Breeding Behavior and Feeding Habits
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BREEDING BEHAVIOR AND FEEDING HABITS OF THE BALD EAGLE (HALIAEETUS LEUCOCEPHALUS L.) ON SAN JUAN ISLAND, WASHINGTON by LASZLO I. RETFALVI B.S.F. (S), University of British Columbia, 1961 A thesis submitted in partial fulfilment of the requirements for the degree of MASTER OF FORESTRY in the Faculty of Forestry "We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April, 1965 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 per• mission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives.. It is understood that copying or publi• cation of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver 8, Canada . ABSTRACT The breeding behavior and feeding habits of the bald eagle (Haliaeetus leucocephalus L.) were studied during 1962 and 1963 on San Juan Island, Washington. The primary aim of the study was to acquire information which would relate to the general decline in bald eagle numbers. Thirteen bald eagle nests were found on San Juan Island. On the basis of the spacing of these nests, the density of breeding eagles was considered to be low. The number of bald eagles varied throughout the year; the highest numbers were present in February and the lowest numbers in October. The change in eagle numbers was caused by the fluctuating numbers of juveniles. Two active nests were kept under observation, one in 1962 and another in 1963. The breeding eagles showed hostility toward intruders such as adult and juvenile bald eagles and man in the vicinity of their nesting site. The birds showed indifference toward intrusions of red-tailed hawks, crows and gulls. The chronology of breeding activities on San Juan Island was approximated with the aid of local information and with findings of former investigators. Egg-laying occurs between March 4 and 19; hatching between\ April 8 and 14. The young spend 12 to 1 3 weeks in the nest during which time parental attention gradually decreases. During the first 5 weeks of the youngs' life the nest is constantly guarded by one of the parents. Parental attention markedly decreases after the young start their wing exercises at the age of 8 to 9 weeks. In general, the female parent spent three times as much time at the nest as did the male. Food was brought to the nest at irregular intervals by both the female and male parents. The young were fed mainly by the female during the first seven weeks of their life; later the young fed themselves. No appreciable change in the amount of food supplied to the young during their nest life was observed. Young of the same nest were similar in size and indications of maltreatment from the others or from the parents were not observed. Most of the food brought to the nest consisted of rabbit carrion. This type of food item was available throughout the year due to the rabbits'high mortality resulting from collision with automobiles on the roads. Fish was fed to the young during the first six weeks of their life; thereafter, mainly rabbit was consumed. Rabbit carrion was the primary source of food for young eagles in their post-nestling period. The destruction of breeding habitat by real estate developments is the major cause of decline in numbers of bald eagles on San Juan Island. i v TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS iv LIST OF TABLES viii LIST OF FIGURES x ACKNOWLEDGMENTS xii CHAPTER I. GENERAL INTRODUCTION 1 1 . INTRODUCTION 2 2. ENVIRONMENT 4 1 . Geography and Geology 4 2. Climate 6 1 . Precipitation 6 2. Temperature 6 3. Winds 7 3. Vegetation 7 4. Fauna 9 1 . Mammals 9 2. Birds 11 3. STUDY METHODS 12 1 . Observations from the ground 12 2. Aerial surveys 13 3. Time spent in the area 14 CHAPTER II. ABUNDANCE AND MOVEMENTS 16 4. INTRODUCTION 17 5. ABUNDANCE 18 1. Eyries 18 1 . Description of nests 20 2. Territories 27 2. Observations 33 3. Summary and Conclusions 44 6. MOVEMENTS 46 CHAPTER III. BREEDING 49 7. INTRODUCTION 50 8. GENERAL BEHAVIOR 53 1 . The adults 53 2. Vocalization 54 3. Roosts and resting places 56 4. The flight of the eagle 60 1 . Fishing flight 60 2. Soaring 61 3. Defense and attack 63 4. Nuptial flight. 