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. The change in numbers of eagles throughout the year on San Juan Island 40
6. Comparison between the monthly changes in precipitation and in eagle numbers on San Juan Island 43
7 . a Perch-trees used by the eagles of Nest A, on Turn Island during 1962 and 1963 ~~ 57
b Perch-trees used by the eagles of Nest B, at Rocky Bay during 1962 and 1963 ~~ 57
8. a The spatial relationship of perch-trees used by the eagles on Turn Island, 1962 and 1963 59
b The spatial relationship of perch-trees used by the eagles at Rocky Bay, 1962 and 1963 59
9. Steps followed by the female eagle of Nest B to carry a
fish to the nest 62
10. a Crows harassing bald eagle at Rocky Bay 72
b Crow attacks eagle after eagle has robbed crow of its food 72 x i
11 . Seasonal decrease in parental attendance at Nest A on Turn Island, 1962 94
12. The parent eagles' attendance at Nest. B during May, 1963, at Rocky Bay 95
13. Diagrams showing the size of the eaglet in comparison to the size of the female parent at Nest B in 1963 103
14. a Perches used by the eaglets of Nest A, on Turn Island, during 1962 ~ 107
b Perches used by the eaglets of Nest B, at Rocky Bay during 1962 and 1963 " 107
15. The presence of adult and juvenile eagles at Nest A on Turn Island during July, 1962 (expressed as percentage of the observation time) 113
16. The monthly change in fish and rabbit consumption by the bald eagles during the summer months of 1962 and 1963 on San Juan Island 134
17. The monthly change in diet of bald eagles in Alaska (after Imler and Kalmbach, 1955) 135
18. Food habits of bald eagles on San Juan Island. Comparison between data obtained by direct observation and by analysis of debris collected underneath the nest on Turn Island 138
19- Location of baits used to lure bald eagles and other carrion feeders 147
MAPS
Map 1 . The San Juan Islands Following page 4
Map 2. San Juan Island Following page 5 xii
ACKNOWLEDGMENTS
To the members of the Department of Zoology and the
Faculty of Forestry at the University of British Columbia, my thanks are due. Dr. M. D. F. Udvardy, who supervised the study, provided guidance, and the facilities needed. For their advice and helpful criticism
I am indebted to Dean I. McT. Cowan and to Doctors J. R. Adams, J. F.
Bendell, P. G. Haddock, A. Kozak and O. Sziklai.
I wish to express my sincere thanks to the people of San
Juan Island for their friendship and sensitive understanding. It is impossible to thank all individuals by name; but I must express my particular indebtedness to the E. S. Wallings, the H. Rogers, the E. J.
Hudsons, the A. C. Collins, the M. M. Baves, C. Nash, A. Nash and
D. Mortel.
The study was financed by the National Research Council and the Canadian Audubon Society (Grants to Dr. M. D. F. Udvardy). Chapter I.
GENERAL INTRODUCTION 2
1. INTRODUCTION
In recent years much attention has been given to the bald eagle
(Haliaeetus leucocephalus. L.) by North American conservation groups in response to the decreasing numbers of the bird. Drastic reduction in
animal numbers had occurred before when man invaded and changed their
environment, but the case of the bald eagle is special for the bird was
chosen in 1782 as the United States' national symbol, thus the presence of the bird holds a sentimental value to the great majority of the North
American population.
The number of bald eagles has been on the decline (Howell,
1949). In the early part of the century bald eagles were systematically
eliminated from the Alaskan coast by a bounty system. The bounty law was
enacted in 1917 by the Territorial Legislature after the investigations of the
Alaskan fish commission resulted in concluding that the bald eagle is one of
the strongest contributing causes in the destruction of salmon during the
spawning season (Van Kammen, 1916). Prominent biologists of the time had
no objections to this system in the belief that eagles were so numerous and
inhabited so inaccessible an area that a serious reduction in their numbers
could not occur (Brooks, 1922, Willet, 1927). Later it became apparent that
if the bounty law was continued a serious reduction in eagle numbers was
possible. The bounty law was repealed in 1953 after studies revealed that
the damage done to spawning fish populations, formerly attributed to eagles,
was not of consequence. During the time the bounty law was in effect an 3
estimated 100,000 eagles were killed (Imler and Kalmbach, 1955).
Presently the bald eagle is protected throughout the United States. A bill entitled "Bald Eagle Protection Act" was signed by the president and became law in 1940. In 1.959. Alaska also came under this law, Imler and Kalmbach (op, cit.) believed that by the termination of the bounty law the bald eagle could be expected to return to normal numbers. Recently
Sprunt and Cunningham (1961) reported that the largest population of bald eagles is in Alaska.
Due to the controversial economic status of the bald eagle, many investigations of its feeding habits have been undertaken. Literature concerning this phase of the bird's biology is voluminous, while other aspects of its biology have been neglected.
The present study was started in 1962 to investigate the breeding behavior and feeding habits of the bald eagle on San Juan Island,
Washington. The primary aim of the study was to acquire information which would relate to the general decline of the bald eagle populations.
The deciding factor in choosing San Juan Island as the place of study was the presence of several breeding pairs of eagles in a readily accessible area, since it was important that the study area could be visited often during the winter and the spring when the early phases of the bald eagles' breeding biology could be observed, 4
2. ENVIRONMENT
The San Juan Island group is located at the intersection of the
123rd meridian of west longitude with the north latitude line of 48° 30'.
The group is composed of several islands of various sizes of which San Juan
Island is the second largest with an area of 35,448 acres (143.5 sq. km)
(Mapl).
2.1 Geography and Geology
The nucleus of the island group is formed by Orcas, Lopez and San Juan Islands. These large islands are bounded on the west by Haro
Strait and Boundary Pass which serve as the international boundary between
Canada and the United States. The Strait of Juan de Fuca forms the
southern boundary, while Rosario Strait separates the islands from the mainland.
The complex system of channels and harbors of Puget Sound and Washington Sound are supplied with ocean water through the Strait of
Juan de Fuca. Also the larger part of the water that flows in and out of the
Gulf of Georgia is derived from the south, therefore it must pass through the channels of the San Juan Islands. Consequently, very heavy tides are produced in many of the channels.
The islands usually have irregular shorelines, deeply indented by narrow fjord-like harbors. The intervening channels that are commonly narrow and U-shaped due to glacial erosion, often attain depths of 600 feet or more. Map 1 . The San. Juan Island Group 5
According.to McLellan (1927), the San Juan Islands are
composed chiefly of Paleozoic and Mezozoic sedimentary rocks. The islands were completely overridden with glaciers and even the highest peaks are striated and polished by the glacial action. Considerable areas on many of the islands are covered with glacial drift.
San Juan Island (Map 2), is 14 1/2 miles long and 6 1/2 miles
across at its widest point. The long dimension of the island points in a northwesterly direction, while the southern end projects out as a long, narrow arm, culminating in Cattle Point. Excepting the southeast portion, the shores are elevated and rocky. The southwest shoreline is broken only by one large bay, known as False Bay, which is so shallow that the greater part of it is bare at low tide.
At the northwest portion of the island, fjord-like bays known
as Mitchell, Garrison and Wescott Bays break up the shoreline.
The northeast shore is indented by Rocky Bay, whose
shorelines make a 90 degree angle with each other. The innermost part of the bay is shallow but farther out its depth is greater than 100 feet, except in the immediate vicinity of O'Neil Island.
Near the middle of the southwest side of the island, the shores
are formed by the rugged slopes of Mount Dallas, which with an elevation
of 1036 feet, is the highest point on the island. To the northeast of the Mt.
Dallas Range there is a large drift-covered valley, San Juan Valley, which
is noted as good agricultural land. Map 2. San Juan Island 6
2.2. Climate
2.2.1. Precipitation
The San Juan Islands are located within a dry-belt, that is, in a rain shadow of mountains on the Olympic Peninsula and on Vancouver
Island. Along the west coast of Vancouver Island the annual rainfall is
about 150 inches, while in the vicinity of Victoria it drops to only 22 inches.
According to R. D. McLellan (1927), Friday Harbor has an approximate
annual rainfall of 25 inches (635 mm). Records kept by F. Dearborn at
Sunset Point, at the west side of San Juan Island, show an average of 16.83 inches (427 mm) of rainfall in the period from 1958 to 1963. Further
eastward the amount of rain increases again until the maximum is reached
on the western slopes of the Cascade Mountains. The annual rainfall
occurring on Mount Constitution Range on Orcas Island, and on other mountain ranges of the region is considerably greater than that occurring
on the areas of low relief.
The rainfall on the San Juan Islands is largely confined to
fall and winter months, although occasional rain does occur at any time
of the year. The monthly rainfall on San Juan Islands for the years 1962
and 1963 is tabulated in Appendix I.
2.2.2. Temperature
The average yearly temperature on San Juan Island is about
52° F (11 .1°C.). The coldest month of the year is January with an
average temperature close to freezing point; the warmest month is August 7
with, an average temperature over 60° F. Temperatures start to rise above the yearly average in mid-April and drop below it in late October. The warming up process in the spring is generally a slower process than the cooling off in the fall. The extremes for the years 1962 and 1963 were registered on January 11, 1963, when the temperature dropped to 15° F. and on May 20, 1963, when temperatures rose to 90° F. The difference between daytime high and nighttime low seldom exceeds 20° F. Values for the maximum, minimum and average monthly temperatures for 1962 and
1963 are presented in Appendix II. Figure 1 shows the relationship between precipitation and temperature throughout the year, based on the weather data of 1962 and 1963.
2.2.3. Winds
During the winter months the region is swept by strong wind storms. Strong winds from the south and southwest are usually present throughout the. year . The trees on the west side of the island show deformation due to this constant wind effect. The re st of the island is relatively calm due to the wind breaking effect of the Mount Dallas Range on the west side of the island.
2.3. Vegetation
About 70 percent of San Juan Island is covered with trees, predominately conifer s . The primary species are Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), Hemlock (Tsuga heterophylla Raf. Sarg.), Temper atur e Precipitation F°| C° cm I in
Fig. 1 Average monthly temperatures and total monthly precipitation on San Juan Island based on data collected by F. Dearborn in 1962 and 1963. 9
Western Red Cedar (Thuja plicata, Donn) and Shore Pine (Pinus contorta,
Dougl.). Most of the tree-covered parts are situated on the north and west
side of the island. This situation is noticeable on all the islands of the group where the soil is shallow and sandy. This phenomenon has been ascribed partly to the relative conservation of moisture on the northern or
shady side and partly to the fact that the most prevalent wind storms come from the south and uproot the vegetation on the southern slopes, especially when the soil is sandy (McLellan, 1927).
Madrona (Arbutus m enzi e sii, Pursh) is abundant all through the islands, mainly on the western side. Garry Oak (Quercus garryana,
Dougl.) is found in stands along Wescott and Garrison Bays and on the side of Mount Young. Scattered trees are found on the southeast side as well.
