ASPECTS OF FORAGING IN BLACK OYSTERCATCHERS (AVES: HAEMATOPODIDAE) by SARAH GROVES B. A. Biology, Harvard College, 1973 THESIS SUBMITTED IN PARTIAL FULFILMENT THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April, 1982 © Sarah Groves, 1982 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of 7?:OOL.O Gf)/ The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 DE-6 (2/79) i i ABSTRACT I studied foraging ecology of black oystercatchers (Haematopus bachmani) in the rocky intertidal. The aims of this study were: 1) to analyze prey choice and patch choice by adult black oystercatchers and evaluate how well their foraging performance was predicted by foraging theory; 2) to study development of foraging in young oystercatchers; 3) to indirectly examine the relationship between parental foraging performance and fitness by measuring chick growth and chick production. The following conclusions were reached: 1) Prey selection by oystercatchers was generally as predicted by theory, but birds showed partial preferences for prey. Patch choice followed general theoretical predictions, but profitabilities achieved within particular patch types were highly variable. Reasons for this are discussed. 2) Growth and physical maturation are important components in development of foraging. During the period of this study, chicks were heavily dependent on parental feeding, and the ability of chicks to forage independently developed after chicks left their natal area at about 50 days of age. 3) Chick growth varied between one-chick and two-chick broods, and this may be related to parental foraging performance. However, during this study chick production was constrained by weather and predation, and no relationship between parental foraging performance and fitness could "be def ined. TABLE OF CONTENTS Abstract i i List of Tables vi List of Figures vii Acknowledgements ix General Introduction 1 Literature Cited 5 Exploiting a Patchy Environment: a Field Study of Black Oystercatchers Foraging in the Rocky Intertidal ..6 Introduction 6 Study Area . 8 Study Animal 9 Methods 9 Distribution and Abundance of Intertidal Organisms ..9 Black Oystercatcher Foraging Behaviour 12 Results 16 Prey Distribution 16 Durations of Foraging Behaviours 21 Searching Behaviour 23 Prey Selection 25 Prey Profitability 32 Cumulative Prey Consumption 36 Allocation of Foraging Time between Zones 39 Discussion 43 Prey Choice 43 Patch Choice and Time Allocation between Zones 46 i v Conclusions 49 Literature Cited 51 Development of Foraging Skills in Young Black Oystercatchers 55 Introduction 55 Study Areas • 55 Study Animal 56 Methods . 57 Intertidal Prey Organisms 57 Chick Growth -. 57 Foraging Behaviour 58 Results .' 59 Prey Distribution and Abundance 59 Chick Growth 59 Allocation of Foraging Time between Zones 62 Prey Selection by Chicks and their Parents 63 Searching Behaviour 72 Winter Foraging 74 ' ' Prey Stealing ..." 75 Discussion 77 Chick Growth 78 Allocation of Foraging Time between Zones 78 Prey Choice 79 Searching Behaviour and Prey Handling 79 Prey Stealing 81 Important Processes in Development of Foraging Skills 81 Literature Cited • 83 V Growth, Sibling Rivalry, and Chick Production in Black Oystercatchers 86 Introduction 86 Study Area 86 Study Animal 87 Methods 88 Black Oystercatcher Territories 88 Chick Growth and Survival 88 Chick Feeding 89 Results ,.89 Territory Fidelity, Clutch Size, and Breeding Success 90 Brood Size and Chick Growth ..93 Weight Differences in Two-chick Broods ...98 Chick Survival 101 Clutch Size and Chick Production 107 Discussion . 109 Chick Growth and Sibling Rivalry ..109 Chick Survival 111 Chick Production and Parental Investment 112 Literature Cited 114 General Conclusion 116 Literature Cited 120 Appendix A . 121 Literature Cited 122 Appendix B ' 123 vi LIST OF TABLES Table I: Correlations between Event Duration and Time Since Start of a Foraging Bout 23 Table II: Durations of Search in Three Intertidal Zones ...25 Table III: Rate of Effective Search as Calculated by Multi- species Disc Equation for 5 Prey Types in 3 Zones 30 Table IV: Average Bout Durations and Profitabilities in 3 Intertidal Zones 40 Table V: Allocation of Foraging Time between Zones 63 Table VI: Numbers of Different Prey Taken by Chicks of Various Ages 72 Table VII: Initiation of Chases in Winter Foraging Flocks .77 Table VIII: Black Oystercatcher Clutch Size 91 Table IX: Chick Production on 11 Black Oystercatcher Territories, 1975-1978 93 Table X: Resightings of Birds Color-banded as Chicks 107 Table XI: Chick Production from One and Two-egg Clutches ..108 vii LIST OF FIGURES Figure 1: Map of the Study Area 4 Figure 2: Structure of Behaviour Data 15 Figure 3: Abundance and Biomass of Invertebrates in Quadrats 