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Female aggressiveness, breeding density, and monogamy in willow ptarmigan Hannon, Susan Jean 1982

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FEMALE AGGRESSIVENESS, BREEDING DENSITY, AND MONOGAMY IN WILLOW PTARMIGAN by SUSAN JEAN HANNON B. Sc. (Hon), University of Guelph, Guelph, 1974 M. Sc., University of Alberta, Edmonton, 1978 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA July 1982 (c) Susan Jean Hannon, 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date /HJ6. 3 , /9fX-DE-6 (3/81) i i ABSTRACT In this thesis I investigate the influence of aggression by females in setting breeding density and maintaining monogamy in a population of willow ptarmigan (Lagopus lagopus). The aims of the study were: 1) to test the hypothesis that female willow ptarmigan determine their own breeding density by spacing behaviour, independently of male density; 2) to evaluate the effect of interactions between the sexes on f i n a l breeding density; and 3) to examine factors which may constrain willow ptarmigan populations to monogamy. Sex r a t i o of the population was altered in spring by continuous removal of most males and females from separate plots. The effect of removals on numbers of the same and the opposite sex was monitored. The following results were obtained and conclusions reached: 1) Females and males defend t e r r i t o r i e s against individuals of the same sex, and this behaviour prevents some potential r e c r u i t s from breeding. Physiologically mature yearling females and males settled in response to the removal of t e r r i t o r i a l birds of l i k e sex. 2) Density of t e r r i t o r i a l males may not determine the number of females that breed. Females s e t t l e d at high density, despite a reduction in the density of males, defended t e r r i t o r i e s against each other within the enlarged t e r r i t o r i e s of the remaining males, and mated polygynously. 3) Settlement patterns and subsequent t e r r i t o r y sizes of males may affe c t density of both males and females. Females preferred t e r r i t o r i e s of medium to large size, and males with smaller t e r r i t o r i e s often remained unmated. Competition by yearling males for limited space on the breeding area may reduce t e r r i t o r y size below that acceptable to females. Females may also a l t e r settlement patterns of males by ignoring t e r r i t o r i a l boundaries of males and i n c i t i n g interactions between neighboring cocks. 4) Unshared male vigilance is not essential to, but may improve, female reproductive success in years of high predation. Polygynous females had similar breeding success and survived as well as monogamous females, except in a year of high nest predation when they suffered higher nest loss. 5) The aggressive behaviour of females may prevent polygyny from occurring in unmanipulated populations, i f a polygyny threshold e x i s t s . Females are able to defend t e r r i t o r i e s which are similar in size to those of males and thus can prevent secondary females from s e t t l i n g . Results of this study indicate that aggressive behaviour of females in a monogamous species may be an important factor in regulating population density. Future studies should examine physiological and ecological factors influencing agonistic behaviour of females and should attempt to manipulate female aggressiveness to test whether changes in this behaviour can cause changes in population density. iv TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i i LIST OF FIGURES ix ACKNOWLEDGEMENTS xi CHAPTER ONE: GENERAL INTRODUCTION 1 Study animal 3 CHAPTER TWO: FEMALE TERRITORIALITY AND BREEDING DENSITY OF WILLOW PTARMIGAN. 5 Introduction 6 Methods ' 9 Study area 9 General Methods 12 Behaviour of females 14 Removal experiments 14 Slow pulsed removal 15 Rapid pulsed removal 15 Single session female removal 16 Results 19 Female-female interactions 19 Removal experiments 21 Female removal 21 Slow pulsed removal 21 Rapid pulsed removal 21 Single session female removal 24 V Age structure, o r i g i n , and condition of recruit s 29 Male response to removal of females 32 Unmated males 32 Male removal 36 Slow pulsed removal 36 Rapid pulsed removal 36 Age structure, o r i g i n , and condition of recruit s 42 Female response to male removal 45 Slow pulsed removal 45 Rapid pulsed removal 45 Terri t o r y sizes of females and males 49 Discussion 52 Female t e r r i t o r i a l i t y and female numbers 52 Male t e r r i t o r i a l i t y and male numbers 55 Interactions between the sexes 56 Female behaviour and male breeding density 56 Male behaviour and female breeding density 58 Cyc l i c declines in density of grouse 59 Sex-specific population regulation in other grouse species 63 CHAPTER THREE: FACTORS MAINTAINING MONOGAMY IN WILLOW PTARMIGAN POPULATIONS 66 Introduction 67 Methods 71 Results 73 Male vigilance 73 Reproduction of polygynous and monogamous females 76 v i Clutch size 76 Hatch date and hatching success 76 Number of chicks fledged 83 Predation on females in the prelaying period 83 Breeding success of polygynous and monogamous males ... 85 Survival to the next breeding season 88 Discussion 90 Importance of unshared male vigilance 90 Female aggression and the maintenance of monogamy 92 Length of the breeding season 93 Male:female t e r r i t o r y size r a t i o 94 Tests of the Female Aggressiveness Hypothesis 96 Mating systems in grouse 97 CHAPTER FOUR: CONCLUDING REMARKS 101 Literature c i t e d 105 v i i LIST OF TABLES Table I. Schedule of experiments conducted on Haines Road and K e l s a l l Lake plots in 1979, 1980, 1981 17 Table II. Number of females that s e t t l e d on small, medium and large removal t e r r i t o r i e s 27 Table I I I . Mean numbers of females that s e t t l e d on removal t e r r i t o r i e s of adult and yearling males, and males with t e r r i t o r i e s at the edge or in the core of the removal plot 28 Table IV. Number of resident females before and after the female removal, number the next spring, and number of replacement females 30 Table V. Comparison of mean length of keel and r a t i o of pect o r a l i s weight to length of keel among female residents, early replacements, and later replacements . 31 Table VI. Numbers of males on removal plots and Control in the period before the female removal experiment and the period after removals had ceased 33 Table VII. Mean t e r r i t o r y size of males of three mating types on Control 35 Table VIII. Number of resident males present before and after male removal, number the next spring, and number of replacement males 43 Table IX. Comparison of mean length of keel and r a t i o of pectora l i s weight to length of keel among male v i i i residents, f i r s t replacements, and later replacements .44 Table X. Numbers of t e r r i t o r i a l males present before and after the male removal experiment, and numbers of females s e t t l i n g on KL, HR, and Control 46 Table XI. Number of males breeding with one, two, three or four females on the male removal and Control plots. ... 69 Table XII. Percentage of females resident on male removal areas and Control that were subsequently located with brood 82 Table XIII. Mean brood counts of primary, secondary, and monogamous females on male removal plots, and lone and accompanied hens on other areas 84 Table XIV. Survival to the next breeding season of polygynous and monogamous females and males 89 ix LIST OF FIGURES Figure 1. Location of the study area and study plots on the Haines Road, northwestern B r i t i s h Columbia 10 Figure 2. Number of females removed from 15 t e r r i t o r i e s during 11 removal periods on HR in 1980 22 Figure 3. Number of female replacements removed from individual t e r r i t o r i e s during the female removal 25 Figure 4. Number of males removed from 16 t e r r i t o r i e s on KL in 1980, and from 15 t e r r i t o r i e s on HR in 1981, during the rapid pulsed removal 37 Figure 5. Mean t e r r i t o r y size (±SE) of males before (clear) and after (hatched) the male removal, and mean area extending onto the plots of t e r r i t o r i e s of males along the edge (solid) 40 Figure 6. T e r r i t o r i a l boundaries of males ( s o l i d lines) and females (shaded) on HR 1981 after the removal of males 47 Figure 7. Ranges of t e r r i t o r y sizes used by females and males in 1981 50 Figure 8. Postulated influences of within-sex spacing behaviour ( s o l i d lines) and between-sex interactions (dotted lines) on the breeding density of willow ptarmigan 60 Figure 9. Number of polygynous males that were seen during routine census with each of their hens, at least two hens, or with only one hen prior to hatch Figure 10. Di s t r i b u t i o n of dates of hatch for monogamous and polygynous females for 1980, and 1981 Figure 1 1 . Number of days between the hatch dates of primary and secondary females within a polygynous group Figure 12. Date of census in which females that were A) seen during the brood season, and B) not seen during the brood season, were last located on the plots before laying began xi ACKNOWLEDGEMENTS F i e l d conditions in the Chilkat Pass are at best, rigorous. I thank the people who spent many long hours helping me with the f i e l d work, in pa r t i c u l a r Jens Roland, Rich Moses, Mike Sull i v a n , and Joan Morgan who were long-term assistants, and also Kathy Martin, Rick Lewis, L i z Hofer, Mary T a i t t , Sarah Groves, Jamie Smith, Jean Carey, Ken Lertzman, Pat Herzog, Stan Boutin, Scott G i l b e r t , and Keith Lindsey. My supervisor, Dr. Jamie Smith; thesis advisor, Dr. Charles Krebs; and former M. Sc. supervisor, Dr. Fred Zwickel; a l l provided encouragement and c r i t i c a l discussion of the work. I also thank the members of my supervisory committee, Drs. L. Gass, R. L i l e y , and A.' S i n c l a i r ; Dr. D. Chitty; the Institute of Animal Resource Ecology Di r t Lunch discussion group; and Dr. R. Moss, Inst, of T e r r e s t r i a l Ecology, Banchory, Scotland; for their comments and c r i t i c i s m s at various parts of the study. J. Smith, C. Krebs, R. L i l e y , D. Chitty, F. Zwickel, R. Lewis, M. T a i t t , I. McLean, P. Arcese, S. Boutin, J. Roland, and I. Jamieson read part or a l l of the thesis. Dr. A. Watson, Inst, of T e r r e s t r i a l Ecology, Banchory, v e r i f i e d the nomenclature of willow ptarmigan c a l l s . L o g i s t i c support was provided by the Arctic Institute of North America at Kluane Lake, Y. T., and the Mule Creek Highway Camp, Haines Rd, B. C. F. Zwickel and D. Nugent lent me the dogs. D. Mossop allowed me to use his cabin and provided background information about the birds and the l o c a l area. I thank the Canadian National Sportsmen's Fund for awarding me a Conservation Scholarship. Funding for the x i i research came from NSERC grants to J. N. M. Smith, C. J. Krebs, and A. R. E. S i n c l a i r , an A r c t i c Institute of North America Student grant-in-aid, the Ar c t i c and Alpine Research Committee (U. B. C ) , Canadian W i l d l i f e Service University Research Fund, Frank M. Chapman Fund, Josselyn van Tyne Fund, and the Paul A. Stewart Fund. Birds were banded and co l l e c t e d with permission from the B r i t i s h Columbia Fish and W i l d l i f e Branch. 1 CHAPTER ONE: GENERAL INTRODUCTION Female-female agonistic or aggressive behaviour is a widespread but r e l a t i v e l y undiscussed phenomenon. Interfemale aggression has been documented in f i s h (Pottle and Green 1979), l i z a r d s (Rand 1967, Bustard 1970), birds (Howard 1920, Nero 1956, Watson 1965, von Haartman 1969, Smith 1978, Brown 1979) and mammals (Downhower and Armitage 1971, Strandgaard 1972, Floody and Pfaff 1977, Gleason et a l . 1980). Early studies on dominance hierarchies in flocks of domestic hens pointed out the importance of high s o c i a l status to reproductive output (reviewed by Guhl 1962), but, u n t i l recently, the functions and consequences of female agonistic behaviour have largely been ignored. Females may interact a g o n i s t i c a l l y for one or a l l of the following reasons: 1. To acquire resources and to prevent other females from sharing them; 2. To gain access to the 'best' male and to monopolize his vi g i l a n c e , and/or parental care; or 3. To improve r e l a t i v e reproductive output by reducing, delaying, or i n h i b i t i n g reproduction by other females. Whatever the evolutionary reasons for the development of interfemale agonism, this behaviour has the ecological consequence of regulating population size. Since females are usually the l i m i t i n g sex (Trivers 1972), the reproductive output of a population is determined by the number of females that breed. The importance of female spacing behaviour (which 2 includes dominance, t e r r i t o r i a l i t y , and agonistic behaviour) in l i m i t i n g population growth has now been recognized for some promiscuous and polygynous species (e.g. Robel 1970, Zwickel 1972,1980, Redfield et a l . 