San Juan Valley, in the center of the island, is treeless and was probably
cleared by the early settlers. Toward the south, trees become scarcer and large open spaces are found with rocky outcrops and grass. Most of the coniferous stands are second growth as early logging activities and fire have removed the mature stands. Remnants of huge trees are found in the northern parts. Turn Island, separated from San Juan Island only by a narrow passage, preserved a small sample of mature stands, and Douglas fir trees of 6 feet in d.b.h. are still found.
2.4. Fauna
2.4.1. Mammals
A. list of mammals present on the islands is not available, 10
however it is thought that the majority of mammals found in mainland
Washington are also present on the San Juan Islands. The following species of mammals were sighted during the years of the study.
One of the most abundant species on the island is the introduced rabbit (Oryctolagus cunicuius L.). Rabbits are present on all the larger islands, but their numbers on San Juan Island are exceptionally high. These animals were raised commercially some 30 years ago, but great numbers were released when the market for them declined.
The Blacktailed deer, (Odocoileus hemionus columbianus,
Richardson) native to the islands, is fairly abundant on some of the larger ones. The animals are very tame in spite of the pressure which is exerted on them during the hunting season and in spite of the year-round shooting for rabbits.
The Red Fox (Vulpes fulva, Desmarest) was introduced to San
Juan Island to combat the situation created by the rabbits. Foxes are often seen; however, the expected increase in their numbers did not occur, consequently their introduction as a control measure was not successful.
Racoon (Procyon lotor L.)is very abundant and is a native to the island. Mink (Mustela vison, Schreber), Muskrat (Ondathra zibethica L.),
Shrews (Sorex sp.), Mice (Peromyscus) and Bats (Myotis) were also sighted often. Killer Whale (Grampus rectipinna, Cope) Harbor Porpoise (Phocaena vomerina, Gill), Harbor Seal (Phoca vitulina, Gray) and Otter (Lutra canadensis, Schreber) are also abundant invthe waters around the islands. 11
2.4.2. Birds
A publication by Miller et al. (1935) gives an annotated list of the.bird life of San Juan Islands. Later publications by Goodge and
McMannama (195:0) and Retfalvi (1963) record species that are not present in Miller's earlier publication. Altogether, 191 species of birds have been recorded for the San Juan Island group. A list of the bird-records is presented in Appendix II. 12
3. STUDY METHODS
3.1. Observations from the Ground
Since bald eagle nests are usually built on tall trees, often
above the canopy layer of the surrounding stand (see section 5 ), it is
difficult to obtain a clear view. It is of prime importance to obtain a clear
view of the nest in order to observe the activities of the young and the adult
eagles. In the one previous major study of the bald eagle, a tower was
employed which was constructed to the same height as the nest, thus a view .
into the nest was achieved (Herrick, 1924). At the start of this project
such a technique could not be employed as the birds were well advanced in
their breeding cycle. Any attempt to construct a tower would have
endangered the young's survival, and as a result, would delay the rest of
the study. An added factor was the cost involved in the construction of such
a tower. An attempt at the beginning of 1963 to build an observation platform
on a tall tree close to the nest had to be abandoned when the tree was found
to be unsafe.
The only alternative was to select suitable high ground in the
area that offered a view of the nest, and bridge the distance with a high-
powered telescope. During the summer of 1962, A Nest on Turn Island was
kept under observation. The vantage point was set on the other side of the
'channel which separates Turn Island from San Juan Island. The distance
between the vantage point and the nest was in excess of 700 yards. To
observe the nest a Bausch & Lomb telescope was used with a 3QX power 13
eyepiece. The view obtained was good, however, the nest was approximately
80 feet higher than the observation point, thus a clear view into the interior of the nest could not be achieved. This difficulty hindered the observations of the young in their early ages and prevented identification of food remains scattered in the nest, Photographs were taken through the telescope employing a single lens reflex camera. A Bushnell Teleadapter was used for coupling the camera to the telescope. The quality of the pictures obtained is only mediocre due to the low resolving power of the telescope and the great amount of light lost with the coupling of the two pieces of equipment.
In the summer of 1963, Nest B at Rocky Bay was observed.
The distance between nest and vantage point was reduced to 600 yards, however the difference between elevations could not be eliminated.
3.2. Aerial Surveys
Several surveys by airplane were conducted during the study.
At the start of the project, the area was surveyed in order to locate nesting pairs of eagles. Aerial observations were also necessary during the spring when activities in the nest could not be readily observed from the ground.
Later in the summer, flights were made to assess the situations at nests other than those under continuous observation. These airplane flights also served as a means for counting the eagles present in the area. Outside of the birds' breeding season, especially during the winter, the presence of the birds could not be ascertained by a tally made at their nesting sites. 14
The planes used were small, single-engined, and capable of flying at speeds down to 60 miles per hour when passing by the nest.
3.3. Time Spent in the Area
The first aerial survey in search of eagle nests was done by
Dr. M. D. F, Udvardy on March 16, 1962. After a brief ground survey on
April 28, 1962, continuous observations were started on the 2nd day of May and continued until September 4. Several weekend trips were made during the following winter and spring; the observation period in the summer of
1963 lasted from May 8 to August 29. Weekend trips were made again during the winter of 1963 and the last visit was made on April 15, 1964.
Table 1 shows in detail the time spent in the study area during the period from 1962 to 1964. Table 1 . Date and duration of observations on San Juan Island, 1962 to 1964
Year: Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.
1962 16 28-29 2 --- 4
1963 19-20 23-24 13-14 13-14 8 ... 29 21 19-20 21-27 23-24 21
1964 23-25 13-15 Chapter II.
ABUNDANCE AND MOVEMENTS 17
4. INTRODUCTION
The bald eagle (Haliaeetus leucocephalus L.) inhabits the
North American continent; it is confined to it with the exception of birds found on the northeast coast of Siberia and on adjacent islands.
The American Ornithologists' Union (1957) classifies the bald eagle into two geographic subspecies:
H.l. alascanus Townsend, which ranges from northern Alaska and northern McKenzie to northern
Ungava and south to British Columbia and the northern United States, and
H_.l_. . leucocephalus L. , which ranges south across the United States, south to Baja California. The ranges of the two races broadly overlap across the mid section of the United
States.
According to this pattern, the bald eagles of San Juan Island belong to the northern race, contrary to the belief of Miller et al. (1935), who regarded them as southern bald eagles.
There is a difference in size between the northern and southern subspecies, the northern bald eagle being larger than the southern race. The difference between sizes served as a basis to divide the species into two races. Peters (1931) and Friedmann (1950) prefer the name
H.l. washingtoniensis Audubon for the northern subspecies since birds belonging to this race on the basis of their size are found not only in Alaska but also in northern United States. 18
5. ABUNDANCE
The bald eagle is most abundant on the seacoast and by large bodies of fresh water. Concentrations of bald eagle numbers are found on the southeastern coast of Alaska, south on the coast of British Columbia, in the Great Lakes Area and also in isolated points along the Atlantic Coast, such as Chesapeake Bay and Florida (Imler and Kalmbach, 1955). Winter movements also concentrate their numbers in areas along the Mississippi
River (Musselman, 1949 and Southern, 1963) and other large rivers in
Missouri and Iowa (Fawks, 1961) and westward in Oklahoma (Van den Akker,
1954 and Holloran,. I960).
A relatively large number of eagles is present on San Juan
Island during the spring and summer, while outside the breeding season, eagles are scarce on the island. San Juan Island was used by eagles in the past; Miller et al. (1935) call them common residents and write that:
"Pairs have been seen near their nests on the east side of San Juan Island".
Eagles were collected from the island as early as 1858 by David Lyall and by James Hepburn in 1862 (in Miller et al. , 1935).
5.1. Eyries
The number of nests found on San Juan Island gives an indication of the number of bald eagles present as well as their probable abundance in the past.
Bald eagles usually build their nests on the seashore (Broley,
1947). The nests are built of sticks as large as 3 inches in diameter and up 19 to 5 to 6 feet in. length (Oberholser, 1906 and Herrick, 1924b). Smaller sticks, twigs, grass and down are used for the lining of the nest. The size of the nest varies from 3 to 4 feet in diameter and one to two feet deep to
8 1 /2 feet in diameter and 12 feet deep (Herrick11924c). Nests are used year after year by the same pair of eagles. Large nests are the result of the bird's yearly nest building activities as new layers of sticks are added to the old nest (Herrick, 1932 and Broley, 1947). If the old nest is destroyed the birds will build a new one in the same area. Herrick (1924c) records a nest which was used for 35 consecutive years. The nests are built on tall trees and usually are located near the top. The height of nests from the ground varies from 15 feet (Broley, 1947) to 137 feet (Imler and
Kalmbach, 1955) in the eagles' extensive range. Murie (1959) found bald eagles nesting on rocky cliffs and pinacles on Kodiak Island, Alaska. Eagles may have several nests which they use in alternate nesting seasons (Amadon,
1964). Howell (1937) reported a pair of eagles which built five nests in eight years.
The bald eagle nests on San Juan Island were found by means of aerial surveys on March 16 and May 6, 1962, and by obtaining information from local residents. The location of nests is indicated on Map 2, following page 5 . For easier reference the nests were named with the consecutive letters of the alphabet in the chronological order of their discovery, thus the nest on Turn Island is called Nest A, as this nest was first found. 20
5.1.1. Description of Nests
Nest A;
is located on Turn Island, south of the Town of Friday Harbor.
It is built on a Douglas fir tree, the second tallest tree on the island, approximately 130 feet from the ground. The tip of the tree, the portion from the nest to the top and approximately 15 feet, is dead. The nest is a huge structure; its estimated dimensions are 6 to 7 feet in depth and 3 1/2 to 4 feet in diameter.
The nest has a history of over half a century. An old resident of Friday Harbor, Mr. Jensen, recalls the shooting of eagles out of the nest for bounty around the turning of the century. The yearly nest-building activities, the adding of new layers of sticks, has resulted in a huge cylindrical structure.
The nest, being located in the center of Turn Island, provides an open view toward San Juan Channel, as well as the small passage between
San Juan and Turn islands.
Nest B;
is located on the south shore of Rocky Bay, 6 miles north of
Friday Harbor. The nest tree is a Douglas fir, 100 feetin height and 4 feet in diameter at breast height. The tree is among the largest of that area and it leans out toward the water. The nest, that is built 10 feet from the dead tree top, is almost concealed within the surrounding canopies. 21
The nest, about 5 feet in diameter and two feet deep is a platform with a shallow depression in the center. Judged by its size this nest appears to be a more recent structure than Nest A. There is a slight elevation behind the nest tree, therefore the view toward the center of the island is blocked, although toward the sea there is a 180 degree unobstructed view of open water.
