t i s m , host f i t n e s s , and age of the host female i n an i s l a n d song sparrow p o p u l a t i o n . Condor, 83, 152-161. Smith, J.N.M. 1988. L i f e t i m e r e p r o d u c t i v e success i n the song sparrow, i n pre s s , In: Reproductive Success, (Ed. by T.H. C l u t t o n - B r o c k ) , Chicago U n i v e r s i t y Press:Chicago. 126 Smith, J.N.M. & Arcese, P. 1986. How does t e r r i t o r i a l behaviour i n f l u e n c e breeding b i r d numbers? pp.89-94, In: Be h a v i o u r a l Ecology and Po p u l a t i o n B i o l o g y , (Ed. by L.C. Drickamer), P r i v a t : T o u l o u s e . Tompa, F.S. 1964. F a c t o r s determining the numbers of song sparrows (Melospiza melodia, Wilson) on Mandarte I s l a n d , B.C., Canada. Acta Z o o l . Fenn., 109, 3-68. W i n g f i e l d , J.C. 1984a. Environmental and endocrine c o n t r o l of re p r o d u c t i o n i n the song sparrow, Melospiza melodia I. Temporal o r g a n i z a t i o n of the breeding c y c l e . Gen. Comp. E n d o c r i n o l . , 56, 406-416. W i n g f i e l d , J.C. 1984b. Environmental and endocrine c o n t r o l of re p r o d u c t i o n i n the song sparrow, Melospiza melodia I I . A g o n i s t i c i n t e r a c t i o n s as environmental i n f o r m a t i o n s t i m u l a t i n g s e c r e t i o n of t e s t o s t e r o n e . Gen. Comp. E n d o c r i n o l . , 56, 417-424. 127 CHAPTER 6. INTRASEXUAL COMPETITION AND MATING SYSTEM IN A PRIMARILY MONOGAMOUS SPARROW INTRODUCTION A c l a s s i c a l q u e s t i o n i n the study of mating systems has been: how do i n d i v i d u a l s of each sex maximize t h e i r r e p r o d u c t i v e success? (e.g. Wittenberger 1979; Murray 1984). Orians (1969) and T r i v e r s (1972) have suggested t h a t , because of the d i f f e r e n c e i n s i z e of male and female gametes, females should be more c a r e f u l than males i n t h e i r s e l e c t i o n of mates. I t f o l l o w s that males should maximize t h e i r r e p r o d u c t i v e success by defending access to as many f e r t i l e females as p o s s i b l e ; females should do so by mating with males that possess s u p e r i o r phenotypic or ge n e t i c q u a l i t i e s , or males that are able to defend e x c l u s i v e access to resources c r u c i a l to r e p r o d u c t i o n (Orians 1969; T r i v e r s 1972; Bradbury & Vehrencamp 1977; Emlen & Oring 1977; Wittenberger 1979; Oring 1982; Murray 1984). Although a r o l e f o r d i r e c t c o m p e t i t i o n f o r resources between females i n the e v o l u t i o n and maintenance of mating systems i s recog n i z e d (e.g. T r i v e r s 1972:153; Altman et a l . 1977; Wittenberger & T i l s o n 1980; Orin g 1982:14), i t has u s u a l l y been downplayed i n comparison to the r o l e of female c h o i c e of mates (Darwin 1871; Orians 1969, 1980; Emlen & Oring 1977; Searcy 1982; Mock 1983). As a r e s u l t , models of mating systems that emphasize female a g g r e s s i o n and resource defence remain 1 28 under-developed and l a r g e l y u ntested. In t h i s chapter, I co n s i d e r the l e v e l and i n f l u e n c e of i n t r a s e x u a l c o m p e t i t i o n i n the mating system of the p r i m a r i l y monogamous song sparrow p o p u l a t i o n r e s i d e n t on Mandarte I s l a n d , B.C. Although the m a j o r i t y of a l l b i r d s mate i n monogamous p a i r s (Lack 1968), many p r i m a r i l y monogamous s p e c i e s o c c a s i o n a l l y mate in polygynous groups (Verner & W i l l s o n 1969; M o l l e r 1986). Mating i n groups other than monogamous p a i r s can have s u b s t a n t i a l consequences f o r i n d i v i d u a l r e p r o d u c t i v e success even i n p r i m a r i l y monogamous s p e c i e s (e.g. Davies & Houston 1986), and t h i s makes such s p e c i e s i n t e r e s t i n g s u b j e c t s f o r study (e.g. Smith et a l . 1982; Hannon 1984; Dhondt 1987). However, p r i m a r i l y monogamous s p e c i e s have so f a r r e c e i v e d much l e s s a t t e n t i o n than polygynous s p e c i e s (reviews i n T r i v e r s 1972; Wittenberger 1979; Emlen & Oring 1977; Oring 1982; Mock 1983). Two outsta n d i n g q u e s t i o n s concerning p r i m a r i l y monogamous sp e c i e s a r e : 1) how do polygynous groups a r i s e ? And, 2) what b e h a v i o u r a l mechanisms determine which females and males w i t h i n a p o p u l a t i o n mate i n polygynous groups? Here, I f i r s t b r i e f l y d i s c u s s how t e r r i t o r y defence, time budgets and the a v a i l a b i l t y of resources e s s e n t i a l f o r the pr o d u c t i o n of young c o u l d i n f l u e n c e female c o m p e t i t i o n and a f f e c t mating p a t t e r n s and r e p r o d u c t i v e success. I then d e r i v e s e v e r a l p r e d i c t i o n s based on the hypothesis that i n t r a s e x u a l competition i n each sex a f f e c t s mating p a t t e r n s i n my study p o p u l a t i o n , and consequently 129 a f f e c t s the r e p r o d u c t i v e success of males and females. Next, I t e s t these p r e d i c t i o n s using o b s e r v a t i o n a l data c o l l e c t e d over 5 yea r s . F i n a l l y , I present r e s u l t s from a food a d d i t i o n experiment designed to a l t e r the time budgets and t e r r i t o r i a l behaviour of males and females. When Should Females Compete? Because males should r a r e l y s u f f e r reduced r e p r o d u c t i v e success when a d d i t i o n a l females s e t t l e on t h e i r t e r r i t o r i e s (Wittenberger & T i l s o n 1980), females can not r e l y on males to ensure e x c l u s i v e access to t e r r i t o r i a l r e s o u r c e s . Thus, females should attempt to prevent settlement by a d d i t i o n a l females on t h e i r mate's t e r r i t o r y whenever doing so prevents the l o s s of e x c l u s i v e access to resources such as p a r e n t a l c a r e , food or nest s i t e s . When the d i s t r i b u t i o n of females i s i n f l u e n c e d by t e r r i t o r i a l behaviour, v a r i a t i o n i n the c o s t s and b e n e f i t s of resource defence by females w i l l i n f l u e n c e the frequency of polygyny by a f f e c t i n g the a b i l i t y of males to defend more than one female (Orians 1969; T r i v e r s 1972; Bradbury & Vehrencamp 1977; Oring 1982; Davies & Lundberg 1984). The t o t a l time devoted to r e p r o d u c t i o n by females n e c e s s a r i l y equals the time spent i n a c t i v i t i e s d i r e c t l y devoted to r e p r o d u c t i o n ( i . e . nest b u i l d i n g , egg p r o d u c t i o n , i n c u b a t i o n , e t c . ) , p l u s that i n d i r e c t l y devoted to r e p r o d u c t i o n ( i . e . defence of food, nest s i t e s , and/or e x c l u s i v e access to 130 male c a r e ) . Thus, the time that females expend i n defence should depend on the value of the b e n e f i t s d e r i v e d (e.g. e x c l u s i v e access to food, nest s i t e s , male care) and c o s t s i n c u r r e d by engaging i n defence (e.g. e n e r g e t i c expenditure, delay i n breeding, r i s k of i n j u r y ) . P r e d i c t i o n s Given that both females and males defend l i m i t i n g resources a g a i n s t o t hers of the same sex, s e v e r a l p r e d i c t i o n s about the behavior and r e p r o d u c t i v e success of i n d i v i d u a l s f o l l o w . For example, because male a i d i n b r o o d - r e a r i n g approximately doubles the number of young that female song sparrows on Mandarte can produce (Smith et a l . 1982), I p r e d i c t e d that females should behave i n ways that reduce polygyny ( p r e d i c t i o n 1), but that males should behave i n ways that promote i t ( p r e d i c t i o n 2). F u r t h e r , when monogamous females r a i s e more young than those mated to polygynous males, and the p r o b a b i l i t y of polygyny depends on c o m p e t i t i v e a b i l i t y , then the females l e a s t able to engage i n t e r r i t o r y defence should most o f t e n breed i n polygynous groups. In c o n t r a s t , the most c o m p e t i t i v e l y a b l e males should become polygynous most o f t e n whenever unaided females can r a i s e as l e a s t some young. On Mandarte, the c o m p e t i t i v e a b i l i t y of song sparrows v a r i e s with age (Nol & Smith 1987; Chap. 3 & 4). I t h e r e f o r e expected that polygyny would be most frequent among the youngest and o l d e s t females 131 ( p r e d i c t i o n 3), and i n c o n t r a s t , most frequent among middle-aged males ( p r e d i c t i o n 4). In e a r l y s p r i n g , when c o o l weather and short days l i m i t f e e d i n g time and cause high expenditure of energy, b i r d s are probably l i m i t e d i n the amount of time and energy a v a i l a b l e f o r d i r e c t investment i n r e p r o d u c t i o n (Martin 1987; Arcese & Smith 1988). Thus, the amount of time spent i n t e r r i t o r y defence at t h i s time by females and males should depend on the d i r e c t investment of each sex to r e p r o d u c t i o n . Because female song sparrows are r e s p o n s i b l e f o r the bulk of r e p r o d u c t i v e a c t i v i t i e s , while males only feed young, females probably have l e s s time than males to devote to t e r r i t o r y defence. To t e s t i f females a d j u s t t h e i r t e r r i t o r i a l behaviour a c c o r d i n g to e n e r g e t i c c o n s t r a i n t s , I added food on 15 t e r r i t o r i e s i n 1985, p r i o r to and d u r i n g the l a y i n g p e r i o d . I p r e d i c t e d that t e r r i t o r i a l females would be r e l e a s e d from the c o n s t r a i n t s of f o r a g i n g and would thus devote more time to t e r r i t o r y defence ( p r e d i c t i o n 5). F u r t h e r , i f females defend t e r r i t o r i e s p r i m a r i l y to prevent settlement by f l o a t e r females, then fed females should be more e f f e c t i v e at e x c l u d i n g f l o a t e r s than c o n t r o l s ( p r e d i c t i o n 6). In c o n t r a s t , i f settlement by female song sparrows occurs p r i m a r i l y by c h o i c e of s u p e r i o r t e r r i t o r i e s , female f l o a t e r s should s e t t l e p r e f e r e n t i a l l y on t e r r i t o r i e s with added food. Two p r e d i c t i o n s analogous to p r e d i c t i o n s 5 & 6 f o r females were a l s o expected f o r males ( p r e d i c t i o n s 7 & 8). 1 32 Next, because a l l females must s u b s t a n t i a l l y decrease time spent i n defence once i n c u b a t i o n begins, and because the long breeding p e r i o d on Mandarte allows m u l t i p l e breeding attempts (Smith & Roff 1980), I expected a l l females to e v e n t u a l l y s e t t l e . More imp o r t a n t l y , however, I f u r t h e r expected that i f t e r r i t o r y defence determines which females mate i n polygynous groups, then any cases of polygyny that r e s u l t e d from settlement by female f l o a t e r s should occur on t e r r i t o r i e s where p r e v i o u s l y s e t t l e d females had begun i n c u b a t i o n ( p r e d i c t i o n 9). F i n a l l y , i f the r e l a t i v e l e v e l s of i n t r a s e x u a l c o m p e t i t i o n w i t h i n p o p u l a t i o n s i n f l u e n c e the long-term, l e v e l of polygyny i n the p o p u l a t i o n , then polygyny should be l e s s common as the r a t i o of males to females i n c r e a s e s (Emlen & O r i n g 1977; Smith et a l . 1982; p r e d i c t i o n 10). T h i s i s because females should more o f t e n be able to defend t h e i r mate's e n t i r e t e r r i t o r y when comp e t i t i o n f o r space i n males exceeds that i n females. In summary, i f a g g r e s s i v e c o m p e t i t i o n i s the primary means by which breeding t e r r i t o r i e s are occupied, s u p e r i o r competitors should a c q u i r e the best breeding o p p o r t u n i t i e s . With regard to polygyny i n the song sparrow on Mandarte I s l a n d , males should behave i n ways that promote polygyny as long as doing so leads to the p r o d u c t i o n of more young than c o u l d be r a i s e d i n monogamy. The reverse i s expected f o r females. Thus, females of low and males of high c o m p e t i t i v e a b i l i t y should be most 133 l i k e l y to breed i n polygynous groups. RESULTS O b s e r v a t i o n a l Study Frequency of polygyny T h i r t y - s i x of a l l 175 females (21%) that bred from 1982-6 d i d so i n a polygynous group at l e a s t once. However, only 3 females bred e x c l u s i v e l y i n polygynous groups, each breeding f o r one season, and only 2 bred i n polygynous groups i n more than one season. Seventeen of 136 males (13%) bred polygynously d u r i n g t h e i r l i f e t i m e s . Two males bred i n trigamous groups. When ana l y s e d as the t o t a l number of matings, 9.5% of a l l matings (67 of 706) i n v o l v e d polygyny, because most females that bred i n polygynous groups a l s o bred monogamously i n the same season. O r i g i n s of polygyny Polygynous matings arose i n two ways (Table 6.1). Seven cases r e s u l t e d when n o n - t e r r i t o r i a l females s e t t l e d on the t e r r i t o r y of a p a i r or t r i o (1 c a s e ) . In 5 of these 7 cases, f l o a t i n g females s e t t l e d on the t e r r i t o r i e s of females that were i n c u b a t i n g eggs and t h e r e f o r e c o u l d not prevent the incoming female from s e t t l i n g . Another 12 cases o c c u r r e d when mated 134 t e r r i t o r i a l males usurped a neighbouring t e r r i t o r y and female, e i t h e r by e v i c t i n g the neighbour or a f t e r the neighbour disappeared (one case l e d to a trigamous group). T e r r i t o r y takeovers by neighbours most o f t e n r e s u l t e d i n polygyny when they took p l a c e d u r i n g the main breeding p e r i o d , from A p r i l through June (Table 6.1). O v e r a l l , 20% (11 of 56) of takeovers r e s u l t e d i n polygyny when they o c c u r r e d from A p r i l through June, compared to only 3% (1 of 34) o u t s i d e of the breeding p e r i o d (P=0.03, F i s h e r ' s exact t e s t ) . Ages of b i r d s i n polygynous groups Females 1- and 4-years-old were more l i k e l y to be p a r t of a polygynous group than were 2- or 3-year-olds when compared to the numbers expected from the a g e - d i s t r i b u t i o n of monogamous females ( F i g . 6.1a; G =10.00, P<0.01, df=3, 1 - t a i l e d , p r e d i c t i o n 3 ). The opposite p a t t e r n was observed f o r males ( F i g . 6.1b). One-year-old males never became polygynous, 2-y e a r - o l d s d i d so much more f r e q u e n t l y than expected by the age-d i s t r i b u t i o n of monogamous b i r d s , and 3- and 4-year-olds d i d so as o f t e n as expected (X =7.68, P<0.05, df=3, 1 - t a i l e d , p r e d i c t i o n 4 ) . 135 Table 6.1. Seasonal frequency of t e r r i t o r y takeovers by neighbouring owners that d i d or d i d not lead to polygynous matings, and the frequency of polygynous matings that r e s u l t e d from the settlement of f l o a t e r females. Month Takeovers J F M A M J J A S O N D Led to Polygyny 0 1 0 3 5 3 0 0 0 0 0 0 Did Not 4 3 1 3 1 6 1 9 6 1 2 4 2 1 3 Female 0 1 1 4 1 0 0 0 0 0 0 0 Settlement A) F e m a l e s B) M a l e s Figure 6.1. The percent of males and females observed to mate i n polygynous groups i n r e l a t i o n to age, compared to that expected by the number of monogamous breeders of each age over the study. 137 Male a i d and female r e p r o d u c t i v e success Monogamous males t y p i c a l l y gave alarm c a l l s when humans or p r e d a t o r s entered t h e i r t e r r i t o r i e s , e s p e c i a l l y when a c t i v e nests or newly f l e d g e d young were p r e s e n t . These males a l s o fed t h e i r young r e g u l a r l y from h a t c h i n g to independence (between day 25-32). Polygynous males behaved s i m i l a r l y , but t y p i c a l l y spent most of t h e i r time near only one female. The second female i n a t e r r i t o r y was f r e q u e n t l y ignored by her mate, and i n no case d i d polygynous males s i m u l t a n e o u s l y feed more than one brood of young. Thus, females mated to polygynous males f r e q u e n t l y l a c k e d male v i g i l a n c e d u r i n g i n c u b a t i o n and male a s s i s t a n c e while feeding n e s t l i n g s and f l e d g l i n g s . Unaided females produced fewer independent o f f s p r i n g per breeding attempt than a i d e d or monogamous females i n a l l 5 years of t h i s study (Table 6.2). Unaided females produced from 0-34% as many young per n e s t i n g attempt on average as d i d females with male a i d . O v e r a l l , both monogamous and a i d e d females produced s i g n i f i c a n t l y more young per attempt than unaided females (P=0.004 and P=0.005, r e s p e c t i v e l y ; Mann-Whitney U - t e s t s on r e s i d u a l s s t a n d a r d i z e d f o r y e a r l y d i f f e r e n c e s i n r e p r o d u c t i v e success, see Table 6.2; Arcese & Smith 1988). However, monogamous and a i d e d females i n polygynous groups produced approximately equal numbers of young (P=0.48, U-test as above, Table 6.2). 