18 Figure 4: Abundance and Biomass of Invertebrates in Mussels Beds 20 Figure 5: Relative Prey Availability and Prey Choice 28 Figure 6: Average Weight of Prey Taken in Each Zone 32 Figure 7: Profitability of Prey with respect to Prey Weight .35 Figure 8: Cumulative Prey Intake during Foraging Bouts ....38 Figure 9: Bout Duration in Each Zone versus Profitability .42 Figure 10: Chick Bill Lengths and Weights versus Chick Age 62 Figure 11: Proportion of Chick Feedings from Parents and Chicks 66 Figure 12: Average Weights of Prey Taken in Each Zone versus Chick Age 68 Figure 13: Prey Types Selected under 3 Conditons ..71 Figure 14: Inter-peck Intervals for Chicks Foraging in each Zone .74 Figure 15: Chick Growth 95 Figure 16: Sibling Growth in the Two Broods with Minimum and Maximum Weight Differences 98 Figure 17: Sibling Chases and Parental Feedings 101 •Figure 18: Chick Survival to Fly and Weight at 20 Days ....103 viii Figure 19: Chick Survival from Hatching 105 ix ACKNOWLEDGEMENTS It is a pleasure to acknowledge the many people who contributed time, ideas, and energy to this project. Jamie Smith supervised the project, and I am grateful for his patience, help in the field, and critical comments from start to finish of this work. Steve Borden was the principal designer of hardware and software for the event recorder, but more importantly I appreciate Steve's philosophic insights into the world. In the field, Mary Taitt and Patrick Michiel gave generously of their time and energy on many occasions during this project. Bristol Foster of the British Columbia Ecological Reserves Unit provided financial support in 1977, and Wayne Campbell of the British Columbia Provincial Museum suggested Cleland Island as a study site in the first place. Members of my research committee - Larry Dill, Lee Gass, Ray Hilborn, Charley Krebs - and Yoram Yom-Tov provided helpful suggestions at various stages of this research and critically read drafts of this thesis. Don Brandys provided the electronic expertise to debug and maintain the event recorder. Fergus O'Har.a designed a weather-proof housing for the event recorder and frequently serviced .a chronically ailing outboard motor. Teresa Tenisci and Bill Webb helped solve several major computing problems. During the last months of this project Maria Weston acted as my agent at U. B. C, and E. E. Cudby and L. C. Zerr made time 'available for me to complete • this work. Many individuals came on field trips, helped with boat X handling, endured life-threatening attacks by gulls, and shared the unique pleasures and pains of life on Cleland Island. I thank my field assistants L. Dick, R. Jaremovic, and L. Paull, and I also appreciate the help at various times by A. J. Baker, D. Dog, P. Groves, B. Hutchins, S. Krepp, P. Lee, K. Lindsay, S. McCormack, S. McCoy, .J. Myers, R. Olenick, A. Peacock, C. Redsell, L. Richards, B. Stiling, C. Whitney.. Finally, my parents Mary Groves and Laurence Groves contributed to this project in many ways. I am grateful for their understanding and continuing interest in black oystercatchers. 1 GENERAL INTRODUCTION How animals forage in patchy environments is of considerable interest to ecologists. A large and growing body of work in this field (Krebs 1978, Pyke et al. 1977) addresses three major issues in foraging ecology: 1) function - what animals do in their search for food, 2) mechanism - how animals make decisions about foraging and how they locate, handle, and digest prey, and 3) consequences - the effects of foraging performance on an animal's fitness. Functional description is the goal of most data and theoretical studies of foraging, but such studies are not concerned with mechanisms animals use in foraging (Krebs e_t al. 1981). Mechanisms and processes animals use in foraging have, with few exceptions (e. g. Ollason 1980, Waage 1979), been ignored by ecologists. However, the importance of understanding foraging mechanisms, especially those involved in decision making - learning, memory, and perception - has been stressed by Orians (1981). Fitness consequences of foraging are important because foraging performance affects how much energy an animal can invest in reproduction (Schoener 1971). However, the use of foraging performance to evaluate fitness remains 'an -elusive goal. Research I did on foraging ecology of black oystercatchers (Haematopus bachmani) addresses each of these issues in foraging ecology - function, mechanism, and consequences. .Black oystercatchers on Cleland Island (Figure 1) near Tofino, British 2 Columbia, were the subjects of my research. Oystercatchers are large shorebirds with powerful, laterally compressed bills that enable them to exploit a variety of well-armored marine invertebrates.