According to information from local sources, there used to be a large nest at Limestone Point, 1.5 miles north of Nest B. This nest was destroyed several years ago when the nest tree was cut down; it is likely that Nest B was built afterward by the inhabitants of the destroyed nest.
Nest C;
is located in the southeast part of the island, at the foothills of Mount Dallas. It is 1600 yards from the seashore in the center of a second growth stand. The surrounding area was logged 15 years ago and now the dense vegetation of alder makes the area hard to penetrate. The nest tree is a 120 feet tall Douglas fir, that is forked at about 7-5 feet from the ground and the nest, 4 to 5 feet in diameter and 2 feet deep, is located at this point. The nest is well concealed and invisible from the roads that run close by. The location of this nest was not known to local residents until its discovery from the airplane on May 6, 1962.
Nest D;
is located at Point Caution, two miles north of Friday Harbor. 22
The nest tree, 250 yards from the shore, is the tallest dominant tree in the surrounding mature Douglas fir stand, approximately 120 feet tall. The nest is built 30 feet from the top, where the tree forms a fork by the rise of a thick side branch. The nest is small, 4 feet in diameter and about 1 1/2 feet in depth, and is difficult to spot from the sea as it blends into the rising hill behind. The view from the nest is unobstructed toward San Juan
Channel but it is blocked toward the center of the island.
Nest E;
is located only 100 yards from Nest C in the same stand. The nest tree is smaller and is also forked, housing the nest similarly to Nest
C. The nest is in good condition and probably is used as an alternate nest by the Nest C pair of eagles.
Nest F;
is a small nest on the west side of Henry Island, west of
Roche Harbor. It is seen only from airplanes and it appears to be abandoned and in bad condition.
Nests G, H, I_;
are located in the northern shore of Spieden Island, 3 miles north of Roche Harbor . All three nests appear abandoned. Spieden Island is 2.8 miles long and is narrow. The nests are evenly spaced on the northern wooded side. The nests likely were used by the same pair of eagles. 23
According to information from local sources, a former sheep herder on the island shot every eagle over a period of several years. This fact may be the cause of the present abandonment of the nests.
NestJ;
is half finished and is situated on the west side of San Juan
Island. The nest tree is an old, irregularly grown, 80 feet tall Douglas fir and the nest is placed close to its top among branches forming a bowl.
The tree is on the shore of a small bay, facing the open waters of Haro
Strait. The nest is easily spotted from a road which runs only 150 yards away. The eagles, likely the Nest C pair, started to construct this nest in the summer of 1962.
Nest K;
is located at the tip of a small peninsula between False and
Kanaka bays. The nest tree is a small Douglas fir, 80 feet in height and the nest, 4 feet in diameter and 2 feet deep, is situated 10 feet from the top, completely surrounded by dense foliage. The nest provides a view over False Bay as well as over the open waters of Haro Strait, but the view toward the south is blocked by trees.
Nest L;
approximately 4 feet in diameter and 3 feet deep, is situated
20 feet from the top of a 100 feet tall Douglas fir. (These figures are only approximations since the nest could be viewed only by means of flying.) The 24
nest tree is located on Henry Island, 180 yards from the western shore of that island. The view toward the water is blocked by trees.
Nest M;
is located on the shore of False Bay, 800 yards from Nest K to the north. In structure and size this nest is similar to Nest K. The nest provides an open view toward False Bay, the view toward the center of the island is blocked by the surrounding stand. On the basis of this nest's proximity to Nest K , it is assumed that both nests are used by the same pair of eagles.
The chronology of the nests' discovery on San Juan Island is as follows:
March 16, 1962: Nests A and B_,
May .6, 1962: Nests C,D,E,F,G,H and'T,
June 6, 1962: Nest J,
February 7, 1963: Nests K and L,
April 14, 1964: Nest M.
The tabulation of the data concerning measurements of nests and nest trees and their situation to shore and human habitations is presented in Table 2.
Activities of eagles were observed at Nests A, B, C, D_and J_ during 1962 and at Nests_A, B, D, J_, K and L during 1963. Eagles were seen at NestsJB, D, and M in the spring of 1964. Nests E, F_, _G, H and I were not used by eagles during the observation period. Table 2. Measurements of nests and nest trees and their proximity to shore and human habitations
NEST TREE NEST
Distance Built View Di stance from human from tree from Success Height Dbh from shore structure Diameter* Depth* top nest in Nest (feet )* (feet)* (yards) (yards) (feet) (feet) (feet) (degrees) 1962 1963
A 145 6 100 700 4-5 6-7 15 360 + B 100 4 0 500 5 2 10 180 + + C 120 4 1600 1000 4-5 2 45 0 - D 120 - 250 500 4 1 . 5 30 270 + + E 100 - 1500 1000 3-4 2 50 0 - F • - - 50 2000 - • - - - - G - - 100 500 - - - 180 - H - - 100 500 - - - 180 - I - - 100 1000 - - - 180 - J 80 - 0 100 - - 10 270 " - K 80 - 0 650 4 2 10 180 ? + L 100 - 180 2000 4 3 20 0 ? + M 80 - 0 600 4 2 10 180
* Estimated values Note: Nests F, G, H and I were seen only from airplane. 26
All bald eagle nests found on San Juan Island are found on
Douglas fir trees, usually the tallest tree of the surrounding forest stand.
In choosing the nest site the eagles tend.to seek spots which provide an open view of the neighbouring areas. In general, altitude is sought by the birds.
The mean height of nine trees housing bald eagle nests on San Juan Island is 102.7 feet (31.3 m). However, in other areas where tall vegetation is lacking, the eagles may build nests close to the ground (Broley, 1947).
Eagles also tend to seek areas undisturbed by human activities. The
shortest distance between a successful nest and human habitation on San
Juan Island is 500 yards (450 m), as in case of Nests B and D. The success of breeding activities and the nests' situation to human habitations is
discussed later (see section 11).
Many of the eagle nests are built on dead or dying trees. On
San Juan Island Nests A and B are built on trees with dead tops. On the basis of size, these nests are considered to be the oldest structures among the nests found on the island. Since eagles seek nest sites which provide a wide view around, it is possible that the top of trees housing Nest A and B were dead at the time when the nests were built. However, it is more
likely that the tree tops died off as a result of the continuous usage of the nests. Since branches above the nest are used by the adults as well as the fledging eagles for perching, the constant grappling of these branches may
have damaged the bark, which resulted in the die off. 27
Height of tree being an important factor in the eagle's selection of nest sites, Douglas fir and hemlock are species likely chosen by the eagles in the Pacific Northwest. Since these two species of trees are primarily important in the region's forest industry, eagles may exert a
slightly harmful effect on man's commercial interest by contributing to the mortality of tall trees through their nest building. However, considering the small number of trees involved, this damage is negligible. Also, tall trees are sought by the birds only in suitable nesting areas, which areas usually lie along the seacoast or by some large body of water where trees tend to grow deformed and crooked as a result of constant wind effect and growing on steep hillsides. The eagles tend to choose trees which fork and grow irregularly. These types of trees are good nesting sites yet they are commercially less valuable.
5.1.2. Territories
Territorial behavior was seldom observed among bald eagles on San Juan Island. Breeding eagles often warded off red-tailed hawks, crows and juvenile bald eagles which were intruding in the vicinity of the nests. Hostility between adult bald.eagles was observed only on two occasions (see section 8.5.1.). Due to the limited number of observations, territory size as an area defended against competing members of the same
species cannot be discussed on the basis of territorial behavior.
Territory size may be calculated on the basis of distance between neighbouring nests. These territories, however, cannot be 28
regarded as defended areas but rather as areas which are available to the nesting pairs without interference from the closest neighbours.
The linear distances between neighbouring nests are tabulated in Table 3. With the exception of two nests, all the nests on San
Juan Island are located on or in close proximity to the shore. The linear distances between 13 neighbouring nests vary between 2.4 miles (3.86 km) and 5.7 miles (9.16 km) with a me an value of 3.87 miles (6.2 km). This means that an area of 11.7 sq. mi (30^28 sq. km) surrounding the nest site is available to each of the existing nests on San Juan Island. These
"available areas" are drawn on Figure 2 to illustrate their spatial relationship with each other. Information available in literature concerning territoriality of bald eagles is scarce. Broley (1947) writes that: "A pair of nesting eagles appears to require, as a rule, a territory extending about half a mile on all sides of the nest", but provides no information on how he arrived at this estimate. According to this estimate eagle nests are spaced oneimile apart in suitable areas. However, greater densities of nests were also found. Broley (op. cit.) saw three active nests within 1000 feet of one another in Florida and Nicholson (in Bent, 1937) found a concentration of 7 active bald eagle nests in an area of 3 1/2 by 3/4 miles also in Florida. Hensel and Troyer (1964) found that 14 territories in Kodiak
Island, Alaska averaged 57 acres (23.1 hectares). They defined territory as an area defended against other bald eagles during the breeding season and measured it by using the linear distance between the nesting site and Table 3. Linear distances between neighbouring bald eagle nests* on San Juan Island
Between nests Distance (miles)
A - D 3.2 D - B 4.0 B - I 3.0 B - L 4.0 L - G 3.2 C - M 2.4 minimum C - D 5.5 M - A 5.7 maximum
* Groups of nests like Nests C, E, J and Nests G, H, I and Nests K, M were considered to belong to the same pair of eagles. 30
perch trees as a radius.
The spacing of nests on San Juan Island permits much larger
"available areas" than the territory sizes estimated or measured by the mentioned investigators. Since most of the San Juan Island nests are located on the shore, the "available areas" of the different nests include the greater part of the island coastline within their boundaries. The nests are relatively evenly spaced along the coastline. However, no eagle nests were found on the west coast of the island between Nests IJ andJM, yet the linear distance between these is over 7 miles (11 km). Also, the Cattle Point area is outside the "available areas" of Nests A and M and is uninhabited by humans, yet it was not used by eagles for breeding purposes during
1962 and 1963, nor were remnants of old nests found in the area.
The "available area" is not the area defended against competing members of the same species, consequently the term "territory" cannot be applied to it. Its size is based on the measured distances between neighbouring nests and it is more likely that human interference has a greater effect on the spacing of the nests than does the interacting between the eagles.
In Figure 2b the locations of the known bald eagle nests are shown in relation to human habitations. To obtain a map on distribution of human dwellings the island was divided into 1 km by 1 km squares and the number of houses counted in each square. Areas with less than three houses per square kilometers are considered to be suitable eagle-nesting Fig. 2a. The location of bald eagle nests on San Juan Island and the "available areas" belonging to each nest. Fig. 2b. The location of bald eagle nests on San Juan Island in relation to human habitations. 33
areas since the position of two houses in an area of 1 sq. km permits a nest to be 500 yards (450 meters) away (which is the minimum distance found between existing nests and human habitations (see section 5.1.1.). All successful nests on San Juan Island are found in areas with densities of 2 houses or less per square kilometer, which suggests that the existing pattern of nest distribution on the island is a result of human interference.