138 Table 6.2. Mean and SE of the number of independent young r a i s e d per n e s t i n g attempt by monogamous, aided and unaided females. Females known or presumed to have had access to supplemental food i n 1985 are excluded. Year Monogamous Aided Unaided 1982 2.02+0.19 Females, Attempts 26,60 0,0 0.50±0.50 2,2 1 983 1.42+0.12 Females, Attempts 50,121 1.70+0.52 0.20+0.20 3,7 3,5 1984 0.96+0.09 Females, Attempts 53,135 2.00 1,*1 0.33+0.21 5,6 1985 0.45+0.07 Females, Attempts 49,129 0.50+0.50 0.11+0.10 2,2 5,9 1986 Females, Attempts 0.42+0.06 54,144 0.45+0.21 8,16 0.00+0.00 9,15 139 The number of young r a i s e d per n e s t i n g attempt a l s o changed as expected when females gained or l o s t the a i d of males w i t h i n breeding seasons. In 5 of 8 cases female r e p r o d u c t i v e success d e c l i n e d when females l o s t male a i d . In a l l 7 cases where females formerly without help gained the a i d of a male, r e p r o d u c t i v e success i n c r e a s e d (P=0.04, F i s h e r ' s exact t e s t ; there was no change i n r e p r o d u c t i v e success i n 3 cases of each t y p e ) . Male r e p r o d u c t i v e success Polygynous males s i r e d 1.4-4 times more independent young on average than d i d monogamous males, except i n 1984 when only one polygynous male bred (Table 6.3). Although the sample s i z e s f o r each year were s m a l l , the o v e r a l l d i f f e r e n c e i n the r e p r o d u c t i v e success of monogamous and polygynous males was s t a t i s t i c a l l y s i g n i f i c a n t (P<0.01, N =242 male-years, N =14 male-years, U-test as f o r females). F u r t h e r , 11 of the 14 polygynous males known not to have had access to supplemental food r a i s e d more young ( r e l a t i v e to the p o p u l a t i o n mean f o r the year) when polygynous than they had p r e v i o u s l y r a i s e d when monogamous. In comparison, 5 of these males bred monogamously l a t e r , and a l l r a i s e d fewer young (P=0.01, F i s h e r ' s exact t e s t ; 2 males d i e d before the next season, 7 males bred polygynously in the l a s t year of s t u d y ) . l<to Table 6.3. Mean and SE of number of independent young r a i s e d each year by monogamous and polygynous males. Males known or presumed to have had access to supplemental food i n 198 5 are excluded. Year Monogamous Polygynous 1982 4.33+0.83 N 27 0 1983 2.46+0.27 5.00+1.00 N 55 3 1984 2.35+0.25 2.00 N 58 1 1985 0.80+0.14 " 3.00+0.58 N 45 3 1986 1.16+0.14 1.67+0.31 N 58 6 141 Use of space by females Although no q u a n t i t a t i v e a n a l y s i s of o v e r l a p was conducted, the areas u t i l i z e d by males and females i n monogamous p a i r s were i n d i s t i n g u i s h a b l e even a f t e r i n t e n s i v e o b s e r v a t i o n , except dur i n g the e a r l y p e r i o d s of n e s t - b u i l d i n g . During t h i s p e r i o d , females f r e q u e n t l y expanded t e r r i t o r y boundaries around newly l o c a t e d n e s t - s i t e s , which were sometimes beyond t h e i r mate's pre v i o u s boundaries (Table 6.4). Such movements o f t e n preceded vigorous c o n f l i c t s between neighbouring males, e s p e c i a l l y when the male being encroached upon was not occupied with f e e d i n g young. Males responded to females l o c a t i n g new nests near or o u t s i d e a p r e v i o u s t e r r i t o r y boundary by expanding t h e i r t e r r i t o r i e s to i n c l u d e the new n e s t - s i t e . In c o n t r a s t , male boundary s h i f t s r a r e l y o c c u r r e d when females b u i l t new nests w e l l - w i t h i n the l i m i t s of an e s t a b l i s h e d t e r r i t o r y (G =21.44, P<0.001, df=2, Table 6.4). Most females i n polygynous groups overlapped l i t t l e i n the areas they used, and they o f t e n engaged i n a g g r e s s i v e d i s p l a y s ( i . e . puff-wave, c h a t t e r , caterwaul, song; Nice 1943:274; Chap. 5) along borders that c o u l d be mapped e a s i l y . These d i s p l a y s were most obvious when females nested i n c l o s e p r o x i m i t y , and when n e i t h e r was i n c u b a t i n g or c a r i n g f o r young. Males were not observed to p a r t i c i p a t e i n female c o n f l i c t s . In s t r i k i n g c o n t r a s t to the behaviour of most females, 3 1^ 2. Table 6.4. S h i f t s i n male t e r r i t o r y boundaries subsequent to females p l a c i n g t h e i r n e w l y - b u i l t nests beyond ( i . e . o u t s i d e the t e r r i t o r y ) , near ( w i t h i n 6m of P c u r r e n t boundary) or w e l l - w i t h i n (>6m from) t h e i r mate's p r e v i o u s t e r r i t o r y boundaries. Data from 1982 only. Boundary S h i f t <6m £6m D i s t a n c e of new Beyond 0 5 nest from o l d <6m 4 8 boundary ^6m 16 143 females behaved a g g r e s s i v e l y only when t e r r i t o r i a l females entered the area immediately surrounding t h e i r n e s t s . These b i r d s were o f t e n chased by t e r r i t o r i a l female neighbours, but they r e g u l a r l y fed s e c r e t i v e l y throughout t e r r i t o r i e s adjacent to t h e i r n e s t s . Each of these females behaved most prominently (by v o c a l i z a t i o n and posture) when t h e i r neighbour incubated or tended young, and at these times they o f t e n i n t e r a c t e d with the male whose t e r r i t o r y i n c l u d e d t h e i r n e s t - s i t e . One such female was prevented from i n c u b a t i n g a n e w l y - l a i d c l u t c h by an ag g r e s s i v e female neighbour i n c u b a t i n g 3m d i s t a n t . None of these females s u c c e s s f u l l y hatched young i n polygynous groups, but a l l r a i s e d young l a t e r i n monogamous attempts (1 the same year, 2 the f o l l o w i n g y e a r ) . Aggression among females was common when female f l o a t e r s attempted to s e t t l e w i t h i n the boundaries of t e r r i t o r i a l females. On these o c c a s i o n s , f i g h t s i n v o l v i n g p h y s i c a l c o n t a c t were common and females sometimes sang (Chap.5; see a l s o Nice 1937; Tompa 1963). However, t e r r i t o r i a l females seldom engaged in defence a f t e r i n c u b a t i o n began. At t h i s time, over 45 min/hr were u s u a l l y spent on the nest and time o f f the nest was p r i m a r i l y spent f o r a g i n g (see below). Sex r a t i o The y e a r l y p r o p o r t i o n of polygynous males v a r i e d from 0-10.5% d u r i n g t h i s study, and from 0-9.8% duri n g an e a r l i e r study 1 44 by Smith et a l . (1982). Combining these data showed that the frequency of polygyny i n males i n c r e a s e d as the r a t i o of t e r r i t o r i a l females to males i n c r e a s e d d u r i n g 10 years (r=0.68, N=10, P<0.02, 1 - t a i l e d ; p r e d i c t i o n 10). One year, when sex r a t i o was a l t e r e d e x p e r i m e n t a l l y , was excluded (1979, Smith et a l . 1982). Food A d d i t i o n Study Body mass Females with supplemental food outweighed c o n t r o l females by over 3g on average only 2 weeks a f t e r f e e d i n g commenced (means (SE) f o r 6 fed and 7 c o n t r o l females: 26.62 (0.41) and 23.41 (0.23), t=6.76, P<0.001, df=8.1). The d i f f e r e n c e was only s l i g h t l y l e s s among males (8 fed and 10 c o n t r o l s ; 27.46 (0.37) and 25.01 (0.31), t=5.07, P<0.001, df=14.6). Thus, males and females not only used the feeders immediately, but a l s o gained c o n s i d e r a b l e mass as a r e s u l t . Male response to taped song To t e s t the immediate e f f e c t of supplemental food on the response of males to simulated i n t r u s i o n s , I played 1 min of taped song from a recorder p l a c e d at the edge of 14 fed and 27 c o n t r o l t e r r i t o r i e s , 3 days a f t e r f e e d i n g began. I subsequently observed the t e r r i t o r i e s f o r 4 min to determine i f 145 males responded by perching above the shrub canopy, approaching the speaker or v o c a l i z i n g . Males that were not observed or heard were recorded as not responding. Ten of 14 fed males responded to playbacks, compared to 7 of 27 c o n t r o l s (G =7.65, P<0.01, d f = l ) . Responses by females to taped songs are d e s c r i b e d i n chapter 5. Time budgets Supplemental food d r a m a t i c a l l y reduced the p r o p o r t i o n of time females spent f e e d i n g , but i n c r e a s e d t h e i r time spent preening s l i g h t l y and i n a l e r t behaviour g r e a t l y ( a l l P<0.001, U - t e s t s , N =13, N =14; F i g . 6.2). Fed females engaged i n s l i g h t l y more defence and d i s p l a y , and l e s s time out of s i g h t , but these d i f f e r e n c e s were not s t a t i s t i c a l l y s i g n i f i c a n t ( a l l P>0.10, U - t e s t s as above; F i g . 6.2). Fed females spent s l i g h t l y more time n e s t - b u i l d i n g than c o n t r o l s (P=0.05, U-test as above). Two of 14 c o n t r o l s flew to areas out of s i g h t of t h e i r t e r r i t o r i e s d u r i n g t h e i r time budgets (presumably to f e e d ) , but no fed females d i d so. Males responded to food a d d i t i o n i n the same way as females ( F i g . 6.3). Males fed much l e s s , preened more and spent much more time i n a l e r t behavior when given food ( a l l P<0.001, U-t e s t s , N =14, N =14, pooled as above). The p r o p o r t i o n of time spent i n defence or d i s p l a y d i d not d i f f e r with treatment (P>0.10, U - t e s t s ) . However, c o n t r o l males spent much more time A l e r t D e f e n c e D i s p l a y F e e d P r e e n Out of S i g h t F C F Figure 6.2. Median, in t e r - q u a r t i l e range, and the range of the data for the percent of spent i n each of 6 behaviours by fed and control females (see Methods) . C and F denote replicates from each fed (N=6,7) and control plot (N=7,7). 147 out of s i g h t than fed males (P<0.001, U - t e s t ) . F u r t h e r , 5 of 14 c o n t r o l males flew out of s i g h t of t h e i r t e r r i t o r i e s d u r i n g time budgets, whereas none of 14 fed males d i d so (P=0.04, F i s h e r ' s exact t e s t ) . Fed b i r d s of each sex spent a l a r g e amount of time a l e r t . F u r t h e r , more chases of n o n - t e r r i t o r i a l i n t r u d e r s f o l l o w e d p e r i o d s of a l e r t behavior than expected i f chases had f o l l o w e d a l l b ehaviors e q u a l l y (G =87.25, P<0.001, df=3; adjacent c l a s s e s were pooled to a v o i d expected values <5.0; Sokal & Rohlf 1981). Settlement of n o n - t e r r i t o r i a l females Four a d u l t and 32 y e a r l i n g females had not occupied t e r r i t o r i e s when the f e e d i n g experiment commenced. At t h i s time, 49 females occupied t e r r i t o r i e s i n monogamous p a i r s (34 c o n t r o l s and 15 fed p a i r s ) . Four unmated males a l s o defended t e r r i t o r i e s , of which 2 were on marginal s i t e s never s e t t l e d by females i n the 5 years of t h i s study. E g g - l a y i n g commenced i n l a t e - A p r i l , and by mid-May the l a s t n o n - t e r r i t o r i a l female had e i t h e r s e t t l e d or disappeared from the i s l a n d . Table 6.5 shows that 29 of these females e v e n t u a l l y s e t t l e d on Mandarte. One female s e t t l e d on H a l i b u t I s l a n d (1.3 km d i s t a n t ) and 6 females disappeared. Both males on marginal s i t e s abandoned t h e i r t e r r i t o r i e s and became f l o a t e r s , 1 unmated male disappeared and the f o u r t h gained a female. A l e r t D e f e n c e S i n g F e e d P r e e n C F C F Out of S i g h t C F C F Figure 6.3. Percent of time spent i n each of 6 behaviours for 6 and 8 fed, and 7 and 7 control males. Conventions as for Figure 6.3. 149 Of the 29 n o n - t e r r i t o r i a l females that s e t t l e d on Mandarte, s i g n i f i c a n t l y fewer d i d so on fed t e r r i t o r i e s than expected i f the p r o b a b i l i t y of s e t t l i n g on each t e r r i t o r y defended at the out s e t of feed i n g had been equal (G =8.16, P<0.002, df=1, 1-t a i l e d , Table 6.5; p r e d i c t i o n 6). F u r t h e r , a l l 4 cases of polygyny that r e s u l t e d from settlement by n o n - t e r r i t o r i a l females o c c u r r e d on c o n t r o l t e r r i t o r i e s . Settlement of n o n - t e r r i t o r i a l males Three a d u l t and 32 y e a r l i n g males a l s o behaved as f l o a t e r s when the fe e d i n g experiment commenced. However, only 17 (49%) of these males e v e n t u a l l y s e t t l e d , as compared to 29 of 36 (81%) female f l o a t e r s (G =7.97, P<0.005, df=1; Table 6.5). Nine males disappeared from the i s l a n d and 9 remained n o n - t e r r i t o r i a l throughout the breeding p e r i o d . Two males s e t t l e d on fed t e r r i t o r i e s (1 gained access to a feeder) and 15 s e t t l e d on c o n t r o l s . F l o a t e r males s e t t l e d l e s s o f t e n than expected on fed t e r r i t o r i e s , but t h i s d i f f e r e n c e i s not q u i t e s t a t i s t i c a l l y s i g n i f i c a n t (G =2.54, P<0.07, df=1, 1 - t a i l e d , Table 6.5; p r e d i c t i o n 8 ) . T h i s d i f f e r e n c e was h i g h l y s i g n i f i c a n t when both sexes were pooled (G =10.06, P<0.001, df=1). J50 Table 6.5. Observed settlement of male and female f l o a t e r s as compared to that expected i f the p r o b a b i l i t y of settlement on each t e r r i t o r y occupied at the outset of f e e d i n g had been equal. C o n t r o l Experimental T o t a l T e r r i t o r i e s defended 38 15 53 (28 February) Observed female 27 2 29 s e t t l e r s Expected female 20.8 8.2 29 s e t t l e r s Observed male 15 2 17 s e t t l e r s Expected male 12.2 4.8 17 s e t t l e r s 151 DISCUSSION A l l 10 p r e d i c t i o n s that f o l l o w from the h y p o t h e s i s that i n t r a s e x u a l c o mpetition i n each sex a f f e c t s the mating system of song sparrows were at l e a s t p a r t l y supported i n t h i s study. The outcome of such c o m p e t i t i o n i n f l u e n c e s which i n d i v i d u a l s mate i n polygynous groups, and thus a f f e c t s r e p r o d u c t i v e success i n both males and females. In the f o l l o w i n g s e c t i o n s , I d i s c u s s the resources that l i m i t r e p r o d u c t i v e success i n each sex, the s p e c i f i c p r e d i c t i o n s made reg a r d i n g song sparrows on Mandarte I s l a n d and the i n f l u e n c e of female c o m p e t i t i o n i n the maintenance of monogamy. Competition f o r L i m i t i n g Resources For females on Mandarte, the resources that l i m i t r e p r o d u c t i o n are access to a high q u a l i t y breeding s i t e , food and male care (Smith 1988; Arcese & Smith 1988; Hochachka et a l . ms; Table 6.2). Most females i n t h i s study e v e n t u a l l y gained breeding s i t e s (Table 6.5), but the f a t e s of females that disappeared from the i s l a n d remain unknown. In c o n t r a s t , s e v e r a l males that remained i n the p o p u l a t i o n d i d not breed e i t h e r because they c o u l d not g a i n a t e r r i t o r y or c o u l d not h o l d one (Smith & Arcese ms; Chap. 3 & 4). However, many males simultaneously gained access to 2-3 females. T h i s suggests that c o m p e t i t i o n i s more intense among males than females, as expected when females can r a i s e young without male a i d ( T r i v e r s 152 1972). However, the p o t e n t i a l f o r i n t r a s e x u a l competition i n females i s s u b s t a n t i a l because females that defend high q u a l i t y t e r r i t o r i e s , i n a d d i t i o n to e x c l u s i v e access to a male, produce more o f f s p r i n g than those that do not (Arcese & Smith 1988; Hochachka et a l . ms; Table 6.2). Female c o m p e t i t i o n and t e r r i t o r y defence has been documented i n many s p e c i e s i n c l u d i n g monogamous open-nesters (Smith 1978; Hannon & Zwickel 1979; Hannon 1984; Harper 1985) and h o l e - n e s t e r s (Gowaty 1981; Stutchbury & Robertson 1985), and i n polygynous (Yasukawa & Searcy 1981,1982; Hurley & Robertson 1984,1985; D i c k i n s o n & L i e n 1987; but see Searcy 1986) and polyandrous s p e c i e s (Oring 1982; P e t r i e 1983). A p a r t i c u l a r l y s t r i k i n g example of the r o l e s of male and female i n t r a s e x u a l c o m petition i n mating system o r g a n i z a t i o n i s seen i n the dunnock, ( P r u n e l l a modular i s ) . Davies & Lundberg ( 1984) showed that by adding supplemental food i n a garden p o p u l a t i o n of dunnocks, female t e r r i t o r y s i z e was reduced and males became b e t t e r able to defend t h e i r mates a g a i n s t e x t r a - p a i r c o p u l a t i o n s with subordinate, s a t e l l i t e males. Davies (1985) and Davies & Houston (1986) l a t e r concluded that female c o m p e t i t i o n f o r space and e x c l u s i v e access to males was i n t e n s e i n t h i s p o p u l a t i o n . Females that gained the a i d of a d d i t i o n a l males subsequently r a i s e d more young. At the same time, s e v e r a l females mated i n polygynous t r i o s , shared t h e i r mate's a i d and produced fewer young. T h i s suggests that c o mpetition i n c r e a s e d a f t e r the a d d i t i o n of food and caused a r e d u c t i o n i n female t e r r i t o r y s i z e 153 (Hixon 1980; Norton et a l . 1982). Stobo & McLaren (1975) d e s c r i b e d a s i m i l a r s c e n a r i o . f o r a p r i m a r i l y monogamous, n a t u r a l p o p u l a t i o n of savannah sparrows, P a s s e r c u l u s s a n d v i c h e n s i s . Male A i d and Reproductive Success In song sparrows and s e v e r a l other s p e c i e s (Weatherhead 1979; Greenlaw & Post 1985; Hannon 1984; Davies & Houston 1986), females that do not r e c e i v e a i d from t h e i r mate r a i s e fewer young (Smith et a l . 1982; Table 6.2). However, whether females i n polygynous groups r e c e i v e a i d o f t e n depends on n e s t i n g synchrony (Verner 1964; Smith et a l . 1982; t h i s s t u d y ) . In other s p e c i e s , males p a r t i t i o n t h e i r a i d between concurrent nests (Martin 1979; Davies & Houston 1986). Verner (1964) suggested that s e t t l e d females should r e p e l a d d i t i o n a l s e t t l e r s when t h i s prevented the l o s s of male a i d , or that they should stagger nest i n i t i a t i o n so that males might a i d females i n t u r n . Wittenberger (1979) and O r i a n s (1980) used t h i s idea to e x p l a i n why i n r e g u l a r l y polygynous s p e c i e s , females are most a g g r e s s i v e p r i o r to nest i n i t i a t i o n . I m p l i c i t i n t h i s reasoning i s the assumption that the r i s k of l o s i n g male a i d d i m i n i s h e s once a female i n i t i a t e s a nest. However, three f a c t o r s suggest t h a t the r i s k of l o s i n g male a i d does not d i m i n i s h s u b s t a n t i a l l y f o r females that have begun i n c u b a t i o n i n sp e c i e s l i k e the song sparrow. F i r s t , because 1 54 nest f a i l u r e i s frequent (Nice 1943; Arcese & Smith 1988), i t i s probably d i f f i c u l t f o r females to p r e d i c t the r i s k of l o s i n g male a i d when i n polygynous groups. Second, when s p e c i e s are multi-brooded, r e c e i v i n g a i d i n a f i r s t attempt does not ensure i t in l a t e r attempts. T h i r d , i n t h i s study 5 of 7 cases where female f l o a t e r s s e t t l e d onto the t e r r i t o r y of a monogamous p a i r o ccurred a f t e r the r e s i d e n t female had begun to incubate. For song sparrows, a r e d u c t i o n i n defence time i s i n e v i t a b l e once i n c u b a t i o n begins. Thus, when male care i s a key resource at r i s k , females might do b e t t e r to delay nest i n i t i a t i o n u n t i l most female f l o a t e r s have s e t t l e d each s p r i n g , than to i n i t i a t e n e s t i n g e a r l y and t r y to ensure that t h e i r nests hatch before those of subsequent s e t t l e r s . The i n f l u e n c e of female spacing behaviour should be sm a l l e r when r e p r o d u c t i v e success does not depend on male care (Orians 1969; T r i v e r s 1972; Oring 1982; see I n t r o d u c t i o n ) . Yasukawa & Searcy (1982) t e s t e d t h i s p r e d i c t i o n by comparing female aggression and n e s t i n g synchrony i n an e a s t e r n p o p u l a t i o n of red-winged b l a c k b i r d s where males p r o v i s i o n e d n e s t l i n g s , to that i n a western p o p u l a t i o n where they d i d not. Nest i n i t i a t i o n was asynchronous where males fed young, but not where females r a i s e d young alone, suggesting that a g g r e s s i o n by females spaced n e s t i n g attempts i n time i n p o p u l a t i o n s where males provide non-shareable care that improves female r e p r o d u c t i o n . 