1978) but as yet has not been discussed for monogamous species. Predictions of current hypotheses on the evolution of mating systems are based on the d i f f e r e n t i a l energy investment of males and females in the production of germ c e l l s and the development of young (Trivers 1972). The type of mating system that develops depends on whether male or female reproductive interests are favoured by environmental factors (Wittenberger and Til s o n 1980). A second ecological consequence of interfemale agonism, i f i t delays or prevents some females from breeding, i s that i t may reduce the degree of polygyny (Holm 1973), or completely prevent i t s occurrence (Wittenberger 1979). There has been l i t t l e discussion of what factors would allow aggressive behaviour by females to enforce monogamy on males. The purpose of this thesis is to consider the ecological consequences of spacing behaviour in females of a monogamous grouse species, the willow ptarmigan (Lagopus lagopus). In Chapter Two I ask the question, "What form of spacing behaviour do female willow ptarmigan display?", and test the hypothesis that female spacing behaviour determines the number of females that breed, independently of the density of males. I then discuss how interactions between females and males can influence the breeding density. In Chapter Three I evaluate the effects of monogamous versus polygynous mating by females on their 3 reproductive success and survival and discuss the role of female aggression in maintaining monogamy in the population. Chapter Four is a b r i e f , general discussion. Study animal I chose the willow ptarmigan as a study animal because i t is a predominantly monogamous species, individuals are easy to capture and mark, can be observed readily in the f i e l d , and are sexually dimorphic. Ptarmigan males begin to set up t e r r i t o r i e s on the breeding area in A p r i l . Females arrive soon after and move about in small groups before s e t t l i n g and pairing with a male in late A p r i l or early May. Females lay eggs in a rudimentary nest during late May and incubate for about 21 days. Dates of hatch of individual nests span the period from late June, through July. The male remains on the t e r r i t o r y during t h i s time and accompanies the hen and brood when they leave the t e r r i t o r y a few days after hatch. Juveniles begin to f l y at 11 or 12 days of age. Ptarmigan spend the winter in large flocks usually on areas away from the breeding range, and the sexes winter separately (Weeden 1964). The diet in winter and early spring consists primarily of willow (Salix spp.) twigs and buds (West and Meng 1966, Weeden 1969). During summer the birds forage on the catkins, buds, and leaves of willow, as well as flowers and seeds of other herbaceous plants (Williams et a l . 1980). Potential predators of ptarmigan that are present on my study area are the golden 4 eagle (Aquila chrysaetos), gyrfalcon (Falco r u s t i c o l u s ) , goshawk (Accipiter gentilus), marsh hawk (Circus cyaneus), short-eared owl (Falco r u s t i c o l u s ) , coloured fox (Vulpes vulpes), short-t a i l e d weasel (Mustela erminea), wolf (Canis lupus), and wolverine (Gulo luscus). Lagopus lagopus populations undergo c y c l i c fluctuations in abundance (Buckley 1954, Williams 1954, Keith 1963). Numbers of willow ptarmigan in North America and Iceland peak every 10 years (Gudmundsson 1960, Bergerud 1970); the willow grouse (L. 1_. lagopus) in northern Europe every 3 to 4 years (Myrberget 1972), and the red grouse (L. 1. scoticus) in Scotland every 6 years (Watson and Moss 1979). Hypotheses invoked to explain fluctuations in abundance of grouse (Tetraonidae) are similar to those proposed for small mammals (reviews: Keith 1963, Krebs 1964, Keith 1974, Krebs and Myers 1974, Krebs 1978) and are summarized by Watson and Moss (1979). These include c y c l i c changes in abundance of predators (Lack 1954, Rusch and Keith 1971, Myrberget 1972, Bulmer 1974, Hornfeldt 1978), variations in quality of food (Watson and Moss 1972), and changes in behaviour that are i n t r i n s i c genetic (Chitty 1967) or phenotypic (Christian and Davis 1964, Christian 1978, Watson and Moss 1972). Although ptarmigan populations cycle, my study was done at high and constant density (varying from 32 to 36 breeding pairs per km2), therefore I was not able to test any of these general hypotheses. CHAPTER TWO: FEMALE TERRITORIALITY AND BREEDING DENSITY WILLOW PTARMIGAN. 6 INTRODUCTION Spacing behaviour and i t s e f f e c t s on the regulation of vertebrate population density have been discussed extensively (e.g. Brown 1969, Watson and Moss 1970, Clarke 1970, Klomp 1972, Philobosian 1975, Krebs 1979, Watson and Moss 1979, Patterson 1980). H i s t o r i c a l l y , descriptive and experimental studies have centered on male aggressiveness, implying that spacing behaviour of males determines both male and female breeding density. This emphasis was based f i r s t on the observation that males of many species are often more aggressive than females, and second, on the conclusion of early removal experiments (e.g. Stewart and A l d r i c h 1951, Hensley and Cope 1951) that "surplus" females ( i . e . p o t e n t i a l l y reproductive females prevented from breeding by t e r r i t o r i a l behaviour of residents) did not e x i s t . However, females behave aggressively toward one another during the breeding season (e.g. Howard 1920, Welter 1935, Nero 1956, Watson and Jenkins 1964, MacDonald 1970, Crawford 1977, Morton et a_l. 1978), and surplus females have now been i d e n t i f i e d for several species (e.g. Watson 1965, Holmes 1966, Watson and Jenkins 1968, Harris 1970, Young 1970, Zwickel 1972, 1980, Manuwal 1974), which suggests that an evaluation of the effect of female spacing on breeding density i s overdue. The main purpose of t h i s study was to determine whether the spacing behaviour of females in a monogamous species could determine the number of females that breed, independently of male density. In some promiscuous or polygynous species, 7 spacing behaviour i s sex-specific. For example, in voles interaction with mature animals of the same sex i n h i b i t s sexual maturation of juveniles (Bujalska 1973, Boonstra 1978, Saitoh 1981), and females of some bird species defend t e r r i t o r i e s against intruders of the same sex (e.g. Nero and Emlen 1951, Nero 1956, Herzog and Boag 1977, 1978). Sex-specific removal experiments on Microtus townsendii indicate that juvenile survival and recruitment in both sexes are inversely related to the density of mature females (Redfield et a l . 1978, Boonstra 1978), which suggests that female density is more important than male density in determining population size. Manipulations of sex r a t i o to test i f spacing behaviour is sex-specific in birds have not previously been attempted. The experiments presented here were designed to test whether spacing behaviour in spring determines breeding density of both sexes in a sex-specific way. I altered the sex ra t i o in spring of a population of willow ptarmigan, Lagopus lagopus  alexandrae G r i n n e l l , by simultaneously removing most males and females from separate p l o t s . I then monitored the effect on the same and opposite sex. The predictions tested were: 1) that i f resident birds prevented potential r e c r u i t s from breeding, then when residents are removed, individuals of the same sex w i l l s e t t l e and attempt to breed (Watson and Moss 1970); and 2) that i f each sex sets i t s density independently of the density of the other, then reduction in density of one sex w i l l not lower density of the other. 8 I had observed that females were aggressive to one another and thus was p a r t i c u l a r l y interested in testing whether female willow ptarmigan are t e r r i t o r i a l during the breeding season, and whether t h i s behaviour could determine numbers of breeding females and males. 9 METHODS Study area The study was conducted in the Chilkat Pass area, northwestern B r i t i s h Columbia, Canada (59° 50'N, 136° 20'W) from A p r i l to August each year from 1979-1981. Study plots were situated on the f l a t s of the pass at an elevation of approximately 880m, near km 128, Haines Road (Fig. 1). Vegetation here i s composed of an overstory of dwarf shrubs, mainly willow (Salix spp), birch (Betula glandulosa) and shrubby ci n q u e f o i l ( P o t e n t i l l a f r u t i c o s a ) ; and an understory of bryoids, graminoids and perennial forbs (Weeden 1959,1960). Vegetation on the plots was generally very open, and willows, the predominant shrubs, averaged about 1m t a l l . Experimental work was conducted in A p r i l and May when snow covered the study area. Snow was approximately 1.5 m deep in A p r i l and usually melted completely by the last week of May. Vegetation provided l i t t l e cover when snow was deep, thus birds were r e l a t i v e l y easy to observe and count. The climate i s characterized by heavy snowfall in winter, frequent l i g h t to moderate rain the rest of the year, and moderate to high winds year round. A more detailed description of the weather and vegetation of the area can be found in Weeden (1959, 1960). 10 Figure 1. Location of the study area and study plots on the Haines Road, northwestern B r i t i s h Columbia. Shaded areas indicate plots used in 1979 (C=Control, HR=Haines Road plot, KL=Kelsall Lake p l o t ) . 11 1 2 General Methods In 1979 three 50ha rectangular plots ( K e l s a l l Lake plot (KL), Haines Road plot (HR), and a Control), each at least 1 km apart, were gridded into 100m squares (Fig. 1). In 1980, I established a new 50ha Control plot because of low density of birds on the 1979 Control. KL and HR were enlarged in 1980 to 80 and 90 ha respectively in order to increase sample size for the experiments. Birds were captured by gently driving them into 1m high white g i l l nets strung through the willows or by noosing with a 6m c o l l a p s i b l e noose pole. Sexes were distinguished by differences in plumage, voice, and wing length (Bergerud et a l . 1963). Birds were categorized as yearling (hatched the previous summer) or adult, by comparing pigmentation of the ninth and eighth primaries (Bergerud et al. 1963). Each bird received a numbered aluminum band and 3 coloured p l a s t i c bands (2 bands per leg) in a unique combination. Density of t e r r i t o r i a l birds was determined by skiing or walking in a zig-zag pattern between grid lines and marking the position of each bird on a map. Two observers were present throughout most of the study, 3 were present in May. In spring of 1979 before experiments began, few birds were colour-marked, so density was estimated from 3 complete censuses in which the entire plot was surveyed in one day. In 1980 and 1981, a l l birds on t e r r i t o r i e s were either marked or recognized i n d i v i d u a l l y by plumage c h a r a c t e r i s t i c s before experiments began, so a complete count was made. Birds were also censused 13 on a 100m wide s t r i p around each p l o t . We pushed birds to the edges of their t e r r i t o r i e s to i n i t i a t e boundary disputes or to determine the point at which the bird turned back. T e r r i t o r i e s were plotted by marking these positions and by noting the places where birds c a l l e d or engaged in natural border disputes on a map. Boundaries were drawn by connecting the outermost points. A b i r d (male or female) was considered to be t e r r i t o r i a l i f i t was consistently seen on the same area and i f i t advertised vocally in the a i r or on the ground within that area, or i f i t engaged in border disputes with neighbours, or chased intruders from the area. I defined a female to be t e r r i t o r i a l or to be mated with a male i f she was seen on the t e r r i t o r y in more than two spring censuses; advertised vocally, engaged in border disputes, or chased other females from the t e r r i t o r y ; was found on a nest or with one-day-old chicks on the t e r r i t o r y ; or was seen with a brood and accompanied by the male on or off the t e r r i t o r y . Hatch dates were determined by estimating age of chicks in each brood by the method of Bergerud et a_l. (1963), and backdating. Date of laying the f i r s t egg was estimated by subtracting incubation time (21 days) and time to lay a clutch of mean size (8 days) from date of hatch. Nonparametric s t a t i s t i c a l tests (Siegel 1956) were used i f the assumptions of parametric tests were violated, or i f the sample size was very small. P r o b a b i l i t i e s greater than 0.05 were judged not s i g n i f i c a n t . A l l tests were two-tailed unless otherwise s p e c i f i e d . 1 4 Behaviour of females To determine whether females were aggressive towards t e r r i t o r i a l intruders, the behaviour of females in interactions with other hens or with a female model was observed. In 1981, we described a l l natural cases of female-female interaction that were observed at close quarters. In spring 1980 and 1981 I placed a stuffed female model mounted in a l e r t posture (Watson and Jenkins 1964) onto 23 t e r r i t o r i e s and as a control, a stuffed female bufflehead (Bucephala albeola) onto 5 t e r r i t o r i e s . I pushed the female in the di r e c t i o n of the model u n t i l she had seen i t . The behaviour of the female in response to the model was recorded for 5 minutes. Removal experiments The removal experiments were designed to answer the following questions: 1) Is there a surplus of birds of both sexes that is prevented from breeding by the behaviour of residents in spring, and that can s e t t l e and breed when resident birds are removed? 2) Is the f i n a l breeding density of either sex determined by the density of the opposite sex, or are numbers determined by within-sex influences? To address these questions simultaneously, I removed males and females from separate plots as outlined below. 