5.2. Observations
It was not of primary importance in the study to follow the changes in bald eagle numbers during the year, therefore no systematic surveys were conducted. However, regular counts were made by means of aerial and ground surveys during the spring and fall months when the area was visited to obtain information on breeding activities. Also, sightings of eagles were recorded throughout the summers of 1962 and 1963.
The counts made by means of aerial and ground surveys are presented in
Table 4, while Table 5 presents the occasional sightings of eagles during
1962 and 1963. The figures presented in these tables do not include unfledged eagles.
The counts of eagles made by means of aircraft give a better approximation of the actual number of eagles present on the island than do the ground surveys. The coastline, which tends to attract eagles, can be surveyed by means of airplanes, while the ground survey is limited to the roads. Also, alarger area along the flight path is visible from the airplane; a narrower area is visible from the roads, especially in the northern part of Table 4. Bald eagle counts made during 1962 - 1964 on ground and by- air on San Juan Island.
Date Numbers seen Type of survey Year Month Day Adult Juvenile Total
1962 March 16 aerial 13* 2 15 1962 May .6 aerial 9 11 20 1963 Jan. 20 ground 2 - 2 1963 Feb. 7 aerial 21 11 32 1963 Feb. 23 ground 9 2 11 1963 March 13 ground 4 2 6 1963 Mar ch 14 ground 5 3 8 1963 March 23 ground 4 4 8 1963 April 21 aerial 8 1 9 1963 May 21 aerial 2 3 5 1963 Sept. 21 aerial 3 1 4 1963 Oct. 20 ground 2 1 3 1963 Dec. 27 ground 5 3 8 1964 Jan. 2 ground 4 - • 4 1964 Feb. 24 ground 7 14 21 1964 Feb. 25 ground 6 6 12 1964 April 14 aerial 8 3 11 1964 April 16 aerial 3 1 4
5*. Underlined numbers are the highest numbers of birds seen in any particular month and were used to construct Figure 5. Table 5. Occasional sightings of bald eagles on San Juan Island during 1962 and 1963
Date Numbers seen
Year Month Day Adult Juvenile Total
1962 May- 15 7 3 10 1962 May 26 6 2 8 1962 June 3 6 2 8 1962 July 27 2 4 6 1962 Aug. 6 2 - 2 1962 Aug. 23 2 2 4 1962 Sept. 1 - 2 2 1963 May 26 10* 4 14 1963 June 6 10 1 11 1963 June 27 3 - 3 1963 July 1 8 3 11 1963 July 7 3 2 5 1963 July 11 4 7 11 1963 July 16 2 - 2 1963 July 31 2 2 1963 Aug. 8 1 3 4 1963 Aug. 16 3 — 3
Underlined numbers are the highest numbers of birds seen in that particular month and were used to construct Figure 5. 36
the island where both sides of the road are cover ed with trees . Figure 3 provides a comparison between the areas covered in the count by means of aerial and ground surveys.
Both aerial and ground surveys tend to underestimate the number of eagles actually present on the island because of insufficient coverage and because many birds are missed due to the difficulties in spotting them.
The largest number of bald eagles were seen during February.
On February 7, 1963, 21 adult and 11 juvenile birds were recorded in an aerial count. On February 24, 1964, 7 adults and 14 juveniles were spotted during a ground survey. The number of birds in the count markedly decrease during March and April, however the figures for these months are very low in comparison to the reports by local farmers. A flock of 14 eagles (adults and juveniles in equal numbers) were seen by W. Sundstrom on his farm (1 .5 miles southeast of Friday Harbor) for several days during
April, 1964. Ten eagles and more were also seen by E. J. Hudson during the same period.
Twenty birds were counted on May 6, 1962, 9 adults and 11 juveniles. This was the only count in which the number of juveniles exceeded that of adults. During the summer months eagles numbered around the yearly average figure of 13 birds of more. Numbers markedly decreased again from August on. The lowest number of eagles, 2 adults and 1 juvenile was seen on October 20, 1963. No surveys were conducted during November 1962 and 1963, nor were sightings by local residents Fig. 3 The area covered by means of aerial and ground surveys on San Juan Island, 1962 - 1964. 38
available. On December 27, 1963, 5 adults and 3 juveniles were seen during a ground survey. A count in January 1962, conducted by local bird watchers recorded 48 eagle sightings over a two week period. In this count however, the same birds must have been recorded several times during the survey. The highest number recorded for a single day was 15 eagles (8 adults and 7 juveniles).
Due to the lack of sufficient data, the changes in eagle numbers in the consecutive years of 1962, 1963 and 1964 cannot be related to one another; however a comparison of the data obtained in these years is provided in Figure 4. Combining the data of Table 4 and Table 5, Figure
5 was constructed to indicate the seasonal fluctuations in bald eagle numbers.
The numbers of adult and juvenile bald eagles plotted in the figure are the highest numbers seen in each month during the period the data was collected. The curve representing the changes in numbers of adult bald eagles follows a smoother path than does that of the juvenile birds, consequently the month to month fluctuations in total numbers is caused by the fluctuations of juvenile eagle numbers to a greater degree than by that of the adults. The changes in eagle numbers shown in Figure 5 are only an approximation of the actual changes which take place during the year.
The sudden decline during March and April as shown in the figure is probably an exaggeration of the actual situation. Since the surveys were not conducted at regular intervals, the representative numbers of birds for each particular month is biased, due to the day to day fluctuations in eagle number s. 39
35-
962 3C- 1 • 1 963
196 4 25-
CO 20- V/. 1—I 1 cu 15 u 1 3 2 1C! 2 1 §1
tzil 21 .21 n M M I I O N D Months
Fig. 4 Number of bald eagles seen on San Juan Island in 1962, 1963 and 1964. 40
J FMAMJ J ASO N D Months
Fig. 5 The change in numbers of eagles throughout the year on San Juan Island. The highest number of adult and juvenile eagles seen in any month of the period 1962 - 1964. 41
It appears that more birds are present on the island during
February than during any other month of the year . The sightings of 21 adult eagles during this month indicate that the number of potential breeding birds is greater at the start of the breeding season than the eventual number that attempts raising a brood later. A count of 14 juvenile eagles in this month is also the highest figure for the year. This peak in eagle numbers appears to coincide with the lambing time of most of the island's sheep.
The sightings of 11 eagles on February 23, 1963, 21 eagles on February 24, and 12 eagles on February 25, 1964 were recorded on or near fields that contained sheep with newborn lambs. The relationship between eagles and sheep is discussed later (see sections 8.5.7. and 13.2.1.).
There is an apparent decrease in numbers during March and
April which is probably due to the increased hostility of breeding eagles toward the non breeding ones (see section 8.5.1.). During the breeding season the number of adults remained at 10 and less birds, while the number of juvenile birds fluctuated. On May 6, 1962, 11 juvenile birds were seen,
7 of them perching on the same tree on the shore of Trout Lake, while during June the highest number of juveniles seen was 4 birds. During July juveniles were more frequently sighted when the young of the year started flying. In this month adult birds which raised young during the breeding season were no longer found by their nest and were sighted only occasionally.
Golden eagles (Aquila crysaetos L.) and turkey vultures (Cathartes aura L.) were visiting the island during this period, which by the fact that they are easily confused with juvenile bald eagles, made the reports of local bird 42
watchers less reliable.
Eagle numbers start to diminish from September on, but the birds do not disappear from the area. Sightings of 5 and 3 birds were recorded for September and October respectively. During these months, eagles are virtually absent from the neighbouring small Canadian islands as well as from the west coast of Vancouver Island (D.Hancock - pers. comm.).
More eagles start to appear in December and the rise to the peak in eagle numbers in February is continuous throughout January.
The highest concentrations of non breeding eagles were usually located in the Trout Lake area. Unfortunately, the area seldom could be surveyed due to limited accessibility. The area was mainly used by juveniles, but single adults were also found among them. Fields used for
sheep grazing in the San Juan Valley also attracted eagles. Fields lying to the north of False Bay, towards the center of the island were most often visited by the birds. Large numbers were seen during the spring lambing
season in these vicinities although birds up to 10 in number were seen throughout the summer.
Figure 5 provides only an approximation of the numerical changes in the bald eagle population of San Juan Island. To obtain a better knowledge of the seasonal and yearly fluctuations in numbers of eagles, the conduction of a systematic aerial survey would be necessary. Such a
survey was beyond the means of the present study.
A comparison between the seasonal changes in eagle numbers
and the seasonal changes in precipitation and temperatures (Figure 6) 43
Temperature (F°)
Number s 35 J U 80
30 ^ 70
25
20 60
15 50
10
40
O N D Months Breeding r Season
Fig. 6 Comparison between the monthly changes of eagle numbers and the monthly changes in precipitation and in temperature .on San Juan Island. 44
reveals that the peak in eagle numbers occurs in the dry and cold spring.
Numbers of eagles start to decrease as temperatures rise over 60°F and reach the lowest point during the wet fall. This comparison suggests that eagles tend to avoid excessive heat and moisture.
5.3. Conclusions and Summary
Thirteen bald eagle nests were found on San Juan Island.
Breeding activities of the birds were observed at eight nests, the remaining
six nests were not used by the birds. Breeding activities were successfully completed at three nests in 1962 and at four nests in 1963.
The nests are built on tall Douglas fir trees close to the
seashore. The mean distance from the shore of five successful nests is 106 yards (96 m). The nests are built in areas which are not frequented by humans. The distance between successful nests and human habitations ranges between 500 and 2000 yards, with a mean value of 870 yards (790 m).
The nests are spaced 2 to 4 miles apart along the coast line.
The pattern of nest spacing is not considered as a result of the birds' territorial behavior since such behavior was not observed, but rather as the result of human interference.
The seasonal changes in bald eagle numbers can only be approximated because no systematic counts were conducted. Counts of eagles, however, are available for every month of the year with the exception of November. These counts were obtained by means of aerial and ground surveys conducted during the spring and by means of occasional 45
sightings of birds during the summer. The analysis of the available data reveals that the numbers of bald eagles on San Juan Island change throughout the year. A yearly average of 14 birds or less are present during the summer, while numbers more than twice the average are present during the early spring.