1 55 Polygyny and Male Reproductive Success In c o n t r a s t to the case f o r females, polygyny i n c r e a s e s male r e p r o d u c t i v e success i n s p e c i e s with a v a r i e t y of mating systems (Orians 1980; Davies & Houston 1986; Clutton-Brock et a l . 1982; Freed 1987; Table 6.3). T h i s i s expected, given that females can r a i s e some young without male a i d , and i t h i g h l i g h t s an important d i f f e r e n c e between males and females that probably a f f e c t s behaviour ( T r i v e r s 1972). In p r i m a r i l y monogamous s p e c i e s , the p o t e n t i a l f o r a d d i t i o n a l matings probably e x p l a i n s the a g g r e s s i v e behaviour of males that i n c r e a s e t e r r i t o r y s i z e by e v i c t i n g neighbours and p r e v e n t i n g settlement by f l o a t e r s (e.g. McLaren 1972; Ueda 1986; Freed 1987; Chap. 4; Table 6.1). Frequency of Polygyny Polygyny i s an uncommon but r e c u r r e n t f e a t u r e of both the song sparrow p o p u l a t i o n on Mandarte and p a s s e r i n e p o p u l a t i o n s i n gene r a l (Verner & Wilson 1969; M o l l e r 1986). The low annual l e v e l of polygyny observed i n t h i s study confirms a general p r e d i c t i o n made i n the I n t r o d u c t i o n , given the s u b s t a n t i a l c o n t r i b u t i o n of male a i d and t e r r i t o r y q u a l i t y to female r e p r o d u c t i v e success (Smith et a l . 1982; Arcese & Smith 1988; Hochachka et a l . ms; Table 6.2). Ne v e r t h e l e s s , over 5 years 21% of females and 13% of males bred i n polygynous groups at l e a s t once. Although song sparrows d i s p l a y a low annual rate 1 56 of polygyny (0-10.5% of males; Nice 1943; Smith et a l . 1982; Table 6.2 & 6.3), I suggest that the r e p r o d u c t i v e consequences of polygyny i n f l u e n c e s the behaviour of both males and females c o n s i d e r a b l y . Proximate O r i g i n s of Polygyny Polygyny occurred most o f t e n when males annexed neighbouring t e r r i t o r i e s , but a l s o a f t e r f l o a t e r females s e t t l e d onto occupied t e r r i t o r i e s (Table 6.1). In 5 of 7 cases, f l o a t e r females s e t t l e d onto the t e r r i t o r i e s of i n c u b a t i n g females. These r e s u l t s support the idea that the a b i l i t y t o engage in t e r r i t o r y defence i s an important determinant of which b i r d s become polygynous i n t h i s p o p u l a t i o n (McLaren 1972). T h i s i s a l s o true f o r males i n r e g u l a r l y polygynous t e r r i t o r i a l s p e c i e s , where co m p e t i t i v e a b i l i t y determines access to h a b i t a t and mates (e.g. Robinson 1986; review i n Searcy & Yasukawa 1983) . Options of Females i n Polygynous Groups F o l l o w i n g takeovers by neighbouring males or settlement by a d d i t i o n a l females, females c o u l d respond i n four ways: (1) e v i c t a female neighbour, (2) d i s p e r s e , (3) a t t r a c t an unmated male, or (4) remain i n a polygynous group. The c o s t or b e n e f i t of each o p t i o n w i l l vary depending on a females c u r r e n t commitment of time and energy to r e p r o d u c t i o n , and with the 157 a v a i l a b i l i t y of a l t e r n a t i v e t e r r i t o r i e s and males. E v i c t i n g t e r r i t o r y owners i s e n e r g e t i c a l l y c o s t l y and p h y s i c a l l y r i s k y (Chap.3 & 4). D i s p e r s a l d u r i n g the breeding p e r i o d occurs (Smith et a l . 1982), but i s uncommon, probably because unmated t e r r i t o r i a l males are u s u a l l y r a r e and f r e q u e n t l y defend patches of h a b i t a t r a r e l y used f o r breeding. F u r t h e r , females that abandon t e r r i t o r i e s to search f o r unmated males r i s k l o s i n g t h e i r o l d t e r r i t o r y should they f a i l to l o c a t e a s u i t a b l e s i t e (see Krebs 1983; Chap. 3 & 4 f o r analogous examples i n males). Mate a t t r a c t i o n w i l l only be s u c c e s s f u l i f the present male cannot r e p e l p o t e n t i a l s e t t l e r s , which i s u n l i k e l y i f polygyny i n i t i a l l y r e s u l t e d from a takeover. Females are t h e r e f o r e l i m i t e d i n t h e i r o p p o r t u n i t i e s to a v o i d polygyny once breeding commences. In c o n t r a s t , females should be most able to defend e x c l u s i v e t e r r i t o r i e s i n f a l l , when feeding c o n d i t i o n s are r e l a t i v e l y good and there are no time or energy commitments to r e p r o d u c t i o n . E a r l y s p r i n g and f a l l are the times when female t e r r i t o r i a l behaviour i s most o f t e n observed i n song sparrows (Nice 1937; Tompa 1964; Smith et a l . 1982; Chap. 5). T e r r i t o r y defence by females at these times c o u l d minimize f u t u r e competition i f i t caused u n s e t t l e d y e a r l i n g females to d i s p e r s e . In t h i s study, the p r o p o r t i o n of y e a r l i n g females that r e c r u i t e d to the po p u l a t i o n d e c l i n e d as the d e n s i t y of t e r r i t o r i a l females i n f a l l i n c r e a s e d (range=66-32%; R =-0.94, N=6, P<0.05, 1981-86 c o h o r t s ) . F u r t h e r , l a t e summer, f a l l and e a r l y s p r i n g are the 158 p e r i o d s d u r i n g which most y e a r l i n g s disappear from the p o p u l a t i o n (Tompa 1964; Chap. 7). T h i s suggests that f a l l t e r r i t o r i a l i t y reduces p o p u l a t i o n s i z e and consequently reduces competition f o r breeding t e r r i t o r i e s the f o l l o w i n g s p r i n g (Tompa 1964; Brown 1969; Watson & Moss 1970; P a t t e r s o n 1980). Female t e r r i t o r i a l i t y has been shown to prevent recruitment i n the song sparrow i n l a t e winter (Knapton & Krebs 1974), and i n s e v e r a l other s p e c i e s d u r i n g the breeding p e r i o d ( V i l l a g e 1983; Hannon 1983,1984; Hurley & Robertson 1985; Stutchbury & Robertson 1985). Age and Polygyny The p r o b a b i l i t y of breeding i n a polygynous group depended on age ( F i g . 6.1). T h i s confirms two important p r e d i c t i o n s concerning the r e l a t i o n s h i p between polygyny and c o m p e t i t i v e a b i l i t y i n song sparrows ( p r e d i c t i o n s 3 & 4), and suggests that polygyny arose most o f t e n among the females l e a s t able to engage in t e r r i t o r y defence and the males most able to do so. H a b i t a t S a t u r a t i o n , Sex R a t i o and Polygyny A l l breeding h a b i t a t on Mandarte was defended even when d e n s i t y was low (e.g. 25 p a i r s ) . As d e n s i t y i n c r e a s e d , so d i d the p r o p o r t i o n of males that f l o a t e d f o r p a r t or a l l of the breeding season (Smith & Arcese ms). Female f l o a t e r s o c c a s i o n a l l y e x i s t e d i n the e a r l y breeding season when d e n s i t y 159 was high ( i . e . 60-70 p a i r s ; Chap. 7), but these b i r d s e i t h e r s e t t l e d or disappeared soon a f t e r breeding commenced. T h i s c o u l d mean th a t i n g e n e r a l there are more breeding p o s i t i o n s a v a i l a b l e to f l o a t i n g females than to males. T h i s might a r i s e i n four ways. F i r s t , because y e a r l i n g s u r v i v a l i s s l i g h t l y lower i n females than males (Smith et a l . 1980; Arcese & Smith 1985), the number of candidate males o f t e n exceeds that f o r females. Second, females t h a t might otherwise f l o a t sometimes s e t t l e i n t o polygynous groups. T h i r d , the m o r t a l i t y r a t e among a d u l t females s l i g h t l y exceeds that f o r males (Nol & Smith 1987), which r e s u l t s i n more empty t e r r i t o r i e s a v a i l a b l e f o r females than males. F i n a l l y , females from Mandarte sometimes breed elsewhere and an immigrant female s e t t l e s on Mandarte n e a r l y every year (Chap. 7). However, j u s t 2 immigrant and no emigrant males are known to have bred. Because the sex r a t i o of unbanded d i s p e r s e r s to Mandarte i s roughly e q u a l , t h i s suggests t h a t n o n - t e r r i t o r i a l females are more l i k e l y to s e t t l e o u t s i d e the n a t a l p o p u l a t i o n than males. These f a c t o r s c o n t r i b u t e s l i g h t to moderate b i a s e s i n the r a t i o of t e r r i t o r i a l females to t e r r i t o r i a l males that has favored males i n 14 of 15 years (range=0.64-1.07, mean=0.92, estimated each s p r i n g ; Tompa 1963; Smith 1988; t h i s s t u d y ) . Polygyny i n males i n c r e a s e d with the r a t i o of t e r r i t o r i a l females to males, as expected i f the r e l a t i v e l e v e l s of i n t r a s p e c i f i c c o m petition i n f l u e n c e the p o p u l a t i o n l e v e l of 160 polygyny (Emlen & Oring 1977; Smith et a l . 1982; p r e d i c t i o n 10). However, the r a t i o of t e r r i t o r i a l females to males f r e q u e n t l y exceeded the t o t a l a d u l t sex r a t i o because s e v e r a l males o f t e n behaved as f l o a t e r s (adult sex r a t i o : range=0.49~ 1.07, mean=0.78, 1975-8 and 1981-6; Smith 1988; t h i s s t u d y ) . N e v e r t h e l e s s , these measures of sex r a t i o were p e r f e c t l y c o r r e l a t e d (R =0.99, N=10). Emlen & Oring (1977) p r e d i c t e d that monogamy should predominate when the r a t i o of s e x u a l l y a c t i v e males to females (the ' o p e r a t i o n a l sex r a t i o ' , OSR) equals 1, but that polygamy should occur when the OSR d e v i a t e s from 1. In p a r t i a l support of t h i s h y p o t h e s i s , Smith et a l . (1982) i n c r e a s e d the l e v e l of polygyny a f t e r r a i s i n g the OSR by removing t e r r i t o r i a l males and t h e i r replacements d u r i n g breeding. F u r t h e r , i n the dunnock, a s p e c i e s with s e v e r a l l i f e h i s t o r y c h a r a c t e r i s t i c s , h a b i t a t p r e f e r e n c e s and behaviours i n common with song sparrows, polyandry o c c u r r e d i n 3 years when the OSR ranged from 0.67-0.89 (Davies & Lundberg 1984). In c o n t r a s t , although sex r a t i o s on Mandarte were o f t e n even more b i a s e d towards an excess of males than i n the dunnock, simultaneous polyandry has never been observed. I suggest that the absence of polyandry i s due the extreme i n t o l e r a n c e towards i n t r u d e r s d i s p l a y e d by t e r r i t o r i a l male song sparrows (compare Davies 1985 and Chap. 3 & 4). 161 Use of Space by Females D e t a i l e d o b s e r v a t i o n s over 5 breeding seasons s t r o n g l y suggested that male and female t e r r i t o r y boundaries were contiguous, except when males defended the t e r r i t o r i e s of more than one female, or t e m p o r a r i l y when females b u i l t t h e i r nests near or beyond t h e i r mate's t e r r i t o r y boundaries. In the l a t t e r case, males n e a r l y always a g g r e s s i v e l y expanded t h e i r t e r r i t o r y to surround the new n e s t - s i t e (Table 6.4). Such o b s e r v a t i o n s are common i n e a r l y accounts of t e r r i t o r y e s tablishment i n p r i m a r i l y monogamous b i r d s (Lanyon 1956:486; Marl e r 1956:498; Snow 1956:441; Young 1956:449; F i c k e n 1962:626). More r e c e n t l y , D i c k i n s o n & L e i n (1987) showed q u a n t i t a t i v e l y t hat t e r r i t o r y boundaries and the r a t e of a g g r e s s i v e i n t e r a c t i o n between male red-winged b l a c k b i r d s depended on female nest placement. These r e s u l t s suggest that male and female t e r r i t o r y boundaries are o f t e n contiguous because males defend the areas u t i l i z e d by t h e i r mates, r a t h e r than because females r e s t r i c t t h e i r movements w i t h i n the areas t h a t males defend (Stobo & McLaren 1975; Smith et a l . 1982). E f f e c t of Food on Time Budgets Adding high q u a l i t y food on 15 t e r r i t o r i e s d r a m a t i c a l l y decreased the amount of time that b i r d s of each sex devoted to f o r a g i n g , the main a c t i v i t y of c o n t r o l song sparrows d u r i n g the same p e r i o d ( F i g . 6.2 & 6.3). Even so, fed b i r d s outweighed 162 c o n t r o l s of t h e i r r e s p e c t i v e sex by 10-13% on average. Fed b i r d s a l l o c a t e d s l i g h t l y more time to self-maintenance (e.g. preening) and r e p r o d u c t i o n (e.g. nest b u i l d i n g ) , but the most s t r i k i n g i n c r e a s e i n each sex o c c u r r e d f o r a l e r t behavior. A l e r t b i r d s perched prominently with f e a t h e r s sleeked and wings e r e c t and surveyed t h e i r surroundings. Chases of non-t e r r i t o r i a l i n t r u d e r s f o l l o w e d p e r i o d s of a l e r t behaviour s i g n i f i c a n t l y more o f t e n than p e r i o d s of a l l other behaviours. Fed females spent even more time a l e r t than c o n t r o l males (P<0.05, U - t e s t , F i g . 6.2 & 6.3). I conclude t h a t , as p r e d i c t e d , b i r d s responded to supplemental food by d e v o t i n g a g r e a t e r e f f o r t to p r e v e n t i n g the settlement of a d d i t i o n a l l o c a l breeders. Food D i s t r i b u t i o n , T e r r i t o r i a l i t y and Mating System C l a s s i c a l mating system t h e o r i e s p o s i t female mate or t e r r i t o r y c h o i c e as the p r i n c i p l e mechanism determining mating p a t t e r n s , while competition between females p l a y s a minor r o l e (Verner & Wilson 1966; Orians 1969; Bradbury & Vehrencamp 1977; Emlen & O r i n g 1977; Weatherhead & Robertson 1979; Wittenberger 1979; Murray 1984). A c c o r d i n g l y , f l o a t i n g females i n t h i s study should have s e t t l e d p r e f e r e n t i a l l y on fed t e r r i t o r i e s , assuming that they were able to sample t e r r i t o r y q u a l i t y , because r e p r o d u c t i v e success there was four times g r e a t e r than on c o n t r o l t e r r i t o r i e s (Arcese & Smith 1988). If one assumed no sampling by f l o a t e r s , random settlement should have o c c u r r e d . 1 6 3 However, female f l o a t e r s showed a strong tendency to s e t t l e i n areas where they subsequently reproduced p o o r l y (Table 6.5). T h i s s t r o n g l y supports the hypothesis that i n t r a s p e c i f i c t e r r i t o r i a l i t y a f f e c t s the maintenance of monogamy i n song sparrows, and suggests that models that do not inc l u d e the i n f l u e n c e of female c o m p e t i t i o n and spacing behaviour on mating systems can be r e j e c t e d . However, when female c o m p e t i t i o n and i n d i v i d u a l v a r i a t i o n i n c o m p e t i t i v e a b i l i t y are i n c o r p o r a t e d i n t o c u r r e n t models of the o r g a n i z a t i o n of mating systems (Orians 1969; Emlen & Orin g 1977; Wittenberger 1979; Oring 1982, Murray 1984), s e v e r a l new p r e d i c t i o n s are generated (see I n t r o d u c t i o n ) . A l l of these are supported by t h i s study. McLaren (1972) and Stobo & McLaren (1975) were the f i r s t to emphasize the importance of female spacing behaviour and the a b i l i t y of males to defend t e r r i t o r i e s i n the maintenance of monogamy i n b i r d s . More r e c e n t l y , Davies (1985) and Davies & Houston (1986) have s t r e s s e d that the nature and i n t e n s i t y of i n t r a s e x u a l c o mpetition i n each sex determines mating p a t t e r n s . They suggested i n p a r t i c u l a r that a lack of c o n s i d e r a t i o n f o r female competition has hampered our understanding of mating systems. Although the e v o l u t i o n of monogamy as the predominant mating system of b i r d s remains p o o r l y understood, t h i s study demonstrates how female resource defence p l a y s a r o l e i n the maintenance of monogamy. 1 64 REFERENCES Altmann, S. A., Wagner, S.S., & Lenington, S. 1977. Two models fo r the e v o l u t i o n of polygyny. Behav. E c o l . S o c i o b i o l . , 2, 397-410. Arcese, P. & Smith, J.N.M. 1985. Phenotypic c o r r e l a t e s and e c o l o g i c a l consequences of dominance i n song sparrows. J . Anim. E c o l . , 54, 817-830. Arcese, P. & Smith, J.N.M. 1988. The e f f e c t s of p o p u l a t i o n d e n s i t y and supplemental food on r e p r o d u c t i o n i n song sparrows. J . Anim. E c o l . , 56, i n p r e s s . Brown, J.L. 1969. 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Norton, M.E., Arcese, P. & Ewald, P.W. 1982. E f f e c t of i n t r u s i o n p r e s s u r e on t e r r i t o r y s i z e i n black-chinned hummingbirds ( A r c h i l o c h u s a l e x a n d r i ) . Auk, 99, 761-764. O r i a n s , G.H. 1969. On the e v o l u t i o n of mating systems i n b i r d s and mammals. Am. Nat., 103, 589-603. Or i a n s , G.H. 1980. Some Adaptations of Marsh N e s t i n g B l a c k b i r d s . New J e r s e y : P r i n c e t o n U n i v e r s i t y Press. Oring, L. W. 1982. Avian mating systems. In Avian B i o l o g y VI (Ed. by D.S. Farner, J.R. King, & K.C. Parkes), pp.1-85. New York: Academic P r e s s . 168 P a t t e r s o n , I. J . 