1 5 Slow pulsed removal On 5 and 6 May 1979, 22 of 29 males on KL and 15 of 20 females on HR were removed by shooting. Removals were from t e r r i t o r i e s located uniformly across the pl o t . Since most birds were unmarked at the time of the experiment, a numbered card was placed on each removal t e r r i t o r y , and any unmarked birds seen within 50m of the card were removed on 14 and 15 May and again on 23 May. The male removal was i n i t i a t e d after females had se t t l e d on the plot. Rapid pulsed removal In 1980 the experiments were repeated on the same areas, but the rate of removal was increased to every 2 or 3 days. A l l resident birds were i n d i v i d u a l l y i d e n t i f i e d , and the t e r r i t o r i a l boundaries of the males were mapped before the removal began. Birds on t e r r i t o r i e s in the 100m s t r i p around the plots were also marked. Females were removed from 15 of 28 randomly chosen t e r r i t o r i e s of males on HR on 12 May 1980. Males were removed randomly from 16 of 32 t e r r i t o r i e s on 8 and 9 May 1980 on KL. Replacements, except for neighbouring males expanding their t e r r i t o r i e s , were removed every 2 or 3 days u n t i l the f i r s t week of June. Replacement birds were only removed i f they appeared to have set t l e d on the pl o t : i . e . for males i f they moved about openly on the t e r r i t o r y and did not f l y off i t when approached; for females i f they were accompanied by the male and did not f l y 16 off the t e r r i t o r y when approached. The rapid pulsed male removal experiment was repeated on 15 of 31 t e r r i t o r i e s on HR in 1981 sta r t i n g 30 A p r i l . The male removals in 1980 and 1981 were i n i t i a t e d before females had s e t t l e d on t e r r i t o r i e s . Single session female removal During the pulsed removals, replacement females were not allowed to s e t t l e long enough to breed. In order to test whether replacement females were ph y s i o l o g i c a l l y capable of breeding, 10 females were removed from 9 t e r r i t o r i e s on KL on 14 and 15 May 1981. Replacement females were allowed to s e t t l e and were then captured and colour-marked. The plot was censused regularly to determine whether these females nested and produced broods. Table I summarizes the schedule and location of each removal experiment. To compare body size and condition of replacement birds to those of residents, a necropsy was performed on each bird that was removed. I predicted that residents would be larger or in better condition since they were able to maintain t e r r i t o r i e s against intruders. Pectoral muscle mass and volume have been used as indicators of body condition in birds (Evans and Smith 1974). I used length of keel as an index of body size because i t i s not subject to wear (as i s wing length), and was rarely damaged by shot (as were the long bones). Pectoralis muscles were excised and weighed to the nearest g. Length of keel was measured with vernier calipers to the closest 0.1mm. To correct 17 ; ,. Table I. Schedule of experiments conducted on Haines Road and K e l s a l l Lake plots in 1979, 1980, 1981. Haines Road (HR) K e l s a l l Lake (KL) 1979 slow pulsed female removal slow pulsed male removal 1980 rapid pulsed female removal rapid pulsed male removal 1981 rapid pulsed male removal single session female removal 18 f o r p o s s i b l e d i f f e r e n c e s i n body s i z e , I e x p r e s s e d p e c t o r a l i s weight as a f u n c t i o n of l e n g t h of k e e l and used t h i s as an index of body c o n d i t i o n . 19 RESULTS Female-female interact ions Female and male willow ptarmigan advertise their presence on t e r r i t o r i e s by f l y i n g up in an arc, and at the apex singing or "becking" (nomenclature of Watson and Jenkins 1964). They give a similar c a l l from the ground as advertisement or threat: ground becking or the "ko-ko-ko-ko-krrr". The "krrow" and "kohway" are used as threat c a l l s in interactions with other birds (Watson and Jenkins 1964). C a l l s of willow ptarmigan are similar to those of the conspecific red grouse, L. 1. scoticus. Interactions among females were recorded during 15 intruder chases and 16 border disputes. Actual fight i n g occurred in only one intruder chase, usually the interloper ran or flew off the t e r r i t o r y when chased by the resident. Vocalizations commonly given during chases were the krrow (9 cases), ground becking (5), and the kohway (2). Resident males interfered in 5 chases: in 3 of these, males ran between the 2 females and gave krrow c a l l s to their mates; in one case the male attempted to display to the intruder; and once the male helped his mate chase the intruder. Females also fought infrequently during border disputes with neighbouring hens: in only 2 of 16 disputes. The usual pattern was for females to stand side by side (11) or to walk in l i n e (5) while giving repeated krrow c a l l s (12), ground becks (3), or kohways (1). Males interfered in 10 border disputes; 7 of these were between 2 mates of polygynously mated males. In 20 the polygynous groupings, the male attempted to stay between the 2 females, giving krrow c a l l s i f one of them approached the other. In the border disputes between monogamously mated females, the male either chased the neighbouring hen, chased his own mate back onto the t e r r i t o r y , or displayed between the 2 females. Twenty-three resident females were presented with the stuffed female model between 11 and 24 May 1980, 1981. A l l advanced towards i t , thirteen gave an a e r i a l beck or ground beck, and 17 of 23 gave repeated krrow c a l l s . Ten of the females attacked the dummy by f l y i n g at i t and h i t t i n g i t with the feet, running toward i t and bumping i t with the shoulder or wing, and/or pecking at the head and neck. One female neither vocalized nor attacked the model. Males were present during 20 experiments and 16 of them interfered with the female by keeping between her and the dummy or chasing her while giving krrow c a l l s . In these cases, 9 of the females attempted to lead the male away from the dummy by crouching and giving a head-wag accompanied by krrow c a l l s . Twelve males attacked the dummy by pecking i t and 4 displayed and attempted to copulate with i t . In one case where the female was unable to interact physically with the dummy because of male interference, after I noosed the male, the female approached the dummy and attacked i t . In the five control experiments with the model of the duck, females looked at the model but showed no further interest in i t . Clearly, these observations and experiments suggest that female willow ptarmigan do defend t e r r i t o r i e s by chasing or 21 attacking female intruders, and engage in border disputes with neighbours in a similar way to males. I did not c o l l e c t detailed information to compare amount of time spent in t e r r i t o r i a l advertisement and defence by males and females, a question which requires further study. However, aggressive behaviour of females was less apparent than i t was in males, partly because in females i t was less frequent d i u r n a l l y and more r e s t r i c t e d seasonally. A complete description of male t e r r i t o r i a l behaviour is given in Watson and Jenkins (1964). Removal experiments Female removal Slow pulsed removal: At least 11 hens settled on the 15 removal t e r r i t o r i e s within a week after the f i r s t removal period. Ten of these were shot. Six additional females settled on the plot and were not removed. The f i n a l breeding density of females on HR was 12, and eight of these were later located with broods, indicating that at least some of the replacement females had bred. The o r i g i n of the replacements was unknown because not a l l females were banded prior to the experiment. Rapid pulsed removal: F i f t y - s i x females se t t l e d and were subsequently removed from the 15 removal t e r r i t o r i e s between 12 May and 4 June 1980 (Fig. 2). Some females appeared on t e r r i t o r i e s within 4 to 6 hours of the previous removal. 22 Figure 2. Number of females removed from 15 t e r r i t o r i e s during 11 removal periods on HR in 1980 (inset i s a histogram of date of f i r s t egg l a i d for non-experimental hens on the p l o t ) . Dots represent removal periods when no hens were removed. 24 Replacement rate was high u n t i l 26 May (Fig. 2), then declined suddenly, coinciding with the period when hens on unmanipulated t e r r i t o r i e s began to lay eggs. Some t e r r i t o r i e s had more replacements than others (Fig. 3). Number of replacements was p o s i t i v e l y correlated with t e r r i t o r y size of the male (Spearman's rank c o r r e l a t i o n coefficient=0.69, N=15, p<0.0l). To investigate whether females chose larger male t e r r i t o r i e s or simply settled randomly, I tested the n u l l hypothesis that females settled in proportion to the amount of area taken up by a l l males with small (l-1.9ha), medium (2-3.4ha) and large (3.5-6ha) t e r r i t o r i e s . The n u l l hypothesis was rejected (^2=6.08, p<0.05). Females settled less frequently on small t e r r i t o r i e s , most frequently on medium t e r r i t o r i e s , and as expected on large t e r r i t o r i e s (Table I I ) . Neither age of male (adult or yearling) nor position of his t e r r i t o r y (either on the outer edge of the plot or in the core) had a s i g n i f i c a n t effect on the number of females that s e t t l e d on his t e r r i t o r y (Table III) (Mann-Whitney U-test, U=17.5, p=0.57; U=18.0, p=0.23 resp e c t i v e l y ) . Single session female removal: Seven new females se t t l e d on the 9 t e r r i t o r i e s after the removal on 14 and 15 May 1981. Four of these were la t e r seen with broods, one other was probably in laying condition at capture as she had an enlarged cloaca and wide pubic symphysis, and two were not seen during the brood season. This experiment indicated that at least half of the replacement females were p h y s i o l o g i c a l l y able to breed. 25 F i g u r e 3. Number of female r e p l a c e m e n t s removed from i n d i v i d u a l t e r r i t o r i e s d u r i n g the female removal. Number of terr i tories O — rv> O J -f> 0 1 3 or o o n> 3 3 (A 0 4 H 0> 27 S Table II. Number of females that settled on small, medium, and large removal t e r r i t o r i e s compared with the expected number that would have set t l e d at random. Ter r i t o r y size Number of female replacements Observed Expected small 6 10.9 (N=5) medium 23 15.5 (N=5) large 27 29.7 (N=5) 2.2 3.6 0.3 6.1 Table I I I . Mean numbers of females that settled on removal t e r r i t o r i e s of adult and yearling males, and males with t e r r i t o r i e s at the edge (outer) or in the core (inner) of the removal plot. Mean Adult males 4.4 Yearling males 3.4 Inner t e r r i t o r i e s 3.2 Outer t e r r i t o r i e s 4.1 SE N range 1.08 5 1-7 0.70 10 0-6 0.79 6 0-5 0.82 9 0-7 29 Age structure, o r i g i n , and condition of r e c r u i t s : Most replacement females in a l l removals were yearlings (Table IV). Of the 63 females i d e n t i f i e d as replacements in 1980 and 1981, 3% moved from other t e r r i t o r i e s on the plots, 10% were n o n t e r r i t o r i a l banded birds (seen in small groups of females, or alone and not regarded as t e r r i t o r i a l or mated (p. 13)), and 87% were of unknown o r i g i n . Clearly the vast majority of birds came from areas off the plot and not from the t e r r i t o r i e s d i r e c t l y surrounding i t ( i . e . from the 100m wide s t r i p ) . Removal of females did not affect the density of females the next spring nor did the age r a t i o (% yearlings) change s i g n i f i c a n t l y from one spring to the next (Table IV). I compared a body size (length of keel) and condition index (pectoralis weight divided by length of keel) among resident females, females who replaced birds shot during the f i r s t removal period, and females replacing birds removed in later removal periods (see Table V and F i g . 2 for dates of removal periods). A l l classes of females were similar in terms of body size (ANOVA, F=0.35, p=0.79) (Table V). Mean condition indices of females in the four groups varied (F=3.19, p=0.03). The condition index of replacement females in the f i r s t group was not s i g n i f i c a n t l y d i f f e r e n t from that of residents, although the index was higher than those of later replacements (Student-Newman-Keuls test, p<0.05, Sokal and Rohlf (1969), p. 242). This analysis indicates that replacements are of the same body size as residents, but, except for the e a r l i e s t replacements, were in poorer body condition. Table IV. Number of resident females before and a f t e r the female removal, number breeding the next spring, and number of replacement females (% yearlings in brackets). 1979 HR 1979 Con. 1980 HR* 1980 Con. 1981 KL 1981 Con. Number before 20 13 removal (60°/.)' (64%) 28 26 23 22 (59%) (39%) (57%) (50%) Number of 11 replacements (91%) 56 (87%) 7 (67%) Number a f t e r remova1 12 (64%) 14 13 (38%) 26 20 (63%) 22 Number the 19 next spring (59%) 14 + 28 (56%) 22 (50%) 0.04 p>0.8 0 p>0.9 0.26 p>0. 5 * in 1980 HR plot s i z e was increased to 90 ha. + birds were not banded, r e s u l t of two censuses. 31 Table V. Comparison of mean length of keel and r a t i o of pectora l i s weight to length of keel among female residents, early replacements, and later replacements. Length of keel (mm) Mean 66.5 65.5 65.5 66.2 SE 0.84 1.1.9 0.83 0.61 N 9 8 14 10 Pectoralis to keel r a t i o (g/mm) Mean 0.68 0.71 0.65 0.63 SE 0.011 0.021 0.012 0.023 N 9 8 14 10 * first=removal sessions 2,3; second=removal sessions 4,5; third= removal sessions 6-10 (see F i g . 1). 32 Male response to removal of females Removal of females from HR in 1979 and 1980 had l i t t l e e f f e c t on the number of males (Table VI). Eleven males that had their mates removed in 1979 were banded: four of these l e f t the p l o t . In 1980, despite the more rapid rate of removal, only 4 of 15 males on female removal t e r r i t o r i e s l e f t the plot. Thus a t o t a l of 11 banded males in the 2 years l e f t HR: 7 were not seen again, 2 moved to the male removal area (KL), one gained a t e r r i t o r y on the edge of HR and mated with a female, and one remained n o n t e r r i t o r i a l and unmated. A l l of these males had remained on their t e r r i t o r i e s up to the end of the second week of May before leaving. Most l e f t during the l a s t week of May, a period when resident hens had begun to lay eggs, and when female replacement rate had dropped (Fig. 2). Unmated males During a l l years, some t e r r i t o r i a l males were unmated, and hence served as natural counterparts to males on female removal t e r r i t o r i e s . These males remained unmated despite an excess of females being available in the population (as indicated by the female removal). On the Control in 1981, 3 males were unmated, 2 males with adjacent t e r r i t o r i e s shared a female ( i . e . she was seen with both males throughout the spring), and one male lost his female to a previously unmated male with an adjacent t e r r i t o r y . On KL in 1981, one male lost his mate and did not a t t r a c t another, one remained unmated, and one took over an 33 Table VI. Numbers of males on removal plots and Control in the period before the female removal experiment and the period after removals had ceased. 