During the fall the numbers of eagles decrease to less than a third of the yearly average. Adult birds are present in numbers twice as large as the number of juveniles. The number of adult birds remains near constant during the breeding season while the number of juveniles fluctuates. San
Juan Island is a relatively small area in comparison to the large area in which non breeding eagles move about. Thus the fluctuation in juvenile numbers may be the result of a sampling bias occasioned by the fact that the area of San Juan Island is not an adequate sample size to assess the abundance of a wide ranging species. In contrast, breeding adults are always found in the vicinity of their nesting site, thus their numbers remain relatively constant in the counts. 46
6. MOVEMENTS
A definite migratory pattern has not been observed, nevertheless, the band returns from juvenile bald eagles banded in Florida proved that the young, after leaving the nest in the fall, travel great distances (Broley, 1947).
The disappearance of the adult bald eagles from the nesting areas during this time of the year also suggests a trend to move away from the breeding ground for a certain length of time.
A similar situation was observed with the bald eagles on San
Juan Island (see Figure 5) and with the large population of bald eagles on the west coast of Vancouver Island (D. Hancock, pers. comm.). The near total disappearance of juveniles from San Juan Island in the fall, as indicated by the counts, supports this trend. But the presence of adult birds in the
September and October, 1963 counts, indicates that some birds do not leave the area.
The movements of local eagles outside of the study area could not be followed because of lack of marking; their movements, therefore, can only be assumed. The sharp decline in eagle numbers occurs before temperatures reach the yearly maximum in mid August, thus indicating that the movement of eagles is away from heat, possibly in a northward direction. The movements of the Florida bald eagles is similar in trend
(Broley, 1947). A southward movement of northern bald eagles is reported to take place in the central part of the continent during the second half of
December (Sprunt and Cunningham, 1961), however, band returns from 47
eagles banded in Alaska do not indicate such movement (Robbins, I960).
Presuming that the San Juan Island birds move northward; eagles seen on
San Juan Island during the fall may be birds of southern origin. The
sightings of adult eagles in September were of single birds not in the
vicinity of nests; however, the birds seen perched on Nest B tree in
October were positively identified as the resident adult pair of the preceding summer. Also, the resident pair of Nest A was seen perched
at their nest site in January and December, 1963. On the basis of these
sightings, it is believed that the adult breeding eagles of San Juan Island
do not leave the area. However, they may be absent in November, for which month no observations are available.
Bald eagles are noted for their strong site tenacity.
Encounter s between breeding adults and juvenile eagles (see section 8.5.1.)
are usually between parents and offspring of the previous year who return
to their place of birth (Bent, 1937 and Broley, 1947). These encounters
were most often observed during May, when the number of juveniles
present on the island is at a high level. It is probable that the young return
to the nest for several years after first leaving it. Juvenile eagles in
plumages which showed much white, characteristic of two to three year old
birds, were also seen in conflict with the breeding adults of Nest B several
times in May, 1963.
The number of juveniles fluctuates during the summer while
that of the adults is more constant. Non breeding adults are seldom seen
on the island (see section 8.5.1.). The presence of an abundant food supply 48
seemingly acts to motivate the movements of non breeding birds during the
summer .
The daily movements of breeding birds are confined to the general area of the nests. The birds spend most of their time perched on a tree close to the nest (see section 8.3) as was the case with the birds of
Nests A, B and D. The daily sightings of adult eagles perched on the tip of a peninsula between Mitchell and Garrison Bays during the summer of
1963 indicate that the breeding birds of Nest L on Henry Island use this
spot for a vantage point. Nest L is built on the west side of Henry Island, which side of the island is facing the open waters of Haro Strait where fishing is difficult, while their vantage point, more than 2 miles away, overlooks Mosquito Pass, a narrow and shallow passage where the catching of fish is easier .
The movements of breeding birds in search of rabbit carrion in the interior of the island could not be followed due to the difficulty of individual identification. Chapter III.
BREEDING 50
7. INTRODUCTION
Since Oberholser1 s (1906) account of the bald eagle's economic relations, information has appeared extensively on the pages of various
scientific journals. Most of the earlier papers deal with two aspects of the bird's biology: its nesting habits in the description of the "eyries", and its feeding habits. The validity of such information gathered from various
sources is often questionable (e. g. Dixon, 1909, Hoxie, 1910). Many of the former observers were egg collectors, consequently measurements and
descriptions of eggs recorded are accurate and abundant. On the other hand, information on breeding behavior, which requires lengthy observations, is
scarce.
Herrick's (1924, 1932, 1933) studies in Ohio provide abundant information on the everyday life throughout the bald eagle's breeding season, while the banding efforts of Broley (1947) supply information on the movements of the birds outside the breeding season. The majority of our present knowledge on the habits of the bald eagle is based on the works of these two investigators.
A common belief is that bald eagles mate for life and use the
same nest year after year. On the loss of the mate, the bird promptly
acquires a new one (Herrick, 1932), The characteristic white head and tail are attained after the fourth year in the bird's life, but sexual maturity may occur sooner; consequently, it is possible to have a bird in adult plumage mated with a bird in juvenile plumage. Hoxie (1910) records a pair 51
of eagles both in juvenile plumage nesting successfully in Georgia.
The clutch size of the bald eagle may vary from 1 to 3 eggs; two being the most common number. The incubation of the eggs takes 34 to 35 days (Herrick, 1932, Nicholson, 1952). Egg-laying dates vary from the north to the south in the bald eagle's extensive range. Bent (1937) provides dates for egg-laying throughout the continent (Table 6.).
A well-documented phase of the bald eagle's biology,.due to the egg collectors' experiences, is the laying of a second set of eggs if the first set is removed (Bent, 1937). However, these second attempts of the
birds are seldom successful in the northern part of the eagle's breeding
r ange.
After hatching, the young spend 10 to 13 weeks in the nest
(Herrick, 1932). Family ties between parents and offsprings are still
apparent two to three weeks after the young leave the nest, after which
time the young are entirely independent. The movements of-the adults
outside the breeding season are limited to within 100 to 200 miles of their
nesting site. The young, however, cover extensive ranges, often in excess
of 1000 miles. Broley (1947) has shown that young bald eagles banded in
Florida travelled northward and many of them were recovered as far as
Ontario and Nova Scotia.
The eagles, juveniles and adults, reappear on the breeding
ranges two to three months before the laying of eggs. Young of the
previous year are driven from the vicinity of the nest by the parents.
Territorial fights between adults of different pairs are seldom observed. Table 6. Egg-laying dates for the bald eagle in various parts of its breeding range (after Bent, 1937).
Number of Mean egg-laying Breeding Area records date
Alaska and Arctic May 10 America 62 March 6 Oregon and Mexico 40
Georgia, Florida January 2 and Texas 62 New Jersey and Virginia 7 5 March 3
Maine and Michigan 6 April 10 8. GENERAL BEHAVIOR
Since the majority of the observations were carried out
during the breeding season, both in 1962 and 1963, observed behavior not necessarily connected with breeding activities is also included in this chapter.
8.1. The Adults
Among the diurnal birds of prey, the female bird is
considerably larger than the male. In the case of bald eagles this is true
to the extent that after several days of observation the sexes can be
identified by size alone. While watching the nest with a telescope, always
from the same observation point, the eye of the observer becomes
accustomed to the size differences and sexes are identified according to
the relative proportions they take up in the field of vision.
In addition to size, the general shape and the physique of the
birds also provide distinguishing marks of the sexes. The male bird besides
being smaller is always slimmer than the female.
The parent birds of Nest B at Rocky Bay were identified as to
their sexes with little difficulty. The female bird possessed a dark spot on her head, possibly caused by missing feathers, which enabled the observer
to identify her even when she was sitting deep in the nest.
Certain behavioral differences also aided in distinguishing
between sexes. The male and female eagle of Nest A used separate perches,
a certain branch of the nest tree, which they used consistently throughout
the breeding season. 54
Generally the female remained longer by the nest and demonstrated a persistence which was in sharp contrast to the nervousness of the male bird. The male was always active while perched, stepping sideways, turning around or simply following the movements of the surrounding wildlife with his head and often vocalized.
8 . 2 Vocalization
The repertoire of the bald eagle is limited. Bent (1937) describes it as "ridiculously weak and insignificant, more of a squeal than a scream, quite unbecoming a bird of its size and strength", while
Oberholser (1906) calls it a "loud harsh scream".
There are basically three different sounds uttered by the bald eagles on San Juan Island:
(a) Hoarse sounds given in quick succession from the throat,
sounding like; kah - kah - kah. Brewster (in Bent, 1937)
describes it as "notes given quickly but haltingly and with
apparent difficulty as if their author were choking or grasping
for breath" .
(b) A chuckling sound, similar to horse-neighing, sounding like;
ye-ha-ha ye-ha-ha ha-ha-ha. In Brewster's (op. cit.) words:
"it is rather a snickering laugh expressive of imbecile derision".
(c) A gull-like yaap-yaap-yaap, heard mainly from the young and
female. 55
Loud screams, mentioned by Oberholser (op. cit.), Dixon
(1906) and Herrick (1932) were never heard on San Juan Island. The birds
of Nest B at Rocky Bay vocalized more often than did the birds of Nest A,
especially on the close approach of humans.
The intensity and frequency of the calls (a), (b) and (c) varied with situations and the time of the day. Interpretive names were given to
the different calls, uttered by the birds under certain circumstances.
The role of these calls in the communication of the birds, however, was not studied further .
(i) "threat" - often uttered on approach of humans or adult and
juvenile bald eagles. Consists of the utterance of (a)
repeatedly in a harsh manner, followed by (b).
(ii) "mild threat" - uttered on approach of red-tailed hawks and
turkey vultures. It consists of a sharp (b) without the final
ha-ha-ha sounds.
(iii) "annoyance" - more often uttered by the female bird, when
on guard by the nest alone. Seabirds, crows and passing
airplanes induced the call. It consists of (b) with the final
ha-ha-ha sounds uttered slowly, often combined with (c).
(iv) "greeting" - expressed on arrival of mate. A. highly pitched
(b) uttered in a laughing manner . It is often heard during
courtship from both sexes. Usually the vocalizing bird draws
its head backward till it almost touches the back. 56
(v) "hunger call" - or gull-call (c) - most often heard from the
young, in particular during the last two weeks of family
disintegration.
8.3. Roosts and Resting Places
During the first part of the breeding season, one of the parent birds is always in the near vicinity of the nest. The birds usually perch on the nest tree, which is the tallest dominant tree in the stand, or take positions on one of the surrounding tall trees, which frequently provide a better view of the neighbouring areas than does the nest tree itself. Later in the breeding season, the parents visit the nest only when food is to be brought in. They spend the rest of the time farther away from the nest.
Figure 7a presents a view of the dominant trees of Turn Island which were used for perching by the adult birds of Nest A. The trees are lettered for easier reference,.. The nest tree is "a". Tree "b" in the center of the island provided the tallest perching position while tree "d" on the shore allowed a view of the shallow channel between Turn and San Juan
Islands. Tree "c" was often used.by the fledglings before they left the area.