1980. T e r r i t o r i a l behaviour and the l i m i t a t i o n of p o p u l a t i o n d e n s i t y . Ardea, 68, 53-62. P e t r i e , M. 1983. Female moorhens compete f o r small f a t males. Science, N.Y., 220, 413-414. Robinson, S.K. 1986. B e n e f i t s , c o s t s and determinants of dominance i n a polygynous o r i o l e . Anim. Behav., 34, 241-256. Searcy, W.A. 1982. The e v o l u t i o n a r y e f f e c t s of mate s e l e c t i o n . Ann. Rev. E c o l . S y s t . , 13, 57-85. Searcy, W.A. 1986. Are female red-winged b l a c k b i r d s t e r r i t o r i a l ? Anim. Behav., 34, 1381-1392. Searcy, W.A. & Yasukawa, K. 1983. Sexual s e l e c t i o n and red-winged b l a c k b i r d s . Amer. S c i . , 71, 166-174. Smith, J.N.M. 1988. Determinants of l i f e t i m e r e p r o d u c t i v e success i n the song sparrow. In Reproductive Success (Ed. by T.H. C l u t t o n - B r o c k ) , Chicago: Chicago U n i v e r s i t y P r e s s . Smith, J.N.M. & Roff, D.A. 1980. Temporal spacing of broods, brood s i z e , and p a r e n t a l care i n song sparrows (Melospiza  melodia). Can. J . Zoo l . , 58, 1007-1015. Smith, J.N.M. & Arcese, P. How f i t are f l o a t e r s : consequences of a l t e r n a t e t e r r i t o r i a l behaviors i n a non-migratory sparrow. Submitted ms. Smith, J.N.M., Yom-Tov, Y., & Moses, R. 1982. Polygyny, male p a r e n t a l c a r e , and sex r a t i o i n song sparrows: an experimental study. Auk, 99, 555-564. Smith, J.N.M., Montgomerie, R.D., T a i t t , M.J. & Yom-Tov, Y. 1980. A winter fe e d i n g experiment on an i s l a n d song sparrow p o p u l a t i o n . O e c o l o g i a , 47, 164-170. 169 Smith, S.M. 1978. The 'underworld' i n a t e r r i t o r i a l sparrow: ada p t i v e s t r a t e g y f o r f l o a t e r s . Am. Nat., 112, 571-582. Snow, D.W. 1956. T e r r i t o r y i n the b l a c k b i r d Turdus merula. I b i s , 98, 438-449. Stobo, W.T. & McLaren, I.A. 1985. The Ipswich Sparrow. H a l i f a x : Nova S c o t i a I n s t , of Science. Stutchbury, B.J. & Robertson, R.J. 1985. F l o a t i n g p o p u l a t i o n s of female t r e e swallows. Auk, 102, 651-653. Sokal, R.R. & Rohlf, F . J . 1981. Biometry. San F r a n c i s c o : W.H. Freeman & Company Tompa, F.S. 1963. F a c t o r s determining the numbers of song sparrows Melospiza melodia (Wilson) on Mandarte I s l a n d , B.C. Ph.D. d i s s . U n i v e r s i t y of B r i t i s h Columbia, Vancouver. Tompa, F.S. 1964. F a c t o r s determining the numbers of song sparrows Melospiza melodia (Wilson) on Mandarte I s l a n d , B.C., Canada. Acta Z o o l . Fenn., 109, 3-73. T r i v e r s , R.L. 1972. P a r e n t a l investment and sexual s e l e c t i o n . In Sexual S e l e c t i o n and the Descent of Man (Ed. by B.G. Campbell), pp.136-179. Chicago: A l d i n e P r e s s . Ueda, K. 1986. A polygamous s o c i a l system of the f a n - t a i l e d warbler C i s t i c o l a j u n c i d i s . Ethology, 73, 43-55. Verner, J . 1964. E v o l u t i o n of polygamy in the l o n g - b i l l e d marsh wren. E v o l u t i o n , 18, 252-261. Verner, J . & W i l l s o n , M.F. 1966. The i n f l u e n c e of h a b i t a t s on mating systems of North American p a s s e r i n e b i r d s . Ecology, 47, 143-147. Verner, J . & W i l l s o n , M.F. 1969. Mating systems, sexual dimorphism, and the r o l e of male North American p a s s e r i n e b i r d s i n the n e s t i n g c y c l e . O r n i t h o l . Monogr., 9, 1-76. 170 V i l l a g e , A. 1983. The r o l e of n e s t - s i t e a v a i l a b i l i t y and t e r r i t o r i a l behaviour i n l i m i t i n g the breeding d e n s i t y of k e s t r e l s . J . Anim. E c o l . , 52, 635-645. Watson, A. & Moss, R. 1970. Dominance, spacing behaviour and aggre s s i o n i n r e l a t i o n t o p o p u l a t i o n i n v e r t e b r a t e s . In Animal P o p u l a t i o n s i n R e l a t i o n to T h e i r Food Resources (Ed. by A. Watson), pp.167-218. Oxford: B l a c k w e l l S c i e n t i f i c P r e s s . Weatherhead, P.J. 1979. E c o l o g i c a l c o r r e l a t e s of monogamy i n tundra-breeding savannah sparrows. Auk, 96, 391-401. Weatherhead, P.J. & Robertson, R.J. 1979. O f f s p r i n g q u a l i t y and the polygyny t h r e s h o l d : "The sexy son Hypothesis". Am. Nat., 113, 201-208. Wittenberger, J.F. 1979. The e v o l u t i o n of mating systems i n b i r d s and mammals. In Handbook of B e h a v i o r a l Neurobiology. Vol..3, S o c i a l Behavior and Communication (Ed. by P. Marler & J . Vandenbergh), pp.271-349. New York: Plenum. Wittenberger, J.F. & T i l s o n , R.L. 1980. The e v o l u t i o n of monogamy: hypotheses and t e s t s . Ann. Rev. E c o l . S y s t . , 11, 197 232. Yasukawa, K. & Searcy, W.A. 1981. N e s t i n g synchrony and d i s p e r s i o n i n red-winged b l a c k b i r d s : i s the harem co m p e t i t i v e or c o o p e r a t i v e ? Auk, 98, 659-668. Yasukawa, K. & Searcy, W.A. 1982. Aggression i n female red-winged b l a c k b i r d s : a s t r a t e g y to ensure male p a r e n t a l investment. Behav. E c o l . S o c i o b i o l . , 11, 13-17. Young, H. 1956. T e r r i t o r i a l a c t i v i t i e s of the American r o b i n Turdus m i g r a t o r i u s . I b i s , 98, 448-453. 171 CHAPTER 7. INTRASEXUAL COMPETITION, MATING SYSTEM AND NATAL DISPERSAL IN SONG SPARROWS INTRODUCTION Pa t t e r n s of n a t a l d i s p e r s a l ( i . e . d i s t a n c e from p l a c e of b i r t h to f i r s t s ettlement; Howard 1960; Greenwood 1980) are l i n k e d to a wide range of other l i f e h i s t o r y and demographic t r a i t s (Comins et a l . 1980, Greenwood 1980; Murray 1984). D i s p e r s a l a l s o i n f l u e n c e s p o p u l a t i o n dynamics ( L i d i c k e r 1975; Krebs et a l . 1976; Tamarin 1980), the g e n e t i c s t r u c t u r e of p o p u l a t i o n s (Wright 1977; S h i e l d s 1982) and the e v o l u t i o n of s o c i a l behaviour (Hamilton 1964; Woolfenden & F i t z p a t r i c k 1984; Brown 1987). For these reasons, d i s p e r s a l i s of" int e n s e i n t e r e s t to b e h a v i o u r i s t s and p o p u l a t i o n and e v o l u t i o n a r y e c o l o g i s t s (e.g. Gaines & McClenaghan 1980; Greenwood & Harvey 1982; S h i e l d s 1982; Dobson 1982; Waser & Jones 1983; Boonstra et a l . 1987; Buechner 1987). In t h i s chapter, I c o n s i d e r s e v e r a l c o r r e l a t e s , causes and consequences of v a r i a t i o n i n d i s p e r s a l behaviour i n a r e s i d e n t and t e r r i t o r i a l p o p u l a t i o n of song sparrows. One area of p a r t i c u l a r l y s p i r i t e d debate i s the cause of i n d i v i d u a l v a r i a t i o n i n d i s p e r s a l behaviour (e.g. Moore & A l i 1984; Dobson & Jones 1985; Packer 1985; Waser 1985; Buechner 1987). Packer (1985) and Dobson & Jones (1985) have suggested that mate s e l e c t i o n , r e l a t e d n e s s and c o m p e t i t i o n f o r resources 172 have too o f t e n been t r e a t e d as mutually e x c l u s i v e causes of d i s p e r s a l . However, few b i r d or mammal s p e c i e s have been s t u d i e d i n s u f f i c i e n t d e t a i l to i n v e s t i g a t e the p o t e n t i a l i n t e r p l a y of these f a c t o r s . Moreover, c o m p e t i t i v e a b i l i t y , sex, i n h e r i t a n c e , the mating system, demography and resource a v a i l a b i l i t y are a l l i m p l i c a t e d as e i t h e r c o r r e l a t e s , causes or consequences of d i s p e r s a l (Krebs et a l . 1976; Greenwood 1980; Dobson 1982). Few s p e c i e s are w e l l - s u i t e d to a comprehensive study of these i s s u e s (e.g. Howard 1949; Myers & Krebs 1971; Greenwood et a l . I979a,b). The song sparrows on Mandarte I s l a n d , B.C., o f f e r a r a r e chance to study d i s p e r s a l in p a r t i c u l a r d e t a i l . Nearly a l l b i r d s that breed there hatch l o c a l l y and are banded as n e s t l i n g s . I n d i v i d u a l s of known r e l a t e d n e s s can thus be f o l l o w e d throughout t h e i r l i v e s , and v a r i a t i o n i n d i s p e r s a l can be r e l a t e d to i n d i v i d u a l c h a r a c t e r i s t i c s , p o p u l a t i o n demography, in b r e e d i n g and l i f e t i m e p r o d u c t i o n of young. T h i s allowed me to examine s e v e r a l assumptions and hypotheses about d i s p e r s a l . Here I d e s c r i b e : (1) p a t t e r n s of n a t a l d i s p e r s a l i n f i v e c o h o r t s of young, (2) r e l a t i o n s h i p s between d i s p e r s a l d i s t a n c e and i n d i v i d u a l c h a r a c t e r i s t i c s ( i . e . sex, s o c i a l dominance and age), p r o b a b i l i t y of i n b r e e d i n g , age of the a c q u i r e d mate, and p o p u l a t i o n d e n s i t y , (3) the e f f e c t of s e t t l i n g with a c l o s e r e l a t i v e on subsequent r e p r o d u c t i v e success and (4) the resemblance between s i b l i n g s i n d i s p e r s a l d i s t a n c e . I a l s o 173 present r e s u l t s from two experiments. F i r s t , I t e m p o r a r i l y removed n e a r l y a l l song sparrows from the c l o s e s t i s l a n d adjacent to Mandarte ( H a l i b u t I s l a n d , 1.3 km d i s t a n t ) i n l a t e summer to t e s t i f i n c r e a s i n g the a v a i l a b i l i t y of unsaturated h a b i t a t near Mandarte caused an i n c r e a s e i n the number of s u c c e s s f u l emigrants from Mandarte. Second, I conducted a f e e d i n g experiment throughout the breeding p e r i o d i n 1985 to t e s t i f d i s p e r s a l i s a f f e c t e d by the q u a l i t y of the n a t a l t e r r i t o r y . L a s t l y , I compare my r e s u l t s with 5 p r e v i o u s s t u d i e s of d i s p e r s a l i n song sparrows. Hypotheses About D i s p e r s a l The hypotheses, assumptions and p r e d i c t i o n s addressed i n t h i s study are summarized i n Table 7.1 and d i s c u s s e d b r i e f l y below. I t i s g e n e r a l l y accepted that the mating systems of t e r r i t o r i a l s p e c i e s i n f l u e n c e p a t t e r n s of d i s p e r s a l by males and females, and that these d i f f e r e n c e s are l i n k e d to the amount of e f f o r t each sex spends competing f o r t e r r i t o r i e s or mates versus, the e f f o r t spent i n care of o f f s p r i n g and mate c h o i c e (Bradbury & Vehrencamp 1977; Greenwood 1980; Dobson 1982; Waser & Jones 1983). Greenwood (1980) and Greenwood & Harvey (1982) argued that female p a s s e r i n e s o f t e n d i s p e r s e d f u r t h e r than males because the c o s t of i n b r e e d i n g outweighs the c o s t of d i s p e r s a l i n females, but not i n males. They assumed that the c o s t of (I Table 7.1. Hypotheses, p r e d i c t i o n s and assumptions about i n t e r a c t i o n s between d i s p e r s a l and inbreeding, competition f o r breeding resources and demography. Numbers in parentheses i n d i c a t e the references l i s t e d below. Class of Hypotheses S p e c i f i c Mechanism Predictions/Assumptions Mating System Competition f o r mates (1-3) 1) Sex-bias depends on the e f f o r t expended by each sex i n mate defence. 2) Distance r e l a t e d to mate q u a l i t y (4). 3) Distance unaffected by food supplementation. Competition f o r environmental resources (1-3,5,6) 1) Sex-bias depends on the e f f o r t expended by each sex i n resource defense. 2) Distance reduced when t e r r i t o r y q u a l i t y i s enhanced by food supplementation. Intrasexual competition between parents 1) No sex bias i n d i s p e r s a l when the mating system i s monogamy. and o f f s p r i n g 2) Strong competition between (7) parents and o f f s p r i n g over mates. 3) D i s p e r s a l depends on sur-v i v a l of parent of the same sex. 4) Food supplementation has no e f f e c t on d i s p e r s a l . Random Population movement demography models (8-10) 1) Frequency of inbreeding decreases with distance dispersed. 2) Presence and d i r e c t i o n of-sex-bias depend on m o r t a l i t y rate of males and females. 3) Distance and time of settlement should be p o s i t i v e l y c o r r e l a t e d . 4) Food supplementation a f f e c t s d i s p e r s a l only i f i t a f f e c t s adult m o r t a l i t y . 5) Increasing open ha b i t a t increases d i s p e r s a l . S o c i a l dominance 1) Subordinates disperse f u r t h e r than dominants. 17 Q> (11-12) 2) Subordinates s e t t l e l a t e r than dominants. 3) Sex-bias depends on sex-r e l a t e d d i f f e r e n c e s i n dominance. Inbreeding Avoidance 1) D i s p e r s a l depends on sur-(13-18) v i v a l of parent of the same sex. 2) Reproduction s t r o n g l y de-pressed by clo s e inbreeding. 3) Sex-bias depends on costs of d i s p e r s a l i n each sex, r e l a t i v e to the costs of inbreeding. 4) Food supplementation does not a f f e c t d i s p e r s a l . Optimal 1) P r i m a r i l y short-distance outbreeding d i s p e r s a l . (19-20) 2) Reproduction s t r o n g l y de-pressed by clo s e inbreeding, enhanced by weak inbreeding. 3) Food supplementation does not a f f e c t d i s p e r s a l . Inheritance Genetic tendency 1) High f a m i l i a l s i m i l a r i t y i n distance dispersed. (21-25) 2) Frequency of inbreeding decreases with distance dispersed. 3) Bimodal d i s t r i b u t i o n of d i s p e r s a l d i s t a n c e s . 4) Food supplementation does not a f f e c t d i s p e r s a l . References: (1) Dobson 1982; (2) Greenwood £> Harvey 1982; (3) Waser & Jones 1983; (4) Greenwood et a l . 1979b; (5) Dobson 1980; (6) Krebs & Delong 1965; (7) Lieberg & von Shantz 1985; (8) Murray 1967; (9) Waser 1985; (10) Buechner 1987; (11) Dhondt & Huble 1968; (12) Gauthreaux 1978; (13) Greenwood et a l . 1978; (14) Packer 1979; (15) Koenig & P i t e l k a 1979; (16) Pusey 1980; (17) Hoogland 1982; (18) Woolfenden & F i t z p a t r i c k 1984; (19) Shields 1982; (20) Bateson 1983; (21) Howard 1960; (22) Johnston 1961; (23) Myers & Krebs 1971; (24) Greenwood et a l . 1979a; (25) Comins et a l . 1980. 1 78 g a i n i n g a t e r r i t o r y i n males exceeds that f o r females, and that males that remain i n t h e i r f a m i l i a r , n a t a l area are most l i k e l y to gain t e r r i t o r i e s . Among song sparrows on Mandarte, i n t r a - s e x u a l competition i n both males and females s t r o n g l y i n f l u e n c e s the p a t t e r n of matings (Smith et a l . 1982; Chap. 3-6). Because t e r r i t o r y establishment i s h i g h l y c o m p e t i t i v e i n females and males, and each sex defends i t s t e r r i t o r y a g a i n s t members of the same sex, monogamy i s the most frequent mating arrangement. For song sparrows on Mandarte I s l a n d , Greenwood's (1980) hy p o t h e s i s t h e r e f o r e p r e d i c t s that sex-biases i n d i s p e r s a l d i s t a n c e should be small or absent. F u r t h e r , i f d i s p e r s a l i s p r i m a r i l y the r e s u l t of competition f o r good breeding s i t e s , I p r e d i c t e d that supplementing areas with food would reduce d i s p e r s a l d i s t a n c e approximately e q u a l l y i n each sex compared to c o n t r o l s . Two r e l a t e d hypotheses based on the o r g a n i z a t i o n of the mating system make s i m i l a r p r e d i c t i o n s to those of Greenwood (1980). Dobson (1982) emphasized co m p e t i t i o n f o r mates as opposed to t e r r i t o r i e s . In song sparrows, however, t h i s would be hard to t e s t because these a l t e r n a t i v e s are d i f f i c u l t to d i s t i n g u i s h (Smith et a l . 1982; Chap. 6). L i e b e r g & von Schantz (1985) proposed a v a r i a t i o n of Dobson and Greenwood's hypotheses that emphasize i n t r a s e x u a l competition between parents and o f f s p r i n g , and p r e d i c t s that o f f s p r i n g d i s p e r s a l should depend on s u r v i v a l and p h i l o p a t r y i n the parent of the 179 same sex. S e v e r a l s i m u l a t i o n models of d i s p e r s a l based on competition f o r breeding s i t e s , demography, and movement r u l e s f o r d i s p e r s e r s p r e d i c t that sex-biases i n d i s p e r s a l a r i s e through d i f f e r e n c e s i n the m o r t a l i t y r a t e s of t e r r i t o r i a l males and females (Murray 1967; Waser & Jones 1983; Waser 1985; Buechner 1987). I t h e r e f o r e expected d i s p e r s a l to be r e l a t e d to d i f f e r e n c e s i n a d u l t s u r v i v a l i n song sparrows. For mathematical convenience, d i s p e r s e r s are t y p i c a l l y modelled as moving i n s t r a i g h t l i n e s to the f i r s t empty t e r r i t o r y encountered (Murray 1967), or s e a r c h i n g i n c o n c e n t r i c r i n g s around the n a t a l s i t e i n t h e i r path outwards from i t (Waser 1985). Such models p r e d i c t p o s i t i v e c o r r e l a t i o n s between d i s t a n c e d i s p e r s e d and time of s e t t l e m e n t . S e v e r a l models p o s i t s o c i a l dominance as the mechanism cau s i n g d i s p e r s a l and s e t t l i n g c o n f l i c t s over breeding resources (Dhondt & Huble 1968; Gauthreaux 1978; Waser 1985; De Laet 1985; Beuchner 1987). Three p r e d i c t i o n s of these models are t e s t e d here: (1) d i s p e r s a l d i s t a n c e should i n c r e a s e as dominance decreases, (2) settlement should be delayed i n s u b o r d i n a t e s , and (3) females should d i s p e r s e f u r t h e r than males because they are u s u a l l y subordinate to them d u r i n g the non-breeding p e r i o d (Smith 1982; Arcese & Smith 1985). Inbreeding avoidance i s a primary cause of d i s p e r s a l i n 180 some g r o u p - l i v i n g s p e c i e s (Koenig & P i t e l k a 1979; Packer 1979; Pusey 1980; Hoogland 1982; Woolfenden & F i t z p a t r i c k 1984; Pusey & Packer 1987). However, i n b r e e d i n g can be e i t h e r a r a r e or a r e g u l a r event ( S h i e l d s 1982), and i t occurs even i n s p e c i e s with sex-biased d i s p e r s a l (Bulmer 1973; van Noordwijk & Scharloo 1981; Moore & A l i 1984; Rowley et a l . 1986). F u r t h e r , both t h e o r e t i c a l and f i e l d s t u d i e s show that i n b r e e d i n g i s not always d e l e t e r i o u s (Richdale 1949; Bengtsson 1978; Smith 1979; van Noordwijk & Scharloo 1981; S h i e l d s 1982; Rowley et a l . 