64 5. Relations with birds, mammals and man 65 1 . Relations with other bald eagles 65 2. Relations with red-tailed hawks 68 3. Relations with ospreys 69 vi 4. Relations with crows 7 0 5. Relations with gulls 7 3 6. Relations with other birds 74 7. Relations with mammals 75 8. Relations with man 76 6. Summary and Conclusions 78 9. BREEDING SEASON 80 10. BREEDING BEHAVIOR 85 1 . Pre-nestling period 85 1. Nest-repair activities 85 2. Mating s 86 3. Parental attention to eggs 88 4. Hatching 89 2. Nestling period 90 1 . Parental attention to the young 90 1. Care of the young 90 2. Feeding of the young 96 2. Development of the young 102 1. Changes in size 102 2. Changes in plumage 104 3. Wing exercises . 105 3. Relationship between young in the same nest 108 3. Post-nestling period 109 4. Summary and Conclusions 114 vii 11. BREEDING SUCCESS 116 CHAPTER IV. FEEDING HABITS 123 12. INTRODUCTION 124 1. Methods of collecting food data 124 2. Former food habit studies 126 13. FEEDING HABITS OF BALD EAGLES ON SAN JUAN ISLAND 129 1 . Sources of information 129 1. Direct observations 129 2. Debris analysis 133 2. Sources of food 139 1. Mammals 139 2. Fish 144 3. Birds 144 4. Other items 145 3. Baiting experiments 146 4. Summary and Conclusions 150 CHAPTER V. GENERAL CONCLUSIONS AND SUMMARY 154 14. CONCLUSIONS 155 15. SUMMARY 161 APPENDICES 163 LITERATURE CITED 176 viii LIST OF TABLES 1 . Date and duration of observations on San Juan Island, 1962 to 1964 . 15 2. Measurements of nests and nest trees and their proximity to shore and human habitations . 25 3. Linear distances between neighbouring bald eagle nests on San Juan Island • 29 4. Bald eagle counts made during 1962 to 1964 on ground and by air on San Juan Island. 34 5. Occasional sightings of bald eagles on San Juan Island during 1962 and 1963 35 6. Egg-laying dates of the bald eagle in various parts of its breeding range (After Bent, 1937). 52 7. Breeding activity of bald eagles on San Juan Island during the spring of 1963 82 8. The approximate chronology of breeding activities of bald eagles on San Juan Island 84 9. Observed copulation of bald eagles on San Juan Island, 1962 and 1963 87 10. The male and female bald eagle's participation in guarding the young of Nest A during 1962 on Turn Island 91 11 . The male and female bald eagle's participation in guarding the young of Nest B during May 1963 at Rocky Bay 93 12. Number and date of food items brought to the nest by the male and female parent eagles during 1962 and 1963, on San Juan Island 97 13. Food brought to the nest by the parent eagles during the morning, day and evening hours, during 1962 and 1963 on San Juan Island 100 1 X 14. Number of food items brought to the nest by the female and male parent bald eagles in relation to the young's age, during the breeding season on San Juan Island 101 15. Family disintegration and nest abandonment at Nest A, on Turn Island, 1962 111 16. Breeding success at active bald eagle nests on San Juan Island during 1962 and 1963 117 17. Nesting success of bald eagles on San Juan Island during 1962 and 1963 122 1.8. Diet of the bald eagle in different localities of its range (numbers represent percentages) 127 19. Diet of bald eagles on San Juan Island, based on observations of food items brought to the nest during 1962 and 1963 130 20. The monthly constitution of diet of bald eagles on San Juan Island, based on observations of food items brought to the nest during the summers of 1962 and 1963 132 21 . The constitution of diet of bald eagles on San Juan Island, based on analysis of debris collected underneath the nest tree on Turn Island 136 22. Rate of disappearance of car-killed rabbits on San Juan Island. Numbers of rabbits counted on roads at the beginning and end of 1 2 hour periods, during the summer of 1963 (d is number of disappeared carcasses) 141 23. The participation of different species of birds and mammals in consuming eagle baits during the months of July and August', in 1962 and 1963 149 X LIST OF FIGURES 1 . Average monthly temperatures and total monthly precipitation on San Juan Island, based on data collected by F. Dearborn in 1962 and 1963 8 2. a The location of bald eagle nests on San Juan Island and the "available areas" belonging to each nest 31 b The location of bald eagle nests on San Juan Island in relation to human habitation 32 3. The area covered by means of aerial and ground surveys on San Juan Island, 1962 to 1964 37 4. Number of bald eagles seen on San Juan Island during 1962, 1963 and 1964 39 5.