In Rocky Bay there were fewer available perch trees (Figure
7b) and the adult birds of Nest Bwere usually perched on "n" and "p" trees.
When the birds were disturbed by humans walking around the base of the nest tree, they perched farther away on trees which were normally not used, and remained on these perches for 30 to 60 minutes after the departure of the intruder . 57
Figure 7a Perch trees used by the eagles of Nest A on Turn Island in 1962 and 1963-
Figure 7b Perch trees used by the eagles of Nest B at Rocky Bay during 1962 and 1963. 58
The male parents of Nests A and B spent the night on the nest tree or on "b" and "p" trees respectively, while the female stayed in the nest. Later, when the male did not spend the night at the nest site, the female used the perches formerly used by the male.
It is believed that the birds, especially the male, used perches close to the nest which were not visible from the observer's vantage point.
This was often indicated in instances of danger to the nest by the sudden appearance of the male bird. Also, calls of the adults often were heard when no mature eagles appeared in sight at Nest A.
Outside the breeding season, the birds roam about the island and perches are usually taken on tall trees surrounding fields heavily populated with rabbits and fields where sheep are raised. The trees surrounding Trout Lake had eagles perched on them every occasion the area was visited. Trout Lake being the water reservoir of Friday Harbor is closed to the public, thus it provides an undisturbed roosting place for eagles not participating in breeding activities.
It was noted that during adverse weather conditions, in steady rain or during rain storms, the birds sought shelter. Although they used the same perch trees, they roosted farther down from the top, almost hidden by the foliage. In strong wind the birds moved more often from perch tree to perch tree.
Figure 8 indicates the spatial relationship of perch trees used most often by the parent birds of Nest A and Nest B during and outside the breeding season. 59
Fig. 8 The spatial relationship of perch trees used by the breeding adults of Nests A. and B during 1962 and 1963. 60
8.4. The Flight of the Eagle
Bent (1937) describes the flight of the bald eagle as:
"powerful and impressive, but not so graceful or inspiring as that of the golden eagle. Its ordinary travelling flight appears heavy and labored as it moves steadily along with slow beats of its great wings, but it is really much swifter than it seems, as is often the case with large birds". Often it attains great speeds (Herrick, 1933). Herrick (op. cit.) found that 1/400 to 1/500 of a second exposure time was needed when photographing from a distance of 38 feet in order to stop the motion of an eagle approaching its nest. When travelling on a level course the eagle often sails with motionless wings.
Broley's (1947) banding records revealed that some of these birds travel long distances. A young eagle was found to cover more than
1600 miles in 32 days.
During the breeding season on San Juan Island, the flights of the adults were confined to fishing trips over the water, hunting trips inland and occasional chasing flights against intruders.
8.4.1. Fishing Flight
The most often practiced fishing method of eagles on San Juan
Island consists of a steep dive from a tree perch by the shore, usually with a break in its speed as the water surface is approached. Often they fly low over the water and snatch the prey without any break in their flight. A fishing method, performed similar to the manner of the osprey (Oberholser,
1906) was also observed. With powerful downward thrust of the wings, the 61 cruising bird suddenly turns in the manner of a twisted sommersault and dives headlong toward the water with wings held close to the body. Plunges into the water with partial or complete submergence (Oberholser, op. cit.) were never seen, rather the dives usually ended in "wetting the legs" and the eagle's return to its former height with quick beats of the wing. If a fish is captured, it is carried in the talons to the nest with heavy and laborious wingbeats.
Certain steps were often followed by the birds carrying fish in order to reach the level of the nest which is more than 150 feet from the water at Rocky Bay. On May 9, 1963 the female of Nest B captured a large fish, weighing an estimated 5 pounds. First the fish was hauled out on the rocky shore where the bird climbed higher up, then flew over to a short tree 50 yards from the nest tree. After resting for a few minutes she jumped off and flew toward the sea, then turned back toward the nest. Her first attempt to reach the nest failed and she landed again on tree p. Upon leaving tree p the bird reached the nest (Figure 9).
The birds usually hold the fish in one talon, the other is used for landing on the perch. Naturally the landing is more difficult in such cases.
Once the male bird of Nest A was seen falling amongst the branches some 20 feet after the initial landing effort was unsuccessful; however, he held onto the fish.
8.4.2 Soaring
Soaring at great heights, characteristic of eagles, was often 62
shoreline
shor eline
Fig. 9 Steps followed by the female eagle of Rocky Bay to carry a fish to the nest. 63
observed during the second part of the summer. Soaring usually was
performed above the nest containing the fully fledged young. On June 30,
1963 in Rocky Bay a dark object was spotted above Nest B, which after coming
down to an estimated 2000 feet was identified as an adult eagle.
Soaring flights always occurred on warm, clear sunny days
during the afternoon hours.
8.4.3. Defense and Attack
The eagle's flight is most impressive when the bird is
attacked by or is attacking some avian foe. While adult eagles are seldom
forced to def end themselve s, juvenile eagles often are in situations where
defensive maneuvering is necessary. These situations usually are encounters
with breeding adult eagles, ospreys and rarely with red-tailed hawks.
The characteristic "flip", or turn on the back while presenting
the claws to the attacker is performed by the eagles without effort, even without disturbing the continuity of their flight. The "flip" consists of a full
turn around the anterio-posterior axis of the bird and is done in great speed.
The eagle, with head turned sideways, watches the attacker above and
performs the flip when the attacker reaches striking distance. Such
performance was observed often,, once on April 28, 1962 on top of Mount
Dallas between a red-tailed hawk and a juvenile bald eagle, only a few yards
away from the observer. Similar defensive action is taken by the golden
eagle, as observed on August 12, 1962 near Sportsmans Lake, on the attack
of an osprey (Retfalvi, 1963). 64
An aerial maneuver previously not observed was seen on May
11 , 1963 exhibited by a juvenile bald eagle which was passing through the
osprey's territory at Rocky Bay. The osprey, a good flier, does not break its dive when it nears the intruder but continues to descend beyond the former's altitude, thus blocking its way while passing in front of it. The osprey quickly ascends and similarly repeats the attack. On one such
approach the young eagle suddenly thrust down its wings which made the bird turn backward in a half-completed somersault bringing the talons forward to grasp. The osprey escaped with great difficulty.
When attacking, the eagle approaches its intended prey from below and seizes it with the talons, which are brought into striking position by the eagle's flip performed at the moment when the prey is in
reach. Prey was captured in this way by golden eagles (Meinertzhagen,
1959) and by bald eagles (Brewster in Meinertzhagen, 1959). The male
adult of Nest A attacked a juvenile red-tailed hawk which ventured close to the eyrie in this manner on June 5, 1962.
8.4.4. Nuptial Flight
Probably the most spectacular among the aerial maneuvers is the so called "cartwheel-display" (Whitman in Armstrong, 1942). Such
a display between two mature eagles was observed over False Bay on May
6, 1962. An adult bird was seen sitting on a rock in the center of the bay.
The bird took wing on the approach of another adult eagle and the birds
swooped alternately at each other, avoiding contact by side slips and mounting fast in the air . Suddenly one of the birds turned on its back and grasped the extended talons of the oncoming bird. The two birds fell together towards the water in a spinning, turning cartwheel fashion. The birds released their hold only a few yards from the water surface. The birds then departed towards
Kanaka Bay but continued to swoop at each other. On June 2, 1962 two golden eagles were observed exhibiting a similar display (Retfalvi, 1963).
8.5. Relations with Birds, Mammals and Man
8.5.1. Relations with Other Bald Eagles
Encounters between adult bald eagles are seldom observed.
Information in this regard is also scarce in the literature. Conflicts between adult bald eagles on San Juan Island were observed only on three occasions. On July 2, 1962, the female parent of Nest A warned off an adult bald eagle from the vicinity of the nest. The intruding eagle approached
Turn Island from the north and was promptly attacked by the local bird. The attack was observed somewhat as follows; the female bird of Nest A left her perch and with quick wing beats approached the altitude of the intruder causing the intruding bird to increase its speed. Maneuvering for positions was not apparent and there was never less than 10 feet distance between the two birds. After chasing the intruding bird for approximately 100 yards the female returned to her mate who was uttering the "threat" call throughout the incident. The two nearly full grown young in the nest did not react in any perceivable way. A similar incident took place on March 14, 1963, at
Nest A , when the female bird took the offensive again and on her return to 66 the nest engaged in a chasing flight with her mate. In both occasions the male bird stayed on his perch by the nest.
A more complex encounter was observed on March 23, 1963, in San Juan Valley. Three adult birds, two side by side and one farther away, were seen perched on trees in the center of a field when the single bird left its perch, flew over to the other two birds and engaged in a chasing flight with one of them, while the other bird remained on its perch.
The two birds in the air left together toward Mount Dallas. Sexes were not distinguishable because of the distance involved.
The relation between breeding adults and immature birds is one of hostility, and conflicts between them were often observed. Broley
(1947 ) suggests that the juveniles intruding in the territory of the breeding pairs are the previous year's young. This might be the case with the San
Juan Island birds as most intruding juveniles were in the dark brown first year plumage. On May 7, 1962, at Nest C the adult bird's warning call was sufficient to turn the intruding juvenile away, but on May 10, the adult was seen pursuing the juvenile in the manner described in the case of the adult eagles' intrusion. On May 15, 1962, at Nest A, the female adult took quick action on the approach of a juvenile and chased it from the area.
Later in that day the male bird of Nest A "followed" an intruding juvenile from the nest's vicinity.
Generally, the breeding eagles of Nest B at Rocky Bay were more tolerant of intruding juveniles in spite of the latter's frequent visits to the area. An unusual episode occurred on May 14, 1963. Before dark 67
in the evening a high pitched "gull-call" was heard when a juvenile bald eagle appeared flying straight toward Nest B. The female adult in the nest took no apparent interest when the intruding juvenile landed among trees well below the nest level, however the male made a sudden fast dive at this young bird who by this time disappeared from the observer's view.
The adult bird ascended again and repeated the dive into the trees and also disappeared from sight. No sounds were heard and shortly after the male bird returned to his former perch to spend the night. A visit to the area around the base of the nest tree on the following morning revealed no signs of a fight. Presumably the male bird tolerated the juvenile's presence.
The interrelation of bald eagles not engaged in breeding activities is one of mutual tolerance or indifference. Common interest in some food items makes them gather in great numbers at certain locations.
Dixon (1909) and Willet (in Imler and Kalmbach, 1955) record 20 to 40 eagles occupying one tree. On San Juan Island 8 eagles, all juveniles except one, were seen perched on a snag by Trout Lake on May 6, 1962. The same number of eagles, 2 adults and 6 juveniles, were counted around a four acre field containing sheep and newly born lambs on February 24, 1964. Juvenile bald eagles were never antagonistic toward their kind; the occasional swoops at each other were made rather playfully resembling the aerial play of the mated adults.