1979 HR 1979 Control 1980 HR 1980 Control Before 26 15 28 24 After 19 14 24 24 34 adjacent t e r r i t o r y and mated with that female when her previous mate was k i l l e d by a predator. In 1979 on KL, 2 previously unmated males moved onto removal t e r r i t o r i e s , mated, and raised broods, which indicates that t h i s class of males can breed. Unmated, and monogamously or bigamously mated males on the Control had t e r r i t o r i e s of d i f f e r e n t sizes (Kruskall-Wallis one way analysis of variance by ranks, H=13.94, N=7,9,3, p=0.00l) (Table VII). Unmated males appeared to have smaller t e r r i t o r i e s than other males (Table V11). They also had shorter wings (unmated: x=l83.9mm, SE=1.09, N=9; mated: X=188.1, SE=1.09, N=17, unpaired t - t e s t , T=2.5, p<0.05)) and weighed less (unmated: x=486.7g, SE=7.15, N=6; mated: x=525.0, SE=3.35, N=6, T=2.5, p<0.05) than mated yearling males (weight comparison made between birds caught between 24 and 29 A p r i l 1981). A l l unmated males except 2 remained on t e r r i t o r y at least u n t i l females were on nests: one was k i l l e d by a predator, another l e f t his t e r r i t o r y around 15 May and was seen skulking on another's t e r r i t o r y . A l l unmated males that were i n d i v i d u a l l y i d e n t i f i e d were yearlings (N=15). The female removal experiments indicated that a large number of females, primarily yearlings, were available to replace residents. These females were ph y s i o l o g i c a l l y able to breed. Most replacements did not come from t e r r i t o r i e s on or d i r e c t l y surrounding the removal plo t . This large reduction in female density did not result in a similar reduction of male density. "Widowed" males and naturally unmated males stayed on Table VII. Mean t e r r i t o r y size (ha) of males of three mating types on Control. unmated* Mean SE N 1 .5 0.11 7 monogamously mated 2.2 0.08 9 bigamously mated 4. 1 0.34 3 * includes males that 'shared' a female. 36 their t e r r i t o r i e s through most of May and the few that l e f t did so after female replacement rate had dropped. Male removal Slow pulsed removal: After the f i r s t removal of 22 males from KL, 16 males settled on vacant areas by 13 May 1979. After a second removal of 8 males on 14 and 15 May, 5 more males settled and were subsequently removed on 23 May. One more set t l e d but did not mate. Sixteen t e r r i t o r i a l males remained after the experiment. At least 8 of these were replacements that settled, mated, and raised young (only 7 residents remained after the f i r s t removal). Rapid pulsed removal: Sixteen males replaced removed birds on 16 t e r r i t o r i e s on KL in 1980, and 10 on 15 t e r r i t o r i e s on HR in 1981 (Fig. 4). Replacement rates for males were much lower than those for females (Fig. 2 and 4) (females: 7 birds per removal session; males: 1.6 per session). Remaining males quickly moved onto the vacant ground, in some cases within a few hours, and began to defend i t , thus probably excluding potential replacements. For example, on KL in 1980 on the day following the removal, 5 males whose t e r r i t o r i e s abutted removal t e r r i t o r i e s were sighted on these removal t e r r i t o r i e s . In 1981, on the Control, a t e r r i t o r i a l male was accidentally k i l l e d during capture, and within 30 minutes his neighbour was c a l l i n g on his t e r r i t o r y . 37 Figure 4. Number of males removed from 16 t e r r i t o r i e s on KL in 1980, and from 15 t e r r i t o r i e s on HR in 1981, during the rapid pulsed removal. 38 Kelsall Lake 1980 Haines Road 1981 4—+ i i 3 0 2 4 6 8 10 12 14 16 18 2 0 22 2 4 2 6 May 39 T e r r i t o r i a l boundaries were plotted for each male on the plot, before and after the removal. Ter r i t o r y size of the remaining males expanded dramatically when male density was reduced (Fig. 5) (Wilcoxon's matched pairs signed ranks test, KL1980: T=0.0, N=9, p<0.0l; HR1981: T=0.00, N=8, p<0.01). Te r r i t o r y size on Control remained constant during the same time period (1980: T=55.0, N=16, p>0.05; 1981: T=16.0, N=8, p>0.05). Males situated at the edge of the removal plots expanded s i g n i f i c a n t l y their t e r r i t o r i a l boundaries onto the plot after the removal (Fig. 5) (1980: T=0.0, N=8, p<0.0l; 1981: T=0.0, N=9, p<0.0l) but those the Control kept within their former boundaries (1980: T=4.0, N=4, p>0.05; 1981: T=3.0, N=4, p>0.05). Some males increased their t e r r i t o r y sizes more than others. Percent increase in size ranged from 11% to 449%. The number of females mated to a male was correlated with his t e r r i t o r y size after the removal (Spearman's rank correlation coefficient=0.64, N= 1 7, p < 0 . 0 l ) but not (rs=0.26, N=17, p>0.20) with the percent increase in his t e r r i t o r y size ( i . e . his capacity to gain more area). N o n t e r r i t o r i a l males were also seen on the other plots throughout spring u n t i l late June, alone or in small groups (2-7 bird s ) , skulking on t e r r i t o r i e s and being chased by t e r r i t o r i a l males. 40 Figure 5. Mean t e r r i t o r y size (±SE) of males before (clear) and after (hatched) the male removal, and mean area extending onto the plots of t e r r i t o r i e s of males along the edge ( s o l i d ) . 41 42 Age structure, o r i g i n , and condition of r e c r u i t s : Replacement males in a l l years were mainly yearlings (Table VIII). For 1980 and 1981 combined, 3 replacements were n o n - t e r r i t o r i a l banded birds, one came from the female-removal plot, and 18 were unbanded and of unknown o r i g i n . Removal of males in 1979 did not a l t e r the age r a t i o (% yearlings) of resident males in 1980. However, the age r a t i o in 1981 on KL was biased towards yearlings and was d i f f e r e n t from the age r a t i o in 1980 (Table VIII). This bias may have occurred because 12 of the 17 males l e f t on the plot after the 1980 removal were adults and 8 of these were at least 3 years old. The high proportion of older males contributed to a low overwinter survival of adult males from KL in 1980-1981 (16% compared to 54% on the Control (N=24)). The number of males holding t e r r i t o r i e s on KL decreased from 1979 to 1981. Males resident on the plot before the removal were not s i g n i f i c a n t l y larger than males that immediately replaced them (before the next removal session) nor than males who replaced them later in the season (Table IX) (ANOVA, F=0.08, p=0.92). However, resident males were in better condition than either of the groups of replacement males (Table IX) (F=3.77, p=0.03, Student-Newman- Keuls t e s t ) . Data from a l l years of the study were lumped, since p e c t o r a l i s to keel ratios for residents were id e n t i c a l for the 3 years. This analysis suggests that replacement males were prevented from obtaining a t e r r i t o r y because they could not compete e f f e c t i v e l y with males in better condition. Table VIII. Number of resident males before and af t e r the male removal, number breeding the next spring, and number of replacement males (% yearlings 1n brackets) . 1979 KL 1979 Con. 1980 KL* 1980 Con. 1981 'HR 1981 Con. Number before 29 15 removal (42%) (69%) 32 24 31 24 (31%) (50%) (48%) (46%) Number of 21 replacements (73%) 16 (87%) 10 (90%) Number a f t e r 16 removal (88%) 14 17 (29%) 24 17 (35%) 24 Number the next spring 20 (35%) 14+ 26 24 (80%) (46%) X 1 0.02 p>0. 9 12.22 p<0.001 p>0.99 * In 1980 KL plot s i z e was Increased to 80 ha. + birds were not banded, r e s u l t s of two censuses. 44 Table IX. Comparison of mean length of keel and ra t i o of pec t o r a l i s weight to length of keel among male residents, f i r s t replacements, and later replacements. Residents F i r s t replacements Later replacements Length of keel (mm) Mean S E N 70.4 0.59 24 70.8 0.93 1 1 70.4 0.89 1 3 Pectoralis to keel r a t i o (g/mm) Mean S E N 0.72 0.011 24 0.67 0.014 1 1 0.67 0.024 1 3 45 Female response to male removal Slow pulsed removal: Females did not leave their t e r r i t o r i e s when male density was reduced. Twenty-two females had set t l e d on KL prior to the removal experiment and 21 remained after 23 May when removals ceased. On Control, 13 females were i d e n t i f i e d before the removal period, 14 a f t e r . Females on KL were banded and their mating status was assessed. Thirteen females mated polygynously: 6 of these were later seen with their broods accompanied by males, and seven reared broods without male assistance. Eight of the 21 were monogamously mated: 2 of these to formerly unmated t e r r i t o r i a l males that had moved onto removal t e r r i t o r i e s . Rapid pulsed removal: Females settled at high densities on both areas in both years even though male density had been substantially reduced e a r l i e r (Table X). Breeding sex r a t i o was skewed on the removal plots but not on the Control. Since the removal of males caused an increase in polygynous matings, males on removal plots divided their attention between their mates. This allowed me to plot the t e r r i t o r i a l boundaries of females with a minimum of male interference. On Control t h i s was impossible because males chased their mates when I attempted to plot their t e r r i t o r i e s . On HR in 1981, females occupied exclusive areas that overlapped l i t t l e with other females and these areas were not always congruent with the t e r r i t o r i e s of males (Fig. 6) . 46 Table X. Numbers of t e r r i t o r i a l males before and after the male removal experiment, and numbers of females s e t t l i n g on KL, HR and Control. 1980 1981 KL Control HR Control No. males before* 32 24 31 24 NO. males after 17 24 1 7 24 No. females 29 26 28 22 Sex ra t i o 1:1.7 1:1.1 1:1.6 1:0.9 * males that had at least part of their t e r r i t o r y on the plot before the removal. 47 Figure 6. T e r r i t o r i a l boundaries of males ( s o l i d lines) and females (shaded) on HR 1981 after the removal of males. 100m 49 The male removals indicated that excess males, primarily yearlings, were available to replace residents. However, there were substantially fewer male replacements compared with female replacements in the p a r a l l e l female removal, even though n o n t e r r i t o r i a l males were seen on other areas. Replacements were ph y s i o l o g i c a l l y able to breed. Remaining residents expanded their t e r r i t o r i a l boundaries soon aft e r the removal and obtained much larger t e r r i t o r i e s . The density of females was not reduced whether males were removed before or after hens had s e t t l e d . When male numbers were low, females mated polygynously and defended t e r r i t o r i e s against each other within the larger t e r r i t o r i e s of the males. Te r r i t o r y sizes of females and males The d i s t r i b u t i o n of t e r r i t o r y sizes of females had the same mode, but a more limited range than that of males (Fig. 7). No females on HR in 1981 defended t e r r i t o r i e s smaller than 1.5ha, and the smallest t e r r i t o r y of a mated male on a l l plots in 1980 and '1981 was 1.6ha. Nor did a female defend a t e r r i t o r y larger than 5ha. Females appeared to prefer t e r r i t o r i e s of medium size : as reported previously, more females s e t t l e d on medium than on small or large t e r r i t o r i e s during the female-removal experiment. Thus males on small t e r r i t o r i e s l i k e l y remain unmated, while those on larger t e r r i t o r i e s could attract two females. The 3 bigamous males on Control in 1981 had t e r r i t o r i e s of 3.6, 3.9,and 4.8ha in size. 50 Figure 7. Ranges of t e r r i t o r y sizes used by females and males in 1981 (males on HR hatched). Females HR 1981 Males all a reas 0 I 2 3 4 5 6 7 8 9 T e r r i t o r y size (ha) 52 DISCUSSION Female t e r r i t o r i a l i t y and female numbers Wilson (1975, p. 256) defined t e r r i t o r y as "an area occupied more or less exclusively by an animal or group of animals by means of repulsion through overt defense or advertisement." Female willow ptarmigan are t e r r i t o r i a l : they occupy and vocally advertise on areas from which they repel intruders of the same sex, and defend borders against encroachment by neighbours. Does the t e r r i t o r i a l i t y of females l i m i t the number of females that can breed? To determine t h i s I had to demonstrate that a substantial part of the population was prevented from breeding by the t e r r i t o r i a l behaviour of residents and that these surplus birds were p h y s i o l o g i c a l l y able to breed (Watson and Moss 1970). When I removed females from 15 t e r r i t o r i e s , 56 females s e t t l e d and were subsequently removed (Fig. 2). These replacement birds did not come from other t e r r i t o r i e s on the plot, nor from the 100m wide s t r i p around the plot, indicating that they were not immediate neighbours. I conclude that these birds had been prevented from breeding on my study plot by the t e r r i t o r i a l behaviour of the resident females. Replacement females were able to breed: most of those that se t t l e d and were not removed, were later seen with broods. Replacement birds were similar in body size to residents and the e a r l i e s t replacements were equal in condition to residents. Thus i t i s unlikely that body size or condition affected the i n i t i a l p r o b a b i l i t y of gaining a t e r r i t o r y . 