1986, Temple 1986). On the c o n t r a r y , S h i e l d s (1982) and Bateson (1983) have suggested that outbreeding c o u l d be d e l e t e r i o u s i f i n d i v i d u a l s possess h e r i t a b l e t r a i t s that are h i g h l y adapted to the l o c a l h a b i t a t . I t e s t three p r e d i c t i o n s of these hypotheses: (1) c l o s e i n b r e e d i n g ( s i b and p a r e n t - o f f s p r i n g matings; S h i e l d s 1982) depresses r e p r o d u c t i v e success, (2) c l o s e i n b r e e d i n g i s avoided e i t h e r p a s s i v e l y by sex-biases i n d i s p e r s a l , or a c t i v e l y by behaviours that i n h i b i t the settlement or r e p r o d u c t i o n of young i n t h e i r n a t a l area when a parent i s the l i k e l y mate (Koenig & P i t e l k a 1979; Holmes 1984; Cockburn et a l . 1985; Pusey & Packer 1987), and (3) the r e l a t e d n e s s of p o t e n t i a l mates d e c l i n e s as the number of t e r r i t o r i e s d i s p e r s e d i n c r e a s e s . A f i n a l group of models c h a r a c t e r i z e s d i s p e r s a l as a g e n e t i c a l l y h e r i t a b l e t r a i t that i s maintained i n p o p u l a t i o n s i n d i f f e r e n t degrees depending on o p p o r t u n i t i e s t o : 1) c o l o n i z e vacant h a b i t a t s ; 2) i n c r e a s e the vigour of o f f s p r i n g by 181 o u t c r o s s i n g ; or 3) a v o i d competition f o r c r u c i a l resources with c l o s e kin (Howard 1960; Johnston 1961; Greenwood et a l . 1979a; Smith 1979; Comins et a l . 1980; Gaines & McClenaghan 1980). These hypotheses p r e d i c t that d i s p e r s a l d i s t a n c e s w i l l be more s i m i l a r w i t h i n f a m i l i e s than between them, and that these d i f f e r e n c e s are not due to common environment (van Noordwijk 1984). RESULTS D i s p e r s a l W i thin Mandarte I s l a n d Y e a r l y v a r i a t i o n i n d i s p e r s a l There were some d i f f e r e n c e s i n the median d i s t a n c e s moved by males and females between years ( F i g . 7.1), but these were not s t a t i s t i c a l l y s i g n i f i c a n t (P>0.10 and P>0.50, K r u s k a l - W a l l i s t e s t s ; males and females, r e s p e c t i v e l y ) . When comparing d i s t a n c e s i n u n i t s of t e r r i t o r i e s , however, there were s i g n i f i c a n t d i f f e r e n c e s among years i n each sex (P<0.025 and P<0.005, K-W t e s t s ; males and females, r e s p e c t i v e l y ; F i g . 7.2). These d i f f e r e n c e s were p o s i t i v e l y r e l a t e d to p o p u l a t i o n d e n s i t y i n the year of hatch when d i s p e r s a l was measured in t e r r i t o r i e s t r a v e r s e d (r=0.76 and r=0.85, N=5 y e a r s ; males and females, r e s p e c t i v e l y ) , but not when measured in meters (r=-0.13 Males 135 rm nhn n Females 159 1 O z LU Z) o LU rr Li. 102 • hri lnnl l Ihrm 72 'I mn n n m n n nn n 114 150, JIM n n 0 100 200 300 400 114 Mil 108 an n n n n n n n n 114 147 100 200 300 400 D I S T A N C E M O V E D (m) Figure 7.1. Natal distance dispersed i n 5 cohorts hatched from 1982-6 (top to bottom). Arrows and numbers in d i c a t e median distances dispersed by each sex. >-o z 111 ZD o LU-CC Li. 7.9 Males WMk. 12.6 ml n rim om loo 8.8 ml n linn nn n n n 17.6 HUM 21.2^ i m O 10 20 30 40 60 9.3 Females 14.1 13.1 17.6 20.8 M J 1 10 20 30 40 SO T E R R I T O R I E S M O V E D Figure 7.2. Natal distance dispersed i n units of t e r r i t o r i e s moved. Conventions as f o r Figure 7.1. 184 and r=-0.60, as above). T h i s was because b i r d s of each sex d i s p e r s e d approximately equal numbers of meters between years ( F i g . 7.1), but t e r r i t o r y s i z e v a r i e d by roughly 3 times d u r i n g the course of the study. D i s t a n c e s moved each year by males and females were p o s i t i v e l y c o r r e l a t e d (r=0.84 and r=0.97, meters and t e r r i t o r i e s , r e s p e c t i v e l y ) . P o i n t of o r i g i n and d i s t a n c e d i s p e r s e d There were s i g n i f i c a n t d i f f e r e n c e s i n the number of t e r r i t o r i e s d i s p e r s e d by young o r i g i n a t i n g from d i f f e r e n t p o r t i o n s of the i s l a n d i n 2 years ( F i g . 7.3; 1982 and 1984 c o h o r t s , P<0.001, K-W t e s t s ) . The p a t t e r n was s i m i l a r i n 1985 and 1986 but not s i g n i f i c a n t by the r e v i s e d l e v e l of acceptance ( i . e . P<0.01). However, when the data f o r a l l years were pooled, p o i n t of o r i g i n and the number of t e r r i t o r i e s d i s p e r s e d were s t r o n g l y l i n k e d (P<0.001, K-W t e s t ) . T h i s was because b i r d s hatched near the ends of the i s l a n d had a l a r g e r set of d i s t a n c e s over which to d i s p e r s e than those hatched near the c e n t e r . Sex d i f f e r e n c e s i n d i s p e r s a l In 3 years the median d i s p e r s a l d i s t a n c e by females s l i g h t l y exceeded that f o r males, but i n 2 years there was no d i f f e r e n c e ( F i g . 7.1 & 7.2). There were no s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s i n any year, or when the data f o r a l l 5 185 20 10 0 40 CD > 20 o 0 50 CO CD 30 o 0 •4-> 50 l _ k- 30 <D h- 0 40 20 0 I I T 11 i 17 T 11 L T JL L 1 12 5 I I—. T 12 r 10 1 8 1 T T 11 1 2 3 4 5 6 A r e a Figure 7.3. Median, i n t e r - q u a r t i l e range, and the range of the data for the d i s t r i b u t i o n of d i s p e r s a l distances i n 5 cohorts of young hatched i n d i f f e r e n t portions of the i s l a n d , from 1982-6 (top to bottom). 186 years were pooled ( a l l P>0.20, Mann-Whitney U - t e s t s ) . Dominance, hatch date and d i s t a n c e moved Dominance was not s i g n i f i c a n t l y r e l a t e d to d i s p e r s a l d i s t a n c e i n e i t h e r sex i n the 3 years that i t was estimated (range r=-0.26 to +0.40, 4 p o s i t i v e and 2 negative c o e f f i c i e n t s , N=12-38, a l l P>0.05). S i m i l a r r e s u l t s were obtained i n weighted a n a l y s e s . Hatch date i s h i g h l y c o r r e l a t e d with dominance (Arcese & Smith 1985), but was u n r e l a t e d t o d i s t a n c e d i s p e r s e d i n a l l 5 cohor t s (range r=-0.32 to +0.28, 6 p o s i t i v e and 4 negative c o e f f i c i e n t s , N=26-65, a l l P>0.05). To c o n t r o l f o r the i n f l u e n c e of p o i n t of o r i g i n , I repeated the above a n a l y s i s on date of hatch using only b i r d s hatched i n p o r t i o n s of the i s l a n d that allowed s i m i l a r o p p o r t u n i t i e s f o r d i s p e r s a l ( i . e . F i g . 7.3, areas 1 & 6, 2 & 5, 3 & 4; sexes p o o l e d ) . However, none of 15 c o r r e l a t i o n s were s t a t i s t i c a l l y s i g n i f i c a n t (range r=-0.75 to +0.55, 10 p o s i t i v e and 5 negative c o e f f i c i e n t s , N=7-27, a l l P>0.05). Dominance, hatch date and settlement time on Mandarte Dominance was r e l a t e d to the p r o b a b i l i t y of settlement on Mandarte i n males and females d u r i n g the f i r s t two years of t h i s study (Arcese & Smith 1985). Table 7.2 shows that dominant males and females i n the 1984 cohort were a l s o more l i k e l y to 187 s e t t l e by the end of A p r i l the f o l l o w i n g year than were subordinates of the same sex (G =5.04 and G =6.80, males and females, r e s p e c t i v e l y ; df=1, P<0.025 in each c a s e ) . In males, hatch date was a l s o r e l a t e d to t e r r i t o r i a l s t a t u s i n the f o l l o w i n g s p r i n g ( F i g . 7.4a). No females from the 1982 and 1983 cohorts behaved as f l o a t e r s , but a few females d i d so i n the s p r i n g s of 1985 and 1986 when d e n s i t i e s were p a r t i c u l a r l y high (Chap. 1, F i g . 1.1). F i g u r e 7.4b shows that hatch date was r e l a t e d to the t e r r i t o r i a l s t a t u s of females i n l a t e A p r i l when the 1982-5 cohor t s were pooled. In males, where I was able to estimate the date of settlement i n 4 cohorts (1982-85), b i r d s hatched e a r l i e r i n the year o f t e n s e t t l e d e a r l i e r ( F i g . 7.5). However, the e f f e c t of hatch date on settlement date was strong only among the 18 males that s e t t l e d before 1 January of the year f o l l o w i n g h a t c h i n g . Seventeen of these males were a l s o hatched before the mean hatch date of t h e i r cohort, compared to the 10.3 and 7.7 males expected to have s e t t l e d before and a f t e r the new year by the d i s t r i b u t i o n of hatch dates (G= 12.23, df=1, P<0.001). I found no r e l a t i o n s h i p between d i s t a n c e moved and time spent as a d i s p e r s e r (r=-0.l2, N=103, P>0.05). Of males that e v e n t u a l l y s e t t l e d on Mandarte from the 1982-4 c o h o r t s , those of higher s t a t u s o f t e n s e t t l e d e a r l i e r than more subordinate males. However, t h i s r e s u l t was s t a t i s t i c a l l y s i g n i f i c a n t only i n a weighted a n a l y s i s of the pooled data (P<0.01, r=-0.42, N=94). 1$6 Table 7.2. T e r r i t o r i a l s t a t u s at the end of A p r i l , 1985 i n r e l a t i o n to the dominance of y e a r l i n g s hatched the p r e v i o u s year. T e r r i t o r i a l S t atus Sex Dominace s t a t u s Owner F l o a t e r Absent Male Dominant 9 4 5 Subordinate 6 12 14 Female Dominant Subordinate 1 5 19 0 2 2 1 5 M a l e s IX I x CD +-« Q o CO X 7 0 5 0 i 30-1 0-1 0-30" 50-7 0-7 0 5 0 30H 1 0 • 1 0 3 0 5 0 7 0 T 6 2 1 0 1 T 75 ft i 5 6 F e m a l e s 83 1 1— Figure 7.4. T e r r i t o r i a l status of males and females at the end of A p r i l i n the year following hatch versus date hatched the previous year (median, i n t e r - q u a r t i l e range, and the range of the data). Data are from the 1982-5 cohorts. Numbers in d i c a t e sample s i z e . 0 +-» CO Q 0 E 0 0 CO 40 •20 20 •21 40+ Hatch Date (x-x) Figure 7 Settlement date of male r e c r u i t s (mean±SE) i n r e l a t i o n to four categories of date of hatch (F^ ^ ^=2.83, P<0.05). Numbers indic a t e the number of male s e t t l e r s . O 191 S i b l i n g resemblance in d i s p e r s a l I found no evidence of s i b l i n g resemblance i n d i s p e r s a l d i s t a n c e when the young that were compared had been hatched i n p o r t i o n s of the i s l a n d that allowed s i m i l a r o p p o r t u n i t i e s f o r d i s p e r s a l (Table 7.3). When f a m i l i e s from the e n t i r e i s l a n d were compared, s i g n i f i c a n t d i f f e r e n c e s between f a m i l i e s o c c u r r e d because the p o i n t s of o r i g i n were not c o n t r o l l e d f o r (P<0.01, 1982 and 1984, P<0.05, 1985 and 1986; K-W t e s t s ) . Age of the a c q u i r e d mate For both male and female d i s p e r s e r ' s , the age of the d i s p e r s e r s f i r s t mate was u n r e l a t e d to the d i s t a n c e moved (G =2.11 and G= 1.03, males and females, r e s p e c t i v e l y , df=6, P>0.50). T h i s r e s u l t was obtained by p o o l i n g y e a r l y data i n t o three d i s t a n c e c a t e g o r i e s and comparing d i s t a n c e moved with the age of the a c q u i r e d mate ( a f t e r Greenwood et a l . 1979). I n h e r i t a n c e of n a t a l t e r r i t o r i e s Only 4 males and 1 female s e t t l e d on t h e i r n a t a l t e r r i t o r y ( F i g . 7.2), even though a d u l t m o r t a l i t y r a t e s overwinter (27-43%) would have allowed s u b s t a n t i a l l y higher r a t e s of p h i l o p a t r y without i n b r e e d i n g as a consequence. In males, 3 n a t a l settlements took p l a c e a f t e r f l o a t e r sons e v i c t e d t h e i r f a t h e r s . T a b l e 7.3. R e s u l t s of K r u s k a l - W a l l i s ANOVA's on d i f f e r e n c e s between f a m i l i e s i n d i s p e r s a l from end, i n t e r m e d i a t e and m i d d l e p o r t i o n s of the i s l a n d . Low r e c r u i t m e n t from the 1985 and 1986 c o h o r t s p r e v e n t e d u s e f u l c o m p a r i s o n s . No r e s u l t was s t a t i s t i c a l l y s i g n i f i c a n t ( a l l P>0.10). Co h o r t Area F a m i l i e s Young H DF 1982 End 5 15 4.45 4 I n t e r m e d i a t e 7 21 7.41 6 M i d d l e 5 19 1.65 4 1983 End 4 10 1.23 3 I n t e r m e d i a t e 6 17 2.52 5 M i d d l e 9 20 3.30 8 1984 End 4 11 2.49 3 I n t e r m e d i a t e 7 17 9.92 6 M i d d l e 6 20 4.68 5 1 93 I do not know by what method the lone female s e t t l e d on her n a t a l t e r r i t o r y . However, 3 of these 5 s e t t l e r s (1 female, 2 males) subsequently mated with a parent, and two of the males e v i c t e d t h e i r f a t h e r s to do so. M o r t a l i t y , recruitment and sex-biases i n d i s p e r s a l Y e a r l y d i f f e r e n c e s i n d i s p e r s a l bore no c l e a r r e l a t i o n to the p r o p o r t i o n of a d u l t s s u r v i v i n g from the pr e v i o u s year (Table 7.4, F i g . 7.1 & 7.2). D i f f e r e n c e s between a d u l t male and female s u r v i v a l , as w e l l as d i f f e r e n c e s i n s e x - s p e c i f i c recruitment r a t e s , were a l s o u n r e l a t e d to the d i r e c t i o n or presence of a sex-bias (Table 7.4, F i g . s 7.1 & 7.2). Distance and r e p r o d u c t i v e success The t o t a l p r o d u c t i o n of independent young per b i r d ( i . e . a t r u n c a t e d estimate of l i f e t i m e r e p r o d u c t i v e success) was u n r e l a t e d to d i s p e r s a l d i s t a n c e i n a l l 5 c o h o r t s (r=-0.20 to +0.12, 3 negative and 2 p o s i t i v e c o r r e l a t i o n s , N=27-57, a l l P>0.05). Inbreeding, D i s p e r s a l , D e n s i t y and Reproduction Of a t o t a l 178 b i r d s of known pedigree s e t t l i n g with t h e i r f i r s t mate, 8 (4.5%) s e t t l e d with a c l o s e l y r e l a t e d mate, 19 (11%) with a moderately r e l a t e d mate and 151 (84.5%) with Table 7.4. Number of t e r r i t o r i a l a d u l t s at the end of A p r i l each year, t h e i r s u r v i v a l over winter, and the r a t e of recruitment the next year of independent young to the breeding p o p u l a t i o n . Numbers S u r v i v a l Recruitment of Young Year Males Females Males Females Males Females 1982 35 26 0.86 0.81 0.50 0.57 1983 59 54 0.68 0.56 0.23 0.23 1984 64 53 0.77 0.89 0.43 0.39 1985 78 72 0.72 0.58 0.15 0.35 1986 67 63 0.73 0.73 0.40 0.32 1 95 u n r e l a t e d mates. However, the number of t e r r i t o r i e s moved was u n r e l a t e d to the p r o b a b i l i t y of s e t t l i n g with a r e l a t i v e (G =0.51, df=2, P>0.90; Table 7.5). The p r o p o r t i o n of young that s e t t l e d with r e l a t i v e s d e c l i n e d as p o p u l a t i o n d e n s i t y i n c r e a s e d ( F i g . 7.6). However, the number of independent young r a i s e d by i n d i v i d u a l s d u r i n g the study was independent of the r e l a t e d n e s s of t h e i r f i r s t mate (Table 7.6), p o s s i b l y because most b i r d s changed mates r e g u l a r l y as b i r d s d i e d or were e v i c t e d from t h e i r t e r r i t o r i e s (Chap. 3-5). E f f e c t of supplemental food The d i s t a n c e moved by l a t e August 1985 by young p r o v i s i o n e d with supplemental food was much l e s s than that moved by c o n t r o l young (median number of t e r r i t o r i e s moved: 1.7 versus 10.7 f o r 34 fed and 18 c o n t r o l young; P<0.001, U - t e s t ) . T h i s e a r l y d i f f e r e n c e was maintained the f o l l o w i n g year in those young that gained t e r r i t o r i e s ( F i g . 7.7). Fed males and females each s e t t l e d c l o s e r to t h e i r n a t a l t e r r i t o r y than d i d c o n t r o l s (median number of t e r r i t o r i e s moved: 4.4 versus 22.3 f o r 6 fed and 10 c o n t r o l males and 11.2 versus 24.3 f o r 12 fed and 11 c o n t r o l females; both P<0.03, U - t e s t s ) . However, there were no s i g n i f i c a n t d i f f e r e n c e s i n the d i s t a n c e s d i s p e r s e d by males and females w i t h i n treatments (P>0.20, U - t e s t s ) . life Table 7.5. R e l a t i o n s h i p between the number of t e r r i t o r i e s moved and the p r o b a b i l i t y of s e t t l i n g with a c l o s e r e l a t i v e . Relatedness of Mate Cohort G r e a t e r / l e s s than median number Close Moderate Outbred 1982 < 0 2 3 4 1 1 7 1983 < 2 2 2 2 18 20 1984 < 22 22 1985 < 0 0 1 4 1 7 1986 < 0 0 3 1 9 1 1 T o t a l < 3 5 10 9 74 77 ted 20-Expecl 10-5 -2-From 0--ion - 2 -c _ CO — J Devi - 10 --20 -20 • 82 .83 a.86 ,84 ,85 30 40 50 60 70 80 Population Density Figure 7.6. Percent deviation from the expected rate of inbreeding based on the pooled r e s u l t s of the p r o b a b i l i t y of s e t t l i n g with a close r e l a t i v e . 199 Table 7.6. T o t a l p r o d u c t i o n of independent young versus the r e l a t e d n e s s of the f i r s t mate i n 5 co h o r t s of d i s p e r s e r s with known l i n e a g e s (mean+SE). Relatedness Cohort Close Moderate Outbred 1982 N 5.50+3.50 2 1983 N 2.75+0.25 4 1 984 N 0.50+0.50 2 1985 N 1986 N 4.83+1 .97 6 3.00+0.87 18 6.75±2.72 4 3.94+0.58 31 1.50+0.50 2 2.78+0.37 36 3.00+1.00 2 1.77+0.32 31 0.75+0.25 4 1.50+0.25 20 Pooled N -0.18+0.24 8 0.23+0.27 18 0. 13 + 0.09 136 199 The d i f f e r e n c e between fed and c o n t r o l young c o u l d have r e s u l t e d p a r t l y because most c o n t r o l t e r r i t o r i e s were l o c a t e d nearer the ends of the i s l a n d than were fed t e r r i t o r i e s . However, the s t i p l e d area of F i g u r e 7.7 shows that d i f f e r e n c e s in d i s p e r s a l d i s t a n c e due to food a d d i t i o n were maintained when only young hatched in the same p o r t i o n s of the i s l a n d were compared (medians f o r sexes pooled: 13 versus 26 t e r r i t o r i e s f o r 13 fed and 9 c o n t r o l young; P<0.03, U - t e s t ) . A f u r t h e r , unexpected r e s u l t of food a d d i t i o n was a r e d u c t i o n i n the s u r v i v a l of a d u l t s on fed t e r r i t o r i e s (Arcese & Smith 1988). During the winter f o l l o w i n g the food experiment, s i g n i f i c a n t l y more fed than c o n t r o l a d u l t s were a l s o observed as f l o a t e r s , presumably becuase they had been e v i c t e d by y e a r l i n g s that c o n c e n t r a t e d i n t e r r i t o r i e s p r e v i o u s l y s u p p l i e d with food (Arcese & Smith 1988). A d d i t i o n a l e f f e c t s of food supplementation are d e s c r i b e d i n Arcese & Smith (1988). D i s p e r s a l Between P o p u l a t i o n s Emigration Four male and 3 female y e a r l i n g s were s i g h t e d or captured on i s l a n d s other than Mandarte duri n g the f i r s t 3 years of t h i s study, when o c c a s i o n a l v i s i t s were made to most neighbouring i s l a n d s , and r e g u l a r v i s i t s were made to H a l i b u t I s l a n d (1-3 v i s i t s each s p r i n g and f a l l ) . Six of these b i r d s were observed on H a l i b u t I s . and 1 male v i s i t e d a feeder on Vancouver I s . F e d o c <D CJ CD 3 2 n 3 2 C o n t r o l 10 20 3 0 4 0 5 0 Territories Moved F i g u r e 7 . 