Adult bald eagles seen in company of juveniles during the breeding season were considered to be non-breeding birds. Such adults were seen only on two occasions. On May 5, 1962, at Eagle Point two 68 juveniles were seen perched in company of an adult for two hours. On May
6, 1962, 7 juveniles and one adult were seen perching on the same tree by
Trout Lake .
8.5.2. Relations with Red-Tailed Hawks
The conflicts often observed between bald eagles and red- tailed hawks clearly indicate the smaller bird's hostility towards the eagle.
The hostility is one sided because the eagles tend to show an indifference towards the hawks most of the time and will only turn on the offensive if overtempted or if the hawk ventures close to the eyrie.
Red-tailed hawks venturing into the vicinity of eagle nests are easily warded off by the eagle's "mild-threat" call. On June 5, 1962, however, a juvenile hawk proceeded still closer to Nest A after the warning call had been uttered, upon which the male eagle on guard took the offensive, approaching the hawk with a swift flight never .observed previously.
It was of interest to observe that the eagle remained beneath the hawk's altitude and when the smaller bird was overtaken, it turned on its back in preparation for the use of its talons. On the eagle's move the hawk frantically tried to gain altitude, changed the course of its flight and left the area immediately. The eagle did not pursue the intruder.
The majority of the hawk's attacks are directed against juvenile bald eagles who ward off the hawk's persistent dives with a half- completed flip performed without any interruption in their flight. Red- tailed hawks are abundant on San Juan Island and nest in the wooded parts 69
of its interior. The breeding hawks fiercely attack any intruder passing through their territory. Hawks were observed to attack osprey, turkey vulture and great blue heron in a manner similar to the attacks made on the bald eagles.
8.5.3. Relations with Ospreys
An osprey nest at'Rocky Bay only 750 yards from Nest B provided an excellent opportunity to observe the relations between ospreys
and bald eagles.
Generally the breeding adults of Nest B were indifferent towards the ospreys. The actions of the ospreys, however, showed that they regarded eagles as enemies and on the eagles' occasional close approach the smaller birds panicked and took to'the air where they stayed until the
eagles left the vicinity of the osprey's nesting site. Eagles were never
observed robbing ospreys of their prey, although ospreys often caught fish in the bay in view of the eagles perched at Nest B. On one occasion
only was an osprey seen to abandon its prey as a result of interference by
eagles. On May 16, 1963, one of the ospreys which was approaching its nest with a food item in its talons dropped the prey when both eagles of
Nest B came close to the osprey nest. On two other occasions when the
adult eagles flew near the osprey nest both of its occupants panicked but took no offensive action.
The ospreys' attitude towards juvenile bald eagles was in
sharp contrast to that towards the adult eagles. On the appearance of any 70
juvenile on the horizon both ospreys began calling loudly and took to the
air rapidly gaining altitude. If the intruder approached within 200 to 300 yards of their nest it was promptly attacked. The osprey1 s attack on juvenile eagles is similar to that of the red-tailed hawk's, with the
exception that the osprey dives closer to the intruder and retreats faster to its former altitude. The juvenile eagles were observed to perform the flip in response to all the osprey attacks, while among the numerous hawk-
eagle encounters the flip was observed only once. Also, an osprey attack
frequently resulted in the eagle changing its flight direction, which never
occurred on the hawk's attack. On one occasion, on May 11, 1963, a
juvenile eagle was observed to turn on the offensive during an encounter with the osprey (see section 8.4.3.). The osprey's antagonism toward juvenile eagles was also shown when the fledged young of Nest B was fiercely attacked on July 10, 1963 (see section 10.3.).
8.5.4. Relations with Crows
Most annoyance and discomfort which the bald eagles of San
Juan Island suffer is occasioned by crows. Rocky Bay is heavily populated with northwestern crows who were often harassing the adult eagles of Nest
B. It was noted that crows generally respected the immediate vicinity of
the nest tree and attacked eagles only when they were perched farther away
from the eyrie, probably in the crows' own territory. However, on several
occasions the crows harassed the birds even while they perched on "n"
tree. On June 6, 1963, two crows persisted for 4 hours in their attack 71 on the female parent of Nest B. The two crows kept diving at the eagle alternately, sometimes coming as close as 3 to 4 feet from the eagle's head (Figure 10a) . At first the eagle was indifferent towards their actions, then she started to follow the crows' movements by turning her head and occasionally snapping at them with her bill. An interesting feature to observe was the mobility of the eagle's neck, at times almost making full turns. The eagle changed her perch several times and finally, looking disturbed, she gave the "annoyance" call and moved around the perch nervously. The crows ceased their attacks for no apparent reason.
Crows often ventured close to an eagle which had landed on rocks of the shore after a successful fishing trip. On one occasion, on May 10, 1963, a crow actually pecked at the fish held in the eagle's talons. The eagle, while panting heavily, did not react to the crow's action.
On March 24, 1963, a flock of crows came close to Nest A on Turn Island, one crow even landing on top of the nest tree. This prompted the male eagle perching close by to drive into the flock and chase the crows away at high speed. Part of the flock took refuge among the trees. Bent (1937) writes: "Eagles are often attacked by crows, just as these black rascals will attack any large birds of prey; and occasionally the crow pays extreme penalty for its audacity". There are no. records of eagles killing or feeding on crows on San Juan Island (see section 13.2.3.).
On July 11, 1962, the male adult of Nest A robbed a flock of crows which had been feeding on a dead rabbit. The eagle alighted in the middle of 72
Figure 10b Crow mobs eagle after eagle has robbed the crow of its food. 73 the flock upon which the crows scattered in all directions; picked up the rabbit and flew to the nest. One of the crows followed and harassed him for a short distance (Figure 10b).
Brewster (in Bent, 1937) writes of- an incident when a crow alighted on the back of a juvenile bald eagle, forcing the eagle to "plunge headlong into the Lake" in order to escape its tormentor. Juvenile eagles were never observed to be the subject of crows' harassment on San Juan
Island, on the contrary, a juvenile bald eagle was seen on one occasion to feed on a road-killed rabbit in company of crows and in mutual unconcern of the other's presence.
8.5.5. Relations with Gulls
Crows as the primary source of harassment while the eagles are perched are replaced by gulls when the eagles are on wing or engaged in fishing activities. Gulls numbering in dozens followed the eagles of
Rocky Bay whenever fishing sorties were made in the immediate vicinity of the nest. At low tide Rocky Bay appears as a mudflat, attracting gulls from all areas in numbers up to a hundred. When on the water or on the mudflat the gulls fear the eagle and quickly take wing with great commotion and noise. Noise provided by the gulls on O'Neil Island always indicated the eagle's presence or its passing over that island.
Remnants of gulls were found in the debris collected underneath Nest A and also beneath the perch tree used by the Nest D pair at Point Caution (see section 13.1.2.). A mature bald eagle was seen to chase mew gulls on July 23, 1962, in Mosquito Pass. On May 19, 1963, the male eagle of Nest B made a dive at a glaucous-winged gull passing close to the eyrie; the gull managed to escape by means of a sharp turn in its flight.
Several species of gulls are known to be part of the bald eagle's diet (Murie, 1940, Imler and Kalmbach, 1955 and Southern, 1963).
Yeager (1950) records bald eagles attacking a crippled gull. Bald eagles indirectly contribute to egg losses of breeding glaucous-winged gulls on
Mandarte Island, B. C. , by scaring gulls off their nests, which then are robbed by crows (Vermeer, 1964).
8.5.6. Relations with Other Birds
The appearance of eagles never alarmed the numerous ducks, loons and grebes present in the neighbouring waters of Nest A and Nest B.
Remnants of California murre and surf scoter were found in the debris under eagle nests on San Juan Island which indicate their presence in the eagle's diet. The capture of a male bufflehead was seen by local residents in early May, 1963. W . Huntsberry reported to M. D.F . Udvar dy (in litt. ) the sighting of an adult bald eagle feeding on a bird near Dinner Island in
1961 . The remains of the bird were identified to be of a red-breasted merpanser . Eagles are known to feed on many species of waterfowl
(Imler and Kalmbach, 1955). Murie (1940) found that sea birds constitute the largest part of the bald eagles' diet on the Aleutian Islands, Alaska.
Trautman (1942) reported ducks pursuing a mature bald eagle. 75
The adult eagles on San Juan Island were indifferent towards small birds passing by their nest. Tree swallows and starlings were often observed to fly around the eyrie, at times even alighting on the rim of the nest. While the adults took no notice of them, the young eagles showed their interest by moving closer to them, causing the small birds to leave.
8.5.7. Relations with Mammals
Rabbit constitutes the largest portion of the eagles' diet on
San Juan Island (see Chapter IV.). Adult and juvenile eagles were often observed to perch on fence-posts in the middle of fields abundant with rabbits. The rabbits carried on with their feeding activities even when eagles perched above them but promptly headed for their holes when the birds took wing. Eagles were never observed to attack rabbits on the island.
The lambing season of the island sheep extends from the middle of February to the end of April. Eagles often land in fields among grazing sheep during this period. The eagles' landing does not elicit fright reactions from the sheep, only an inquiring look which these animals give any passing object or farm animals. On February 25, 1964, a juvenile eagle was seen in the middle of a field near False Bay, only a few yards away from a recently born lamb accompanied by an adult sheep.
The eagle took to the air suddenly but the action did not evoke any reaction from the sheep.
A small raccoon was seen on April 13, 1964, in Nest K 76 during an aerial survey. The animal was curled up in the nest as if
sleeping, however, it looked up when the aircraft made a close pass.
The raccoon was found in the same spot the following day when another aerial survey was made. Nest K was in use during 1963, however, it appeared abandoned in the spring of 1964; breeding activities of the resident eagles were observed at Nest M only a short distance away.
Raccoons are abundant on the island yet their role in the eagles' diet is not known. The only evidence of raccoon in the bald eagles' diet was reported by Oberholser (1906).
8.5.8. Relations with Man
Bent (1937) writes: "Eagles have few serious enemies except man" . It is known from the egg-collectors' extensive experiences that eagles often will abandon their nest if molested during the incubation period. The birds raise much commotion in order to scare the intruder away but no other action is taken in defence of the nest. Broley (1947) had similar findings when he climbed hundreds of trees containing nests with young at various ages. However, Bendire (in Bent, 1937) cites three instances of eagles actually swooping at nest robbers but no contact was made on these occasions. A more recent record of this nature was made by Murphy (1962) in which he claims the eagle made actual contact with the intruder, causing a superficial laceration on the forehead.