53 Removal experiments on other avian species have also demonstrated that replacement females exist that can breed i f residents are removed (Watson 1965, Holmes 1966, Watson and Jenkins 1968, Harris 1970, Young 1970, Bendell et a l . 1972, Zwickel 1972, Manuwal 1974, Knapton and Krebs 1974, Krebs 1977, Zwickel 1980, Saether and Fonstad 1981). Many population studies, including t h i s one, are conducted in good habitat where density of animals i s high. Replacement individuals may be n o n t e r r i t o r i a l surplus birds or they may have moved from t e r r i t o r i e s in marginal habitat, as in the the Great t i t (Parus  major) (Krebs 1971). Most replacement females in this study appeared within a few hours of the removal, suggesting that i f they had se t t l e d previously in marginal habitat, then th i s habitat must have been nearby. The Control in 1979 was located in poorer habitat than the other p l o t s : breeding density-was much lower here and vegetation was t a l l e r and denser. There was no decrease in density of females on th i s Control during the female removal on HR in 1979 or 1980. This suggests that females did not leave Control and move to HR, or i f they did, they were replaced by other females. If replacement females had not settled prior to their appearance on HR, then i t is unlikely that they would have sett l e d at a l l . Females become t e r r i t o r i a l soon after they a r r i v e : in 1980 a l l females on HR and 86% of those on the Control had settled by 13 May, and by 20 May a l l t e r r i t o r i a l females had settled on my plots. However, I was s t i l l removing females from HR u n t i l 2 June. If they had not settled by 20 54 May, then i t i s unlikely that they would have set t l e d anywhere, at least in the immediate area. Timing of settlement of females on the 1979 Control (poor habitat) was similar to that on KL and HR (at least 80% of t e r r i t o r i a l females had s e t t l e d on each plot by 1 May 1979), indicating that settlement on poor habitat i s similar in time to that on better habitat. Therefore, I conclude that these replacement birds had been prevented from breeding on my study area by the t e r r i t o r i a l behaviour of resident females, and that at least some of them were surplus birds. Replacement of removed females declined markedly in late May, around the period when resident females began to lay eggs. This phenomenon has also been observed in blue grouse, Dendragapus obscurus, (Hannon and Zwickel 1979) and white throated sparrows, Zonotrichia a l b i c o l l i s ( F a l l s 1978). Either the number of surplus individuals has been depleted by frequent removal or by high predation rates (as in surplus red grouse (Jenkins. e_t a_l. 1964), some females may have set t l e d elsewhere, or there may be a physiological block that prevents recrudescence of the gonads after t h i s point in time. For example, i f female willow ptarmigan had settled after 2 June, their eggs would not have hatched u n t i l 13 July at the e a r l i e s t . No f i r s t nests hatched after 8 July in 1980, although renests did hatch aft e r that time. Perhaps the ovaries become refractory i f hens have not set t l e d before a certain time. This could explain why removal experiments conducted on a species after laying has begun have yielded no female replacement (eg. 55 Stewart and Aldrich 1951, Holcomb 1974). Harris (1970), however, found that female oystercatchers (Haematopus  ostralegus) were replaced when removal was done after laying. Male t e r r i t o r i a l i t y and male numbers There were 2 types of nonbreeding males: those with t e r r i t o r i e s on the area and those without. T e r r i t o r i a l males without mates were able to move onto removal t e r r i t o r i e s and breed. Some n o n t e r r i t o r i a l males gained t e r r i t o r i e s during removals, and when l e f t to s e t t l e , were able to breed. Unmated t e r r i t o r i a l males were smaller and younger than mated t e r r i t o r i a l males, which may account for their smaller t e r r i t o r y size (Table VII). Replacement males were in poorer condition than residents (Table IX), but whether this was the cause or effect of not gaining a t e r r i t o r y i s not c l e a r . I do not know whether these males moved from marginal habitat; however, the arguments presented above for females apply to males. Replacement rate of males was much lower than that of females, probably because the experiment was done la t e r in terms of tenure on the breeding range for males than females, and secondly because neighbouring males reacted quickly to removal by expanding their borders, an i l l u s t r a t i o n of Huxley's (1934) " e l a s t i c d i s c " model of t e r r i t o r i a l i t y . When a higher proportion of males on the plot was removed (as in 1979) proportionately more replacements set t l e d , possibly because the few remaining males were unable to expand their boundaries to cover the whole plo t . Replacement by females on the female 56 removal plot was so rapid, that t h i s , and interference by their mates, probably deterred the remaining females from monopolizing adjacent empty t e r r i t o r i e s . Interactions between the sexes The removal experiments suggested that female t e r r i t o r i a l i t y determined the number of females that settled, and male t e r r i t o r i a l i t y , the number of males that s e t t l e d . Did thi s sex-specific t e r r i t o r i a l i t y in any way determine the number of the opposite sex that bred? Female behaviour and male breeding density Females could p o t e n t i a l l y influence density of breeding males at two points: during the i n i t i a l competition for t e r r i t o r i e s ; or after males have s e t t l e d . When females arrive on the breeding area, they may choose a t e r r i t o r y from the existing array of male t e r r i t o r i e s , or they may s e t t l e independently, choose their own t e r r i t o r i e s and the males then readjust their boundaries. The end result is the same: t e r r i t o r i e s of males and females largely overlap. My data are not s u f f i c i e n t to di s t i n g u i s h between these two p o s s i b i l i t i e s , however some evidence indicates that the l a t t e r i s plausible. Females on the male removal plot defended their own t e r r i t o r i e s and did not always remain within the boundaries of their mates' t e r r i t o r i e s (Fig. 6). I have seen both monogmously and polygynously mated females f l y spontaneously, c a l l , and land 57 several metres over the boundaries of their mates' t e r r i t o r i e s . This usually i n c i t e s a dispute between her mate and the neighboring male. An aggressive female may be able to enlarge her mate's t e r r i t o r y in t h i s way, and as a consequence force other males onto smaller t e r r i t o r i e s or out of the area completely. This hypothesis could be tested by increasing a female's aggressiveness by implanting her with testosterone and determining whether t e r r i t o r i a l boundaries changed. A second test would be to map the t e r r i t o r i e s of males before and after females arrived. If a female simply chooses a t e r r i t o r y of a male, after the number of t e r r i t o r i a l males on the area s t a b i l i z e s , then factors influencing the acceptable t e r r i t o r y size to the female could determine the number of males that breed. Unmated t e r r i t o r i a l males occur despite a surplus of females (this study, Jenkins et a l . 1963, Watson and Moss 1980) and have smaller t e r r i t o r i e s than mated males (Table VII, Choate 1963, Watson and M i l l e r 1971). In populations that cycle in density, the proportion of unmated males increases at the peak and during the early decline phase (Jenkins et a l . 1963, Watson 1965, Myrberget 1972) and t e r r i t o r i e s of these males are larger during a decline in numbers than they are in years of high density (Watson and M i l l e r 1971). If spring densities of males and females are similar ( i . e . overwinter mortality of each sex i s equal), then males probably remain unmated because they are rejected as mates by females because their t e r r i t o r i e s are too small. Increases in the minimum t e r r i t o r y size acceptable to females, because of 58 a decrease in resources or an increase in the inherent aggressiveness of females, could p r e c i p i t a t e declines in breeding density. Thus, choice of t e r r i t o r i e s by females could influence the number of males that breed. Male behaviour and female breeding density If females adjust their t e r r i t o r y boundaries to those of males, instead of the other way around, then the settlement patterns of males in spring could influence the number of hens that breed. High competition by r e c r u i t i n g males for limited space on the breeding range may result in many of them holding small t e r r i t o r i e s that w i l l not support a female. Differences in survival and aggressiveness of successive cohorts of males may result in varying amounts of space becoming accessible to potential rec r u i t s and thus aff e c t their t e r r i t o r y size. For example, chicks produced during the peak and decline of a rock ptarmigan (L. mutus) population survived longer and were more aggressive than those hatched the year before the peak (Theberge and Bendell 1980). Watson and M i l l e r (1971) reported that cohorts of males produced during a population decline survived better, were more aggressive, and defended larger t e r r i t o r i e s than cocks produced during the increase phase. A pa r t i c u l a r cohort could defend the majority of the breeding range for one or two years, leaving successive cohorts to compete for the remaining space. High competition could lead to a reduction in t e r r i t o r y size below the minimum acceptable to females and thus these males would be excluded from breeding. Thus the f i n a l 59 breeding density of the population i s determined by the settlement patterns of males, factors a f f e c t i n g minimum acceptable t e r r i t o r y size in females, and within-sex spacing behaviour. In F i g . 8 I summarize the effects of within-sex spacing behaviour and interactions between the sexes on breeding density of the population. Cyclic declines in density of grouse Variations in chick production have been related to fluctuations in population size (Jenkins et a_l. 1963, Bergerud 1970, Gullion and Marshall 1968, Myrberget 1972, Weeden and Theberge 1972). In my removal experiments, I i d e n t i f i e d a surplus of yearlings of both sexes in spring, indicating that survival of juveniles to spring was high. Thus spacing behaviour in spring, not overwinter loss of juveniles, appeared to regulate the breeding density. However, my study was conducted on a population at high and stable density, and whether chick mortality would become a s i g n i f i c a n t factor during a decline in density i s unknown. Low chick production may be caused by increased predation on nests, higher rates of nest desertion, n u t r i t i o n a l l y - r e l a t e d changes in v i a b i l i t y of chicks, i n t r i n s i c phenotypic or genetic changes in vigour or behaviour of chicks, and variations in the quality of parental care (Watson and Moss 1979). Most work has focussed on e x t r i n s i c causes of production loss and overwinter mortality of juveniles. In the future i t would be of interest to determine whether high leve l s of female aggressiveness are related to survival of 60 Figure 8. Postulated influences of within-sex spacing behaviour ( s o l i d lines) and between-sex interactions (dotted lines) on the breeding density of willow ptarmigan. WITHIN-SEX BETWEEN-SEX Females a r r i v e Non-breeding females $ - $ competition " f o r t e r r i t o r i e s <J>'s choose t e r r i t o r i e s independently of males & a l t e r £ settlement Settlement patterns of males l I • s choose <? t e r r i t o r i e s exclude those on -* sma l l t e r r i t o r i e s BREEDING DENSITY WITHIN-SEX Males a r r i v e 11 £-e/competition f o r t e r r i t o r i e s •I F i n a l settlement density Non-breeding males 62 juveniles and their subsequent t e r r i t o r i a l behaviour. If changes in spacing behaviour cause c y c l i c changes in the density of grouse populations as suggested by Watson and Moss (1979, 1980), then l e v e l of aggressiveness must be inherited either due to maternal e f f e c t s , which can be related to n u t r i t i o n of the hen (Watson and Moss 1972) or to overcrowding (Christian 1978), or be genetically inherited (Chitty 1967). Studies on dominance and aggressiveness of red grouse in the f i e l d and in c a p t i v i t y have been devoted solely to males (e.g. Watson and M i l l e r 1971, Moss et a l . 1979, Watson and Parr 1981). These studies indicate that in males aggressiveness and dominance are related to levels of androgen (Moss e_t a_l. 1979) and that they can be inherited (Moss and Watson 1980, Moss et a l . 1982). I have shown that aggressiveness of females may be important in determining breeding density and suggested that changes in female spacing behaviour may be related to changes in population density. Future studies could investigate factors a f f e c t i n g t e r r i t o r y size of females, the inheritance and hormonal basis of aggressiveness in females, and whether changes in female aggressiveness are related to changes in breeding density. 