7 . D i s p e r s a l d i s t a n c e o f f e d and c o n t r o l young f r o m t h e e n t i r e i s l a n d ( a l l b a r s ) , and o n l y t h o s e young h a t c h e d i n t h e c e n t r a l p o r t i o n o f t h e i s l a n d ( a r e a s 3 , 4 ; F i g . 7 . 3 ; s t i p p l e d b a r s ) . 201 (Oak Bay), about 25 km d i s t a n t . A l l 7 b i r d s were subordinates w i t h i n t h e i r sex (of a p o s s i b l e 186). In comparison, none of 121 dominants were observed o f f Mandarte (P=0.028, 1 - t a i l e d F i s h e r ' s exact t e s t ) . No d i s p e r s i n g males were known to have s e t t l e d elsewhere, but one retu r n e d from H a l i b u t to Mandarte and became t e r r i t o r i a l . Two female d i s p e r s e r s s e t t l e d and bred on H a l i b u t I s . , and at l e a s t one of these r a i s e d independent young. The t h i r d female returned to Mandarte and bred s u c c e s s f u l l y f o r 4 ye a r s . Immigration Young d i s p e r s e r s a r r i v i n g on Mandarte i n l a t e summer were common each year, but only one such male s u c c e s s f u l l y s e t t l e d and bred ( i n 1986). In c o n t r a s t , an immigrant female s e t t l e d and subsequently bred on Mandarte i n 1982 and 1984, and 2 females d i d so i n 1985. Four of these immigrants (3 females, 1 male) produced r e c r u i t s that themselves bred on Mandarte. P o p u l a t i o n d e n s i t y was u n r e l a t e d to the p r o p o r t i o n of r e c r u i t s t h a t were immigrants (r=0.23, N=5 y r s ) . H a l i b u t I s l a n d removal experiment From 28 August to 7 September, 1984, 20 song sparrows were ne t t e d and removed from H a l i b u t i s l a n d . Tompa (1963) estimated the t o t a l number of p a i r s on H a l i b u t at 9-11 from 1961-1963. T h i s accords w e l l with my o b s e r v a t i o n s , and suggests that the 202 removals d r a m a t i c a l l y reduced the number of r e s i d e n t s and opened a l a r g e amount of undefended h a b i t a t to p o t e n t i a l s e t t l e r s . Captured b i r d s were r e l e a s e d on the mainland (ca. 45 km d i s t a n t ) , but by 15 October 3 banded r e s i d e n t s r e t u r n e d to H a l i b u t . No subsequent r e t u r n s were observed. In 5 days of n e t t i n g and observing the song sparrows on H a l i b u t , and i n monthly subsequent v i s i t s through March, only 3 male song sparrows from Mandarte were s i g h t e d on H a l i b u t . One of these re t u r n e d to Mandarte and gained a t e r r i t o r y i n l a t e February. A second male f l o a t e d on H a l i b u t and o c c a s i o n a l l y v i s i t e d Mandarte u n t i l d i s a p p e a r i n g i n December. One female that was r e s i d e n t on Mandarte as a f l o a t e r u n t i l mid-February l a t e r s e t t l e d s u c c e s s f u l l y on H a l i b u t . Thus, 4 d i s p e r s e r s i n t o t a l were observed on H a l i b u t subsequent to the removal of r e s i d e n t s . T h i s compares with four b i r d s observed i n s l i g h t l y fewer t o t a l v i s i t s to H a l i b u t from May 1981 to August 1984. T h i s suggests that s u c c e s s f u l settlement by d i s p e r s e r s from Mandarte i s u n a f f e c t e d by the amount of h a b i t a t a v a i l a b l e on H a l i b u t I s . Seasonal p a t t e r n s of disappearance For each of 5 c o h o r t s there were u s u a l l y two main p e r i o d s of disappearance of j u v e n i l e s from the p o p u l a t i o n ( F i g . 7.8). The l a r g e s t and most s i m i l a r number of disappearances ac r o s s years o c c u r r e d from l a t e June, a f t e r the m a j o r i t y of young had 203 reached independence, through l a t e September, d u r i n g the peak p e r i o d of f a l l t e r r i t o r i a l behaviour. A s m a l l e r , more v a r i a b l e p e r i o d of disappearance u s u a l l y c o i n c i d e d with the resurgence of in t e n s e t e r r i t o r i a l a c t i v i t y from mid-January through A p r i l each year (Tompa 1963; Chapters 3 & 4). However, i n 1985-6 most b i r d s disappeared i n November and December. With the exception of 1985-6, major p e r i o d s of disappearance d i d not c o i n c i d e with snow, when seeds, the main winter food, might have been d i f f i c u l t to reach. Late winter and s p r i n g disappearances were, however, o f t e n c o i n c i d e n t with known occurrences of p r e d a t i o n by the hawks, owls and f a l c o n s that were r e g u l a r l y s i g h t e d on the i s l a n d throughout the winter each year. DISCUSSION The song sparrow i s an a r c h e t y p a l monogamous and t e r r i t o r i a l p a s s e r i n e (Nice 1943, Johnston 1956a,b; Tompa 1963; Smith 1988). One might t h e r e f o r e have expected to observe p r i m a r i l y males defending t e r r i t o r i e s , females choosing among mates or breeding s i t e s , and consequently female-biased n a t a l d i s p e r s a l (Greenwood 1980, 1983; Greenwood & Harvey 1982). I found, however, that n a t a l d i s p e r s a l i n song sparrows d i f f e r e d from t h i s e x p e c t a t i o n i n s e v e r a l important ways (Table 7.1). F i r s t , I found no sex-bias i n d i s p e r s a l . Second, young of each sex d i s p e r s e d l e s s f a r when r a i s e d on t e r r i t o r i e s supplemented with food. T h i r d , dominance was a s s o c i a t e d with the time and p r o b a b i l i t y of settlement on Mandarte, and with the p r o b a b i l i t y A w o c 0 ZD CJ d) L L 8 4 r u r u - i PL, • • • • 1982-3 14 6 0 8 4 1983-4 r 12 6 12 6 r n n fT-TU n . _ J 5 4 7 j=n. 1984-5 Jt4 j=d 1985-6 j=d 1986-7 10 Apr 19 Jul 2 7 0 c t 4Feb 15May 13 Aug F i g u r e 7.8. Number o f y e a r l i n g s l a s t s e e n d u r i n g a g i v e n p e r i o d v e r s u s s e a s o n f o r 5 c o h o r t s . A r r o w s i n d i c a t e i n c i d e n c e s o f p r e d a t i o n , d o t s i n d i c a t e d a y s when snow f e l l , and numbers i n d i c a t e c o n s e c u t i v e i n c i d e n c e s o f p r e d a t i o n oJT days o f snow. 205 of e m i g r a t i o n . Fourth, d i s p e r s a l d i s t a n c e was s t r o n g l y a f f e c t e d by po i n t of o r i g i n on the i s l a n d , but not by common parentage. F i n a l l y , the p r o b a b i l i t y of s e t t l i n g with a r e l a t i v e was independent of the number of t e r r i t o r i e s moved, and s e t t l i n g with a r e l a t i v e had no e f f e c t on r e p r o d u c t i v e success. These r e s u l t s suggest that competition f o r high q u a l i t y breeding t e r r i t o r i e s p l a y s a major r o l e i n n a t a l d i s p e r s a l i n t h i s p o p u l a t i o n , but that g e n e t i c a l f a c t o r s (e.g. i n b r e e d i n g , i n h e r i t a n c e ) do not. In the d i s c u s s i o n that f o l l o w s , I d i s c u s s the measurement of d i s p e r s a l , i n t e r p r e t and summarize my r e s u l t s with respect to the hypotheses l i s t e d i n Table 7.1, and draw comparisons from p r e v i o u s s t u d i e s of d i s p e r s a l i n the song sparrow (Table 7.7). Measures of D i s p e r s a l Greenwood & Harvey (1982), S h i e l d s (1982) and others have suggested that i t i s more b i o l o g i c a l l y meaningful to d e s c r i b e d i s p e r s a l i n terms of the number of t e r r i t o r i e s t r a v e r s e d than i n meters f o r two reasons. F i r s t , they assume that the r e l a t e d n e s s of p o t e n t i a l mates d e c l i n e s as d i s t a n c e from the n a t a l s i t e i n c r e a s e s . Thus, the number of t e r r i t o r i e s (or home ranges) t r a v e r s e d i m p l i e s a c l o s e r estimate of the degree of inb r e e d i n g avoidance than does the number of meters. Second, e s t i m a t i n g d i s p e r s a l i n u n i t s of t e r r i t o r i e s f a c i l i t a t e s comparisons between s p e c i e s with d i f f e r e n t p a t t e r n s of Table7.T Studies of natal dispersal in the song sparrow. Median Distance Habitat Density Territory Size (mZ) Moved (m) Territories* S i t e 2 Study Location Type (prs/ha) Males Females Pooled Traversed N F i d e l i t y Nice 1937 Interpoint, OH Urban, riverine scrub 2-3 3300-5600 280 270 275 5 34 High Johnston 1956 San Francisco Bay, CA Tidal marsh and bordering scrub 20-26 380-500 - 185 11 34 High Tompa 1963 Mandarte Is., BC Protected sea island scrub 31-49 200-290 200-240 150-200 ca.200 3 16 120 High Halliburton & Mewaldt 1976 Point Reyes, CA Northern coastal scrub ca. 1 ca. 100004 - 225 3 36 High Weatherhead & Boak 1986 Queen's Univ. Biol . St., ON Northern lakeshore, old-field, gardens ca. 5 ca. 20005 ? ? ? 5 6 1 Low This study Mandarte Is., BC Protected sea island scrub 18-51 200-450 75-150 108-159 114 8 253 High 1) Approximate number of territories traversed, estimated from the diameters of circular t e r r i t o r i e s of average size. 2) Fidelity to the natal site as estimated qualitatively by the authors. 3) Estimated from Tompa (1963:61). 4) Estimated as 'home range size' from Baker & Mewaldt (1979). 5) Estimated average, but density varied greatly with habitat structure (Weatherhead, personal communication). 6) Only one male out of 176 banded young returned to the study site. 207 d i s p e r s i o n (e.g. dense c o l o n i a l versus d i s p e r s e d t e r r i t o r i a l ) . However, measuring d i s t a n c e as t e r r i t o r y u n i t s a l s o r a i s e s some p r a c t i c a l problems. F i r s t , r e l a t e d n e s s was independent of the number of t e r r i t o r i e s t r a v e r s e d i n t h i s study (Table 7.5). T h i s has yet to be t e s t e d i n other p o p u l a t i o n s , but i t c o u l d apply g e n e r a l l y because d i s p e r s a l d i s t a n c e s i n song sparrows are t y p i c a l of s e v e r a l b i r d and mammal sp e c i e s ( S h i e l d s 1982). T h i s suggests that the degree of i n b r e e d i n g avoidance may be a s t e p - f u n c t i o n , r a t h e r than a l i n e a r f u n c t i o n of d i s t a n c e d i s p e r s e d . I f so, s u b s t a n t i a l i n b r e e d i n g avoidance may only occur when i n d i v i d u a l s d i s p e r s e beyond the bounds of l o c a l p o p u l a t i o n s , or when ki n r e c o g n i t i o n f a c i l i t a t e s mate s e l e c t i o n (e.g. Koenig & P i t e l k a 1979; Holmes 1984). Second, mod e l l i n g t e r r i t o r i e s as u n i t s of area that are t r a v e r s e d completely or not at a l l , i m p l i e s t h a t d i s p e r s e r s only s e t t l e i n vacant s i t e s (e.g. Murray 1967; Waser 1985). In song sparrows and a v a r i e t y of other s p e c i e s , d i s p e r s e r s c r e a t e t h e i r own s i t e s of settlement by deposing owners, s p l i t t i n g empty t e r r i t o r i e s and i n s e r t i n g themselves w i t h i n defended areas (Knapton & Krebs 1974; Ueda 1986; Freed 1987; Pieman 1987; Chap. 3 & 4). In growing p o p u l a t i o n s , t h i s w i l l r e s u l t i n s h o r t e r d i s p e r s a l d i s t a n c e s than expected i f t e r r i t o r i e s always remained i n t a c t . F i n a l l y , f i e l d measurements of the number of t e r r i t o r i e s 208 d i s p e r s e d may be i m p r a c t i c a l when d i s p e r s e r s t r a v e r s e fragmented h a b i t a t s that vary i n d e n s i t y (e.g. Newton & Marquiss 1983). Thus, although viewing d i s p e r s a l i n u n i t s of t e r r i t o r i e s f a c i l i t a t e s i n t e r - s p e c i f i c comparisons, i t probably over-s i m p l i f i e s the r e l a t i o n s h i p between d i s p e r s a l and t e r r i t o r y e s t a b l i s h m e n t , and between d i s p e r s a l and i n b r e e d i n g , and may be d i f f i c u l t to achieve i n the f i e l d . P o p u l a t i o n D e n s i t y and D i s p e r s a l Greenwood et a l . (1979a) compared d e n s i t y i n the year of f i r s t breeding to d i s p e r s a l , but n e g l e c t e d t h a t , other e f f e c t s being equal, d e n s i t y i n subsequent years i s a f u n c t i o n of d i s t a n c e d i s p e r s e d and r a t e of recruitment by young hatched the p r e v i o u s year. When measured i n t h i s way, d e n s i t y and d i s p e r s a l are not independent. F u r t h e r , using t e r r i t o r i e s or meters as measures of d i s t a n c e gave d i f f e r e n t r e s u l t s i n t h e i r study and i n mine. T h i s p o i n t s to problems of i n t e r p r e t a t i o n when the mechanisms of d i s p e r s a l , and the ways i n which d i s p e r s e r s p e r c e i v e d i s t a n c e , are not well-understood. For example, I found that young moved s i m i l a r numbers of meters each year, but that t h i s amounted to an i n c r e a s i n g number of t e r r i t o r i e s as d e n s i t y i n c r e a s e d and t e r r i t o r i e s were compressed ( F i g . 7.1 & 7.2). Between p o p u l a t i o n s , song sparrows tended to move longer d i s t a n c e s in l e s s dense p o p u l a t i o n s , but fewer t e r r i t o r y diameters (Table 7.7). 209 Measured in meters, my r e s u l t s suggest that d i s p e r s a l was u n a f f e c t e d by d e n s i t y , but the comparative r e s u l t s suggest a negative r e l a t i o n s h i p between d i s t a n c e i n meters and d e n s i t y (Table 7.7). However, under the assumption that d i s t a n c e i s best measured in t e r r i t o r y u n i t s , o p p o s i t e c o n c l u s i o n s are suggested. Newton & Marquiss (1983) a l s o found no r e l a t i o n s h i p between d e n s i t y and meters d i s p e r s e d , but d i d not convert to t e r r i t o r y e q u i v a l e n t s f o r some of the reasons summarized here. The problem of choosing the a p p r o p r i a t e u n i t s i n which to measure d i s p e r s a l a r i s e s because e x p l a n a t i o n s of d i s p e r s a l d i s t a n c e are p o s s i b l e at d i f f e r e n t l e v e l s , and because d i s t a n c e i n t e r r i t o r y u n i t s depends on p o p u l a t i o n d e n s i t y , the same v a r i a b l e to which i t i s being compared. On average, d i s p e r s e r s must t r a v e l f u r t h e r i n meters i n h i g h l y d i s p e r s e d p o p u l a t i o n s , but t h i s might amount to a s i m i l a r number of t e r r i t o r i e s (e.g. S h i e l d s 1982). In c o n t r a s t , f o r a given p o p u l a t i o n d i s p e r s i o n , d i s p e r s a l d i s t a n c e might be expected to i n c r e a s e with d e n s i t y i f c o m p e t i t i o n causes d i s p e r s a l (e.g. Greenwood et a l . 1979a). Within p o p u l a t i o n s , t e r r i t o r y u n i t s might t h e r e f o r e be c o n s i d e r e d an a p p r o p r i a t e measure of d i s p e r s a l i f competitor d e n s i t y were w e l l - r e p r e s e n t e d by the d e n s i t y of t e r r i t o r y h o l d e r s . However, because the diameter of a t e r r i t o r y u n i t depends on d e n s i t y (Greenwood et a l . 1979a; S h i e l d s 1982), d i s p e r s a l i s no longer independent of the v a r i a b l e to which i t i s being compared. Van Noordwijk (1984) d i s c u s s e s how t h i s interdependece of d i s p e r s a l and p o p u l a t i o n d e n s i t y c o u l d a f f e c t 210 estimates of h e r i t a b i l i t y i n d i s p e r s a l d i s t a n c e when parents and o f f s p r i n g share common environments with respect to d e n s i t y . While i t i s i n t u i t i v e l y reasonable that d i s p e r s e r s would t r a v e l f u r t h e r at h i g h d e n s i t y (Greenwood et a l . 1979a), I suggest that c o n v e r t i n g d i s t a n c e to t e r r i t o r i e s f o r w i t h i n s p e c i e s comparisons presupposes that the mechanisms u n d e r l y i n g d i s p e r s a l , and the ways i n which d i s p e r s e r s p e r c e i v e d i s t a n c e , are well-understood. T h i s i s not yet true f o r b i r d s . I n h e r i t a n c e and Point of O r i g i n Within a given area, d i s t a n c e d i s p e r s e d depends on the range of d i s t a n c e s between a l l p o s s i b l e n a t a l and breeding s i t e s (van Noordwijk- 1984). Thus, young hatched nearer the ends of Mandarte I s l a n d had a l a r g e r set of p o t e n t i a l d i s p e r s a l d i s t a n c e s than those nearer the c e n t e r , and t h i s r e s u l t e d i n p o i n t of o r i g i n having a s t r o n g e f f e c t on d i s p e r s a l d i s t a n c e ( F i g . 7.3). Van Noordwijk (1984) showed how t h i s e f f e c t caused Greenwood et a l . ('1979a) to e r r o n e o u s l y conclude that d i s p e r s a l i n the great t i t was h i g h l y h e r i t a b l e . I avoided t h i s problem by comparing only among broods hatched i n p o r t i o n s of the i s l a n d that allowed s i m i l a r o p p o r t u n i t i e s f o r d i s p e r s a l . Analysed i n t h i s way, s i b l i n g s were not more s i m i l a r i n d i s t a n c e s d i s p e r s e d than were non-sibs (Table 7.3), suggesting that d i s p e r s a l was not s t r o n g l y a f f e c t e d by i n h e r i t a n c e . Keppie (1980) found a high s i m i l a r i t y i n 21 1 d i s t a n c e d i s p e r s e d among s i b l i n g spruce grouse (Canachites  c a n a d e n s i s ) , but d i d not r u l e out p o i n t of o r i g i n as the cause of t h i s resemblance. Newton & Marquiss (1983) re p o r t e d a high m o t h e r - o f f s p r i n g resemblance i n d i s p e r s i n g sparrowhawks, but t h i s i s expected s i n c e d i s t a n c e a l s o depended on n a t a l h a b i t a t ( i . e . 'upland' versus 'lowland': Newton & Marquiss (1983); see van Noordwijk (1984) f o r a summary of problems in parent-o f f s p r i n g comparisons of d i s p e r s a l ) . There i s c o n s i d e r a b l e g e n e t i c evidence from small mammals to i n d i c a t e d i f f e r e n c e s between d i s p e r s e r s and r e s i d e n t s (Myers & Krebs 1971; Krebs et a l . 1976; Gaines & McClenaghan 1980). However, evidence from b i r d s remains i n d i r e c t and c o n s i s t e n t with the h y p othesis that common environment r a t h e r than i n h e r i t a n c e accounts f o r s i m i l a r i t i e s i n d i s p e r s a l among s i b l i n g s and parents and o f f s p r i n g . Sex, Dominance, Distance and Settlement Time Nice (1937) and Tompa (1963) each r e p o r t e d median d i s p e r s a l d i s t a n c e s f o r males that only s l i g h t l y exceeded those f o r females (Table 7.7). These f i n d i n g s are supported by t h i s study ( F i g . 7.1 & 7.2). The l a c k of a l a r g e sex-bias i n d i s p e r s a l i s c o n t r a r y to the much more p r e v a l e n t p a t t e r n of female-biased n a t a l d i s p e r s a l i n b i r d s (Greenwood 1980; note, however, that Greenwood c i t e s Nice (1937) i n c o r r e c t l y ) . D i s t a n c e d i s p e r s e d on Mandarte was a l s o u n r e l a t e d to 212 dominance w i t h i n or between sexes. However, subordinate b i r d s of each sex s e t t l e d l a t e r than dominants and emigrated from Mandarte more o f t e n . T h i s i l l u s t r a t e s an unresolved problem concerning my study area. Subordinates that disappeared from the p o p u l a t i o n may have d i e d or emigrated. However, because known emigrants from Mandarte were su b o r d i n a t e s , i t seems l i k e l y t h a t had the d i s t a n c e s of a l l emigrants (seen and unseen) been i n c l u d e d , dominance and d i s p e r s a l d i s t a n c e . would have been n e g a t i v e l y r e l a t e d . Only a p o r t i o n of a l l independent young r e c r u i t e d to the p o p u l a t i o n (ca. 23-54%, Table 7.4), most of which were dominant b i r d s (Arcese & Smith 1985; Table 7.2). Thus, dominance probably had a l a r g e r i n f l u e n c e on d i s p e r s a l d i s t a n c e than I was able to measure among r e c r u i t s . I t i s i n t e r e s t i n g that even among r e c r u i t s , settlement time was r e t a r d e d i n subordinates and young hatched l a t e i n the season, but was u n r e l a t e d to d i s t a n c e d i s p e r s e d . I suggest that t h i s o c c u r r e d because year-round t e r r i t o r i a l i t y by each sex r e s u l t s i n queues f o r ownership that are ordered by s o c i a l s t a t u s . N o n - t e r r i t o r i a l f l o a t e r s i n these queues occupy home ranges that encompass 1-10 or more t e r r i t o r i e s (Chap. 4). F l o a t e r s become owners when an owner d i e s or when they s u c c e s s f u l l y c h a l l e n g e f o r ownership by i n s e r t i n g i n t o e s t a b l i s h e d t e r r i t o r i e s or deposing v u l n e r a b l e owners (Chap. 3 & 4). Evidence of queues f o r t e r r i t o r y ownership comes from a v a r i e t y of other b i r d s (e.g. Orians 1961; Smith 1978,1987; Jamieson 1983; Jamieson & Zwickel 1983; B e l e t s k y & Orians 213 1987a,b; Eden 1987a,b), and these t e r r i t o r i a l systems are g e n e r a l l y viewed as a step towards the e v o l u t i o n of the r e t e n t i o n of young, h e l p i n g behaviour and c o o p e r a t i v e breeding (Emlen & Vehrencamp 1983; Woolfenden & F i t z p a t r i c k 1984; Brown 1987). An i n t e r e s t i n g q u e s t i o n remains, however, i n how d i s p e r s a l d i s t a n c e r e l a t e s to the establishment of home ranges i n the n o n - t e r r i t o r i a l phase of l i f e . E f f e c t of Supplemental Food Adding food to t e r r i t o r i e s throughout the breeding p e r i o d r e s u l t e d i n reduced d i s p e r s a l of fed j u v e n i l e s as compared to c o n t r o l s i n l a t e August, even though food was not s u p p l i e d a f t e r the end of J u l y . T h i s suggests that young ' had e i t h e r e s t a b l i s h e d home ranges by l a t e J u l y , or were using p r e v i o u s experience as a cue f o r choosing areas in which to l o c a l i z e . Because l a r g e d i f f e r e n c e s between fed and c o n t r o l young were maintained the f o l l o w i n g s p r i n g among r e c r u i t s , food a v a i l a b i l i t y on the n a t a l t e r r i t o r y appears to be an extremely important f a c t o r a f f e c t i n g d i s p e r s a l d i s t a n c e . D i s p e r s a l , Mate Q u a l i t y and Reproductive Success Greenwood et a l . (1979b) suggested that male great t i t s d i s p e r s e d f u r t h e r when doing so allowed them to mate with a d u l t r a t h e r than y e a r l i n g females. In song sparrows, r e p r o d u c t i v e success peaks in middle-age (2-3 y r s , Nol & Smith 1987), but I 214 found that male and female d i s p e r s e r s mated with d i f f e r e n t - a g e d b i r d s independently of d i s t a n c e moved. D i s p e r s a l d i s t a n c e was a l s o u n r e l a t e d to subsequent r e p r o d u c t i v e success, suggesting that remaining c l o s e to the n a t a l s i t e was of l i t t l e consequence as long as a breeding p o s i t i o n was secured. However, remaining c l o s e to the n a t a l s i t e might c o n f e r an advantage p r i m a r i l y by a f f e c t i n g the chances of s e t t l i n g r a t h e r than by a f f e c t i n g r e p r o d u c t i v e success subsequent to se t t l e m e n t . To t e s t t h i s , f u t u r e s t u d i e s should measure the p r o b a b i l i t y of recruitment as d i s t a n c e from the n a t a l s i t e i n c r e a s e s . F a c t o r s A s s o c i a t e d with Inbreeding Song Sparrows r e g u l a r l y s e t t l e d with r e l a t i v e s r e g a r d l e s s of d i s t a n c e d i s p e r s e d (Table 7.5). T h i s might be expected given the smal l and s e m i - i s o l a t e d nature of the p o p u l a t i o n . However, d i s p e r s a l i s a l s o h i g h l y r e s t r i c t e d i n some other song sparrow p o p u l a t i o n s (Table 7.7; S h i e l d s 1982), and t h i s may have c o n t r i b u t e d to c o n s i d e r a b l e geographic divergence i n morphology, c o l o r a t i o n and blood group f r e q u e n c i e s ( M i l l e r 1947; M a r s h a l l 1948a,b; F e r r e l l 1966; A l d r i c h 1984). The frequency of inbred matings d e c l i n e d as p o p u l a t i o n s i z e i n c r e a s e d ( F i g . 7.6). T h i s i s expected by chance, and the p e c u l i a r demography of t h i s p o p u l a t i o n . F i r s t , the p r o b a b i l i t y of s e t t l i n g with c l o s e r e l a t i v e s d e c l i n e s as p o p u l a t i o n s i z e i n c r e a s e s when mating i s random (Bulmer 1973; Falcon e r 1981). 215 Second, because the mean number of young r a i s e d d e c l i n e d from over 4 to l e s s than 1 as d e n s i t y i n c r e a s e d from 26-72 females (Arcese & Smith 1988), the p r o b a b i l i t y of sib-matings d e c l i n e d f a s t e r than expected by p o p u l a t i o n s i z e a l o n e . T h i s suggests a unique p r e d i c t i o n of the hypothesis that sex-biased d i s p e r s a l evolved as a mechanism to a v o i d i n b r e e d i n g : sex-biases should be most pronounced in s p e c i e s where the number of young r a i s e d each year, and thus the p r o b a b i l i t y of sib-matings i n the absence of a s e x - b i a s , i s h i g h . Van Noordwijk & Scharloo (1981), Bulmer (1973) and Greenwood et a l . (1978) a l l found reduced f l e d g i n g success among inb r e d p a i r s of great t i t s . However, van Noordwijk & S c h a r l o o (1981) a l s o found that inbred young r e c r u i t e d to the p o p u l a t i o n at a s l i g h t l y higher r a t e than o t h e r s , and suggested that the e f f e c t of i n b r e e d i n g on l i f e t i m e r e p r o d u c t i v e success might be minimal. Rowley et a l . (1986) r e p o r t e d no d e l e t e r i o u s e f f e c t of i n b r e e d i n g on r e p r o d u c t i o n i n the s p l e n d i d wren (Malurus splendens), where a very high p r o p o r t i o n of a l l matings were between c l o s e r e l a t i v e s (19.4%). In a p r e l i m i n a r y a n a l y s i s , I found no e f f e c t of the r e l a t e d n e s s of the f i r s t mate on subsequent r e p r o d u c t i v e success (Table 7.6). However, i t remains to be determined i f any e f f e c t of i n b r e e d i n g was masked by subsequent mate replacement. I suggest that i n the study p o p u l a t i o n , where com p e t i t i o n f o r t e r r i t o r i e s promotes r a p i d mate replacement (Chap. 2 & 3) and i s perhaps the main determinant of r e p r o d u c t i v e success (Smith 1988; Hochachka et 216 a l . ms; Chap. 8), p a s s i n g over t e r r i t o r i e s occupied by r e l a t i v e s i s probably more d e l e t e r i o u s than i n b r e e d i n g (see a l s o Bengtsson 1978; Smith 1979). Rowley et a l . (1986) reached a s i m i l a r c o n c l u s i o n . D i s p e r s a l Between Po p u l a t i o n s The r e s u l t s of t h i s study suggest that d i s p e r s a l between p o p u l a t i o n s i n the v i c i n i t y of Mandarte takes p l a c e at low f r e q u e n c i e s , but that s u c c e s s f u l immigration i s not o f t e n a c h ieved and i s u n a f f e c t e d by l o c a l f l u c t u a t i o n s i n p o p u l a t i o n d e n s i t y . However, female emigrants may a c q u i r e breeding t e r r i t o r i e s more o f t e n than males. In comparison, Tompa (1963) i d e n t i f i e d 40 j u v e n i l e emigrants (21 males, 19 females) on H a l i b u t I s l a n d from 1961-3, d u r i n g fewer v i s i t s than I made from 1982-5. F u r t h e r , 27 of these young subsequently r e t u r n e d to Mandarte. Immigrants accounted f o r about 1% of the breeding p o p u l a t i o n each year, matching Smith's (1988) estimate of immigration d u r i n g the p e r i o d 1974-9. F i n a l l y , a f t e r Tompa (personal communication) removed h a l f the r e s i d e n t s from H a l i b u t i n the f a l l of 1963, s e v e r a l young hatched on Mandarte subsequently s e t t l e d t h e r e . The c o n t r a s t between Tompa's and my own o b s e r v a t i o n s suggests that the degree of i n t e r - i s l a n d d i s p e r s a l was much lower from 1982-6 than from 1961-4. The reason f o r t h i s remains u n c l e a r . 217 Seasonal P a t t e r n s of Disappearance Tompa (1964) suggested that t e r r i t o r i a l behaviour r e g u l a t e d p o p u l a t i o n d e n s i t y on Mandarte p a r t l y by i n f l u e n c i n g e m i g r a t i o n . T h i s h y pothesis i s i n d i r e c t l y supported by the p a t t e r n of disappearances among y e a r l i n g s each year ( F i g . 7.8). Disappearances occurred most f r e q u e n t l y i n l a t e summer and f a l l , when a g g r e s s i v e i n t e r a c t i o n s among maturing j u v e n i l e s , and l a t e r between t e r r i t o r i a l a d u l t s and y e a r l i n g s , were frequent (Tompa 1964; Arcese & Smith 1985; pe r s o n a l o b s e r v a t i o n s ) . With the exception of 1 year, r e l a t i v e l y few b i r d s disappeared d u r i n g mid-winter, when snow sometimes covered the ground f o r s e v e r a l days. But s e v e r a l b i r d s t y p i c a l l y disappeared i n l a t e winter a f t e r pronounced t e r r i t o r i a l a c t i v i t y resumed (Tompa 1964; Chaps. 3,5). Hypotheses and Proximate Causes of D i s p e r s a l i n Song Sparrows Male and female song sparrows d i s p e r s e approximately e q u a l l y ( F i g . 7.1 & 7.2, Table 7.7), i n accordance with a l l three hypotheses i n v o l v i n g mating system o r g a n i s a t i o n (Table 7.1). Greenwood (1980) and Dobson's (1982) h y p o t h e s i s of competition f o r environmental resources was s t r o n g l y supported by two main r e s u l t s . F i r s t , d i s p e r s a l was d r a m a t i c a l l y reduced when t e r r i t o r y q u a l i t y was enhanced. F u r t h e r , c o n t r a r y to the common p a t t e r n of d i s p e r s a l in b i r d s (Greenwood 1980; Greenwood & Harvey 1982), the lack of a sex-bias i n d i s p e r s a l was 218 c o r r e c t l y p r e d i c t e d a f t e r d e t a i l e d s t u d i e s of the t e r r i t o r i a l and mating systems of song sparrows (Smith et a l . 1982, Chaps. 4-6) . Young u s u a l l y d i s p e r s e d from the n a t a l s i t e r e g a r d l e s s of p a r e n t a l s u r v i v a l . However, the r e s u l t s of the food a d d i t i o n study suggest that high n a t a l - s i t e f i d e l i t y may have reduced a d u l t s u r v i v a l by i n c r e a s i n g c o m p e t i t i o n (Arcese & Smith 1988). I n h e r i t a n c e of the n a t a l s i t e was u s u a l l y achieved by e v i c t i n g a parent of the same sex, and 3 of 5 young that d i d s e t t l e on the n a t a l s i t e subsequently mated with a parent. Thus, L i e b e r g & von Schantz's (1985) hypothesis i s p a r t l y supported by t h i s study. D i s p e r s a l was not sex-biased and o f f s p r i n g may represent a p o t e n t i a l t h r e a t to continued r e p r o d u c t i o n by p a r e n t s , but n a t a l p h i l o p a t r y d i d not depend on the l i k e l i h o o d of i n b r e e d i n g (see Table 7.1). F u r t h e r , the p r o b a b i l i t y of mating with a r e l a t i v e was u n r e l a t e d to d i s t a n c e d i s p e r s e d , suggesting that d i s p e r s i n g f u r t h e r i s not an e f f e c t i v e means of a v o i d i n g i n b r e e d i n g , or of choosing o p t i m a l l y outbred matings, w i t h i n p o p u l a t i o n s . F i n a l l y , the r e l a t e d n e s s of the f i r s t mate had no e f f e c t on subsequent r e p r o d u c t i v e success, showing that s e t t l i n g with a r e l a t i v e had l i t t l e s e l e c t i v e consequence. I t i s p o s s i b l e that small e f f e c t i v e p o p u l a t i o n s i z e ( M i l l e r 1947), p e r i o d i c b o t t l e n e c k s ( S c h l u t e r & Smith 1986; Smith 1988) and low r a t e s of e f f e c t i v e immigration ( M a r s h a l l I948a,b; Johnston I956a,b, Tompa 1963; F e r r e l l 1966; t h i s study) sometimes reduce the negative e f f e c t s of i n b r e e d i n g i n song sparrow p o p u l a t i o n s 219 (Wright 1977; Bengtsson 1978; Smith 1979; S h i e l d s 1982; Falconer 1981). The assumptions and p r e d i c t i o n s of d i s p e r s a l models based s t r i c t l y on demography were a l s o g e n e r a l l y unsupported: d i s p e r s a l d i s t a n c e on Mandarte was u n r e l a t e d to i n b r e e d i n g , time to s ettlement, p o p u l a t i o n t r a j e c t o r y and a d u l t or j u v e n i l e s u r v i v a l (Table 7.1). I n c r e a s i n g the a v a i l a b i l i t y of h a b i t a t on H a l i b u t I s l a n d a l s o had no d e t e c t a b l e e f f e c t on d i s p e r s a l from Mandarte, but t h i s c o u l d have been because few p r o s p e c t i v e s e t t l e r s from Mandarte v i s i t e d H a l i b u t I s l a n d . S o c i a l dominance, a f a c t o r i n c l u d e d i n many demographic models of d i s p e r s a l (Waser 1985; Buechner 1987), c o r r e c t l y p r e d i c t e d which i n d i v i d u a l s emigrated from Mandarte, and settlement time on the i s l a n d , but f a i l e d to p r e d i c t equal d i s p e r s a l among females and males (Table 7.1), because males are dominant to females (Smith et a l . 1980; Arcese & Smith 1985). F i n a l l y , c o n t r a r y to the main assumption of g e n e t i c models of d i s p e r s a l , I found no evidence that d i s p e r s a l d i s t a n c e was a s t r o n g l y h e r i t a b l e t r a i t . I suggest that the process of d i s p e r s a l i n song sparrows r e s u l t s from movements undertaken to sequester resources c r u c i a l f o r r e p r o d u c t i o n (see a l s o Weatherhead & Boak 1986). Because a l l breeding h a b i t a t on Mandarte i s defended even at low breeding d e n s i t i e s (Arcese & Smith 1988), resource a c q u i s i t i o n 220 i s a h i g h l y c o m p e t i t i v e process that has major consequences for l i f e t i m e r e p r o d u c t i v e success (Smith 1988; Smith & Arcese ms; Hochachka et a l . ms). Young may d i s p e r s e from n a t a l s i t e s i n l a t e summer, before most a d u l t m o r t a l i t y occurs, but be c o n s t r a i n e d i n t h e i r a b i l i t y to r e t u r n to n a t a l s i t e s i n the event of p a r e n t a l death because of investment i n home range establishment elsewhere (e.g. Krebs 1982). Success in competition f o r breeding s i t e s on Mandarte, and emigration from the i s l a n d , are determined in part by dominance s t a t u s . Sex-b i a s e s i n d i s p e r s a l behaviour probably depend on strong d i f f e r e n c e s i n the i n t r a s e x u a l c o m p e t i t i v e environment, which were absent i n t h i s p o p u l a t i o n . However, higher success among female emigrants from, and immigrants to Mandarte may r e f l e c t a reduced c o s t of d i s p e r s a l i n females r e l a t i v e to males. T h i s hypothesis can not be t e s t e d , however, u n t i l d i s p e r s a l and m o r t a l i t y are separated. REFERENCES A l d r i c h , J.W. 1984. Ecogeographical v a r i a t i o n i n s i z e and p r o p o r t i o n s of song sparrows (Melospiza melodia). O r n i t h . Monoqr., 35, 1-134. Arcese, P. & Smith, J.N.M. 1988. E f f e c t s of p o p u l a t i o n d e n s i t y and supplemental food on r e p r o d u c t i o n i n the song sparrow. J . Anim. E c o l . , 57, i n p r e s s . Arcese, P. & Ludwig, D. 1986. Improving estimates of dominance based on r a t i o s . 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Woolfenden, G.E. & F i t z p a t r i c k , J.W. 1984. The F l o r i d a Scrub  Jay: Demography, of a C o o p e r a t i v e l y Breeding B i r d . P r i n c e t o n : P r i n c e t o n U n i v e r s i t y Press. Wright S. 1977. E v o l u t i o n and the Ge n e t i c s of P o p u l a t i o n s . Vol.3 Experimental R e s u l t s and E v o l u t i o n a r y Deduct i o n s . Chicago: U n i v e r s i t y of Chicago Press. 230 CHAPTER 8. GENERAL DISCUSSION T e r r i t o r i a l Behaviour and S o c i a l O r g a n i z a t i o n I conclude by d i s c u s s i n g how d i s p e r s a l , t e r r i t o r i a l i t y , the mating system and t h e i r e f f e c t s on r e p r o d u c t i v e success are r e l a t e d to one another i n song sparrows, I f i r s t c o n s i d e r some e f f e c t s of t e r r i t o r i a l i t y on i n d i v i d u a l s and p o p u l a t i o n s . On Mandarte I s l a n d , both male and female song sparrows are t e r r i t o r i a l year-round. However, more young were r a i s e d i n most years than were needed to r e p l a c e a l l breeders that d i e before the subsequent year (Tompa 1964; Arcese & Smith 1988). P o p u l a t i o n d e n s i t y i n c r e a s e d d u r i n g t h i s study ( F i g . 