63 Sex-spec i f ic population regulat ion in other grouse spec ies Can a mechanism, such as I have suggested for willow ptarmigan be generalized to other grouse species? Workers on other species or subspecies of Lagopus have not considered that female t e r r i t o r i a l i t y has an effect on breeding density. However, in these species, females are aggressive towards one another during the breeding season (Watson and Jenkins 1964, MacDonald 1970) and unmated t e r r i t o r i a l males are present (Choate 1963, Watson 1965, Bergerud 1970, Watson and M i l l e r 1971, Myrberget 1972). These s i m i l a r i t i e s among monogamous species indicate that breeding density may be determined in a similar way. A major difference i s that numbers of breeding Scottish red grouse and willow grouse (L. 1. lagopus) in Norway (Blom and Myrberget 1978) appear to be determined proximately by overwinter mortality, which is s o c i a l l y induced by f a l l t e r r i t o r i a l behaviour in red grouse (Jenkins et aJL. 1963). My removal experiments demonstrated an excess of birds available to occupy t e r r i t o r i e s in spring. This difference in the timing of regulation may be related to whether a pa r t i c u l a r population i s migratory or not. Migratory ptarmigan in North America are not t e r r i t o r i a l in f a l l (Weeden 1959) although males of non-migratory willow ptarmigan (L. 1. a l l e n i ) in Newfoundland exhibit t e r r i t o r i a l behaviour at this time (Bergerud 1970). The spacing of females of promiscuous tetraonids has been described in only one species, the spruce grouse (Canachites  canadensis f r a n k l i n i i ) . Females maintain non-overlapping 64 t e r r i t o r i e s separate from males in late spring (Herzog and Boag 1978) and react aggressively to other females (Herzog and Boag 1977). A similar dispersion is possible for female blue grouse (D. o. fuliginosus) which are evenly spaced on the breeding range (Bendell and E l l i o t t 1967) and aggressive in spring ( S t i r l i n g 1968, Hannon 1980), and for female ruffed grouse (Bonasa umbellus) that occupy spring home ranges with l i t t l e overlap (Maxson 1978). Among lekking species, females have been reported to interact aggressively on the lek (Lumsden 1965,1968, K r u i j t and Hogan 1967, Robel 1970) but whether th i s intolerance continues after mating and serves to space hens on the summer range i s unknown. In promiscuous species, females apparently do not depend on resources on a male's t e r r i t o r y , and appear to l i v e independent l i v e s except for a brief period during copulation. In these species, breeding density is determined by the number of females that breed, and this number may be determined by female-female interactions independently of male density. Unfortunately few removal experiments have been attempted to determine whether surplus birds of both sexes exist (but see Bendell et a l . 1972, Zwickel 1972, Zwickel 1980), nor has the effect of removal of one sex on the density of the other been investigated. I have investigated the ef f e c t of female spacing behaviour on the breeding density of the monogamous willow ptarmigan. Resident females are t e r r i t o r i a l during spring prior to incubation, and th i s behaviour prevents n o n - t e r r i t o r i a l females 65 from s e t t l i n g . Spacing behaviour by males determines the i n i t i a l settlement density of males. However the number of females that s e t t l e is not always equal to the number of males on t e r r i t o r i e s , since females do not mate with males on small t e r r i t o r i e s . Since most birds are monogamous, more descriptive and experimental studies are necessary to assess whether the results from th i s study can be generalized to other monogamous spec ies . 66 CHAPTER THREE: FACTORS MAINTAINING MONOGAMY IN WILLOW PTARMIGAN POPULATIONS 67 INTRODUCTION Willow ptarmigan are usually monogamous (Hjorth 1970), but polygyny occurs occasionally: in my study population only 9% (N=57) of males present on Control areas paired polygynously (Table XI). Why is polygyny not more prevalent in willow ptarmigan populations? In a recent review, Wittenberger and Til s o n (1980) presented three hypotheses to explain why most noncolonial b i r d species with multipurpose t e r r i t o r i e s are monogamous: 1. Male parental care i s nonshareable and indispensable to female reproductive success; 2. T e r r i t o r i e s of males do not vary enough in quality to make i t advantageous for a female to mate with an already mated male rather than with an available unmated male ( i . e . the polygyny threshold (Orians 1969) i s not exceeded); and 3. Aggression by females prevents males from acquiring additional mates, even though the polygyny threshold is exceeded. Males of monogamous grouse species provide more investment in female reproductive success than do males of promiscuous species (Wittenberger 1978). In the three ptarmigan species, males defend t e r r i t o r i e s u n t i l the end of incubation (Weeden 1963, Watson 1965, Giesen and Braun 1979) and appear to provide vigilance against predators for the hen during foraging (Choate 1963, MacDonald 1970, Wittenberger 1978). Male rock ptarmigan (L. mutus) (MacDonald 1970) and willow ptarmigan (personal observation) protect the nest from potential predators, and male 68 willow ptarmigan accompany the female and defend the brood against predators. Is thi s male investment essential to female reproductive success and survival in monogamous grouse? In t h i s chapter I test Wittenberger and Tilson's f i r s t hypothesis by comparing the breeding success and survival of polygynous and monogamous female willow ptarmigan. By continuous removal of males from cert a i n t e r r i t o r i e s , I altered part of the population (48%, N=48, Table XI) from monogamy to polygyny (Chapter One). Since mean male t e r r i t o r y size doubled on the male removal area and polygynous females divided the t e r r i t o r i e s of males and defended s u b t e r r i t o r i e s against each other, I concluded that polygynous females had the same access to resources as monogamous females, but had less male investment time a l l o t t e d to them. If males are important as predator detectors in the prelaying and laying periods, then a female foraging without male vigilance should either be more susceptible to predation, or should forage less e f f i c i e n t l y and hence lay a smaller clutch, or lay and hatch eggs later than a monogamous female. Low food a v a i l a b i l i t y has been related to reduced clutch size and later laying date in some species of birds (e.g. Klomp 1970, Moss et a l . 1971, Kallander 1974, Greenlaw 1978). If male vigilance at the nest or with the brood is important, then polygynous females would be expected to suffer higher rates of nest predation and lower brood sizes respect i v e l y . After considering the f i r s t hypothesis, I w i l l then evaluate the a p p l i c a b i l i t y of the second and t h i r d hypotheses to 69 Table XI. Number of males breeding with one, two, three, or four females on the male removal and Control plots. 1979 1980 1981 No. hens KL Con. KL Con. HR Con. One 8 14 9 22 8 16 Two 5 0 4 2 8 3 Three 1 0 4 0 0 0 Four 0 0 0 0 1 0 the maintenance of monogamy in willow ptarmigan using data presented in Chapter One. 71 METHODS Experimental removal of males was conducted during spring of 1979, 1980 and 1981 on plots located in the subalpine tundra of the Chilkat Pass, northwestern B r i t i s h Columbia, Canada (59° 50'N, 136° 20'W). The study area, general methods and removal schedule are described in more d e t a i l in Chapter One. Three plots were used: a 50ha Control, and 2 removal areas: 90ha Haines Road plot (HR); and 80ha K e l s a l l Lake plot (KL). The f i n a l density of breeding males and females, and the number of polygynous matings on each plot are summarized in Table XI. I located nests by searching around roosts of males to a radius of 80 paces. A few nests were found by a pointing dog during routine census. Nests of most females were not located, and hatch dates of nests for these females were determined by estimating age of chicks in the brood by the method of Bergerud et a_l. (1963), and backdating. Counts of fledged chicks were made by locating broods with a trained pointing dog, and then searching the immediate area thoroughly, counting the number of chicks that flew away. Additional counts were made off the plots by searching on foot with or without the dog, and by counting broods along the road from a vehicle. Only those counts in which I was present before chicks began to flush were used to estimate brood s i z e . Chicks began to f l y at about 11 or 12 days of age and those used in the analysis varied from 11 to 33 days of age. Zwickel and Bendell (1967) found that 90% of juvenile mortality occurred prior to fledging in blue grouse and I assumed that once fledged, willow 72 ptarmigan juveniles experienced similar levels of mortality to adults, at least in the period of my study. S t a t i s t i c a l comparisons of breeding success and survival of monogamous and polygynous females were based on the n u l l hypothesis that there was no difference between the two groups, and the alternative hypothesis that polygynous females would do worse. Thus these tests were one-tailed. P r o b a b i l i t i e s greater than 0.05 were judged not s i g n i f i c a n t . 73 RESULTS Male viqilance Males that mate polygynously appear to spend time with more than one of their females during the prelaying and laying periods (Fig. 9). During incubation, a monogamous male s i t s close to the nest (usually within 50m) and d i s t r a c t s intruders by c a l l i n g and bringing attention to himself. Nests are d i f f i c u l t to f i n d , and in only one case were both nests of a polygynous group located. The male was seen close to both nest s i t e s which were located about 80m apart. I did not c o l l e c t data on how much time a male spent with each female during the prelaying, laying, and incubation periods, however, i t i s l i k e l y that each female of a polygynous group receives less male vigilance compared with monogamously mated females. Birds with broods leave their t e r r i t o r i e s 3 or 4 days after the eggs hatch, and males in polygynous groupings accompany and defend the chicks of only one hen. The male usually accompanies the female whose young hatch f i r s t (7 of 8 males where hatch dates were known for both hens). I term polygynously mated females that are accompanied by the male during the brood season, primary females; and those defending the brood alone, secondary females. 74 Figure 9. Number of polygynous males that were seen during routine census with each of their hens, at least two hens, or with only one hen prior to hatch (1980 and 1981) . 9A tn o 6 E •JQ 3 E all T 2/3 I Number of mates 76 Reproduction of polygynous and monogamous females  Clutch size I compared the clutch sizes of polygynous and monogamous females for the 3 years combined and found no difference (U=138.5, N=7,40, p=0.5). Mean clutch size for polygynous females was 7.3 ± 0.56SE and for monogamous females 7.3 ± 0.20. Hatch date and hatching success The d i s t r i b u t i o n s of hatch dates of nests of polygynous and monogamous females were not s i g n i f i c a n t l y d i f f e r e n t (Fig. 10, Kolmogorov-Smirnov Two sample test, X.2 = 3.09, N=30,65, P<0.15) and thus by extrapolation, neither were the dates of laying f i r s t egg. Within polygynous groups, nests usually hatched within 2 days of one another (Fig. 11), the exceptions being one 5 days, one 13, and another 15 days after the date of hatch of the f i r s t female. The l a t t e r two females hatched their eggs much later than other females in the population and were possibly renesters. In 1979 and 1980 most females were seen with broods (75% (N=28) and 85% (N=48) respectively), and the proportion of females with broods did not d i f f e r s i g n i f i c a n t l y between the two years (X2=0.68, p<0.3). However, in 1981 fewer females appeared with broods (60%, N=46), and th i s proportion was s i g n i f i c a n t l y lower than in 1979 and 1980 (X*=5.33, p<0.025). Nest predation was low in 1979 (10%, N=10) and 1980 (0%, N=21) and higher in 77 Figure 10. Dis t r i b u t i o n of dates of hatch for monogamous and polygynous females for 1980, and 1981 (hatch dates in di f f e r e n t years and on di f f e r e n t areas were combined by matching modal dates of hatch for monogamous females). Number of birds CL - I —t *< 79 Figure 11. Number of days between the hatch dates of primary and secondary females within a polygynous group. (ft CD d , 3 7 II 15 1 3 5 9  13 No. of days between hatch dates 81 1981 (26%,N=19) (Fisher exact test, p=0.024 when 1981 was compared to 1979 and 1980 combined). Of the 50 nests found in the 3 years of the study, only one clutch was deserted, and that was by a female that had renested and had incubated for over 29 days (normal incubation i s about 21 days) on i n f e r t i l e eggs. Therefore, i f females do not produce a brood, i t is l i k e l y that their nests have been destroyed by predators. I predicted that predation should occur more frequently on nests of polygynous females since they shared male vi g i l a n c e . In 1979 and 1980 both polygynous and monogamous hens were seen with broods at the same rate (Table XII). However, in 1981, the year of high nest predation, polygynous females were seen less often with broods than monogamous females (Table XII). Polygynous hens in 1981 were seen less often with brood than those polygynous females in 1979 and 1980 combined (")^ 2 = 7.66, p<0.005), whereas monogamous females in 1981 were seen with brood to the same extent as those in 1979 and 1980 (Xf=0.0l, p<0.4). These analyses indicate that in a year of high nest predation, polygynous females may be more vulnerable to having their nests located and destroyed by predators. When nests were destroyed, the entire contents usually disappeared between checks on the nest (within one or two days). The main predators are l i k e l y the coloured fox, Vulpes vulpes, and weasels, Mustela  erminea. Only one egg was found with a hole in i t , which suggests that predation was by corvids. 83 Number of chicks fledged Of females seen with broods, secondary females fledged the same number of chicks as primary females or monogamously mated females on the male removal area (Table XIII). Mean brood sizes of lone females were similar to those of paired females, when a l l birds on the study area were included (Table XIII). In general, i t appears that secondary females (lone females), i f they hatch their nests, can fledge as many chicks as females accompanied by males. I could not compare survival to breeding age of chicks raised by polygynous or monogamous parents because return rate of tagged juveniles to the study area was very low. Predation on females in the prelaying period In the 3 years of the study, only one predator k i l l of a t e r r i t o r i a l female (polygynously paired) was found during the pre-incubation period. Therefore, I have no direc t evidence with which to compare predation on polygynous and monogamous females. Once laying begins, females are rarely seen by observers routinely censusing the plots u n t i l chicks hatch. If females are seen with broods, then they have obviously survived the prelaying and laying periods. But i f females are not seen during the brood season they may be dead, they may have had their nest destroyed and then l e f t the area, or they may have l e f t the area with their brood prior to being censused. Nests of six t e r r i t o r i a l females, not resighted during the brood period, were destroyed by predators. However the disappearance 84 Table XIII. Mean brood counts of primary, secondary and monogamously mated females on male removal plots and lone and accompanied hens on other areas (T values for t-test and le v e l of s i g n i f icance) . Females on male removal plots Secondary Primary Monogamous Mean 4.9 4.0 4.7 SE 0.48 0.50 0.60 N 9 11 12 T 1.25 (p<0.2) 0.28 (p<0.45) Females on whole study area Lone With male Mean 4.8 4.7 SE 0.28 0.22 N 36 98 T 0.24 (p<0.45) 85 of the remaining females is unaccounted for. If females not seen during the brood season were k i l l e d by predators during the prelaying period, I would have expected them to disappear from the plots e a r l i e r than females who were seen with broods. Dates when females were last seen before laying began were similar for both groups of females (Fig. 12) (1980 and 1981 only) (U=206, N=26,18, p=0.25). Unsuccessful polygynous females did not disappear e a r l i e r than unsuccessful monogamous hens (U=31.0,N=5,13, p>0.05). Unless hens are more l i k e l y to be k i l l e d by predators during laying and incubation, I conclude that hens not seen during the brood season were not k i l l e d by predators, but lost their nests, or l e f t the area prior to being censused. Breeding success of polygynous and monogamous males I cannot d i r e c t l y compare the success of polygynous and monogamous males because brood counts were not obtained for a l l females in each polygynous group. However, I can estimate the success of males by using mean brood counts for polygynous and monogamous females. In a year with no nest predation, a bigamous male could produce a mean of 8.8 chicks (mean brood count of polygynous hens=4.4, SE=0.36, N=20) and a monogamous male a mean of 4.7 chicks. In the year of high nest predation (1981), 46% of polygynous hens hatched eggs, thus bigamous males had a prob a b i l i t y of producing a mean of 8.8 X 0.46= 4.0 chicks. Seventy-seven percent of monogamous females hatched young. Thus monogamous males had a prob a b i l i t y of producing a mean of 3.6 86 Figure 12. Date of census in which females that were A) seen during the brood season, and B) not seen during the brood season, were last located on the plots before laying began. A. I l I I I I I B. I 1 16 19 21 23 2 4 26 27 May Date of census 88 chicks. Thus, polygynous males fledged many more chicks on the study area than monogamous males in a good year, and about the same number as monogamous males in a poor year. Survival to the next breeding season Perhaps the rigors of associating with 2 females (for the male), or rai s i n g a brood alone (for the female) could reduce survival to the next breeding season. I compared survival to the next spring of polygynous and monogamous females and males present during the brood season, and found 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 differences (Table XIV). 89 Table XIV. Survival to the next breeding season of polygynous and monogamous females and males (data from 1979 ' and 1980 combined). FEMALES Age Mating status No. surviving N P* Yearling Polygynous 5 1 1 0.50 Monogamous 6 1 1 Adult Polygynous 8 26 0.32 Monogamous 1 1 27 MALES Year1ing Polygynous 4 7 0.54 Monogamous 1 2 24 Adult Polygynous 1 8 0.14 Monogamous 9 21 * Fisher exact test 90 DISCUSSION Importance of unshared male vigilance Unshared male vigilance (Wittenberger and Til s o n 1980, hypothesis 1) i s c l e a r l y not essential to reproductive success and survival of female willow ptarmigan. Polygynous and monogamous females survived equally well, produced similar clutch sizes, l a i d eggs at the same time, and produced broods of similar s i z e . The only difference in reproductive performance appeared to occur during incubation, when polygynous females suffered a higher loss of nests in a year when nest predation was high. These results indicate that the most important contribution of the male may be at the nest, and not during the prelaying and laying periods as suggested by Wittenberger (1978). In years of high nest predation, such as occur during c y c l i c declines of grouse (Weeden and Theberge 1972, Myrberget 1972), the role of the male as sentinel at the nest may become more important to female reproductive success. These results d i f f e r from those of M i l l e r and Watson (1978), who found that secondary females in red grouse produced smaller broods. Bigamous male red grouse did not have s i g n i f i c a n t l y larger t e r r i t o r i e s than monogamous males, and the authors suggested that the t e r r i t o r i e s did not have s u f f i c i e n t resources for two hens. However, i t is also possible that females that share resources on smaller t e r r i t o r i e s may require more male vigilance to succeed, since food a v a i l a b i l i t y per female i s lower. 91 If a polygynous female can raise young successfully by herself, why then are unmanipulated populations of willow ptarmigan not polygynous? Polygyny is c l e a r l y an advantage to males. Males are opportunistically polygynous and polygynous males produce more young than monogamous males in years of low nest predation, and just as many in years of higher nest predation. Some mated males court intruding females, and attempt to separate females engaged in aggressive encounters (Chapter One). However, in order for polygyny to become the prevalent mating system, i t must be advantageous for females as well (Orians 1969). Wittenberger and Tilson (1980) proposed that ptarmigan are monogamous because t e r r i t o r i e s of males do not d i f f e r enough in quality to exceed the polygyny threshold. Thus there i s no advantage to a female to become a second mate of an already mated male, i f unmated males on good quality t e r r i t o r i e s are avai l a b l e . This hypothesis predicts that polygyny occurred on the male removal plot because male t e r r i t o r y size expanded, producing a polygyny threshold where none had existed before. In the absence of data on the quality of t e r r i t o r i e s in willow ptarmigan populations I cannot reject or support the polygyny threshold hypothesis. However, unmated males defend small t e r r i t o r i e s (Chapter One), and in the conspecific red grouse, these t e r r i t o r i e s contain half the weight of green shoots of heather as did those of monogamous males. T e r r i t o r i e s of monogamous and bigamous males appear to produce a similar y i e l d of heather (Miller and Watson 1978). This suggests that for red 92 grouse at least, t e r r i t o r y quality i s variable, though whether a s i g n i f i c a n t proportion of them could support two females i s unclear. I know of no studies that have attempted to measure whether a polygyny threshold exists on t e r r i t o r i e s of a monogamous species, and therefore Wittenberger and Tilson's hypothesis 2 remains untested, and could explain why willow ptarmigan are monogamous. Female aggression and the maintenance of monogamy There is an alternative explanation of the natural and experimentally induced polygyny on large male t e r r i t o r i e s (Chapter One). Female willow ptarmigan chase and attack females that intrude on their t e r r i t o r i e s , thus preventing unmated females from s e t t l i n g , at least on small to medium sized t e r r i t o r i e s (Chapter One). However, the upper l i m i t of female t e r r i t o r y size is lower than that of males (Fig. 7), and defence by females of large t e r r i t o r i e s may become too costly energetically. Thus, when males were removed, and the t e r r i t o r y sizes of the remaining males increased (Fig. 5), females could no longer defend the entire area. Wittenberger and Tilson (1980) dismissed the hypothesis that female aggression prevents polygyny in birds, because females must expend large amounts of energy in egg laying and incubation, and r e p e l l i n g additional females would cost more than to tolerate them. However the cost to females to repel potential second females depends on a number of factors not considered by Wittenberger and Til s o n and which d i f f e r in 93 di f f e r e n t habitats. I propose that polygyny results when a female is unable to defend economically the whole of her mate's t e r r i t o r y against other females. The following factors may influence a female's a b i l i t y to defend her t e r r i t o r y and hence the incidence of polygyny, i f a polygyny threshold e x i s t s . Length of the breeding season As a female's reproductive cycle progresses, the amount of area she i s able to defend decreases markedly. For example, the t e r r i t o r i e s of resident female spruce grouse decrease in size from early spring to late spring as eggs develop and laying begins (Herzog and Boag 1978). During incubation the area defended decreases further. If climatic or other environmental conditions permit a long breeding season, then a second female w i l l be able to se t t l e and breed while the f i r s t i s laying or incubating. Nero (1956) noted that female redwings Agelaius  phoeniceus attempted to defend a large portion of the male's t e r r i t o r y in spring but when nesting began they defended less area. Aggression by females in polygynous species may delay s e t t l i n g of secondary females and hence reduce potential harem size (Holm 1973, Crawford 1977, but see Yasukawa and Searcy 1981) . In blue grouse, few yearlings s e t t l e u n t i l after adults have begun to lay eggs, and thi s has been attributed to s o c i a l i n h i b i t i o n through interactions with adults (Hannon et a l . 1982) . Monogamous ptarmigan species l i v e in areas with short breeding seasons and the hatch of nests is highly synchronous 94 (e.g. Weeden 1963, Giesen e_t a l . 1980). Nests of willow ptarmigan in the Chilkat Pass a l l hatched within a 26-day period, and 88% of the hatch occurred during the f i r s t two weeks. If a female has not settled by a certain date, she may have i n s u f f i c i e n t time to build up reserves for egg-laying, lay her eggs, incubate, and successfully raise a brood. In the continuous female removal experiment, replacement of removed females was rapid u n t i l hens in the unmanipulated population began to lay eggs. After t h i s point few hens attempted to s e t t l e , suggesting that i t was too late in the season to breed successfully (Chapter One). Synchrony of nest i n i t i a t i o n because of the brevity of the breeding season may be a factor which maintains monogamy in other tundra-breeding species (Weatherhead 1979). Male:female t e r r i t o r y size r a t i o If a male i s capable of defending a very large t e r r i t o r y , he may be able to accomodate two females. Obviously the benefits to a male to attr a c t more than one mate are substantial. He w i l l be able to produce many more offspring than a monogomous male. McLaren (1972) has in fact suggested that the adaptive function of t e r r i t o r i a l i t y in males is to attract a second female. However, in some situations, i t may be physically impossible for a male to defend a t e r r i t o r y much larger than the female's. Factors which could decrease malerfemale t e r r i t o r y size r a t i o are high male intruder pressure, low female intruder pressure, low size dimorphism 95 (male equal to or smaller than the female) or a combination of these. Environmental factors, such as severe weather conditions or scarcity of food, could counteract sexual selection for larger body size in males, making them equal in size to females and less able to defend a larger t e r r i t o r y . Male to female t e r r i t o r y size r a t i o in willow ptarmigan i s 0.99 (means for birds on HR 1981: males: 2.79± 0.372, N=23; females: 2.82+ 0.197SE, N=18). Males had a higher upper l i m i t to t e r r i t o r y size than did females, although the most frequently chosen t e r r i t o r y size was the same for both sexes. Male and female t e r r i t o r i e s are probably similar in size because f i r s t , male willow ptarmigan are only s l i g h t l y larger than females (Wiley 1974), and second, because of high energetic costs to the male to defend a t e r r i t o r y larger than the female's. When competitors were removed during the male removal experiment, the size of the remaining males' t e r r i t o r i e s increased dramatically (Chapter One) indicating that high intruder pressure probably increases the energetic costs of defending a t e r r i t o r y and keeps t e r r i t o r i e s small. High intrusion pressure reduces t e r r i t o r y size in other species (Myers e_t a l . 1979, Ewald et al_. 