1.1), but the r a t e of recruitment f e l l by h a l f (Table 7.4). These c o n d i t i o n s suggest that access to t e r r i t o r i e s i s r e s t r i c t e d by the behaviour of t e r r i t o r y owners (Brown 1969; Watson & Moss 1970; P a t t e r s o n 1980). P o p u l a t i o n l i m i t a t i o n by t e r r i t o r i a l behaviour i n p r e f e r r e d breeding h a b i t a t s i s a f e a t u r e of many b i r d and mammal po p u l a t i o n s (e.g. Hannon 1983; Woolfenden & F i t z p a t r i c k 1984; Watson 1985; Dobson 1979; G a l i n d o & Krebs 1987). A r e c u r r i n g f e a t u r e t h a t emerges from s t u d i e s of such s p e c i e s i s the e x i s t e n c e of n o n - t e r r i t o r i a l 'underworlds' (Smith 1978), which can be c o n s i d e r e d as queues f o r t e r r i t o r y ownership. I n d i v i d u a l s w i t h i n these queues are u s u a l l y ordered by dominance and are thought to advance to ownership s t a t u s when t e r r i t o r y 231 owners d i e (Smith 1978; Birkhead & C l a r k s o n 1985; Eden 1987). In song sparrows, however, I have shown that r e l a t i v e l y few f l o a t e r s gain breeding t e r r i t o r i e s by p a s s i v e l y r e p l a c i n g dead owners (Chap. 3, 4, 5 & 6). Instead, f l o a t e r s c r e a t e t h e i r own o p p o r t u n i t i e s by a c t i v e l y e v i c t i n g owners from a l l or p a r t of t h e i r t e r r i t o r i e s . In males, f l o a t e r s a c t i v e l y seek out and c h a l l e n g e those owners l e a s t able to defend themselves a g a i n s t i n t r u d e r s (Chap. 4), and I suspect that the same process occurs i n females (e.g. Chap. 5, 6). T h i s p r o p e n s i t y f o r a g g r e s s i v e a c q u i s i t i o n of breeding s i t e s a f f e c t s a d u l t s u r v i v a l (Kluyver 1970; Caro & C o l l i n s 1987; Arcese & Smith 1988), n a t a l d i s p e r s a l d i s t a n c e (Chap. 7) and p o p u l a t i o n d e n s i t y (Tompa 1964; Knapton & Krebs 1974). I t h e r e f o r e suggest that i t may a l s o be an important f a c t o r a f f e c t i n g p o p u l a t i o n dynamics and s o c i a l o r g a n i z a t i o n that has not p r e v i o u s l y been widely c o n s i d e r e d . Competitive A b i l i t y and Reproductive Success Because a t e r r i t o r y i s r e q u i r e d f o r s u c c e s s f u l r e p r o d u c t i o n i n song sparrows (Smith 1988), I now c o n s i d e r how c o m p e t i t i o n f o r breeding t e r r i t o r i e s a f f e c t s r e p r o d u c t i v e success i n males and females. 232 Males One symptom of i n c r e a s e d p o p u l a t i o n d e n s i t y among males i s a p a r a l l e l i n c r e a s e in the p r o p o r t i o n of n o n - t e r r i t o r i a l males i n the p o p u l a t i o n (Chap. 4; Smith & Arcese ms). In chapters 3 & 4, I showed how t h i s s u r p l u s of males maintained a high l e v e l of competition between f l o a t e r s and t e r r i t o r y owners throughout most of the year, but p a r t i c u l a r l y p r i o r to and d u r i n g the breeding p e r i o d . I suggested that males remained i n t h e i r n a t a l p o p u l a t i o n even when settlement was u n l i k e l y because s u c c e s s f u l immigration by male song sparrows from l o c a l p o p u l a t i o n s near Mandarte i s rar e (Tompa 1964; Chap. 3 & 7). N e v e r t h e l e s s , males hatched i n t o dense p o p u l a t i o n s produced r e l a t i v e l y few young because they r e c r u i t e d l a t e r i n l i f e on average, and r e c r u i t e d l e s s o f t e n o v e r a l l (Smith 1988; Hochachka et a l . ms; Smith & Arcese ms; Chap. 7). By s p r i n g each year, males c o u l d be c a t e g o r i z e d as t e r r i t o r y owners, f l o a t e r s , or as being absent from the p o p u l a t i o n (Chap. 3, 6 & 7 ) . However, the s t a t u s of males i n the two former c l a s s e s changed f r e q u e n t l y as f l o a t e r s s e t t l e d and former t e r r i t o r y owners disappeared or became f l o a t e r s themselves (Chap. 3 & 4). Some deposed t e r r i t o r y owners regained t e r r i t o r i e s l a t e r i n l i f e , s uggesting that male co m p e t i t i v e a b i l i t y v a r i e s i n time and that t h i s leads to changes i n t e r r i t o r i a l s t a t u s . Age was c o r r e l a t e d with changes i n t e r r i t o r i a l s t a t u s and 233 c o m p e t i t i v e a b i l i t y (Chap. 3,4,6 & 7). R e l a t i v e age w i t h i n a cohort had a profound e f f e c t on which males became t e r r i t o r i a l , remained as f l o a t e r s , or disappeared from the p o p u l a t i o n (Arcese & Smith 1985; Chap. 3,4,6, & 7). Between c o h o r t s , c o m p e t i t i v e a b i l i t y i n c r e a s e d from 1- to 2-years of age, but e i t h e r l e v e l e d o f f or d e c l i n e d a f t e r 3-years of age. However, although age was the primary c o r r e l a t e of a b i l i t y that I i d e n t i f i e d , two r e s u l t s suggest that c o n s i s t e n t d i f f e r e n c e s among males p e r s i s t e d throughout l i f e . F i r s t , i n c o n s e c u t i v e years p a r t i c u l a r males experienced s i m i l a r l e v e l s of i n t r u s i o n p r e ssure with respect to the means of t h e i r a g e - c l a s s , even though i n t r u s i o n pressure changed as expected by comparisons a c r o s s a g e - c l a s s e s when males aged (Chap. 4). Second, males that began t h e i r f i r s t breeding season as a f l o a t e r a l s o became f l o a t e r s l a t e r i n l i f e a f t e r g a i n i n g a t e r r i t o r y more o f t e n than males that gained t e r r i t o r i e s e a r l i e r i n l i f e (Chap. 3). I suggest that competition among males was important not only i n r e c r u i t m e n t , but a l s o i n t e r r i t o r y maintenance and l o n g e v i t y as a breeder. Smith (1988) and Hochachka et a l . (ms) have concluded t h a t , a s i d e from chance e f f e c t s that r e s u l t from p o p u l a t i o n s i z e and t r a j e c t o r y at the time of hatch, recruitment and l o n g e v i t y as a breeder are the major f a c t o r s a f f e c t i n g l i f e t i m e r e p r o d u c t i v e success i n male song sparrows. I t h e r e f o r e conclude that c o m p e t i t i v e a b i l i t y i s a key f a c t o r a f f e c t i n g male r e p r o d u c t i v e success. 234 Females Although the data presented f o r females are more l i m i t e d than f o r males, my r e s u l t s s t r o n g l y suggest that competition f o r t e r r i t o r i e s among females a l s o p l a y s a major r o l e i n female r e p r o d u c t i v e success (Chap. 5,6, & 7). As f o r males, recruitment d e c l i n e d as female d e n s i t y i n c r e a s e d (Chap. 6 & 7), and the number of o b s e r v a t i o n s of a g g r e s s i v e c o n t e s t s over t e r r i t o r y ownership among females i n c r e a s e d d r a m a t i c a l l y (Chap. 5). Within c o h o r t s of females, dominance s t a t u s was r e l a t e d to recruitment (Arcese & Smith 1985; Chap. 7), and date of hatch was r e l a t e d to both recruitment and t e r r i t o r i a l s t a t u s i n the f o l l o w i n g year (Chap. 7). However, one p o t e n t i a l l y important d i f f e r e n c e i n the r e l a t i v e l e v e l s of male and female competition concerned the op p o r t u n i t y f o r d i s p e r s a l between p o p u l a t i o n s . Females d i s p e r s e d more s u c c e s s f u l l y between p o p u l a t i o n s than males. T h i s i s p a r t i c u l a r l y i n t e r e s t i n g because the s e x - r a t i o of y e a r l i n g d i s p e r s e r s to Mandarte i n l a t e summer was approximately equal (unpublished r e s u l t s ) , suggesting that females have a g r e a t e r o p p o r t u n i t y to reproduce o u t s i d e t h e i r n a t a l p o p u l a t i o n than do males. T h i s c o u l d occur because females s u f f e r s l i g h t l y higher annual m o r t a l i t y than males (Arcese & Smith 1985; Nol & Smith 1987), which w i l l a f f e c t the instantaneous r a t e of t e r r i t o r y vacancy and the p r o b a b i l i t y of g a i n i n g a t e r r i t o r y without a g g r e s s i v e l y e v i c t i n g owners (Murray 1984; 235 Waser 1985; Chap. 3 & 7). D i s p e r s a l S t r a t e g i e s and Competition f o r Resources When h a b i t a t s are s a t u r a t e d with breeders, as they are on Mandarte I s l a n d year-round, p o t e n t i a l r e c r u i t s must wait u n t i l an owner d i e s or becomes v u l n e r a b l e to a takeover attempt, or d i s p e r s e from the n a t a l area i n search of empty h a b i t a t elsewhere. T h i s problem i s g e n e r a l to a l l r e s i d e n t s p e c i e s , and can be c h a r a c t e r i z e d as the d e c i s i o n to remain and compete f o r r e s o u r c e s l o c a l l y or d i s p e r s e to a v o i d c o m p e t i t i o n (Comins et a l . 1980; Horn 1983). In g e n e r a l , h i g h d i s p e r s a l tendencies are c h a r a c t e r i s t i c of marine s p e c i e s , and of t e r r e s t r i a l s p e c i e s that c o l o n i z e temporary h a b i t a t s or e x p l o i t p e r i o d i c a l l y abundant reso u r c e s (Horn 1983). In b i r d s and mammals, however, d i s p e r s a l i s t y p i c a l l y more r e s t r i c t e d . Most s p e c i e s s t u d i e d so f a r u s u a l l y breed w i t h i n one to s e v e r a l home range (or t e r r i t o r y ) diameters of t h e i r n a t a l s i t e ( S h i e l d s 1982), but l i t t l e i s known about d i s p e r s a l i n h i g h l y v a g i l e , e r u p t i v e s p e c i e s (reviews i n Greenwood 1980; Dobson 1982; Waser & Jones 1983). The problem here i s that i t i s i n h e r e n t l y d i f f i c u l t to study d i s p e r s e r s , as p o i n t e d out i n t h i s study (Chap. 3 & 7). My data do not allow me to come to a c o n c l u s i o n about how t r a d e - o f f s between l o c a l c o m p e t i t i o n and s e a r c h i n g f o r empty s i t e s elsewhere a f f e c t d i s p e r s a l i n song sparrows. A comparison of s e v e r a l s t u d i e s (Chap. 7, Table 236 7.8), however, suggests that i n a l l but s p a r s e l y populated areas (e.g. Weatherhead & Boak 1986) d i s p e r s i n g song sparrows are l i k e l y to encounter moderate to strong c o m p e t i t i o n i n a l l s u i t a b l e breeding h a b i t a t s . If one c o n s i d e r s d i s p e r s a l s t r i c t l y on the l o c a l s c a l e , however, two recent s t u d i e s o f f e r p a r t i c u l a r l y s t r i k i n g p a r a l l e l s to the stay versus d i s p e r s e dilemma. In both magpies ( P i c a p i c a ; Eden 1987) and black-capped chickadees (Parus  a t r i c a p i l l u s ; Smith 1987), most n o n - t e r r i t o r i a l b i r d s l i v e i n f l o c k s ordered by dominance, from which they advance to t e r r i t o r i a l s t a t u s as the breeding season commences and/or t e r r i t o r y openings occur. However, a m i n o r i t y of b i r d s e i t h e r t r a v e l s o l i t a r i l y and attempt to occupy t e r r i t o r i e s o p p o r t u n i s t i c a l l y (magpies) or switch between f l o c k s and assume the p o s i t i o n of more dominant b i r d s a f t e r they disappear ( c h i c k a d e e s ) . In both cases, evidence suggests that the success of f l o c k - s w i t c h e r s and l o n e r s i s equal to that of r e g u l a r f l o c k members. Se v e r a l t h e o r e t i c a l models and e m p i r i c a l s t u d i e s p o s i t c o m p e t i t i o n f o r breeding resources as the primary cause of d i s p e r s a l i n b i r d s and mammals (reviewed i n Chap. 7; Table 7.1), and the r e s u l t s of t h i s study are c o n s i s t e n t with t h i s h y p o t hesis with regard to both i n t e r - and i n t r a - p o p u l a t i o n movements. However, u n t i l i n d i v i d u a l s that leave n a t a l p o p u l a t i o n s can be f o l l o w e d as c l o s e l y as those that remain, one 237 cannot assess the a f f e c t of each o p t i o n on r e p r o d u c t i v e success. C o n f l i c t s of I n t e r e s t Between Males and Females An emerging theme in a few recent s t u d i e s that r e l a t e d i s p e r s a l , t e r r i t o r i a l i t y and mating systems to r e p r o d u c t i v e success has been that males and females o f t e n employ d i f f e r e n t t a c t i c s t o maximize the number of young that they produce, and that t h i s d i f f e r e n c e sometimes leads to a c o n f l i c t of i n t e r e s t between males and females (Clutton-Brock et a l . 1982; Woolfenden & F i t z p a t r i c k 1984; Davies 1985; Pusey & Packer 1987). For song sparrows, I have shown that c o n f l i c t s between males and females are probably r e s t r i c t e d p r i m a r i l y to the mating system, although males and females may a l s o compete d i r e c t l y f o r access to food resources in the non-breeding p e r i o d (Smith et a l . 1980; Arcese & Smith 1985). Although a v a r i e t y of both male and female behaviours c o u l d a f f e c t mating system o r g a n i z a t i o n , r e s e a r c h has h i s t o r i c a l l y c e n t e r e d on the a g g r e s s i v e nature of males, and on the choosiness of females. (Selander 1972; H a l l i d a y 1978; Parker 1978; Mock 1983; Payne 1984). However, s e v e r a l recent s t u d i e s show that a g g r e s s i v e behaviour i n females a f f e c t s spacing p a t t e r n s (e.g. Hannon 1983; Hurley & Robertson 1985), r e p r o d u c t i v e success (Davies & Houston 1986; Clutton-Brock et a l . 1987), d i s p e r s a l (Dobson 1979; G a l i n d o & Krebs 1987) and mating systems (Hannon 1984; Davies & Houston 1986). These 238 p o s s i b i l i t i e s have been recognized i n theory (e.g. T r i v e r s 1972; Oring 1982; Murray 1984), but have only r e c e n t l y been r e g u l a r l y i n v e s t i g a t e d i n the f i e l d (e.g. f o r small mammals see: Dobson 1979; T a i t t & Krebs 1982; Boonstra & Rood 1983; Galindo & Krebs 1987). With the ex c e p t i o n of s t u d i e s of polygynous s p e c i e s (e.g. P e t r i e 1983), female a g g r e s s i v e behaviour has a l s o only r e c e n t l y become a prime t o p i c of study i n b i r d s (Smith 1982; Hannon 1983; 1984; Hurley & Robertson 1985; Stutchbury & Robertson 1985; Searcy 1986). In t h i s study, however, I have shown that a g g r e s s i v e behaviour i n both males and females p l a y s a fundamental r o l e i n determining which i n d i v i d u a l s r e c r u i t to the breeding p o p u l a t i o n , and i n the degree to which males and females are ab l e to monopolize access to mates of opposite sex. In t u r n , these f a c t o r s c o n t r i b u t e to the o v e r a l l v a r i a t i o n i n r e p r o d u c t i v e success among i n d i v i d u a l s hatched i n t o the Mandarte I s l a n d song sparrow p o p u l a t i o n (Chap. 4, 6, & 7). In a p a r t i c u l a r l y s t r i k i n g example of d i f f e r e n c e s i n male and female r e p r o d u c t i v e i n t e r e s t s , Davies (1985) showed how dunnocks of each sex a g g r e s s i v e l y spaced themselves out to maximize the l i k e l i h o o d of o b t a i n i n g e x t r a mates. Males that defended the t e r r i t o r i e s of more than one female i n c r e a s e d the number of young that they s i r e d . However, females that f a i l e d to defend the e n t i r e t e r r i t o r y of t h e i r mate l o s t male care and r a i s e d few young. Females with l a r g e r t e r r i t o r i e s o f t e n 239 r e c e i v e d a i d from more than one male and r a i s e d more young than otherwise p o s s i b l e (Davies & Houston 1986). Davies & Houston's r e s u l t s are remarkably s i m i l a r to my own, with the noteworthy exception that females on Mandarte I s l a n d never became simu l t a n e o u s l y polyandrous. P o s s i b l e e x p l a n a t i o n s f o r t h i s d i f f e r e n c e between dunnocks and song sparrows were d i s c u s s e d i n Chapters 3 & 4. Male Aggression and Female Choice The h i s t o r i c a l emphasis on male a g g r e s s i v e behaviour and general d i s m i s s a l of the importance of a g g r e s s i v e behaviour i n females probably r e s u l t s from two main f a c t o r s . F i r s t , males i n many s p e c i e s are l a r g e r , b r i g h t e r and more e l a b o r a t e l y endowed with weaponry and other f e a t u r e s used i n a g g r e s s i v e and sexual d i s p l a y s than are females (Darwin 1871; Selander 1972; Payne 1984). As a r e s u l t , males are o f t e n more v i s i b l e , s p e c t a c u l a r and t r a c t a b l e s u b j e c t s f o r study than females (Wasser 1983). Second, male, as opposed to female competition i s regarded as more intense and thus more important i n terms of n a t u r a l s e l e c t i o n ( T r i v e r s 1972; Wilson 1975; Wittenberger 1979). N e v e r t h e l e s s , I suggest that r e s e a r c h e r s sometimes f a i l to r ecognize that although l a r g e d i f f e r e n c e s i n the s c a l e of i n t r a s e x u a l c o m petition between males and females may e x i s t , i t does not n e c e s s a r i l y f o l l o w that the i n f l u e n c e of competition i n females i s e i t h e r weak or absent. Because females and males d i f f e r i n l i f e h i s t o r y , c o m p e t i t i o n may manifest i t s e l f 240 b e h a v i o u r a l l y i n d i f f e r e n t ways or at d i f f e r e n t times in each sex. I f so, a thorough knowledge of s e x - s p e c i f i c behaviours and t h e i r consequences i s needed before one can determine the r e l a t i v e c o n t r i b u t i o n s of d i f f e r e n t behaviours to the e v o l u t i o n of present-day b e h a v i o u r a l r e p e r t o i r e s . REFERENCES Arcese, P. & Smith, J.N.M. 1985. Phenotypic c o r r e l a t e s and e c o l o g i c a l consequences of dominance i n song sparrows. J . Anim. E c o l . , 54, 817-830. Arcese, P. & Smith, J.N.M. 1988. E f f e c t s of p o p u l a t i o n d e n s i t y and supplemental food on r e p r o d u c t i o n i n song sparrows. J . Anim. E c o l . , 57, i n p r e s s . Birkhead, T.R. & C l a r k s o n , K. 1985. Ceremonial g a t h e r i n g s of the magpie P i c a p i c a ; t e r r i t o r y probing and a c q u i s i t i o n . Behaviour, 94, 324-332. Boonstra, R. & Rood, F.H. 1983. Re g u l a t i o n of breeding d e n s i t y i n M i c r o t u s p e n n s y l v a n i c u s . J . Anim. 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