1980). To summarize, female willow ptarmigan may prevent polygyny from occurring by aggressively attacking potential secondary females. Although this behaviour may be energetically expensive, a female benefits by having access to unshared male vigil a n c e , which may be p a r t i c u l a r l y important during years of high predation. Two factors enable female willow ptarmigan in unmanipulated populations to defend t e r r i t o r i e s against 96 potential secondary females: 1) breeding seasons are short so that once a female begins to incubate and cannot defend the t e r r i t o r y , i t i s too late for other females to s e t t l e ; and 2) t e r r i t o r i e s of males are rarely larger than those of females, so females can defend the entire area and prevent a second female from s e t t l i n g early in the season. Tests of the Female Aggressiveness Hypothesis To test whether female aggression enforces monogamy for a pa r t i c u l a r species, one must f i r s t determine whether a potential for polygyny exists on t e r r i t o r i e s of monogamous males. This could be done by reducing the aggressiveness of the f i r s t female to allow a second female to s e t t l e , and then assessing whether both females could produce young. A l t e r n a t i v e l y , an a r t i f i c i a l polygyny threshold could be maintained by supplementing p a r t i c u l a r t e r r i t o r i e s with food or nest s i t e s . If more females attempt to s e t t l e on these t e r r i t o r i e s than on Controls, and i f they are prevented from doing so by the behaviour of the resident females, the female aggressiveness hypothesis would be supported. As Wittenberger and Tils o n (1980) pointed out, a single hypothesis i s not s u f f i c i e n t to explain monogamy in a l l situ a t i o n s . Aggression by females may enforce monogamy only under a limited set of environmental conditions. If the costs to a female of defending a t e r r i t o r y outweigh the benefits of exclusive access to resources and male vigilance or parental care, then a female would not be expected to engage in thi s 97 behaviour. Thus, in habitats where breeding seasons are long or where males can economically defend much larger t e r r i t o r i e s than females, primary females would not be expected to react aggressively to secondary females, unless th i s behaviour delays settlement of the secondary female and ensures more male parental investment in the young of the f i r s t female. Mating systems in grouse In the above discussion, I took as given that t e r r i t o r i e s of males and females overlap and that ptarmigan have a long-term pair bond, and then asked why are they not polygynous? Most grouse species are promiscuous, males and females associate only during copulation, and t e r r i t o r i e s or home ranges of males and females do not overlap extensively (Hjorth 1970, Wiley 1974). What ecological factors have selected for a pair bond and overlapping t e r r i t o r i e s in monogamous grouse? Wittenberger (1978) suggested that monogamous grouse species inhabit areas where food is scarce in spring so that females benefit by associating with males and relying on male vigilance during foraging. Promiscuous species apparently l i v e in areas with abundant food, females do not need to forage on t e r r i t o r i e s of males, and avoid them because of increased conspicuousness to predators. Thus food abundance, he suggests, leads to a promiscuous mating system. To support th i s hypothesis, Wittenberger compared spring diets of monogamous and promiscuous species and concluded that monogamous species generally have a polytypic d i e t , and promiscuous species a 98 monotypic d i e t . He equated a monoculture of food resources to a superabundance of food. I suggest that there are no data to support t h i s hypothesis. F i r s t of a l l , spring diets of most monogamous species are monotypic (Peters 1958, West and Meng 1966, Moss 1969, Weeden 1969). For ptarmigan species, i t i s not u n t i l the snow melts and hens have begun to lay that the diet becomes more diverse. Secondly, i t is not the d i v e r s i t y - o f the diet that should be compared among species, but the abundance and dispersion of food plants in the habitat. A knowledge of the n u t r i t i o n a l value of each plant species and plant part would be required since grouse and ptarmigan are selective feeders (Moss et a l . 1973). Thirdly, i f food i s scarce in the habitat of willow ptarmigan, then females that share male vigilance should have lower breeding success than those that do not. I found no differences in s u r v i v a l , clutch size, or date of i n i t i a t i o n of laying between females that shared male vigilance and those that did not. Bradbury (1981) suggested that a key factor in the development of a p a r t i c u l a r mating system type i s the r a t i o of female to male home range si z e . If females have very large home ranges, then males would be unable to defend them, and a strategy of self-advertisement rather than resource defence would be favoured for males. This hypothesis may explain the evolution of leks in certain promiscuous species, since females of these species may occupy extensive home ranges in spring (Robel et a l . 1970, Wallestad 1975). However, the d i s p a r i t y 99 between size of home ranges of females and males of other promiscuous species (those in which males are dispersed over the breeding range), appears to be much less marked, and i s in some cases close to zero (e.g. spruce grouse: Herzog and Boag 1978; ruffed grouse: Maxson 1978; blue grouse: Hannon et a_l. 1982). More detailed studies of the dispersion and home range sizes of females in spring are required for most tetraonid species, however, based on the information c i t e d above, Bradbury's hypothesis does not appear to explain why non-lekking promiscuous species are promiscuous. Females of both monogamous and promiscuous species with dispersed males, appear to be intolerant of other females during spring (Watson and Jenkins 1964, S t i r l i n g 1968, MacDonald 1970, Herzog and Boag 1977, Hannon 1980, Chapter One), and some have been described as t e r r i t o r i a l (Herzog and Boag 1978, Chapter One). If the t e r r i t o r y of the male covers the same ground as that of the female, then he is probably r e s t r i c t e d to breeding with one female, whereas i f he has a separate t e r r i t o r y , he may have access to several females with abutting home ranges. Obviously the l a t t e r situation would be favourable to males unless some feature of the environment makes at least some vigilance or parental investment by the male necessary to the female's reproductive success. In habitats of monogamous grouse this may be high predation pressure in open environments, or scarcity of food as Wittenberger (1978) suggested, or both. Clearly what i s needed i s more information on the a v a i l a b i l i t y of preferred food and predation pressure in the habitats of 100 grouse species of d i f f e r e n t mating system types. By removing males from a monogamous population of willow ptarmigan I tested whether unshared male vigilance or parental care was essential to female reproductive success and su r v i v a l . I found that vigilance by males at nests may be important, but otherwise females that share male vigilance do as well as those that do not. However, each female received some assistance from her mate, and thus the experiment did not test whether t o t a l l y unaided females could survive and produce young. To test f u l l y whether emancipation of the male from vigilance and parental care is possible in habitats of monogamous grouse species, one must remove a l l but a very few males, so that male investment in each female approaches zero. 101 CHAPTER FOUR: CONCLUDING REMARKS A l l b i r d species show some form of spacing behaviour. Whether th i s behaviour can l i m i t population size has been a question that has generated much controversy. Lack (1968) hypothesized that most bird populations are limited through density-dependent mortality of young in the nonbreeding season, primarily through starvation. In t h i s view, t e r r i t o r i a l behaviour simply serves to space out the the remaining individuals on the breeding area. The opposing view i s that, although the f i n a l cause of death of juveniles may be starvation, predation, or disease; t h i s mortality i s s o c i a l l y induced (Watson and Moss 1970). There are two ways to distinguish between these hypotheses: 1) Remove t e r r i t o r i a l birds to determine whether there i s a surplus of individuals that have been prevented from breeding; and 2) Add food in winter to increase overwinter survival of juveniles. If e x t r i n s i c factors l i m i t population size, then the predicted results of these two experiments are: 1) l i t t l e or no replacement of removed birds; and 2) an increase in the breeding density. In Chapter Two I l i s t e d a number of species in which surplus birds had been i d e n t i f i e d . These results reject the f i r s t p rediction. Food has been added in winter to habitats of several passerine populations, with l i t t l e or no increase in the subsequent breeding density (Krebs 1971, YomTov 1974, Samson and Lewis 1979, Smith et a_l. 1 980). These results are not s t a r t l i n g , considering that most population studies are conducted in good habitat, where resources are abundant. 102 E x t r i n s i c factors may be more important in marginal habitats or at the edge of a species' range. An intermediate view i s that changes in e x t r i n s i c factors may be s u f f i c i e n t but not necessary, to cause fluctuations in population size and that i n t r i n s i c mechanisms operate when ex t r i n s i c factors have not reduced juvenile mortality s u f f i c i e n t l y (Boag et a l . 1979, Watson and Moss 1979). Most previous studies have not considered the influence of female spacing behaviour on population regulation. They have assumed that numbers of both sexes are regulated in the same way and have emphasized the role of the male. However, since the sexes must invest d i f f e r e n t i a l l y in the production of young, males and females are expected to have very d i f f e r e n t mating strategies, and may be affected by the environment in d i f f e r e n t ways. For males, females may be a l i m i t i n g resource, and changes in the spacing behaviour of males may be related to the a v a i l a b i l i t y of mates. Females, on the other hand, may be more fi n e l y attuned to changes in resources, and may a l t e r their home range sizes or density in response to variations in a resource faster than males do (Fordham 1971, Watson and Moss 1980, T a i t t 1981). Crowded conditions may also have more immediate and detrimental effects on the reproductive physiology of females (Christian 1971, Myers e_t a l . 1971), which could lower production, or, via maternal e f f e c t s , cause subsequent variations in spacing behaviour of the young (Watson and Moss 1972). Thus changes in the spacing behaviour of females, and the factors which cause them, may be of more importance in 103 regulating population size than those in males. The main contribution of this thesis has been to challenge the ex i s t i n g view that spacing behaviour of males determines numbers of both males and females in a monogamous species. In willow ptarmigan, the number of breeding females may not be limited by the density of males: females can s e t t l e and breed successfully at high density despite a substantial reduction in the number of males. The t e r r i t o r i a l behaviour of females prevents some potential female r e c r u i t s from s e t t l i n g and breeding, and this may constrain the population to a monogamous mating system. Females may also influence the settlement patterns and density of males by i n c i t i n g disputes among cocks and a l t e r i n g their t e r r i t o r i a l boundaries. Can my results be generalized to other species, or are they s p e c i f i c to monogamous t e r r i t o r i a l birds? Aggressiveness can affect population size by deferring maturity of juveniles, reducing reproduction in mature animals, increasing juvenile or adult mortality, and increasing dispersal (King 1973). Many studies on promiscuous and polygynous species have indicated that interfemale agonistic behaviour has obvious effects on numbers in the population. For example, aggressive interactions among females in harems or small s o c i a l groups has increased the mortality of juveniles of low ranking females (e.g. seals: Christian and LeBoeuf 1978, Reiter et a l . 1981, McCann 1982; and monkeys: Wilson e_t a l . 1978). Reproduction by subordinate females has been reduced or inhibited completely in birds (Carrick 1963, Crawford 1977), small mammals (Christian 1971, 1 04 Myers et a l . 1971, Rood 1980), and primate species (Dunbar and Dunbar 1977, Wilson et §_1. 1 978). In microtine rodents, high densities of females have been related to delayed sexual maturation and reduced survival of juveniles (Bujalska 1973, Boonstra 1978, Redfield et a l . 1978, Saitoh 1981). Increased mortality of adults denied access to breeding s i t e s has been documented in squirrels (Carl 1971). On the weight of t h i s accumulating evidence of the importance of interfemale agonistic behaviour in a f f e c t i n g population density, future studies on population regulation should consider female behaviour as an important factor. More work i s required to c l a r i f y the physiological and environmental factors a f f e c t i n g spacing behaviour in females and to manipulate ' th i s behaviour to test hypotheses about i t s role in determining population density, and i t s related effects on the development of p a r t i c u l a r mating system types. 105 LITERATURE CITED Bendell, J . F. and P. W. E l l i o t t . 1967. Behaviour and the regulation of numbers in blue grouse. Can. W i l d l . Serv. Rep. Ser. 4. 76 pp. Bendell, J. F., D. G. King and D. H. Mossop. 1972. Removal and repopulation of blue grouse in a declining population. J. W i l d l . Manage. 36: 1153-1165. Bergerud, A. T. 1970. 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