The eagles on San Juan Island gave the "threat" call upon
every human approach to the base of their nest tree, but further action 77 to drive the intruder away was never taken. On May 7, 1962, when a tree close to Nest C was climbed to obtain a better view of the nest, the adult eagles of that nest never attempted any attack; however, they remained in the area and raised much commotion. Their behavior was interpreted as a "threat display", exhibited to drive the intruder away. Both birds flew in short circles over the nest; their heavy wingbeats creating a swishing sound. The birds often stopped in their course and hovered in one spot with legs extended, then continued in a different direction. The entire display was a combination of swift rushing flights, short dives and quick ascends interspersed with occasional soarings. The birds vocalized strongly throughout the incident and the "threat" call uttered in their flight was incomplete, consisting only of the repeated hoarse "kah-kah-kah" sounds. It was discovered later that Nest Cwas abandoned by the birds, possibly as a consequence of the human disturbance.
Eagles subjected to the presence of humans for a long period of time will become accustomed to their presence and will not panic if the base of the nest tree is visited (Nicholson, in litt. ). This proved true with the breeding eagles on Turn Island during 1962. The shore of this island is a public picnic ground which is often visited by tourists. In contrast, the breeding eagles of Nest B at Rocky Bay panicked when the nest tree was approached by humans within 100 yards and exhibited a display similar to the one observed at Nest C. The effect of the human interference on the eagles' breeding activities is discussed later (see sections 11 and 15). 78
8.6. Summary and Conclusions
The male and female of a mated pair of bald eagles was easily identified due to the species' sexual size dimorphism. Special distinguishing marks as .well as behavioral differences also aided in identifying the birds as to their sexes.
Three basic sounds were found to make up the limited vocal repertoire of the bald eagle. These sounds were uttered with different frequency and intensity under certain circumstances.
Perch trees other than the nest tree were used by the birds in the close vicinity of the nest. The male and female eagle used their own perches consistently throughout the breeding season. Non breeding birds used the Trout Lake area and other uninhabited areas for roosting.
The flight of the eagle is varied. Sudden swoops, glides and fast chasing flights are exhibited during different situations. When fishing, the eagle glides low over the water and swoops down with wings held close to the body if a prey is spotted. Swoops are often made straight from the perch. Soaring with motionless wings was observed during the second part of the breeding season on clear sunny days when rising air thermals provide the lift. A combination of swift rushing flights, short dives and quick asceni is exhibited by the birds on the approach of human intruders. This type of behavior was exhibited until the intruder left the area. It is believed that this threat display involves an extensive expenditure of the birds' energy.
Such strenuous efforts in defence of the nest were exerted by both breeding 79 adults only when man intruded; intrusions by bald eagles and other birds were warded off by one of the resident eagles with a simple "threat" call or by flying towards the intruder. The female bird took the offensive in both observed instances when the intrusion involved another bald eagle,
Intrusions by juvenile bald eagles were warded off alternately by the male and female birds. It may be an adaptive phenomenon to impress the intruder by size; the female, being the larger bird of the mated pair , attacks all formidable opponents, while the smaller male attacks juvenile eagles and other intruding bird species.
The breeding birds were often harassed by crows and gulls but the eagles were indifferent to such harassments in the majority of the cases. Since crows are very abundant in Rocky Bay it is probable that Nest B and its immediate vicinity were included in territories of crows and the crow's antagonism toward the eagles was a manifestation of interspecific territorialism. Grows spent a considerable time harassing the eagles, however, the eagles seldom responded to the crow's attacks which suggests that the crows suffered more from this relationship through the phenomenon known as aggressive neglect (Ripley, 1961). 80
9- BREEDING SEASON
At the start of the study, the chronology of breeding activities of the bald eagles on San Juan Island was approximated with the available information in literature.
Bent (1937) provides information concerning the mean egg- laying dates of the bald eagle in the different parts of its range (see Table 6).
According to this source of information, the mean egg-laying date of the bald eagle in Alaska is May 10 and in the Oregon-Mexico region it is March
6. The length of incubation has been established by various investigators to be 34-35 days (Herrick, 1932, Nicholson, 1952). Since the San Juan Islands are situated between the two regions mentioned above, it was presumed that egg-laying of the island's eagles occurs in early April and the eggs are hatched during the first half of May. This presumption proved to be faulty.
Field work on San Juan Island commenced on May 2, 1962, and at this time the active bald eagle nests contained young three to four weeks old. Nest A on Turn Island and Nest B at Rocky Bay (see Map 2) were kept under observation during the summer of 1962. Both nests contained two young which departed from the nest for the first time on July 7 and 10 and on July 1 and 2, respectively. On August 2, one of the young of Nest A was seen at the nest for the last time, the other young was found shot on
August 10.
In 1963, efforts were concentrated on obtaining data for the period prior to the hatching of eggs. Obtaining this information necessitated 81 several week-end trips on the island early in the spring. The limited number of these reconnaisance visits (see Table 1) along with a limited view into the nest prevented the recording of exact dates for egg-laying and hatchings. Table 7 lists the activities of adult eagles during days the island was visited.
Pairs of adult eagles were observed in the vicinity of nests in January and February. Nest building activities were observed at Nest A and Nest B on February 24. The earliest observation of an adult bald eagle sitting in the nest was on March 3 at Nest A by C. Nash of Friday Harbor.
On March 9 a single egg was sighted in Nest A by R. Franklin of Friday
Harbor, an "Island Sky Ferries" pilot. The single egg disappeared from the nest by March 19 (sighting by R. Franklin). No egg was seen in Nest A on
March 21 when an aerial survey was conducted. Adult birds were seen by
Nest A during March and the first half of April, but from April 21 on, the nest appeared abandoned.
An adult bird was seen sitting in Nest B on March 14 and thereafter. On April 21, the birds sitting in Nests B, D, K, and L refused to move in spite of the repeated passes of the aircraft.
Due to the Nest A pair's failure to breed, Nest B was kept under observation during the summer of 1963. The single young first departed from Nest B on July 9 and was not sighted at the nest site after
July 24.
Combining the available local information with the findings of earlier investigators, the chronology of breeding activities of the bald 82
Table 7. Breeding activities of bald eagles on San Juan Island during the spring of 1963. Observed Date from Activities
Jan. 20 ground Nest A: Adult pair perched in vicinity of the ne st.
Feb. 7 airplane Nests A, B,D, J,K and L: Pairs of adults in vicinity of the nests.
Feb. 23 ground Nests A, J and K: Pairs of adults at the nests
Feb. 24 ground Nests A and B: Nest building activities.
Mar . 3 ground Nest A: One adult on nest, other nearby. Mar . 6 ground Nest A: Adult pair perched on nest tree.
Mar . 8 ground Nest A: Adult sits on nest.
Mar . 9 airplane Nest A: One egg in nest, unguarded Mar . 10 ground Nest A: Adult sits in nest.
Mar . 13 ground Nest A: Incubation.
Mar . 14 ground Nest A: Incubation. Change-over. Nest left unguarded for 30 minutes. Nest B: Incubation. Mar . 21 airplane Nest A: No egg in nest.
Mar . 23 ground Nest A: Adult pair at nest. No incubation. Nest B: Incubation.
Mar . 24 ground Nest A: Male chases crows from nest. Copulation. Apr . 13 ground Nest A: Adult pair at nest. No incubation. Nest B: Incubation. Change-over. Arrangin of twigs in nest.
Apr . 14 ground Nest A: Adult pair at nest. Copulation. No incubation.
Apr . 21 airplane Nest A: Abandoned. Nests B,D,K,L: Adult sits in nest, refuse to move. 83 eagles on San Juan Island has been approximated and is presented in Table
8. Nest repair activities start in the middle of February. Eggs are layed between March 4 and March 10; the young hatch between April 8 and April
14. The young spend 12 to 13 weeks in the nest and depart for the first time between July 1 and July 10. The eagle family disintegrates in 1 5 to
32 days after the young's first flight. After August 2, only adult birds are present in the nest area.
The chronology of the bald eagle's breeding activities on
San Juan Island is similar to that of the "Oregon-Mexico" region (egg dates by Bent, 1937). The timing of the breeding season on San Juan Island is such that all activities associated with the raising of young occur between the beginning of March and the end of July, during which period the precipitation is at its lowest point (see Figure 6). Eggs hatch before temperatures start to rise above the yearly average and the young are fully fledged before temperatures reach the yearly maximum in August.
The timing of breeding season in relation to yearly changes in precipitation and temperatures occurs similarly in Africa for the Verreaux eagle (Rowe,
1947) and in Alaska for the golden eagle (Hobbie and Cade, 1962). Table 8. The approximate chronology of breeding activities of bald eagles on San Juan Island
Activity Date Length
Nest repair From February 15 on -
Egg laying March 4 to March 10 -
Incubation - 35 days
Hatching April 8 to April 14 -
Young in nest - 12 to 13 weeks
Young first depart nest July 1 to July 10 -
Family disintegration - 1 5 to 32 days
Young last seen at nest site July 24 to August 2
oo 85
10. BREEDING BEHAVIOR
10.1. Pre-nestling Period
This period includes all activities of the birds associated with breeding up to the hatching of the eggs. The breeding adults are found in the vicinity of their nests as early as the end of October, but do not continuously occupy it until mid December. During this, time the birds are inactive, perching side by side or hunting and fishing together.
10.1 .1. Nest Repair Activities
It is an established and well substantiated belief that a pair of eagles use the same nest year after year (Herrick, 1924, Nicholson,
1952, Smith, 1936 and Howell, 1937).
The birds, presumably to satisfy their nest building instinct, add new sticks to the nest every year or simply rearrange the old ones and add dried grass for the inner lining. Such activities were observed at Nest
A and Nest B on February 24, 1963 with both adults participating. The birds moved about in the nest, picking up small sticks and putting them aside, often giving the faint "gull-call". These activities were performed often during the day but only for short periods as the birds would leave the nest often and return to it after a short while. No new material was observed to be brought to the nest on any of the visits. Herrick (1932) found that the building instinct of the birds remained activated for two to three weeks after the eggs are hatched, culminating in "bringing several bushels of dried grass" to the nest. Dried grass was taken to Nest A on 86
May 28 and May 29 in 1962 and was arranged by the parents while the young stood by attentively. Food was brought to the nest at the same time by the other parent but feeding started only after the adult finished arranging the grass. The male bird of Nest B also took grass to the nest along with a fish caught on May 10, 1963. The grass may have been picked up accidentally from the shore where the eagle landed for a rest.
10.1.2. Mating s
Generally the mated birds stay close together during this period. They often engage in chase-flights, the most spectacular being the cartwheel display, observed on May 6, 1962 (see section 8.4.4.).
Successful sexual union must occur before the eggs are layed, however, attempts to copulate were not observed during this period due to the short observation time. Table 9 presents the information on the observed matings. A copulation attempt by the pair of Nest A in
1963 took place after the egg, possibly layed on March 9> disappeared from the nest. The matings of the Nest A pair on May 26, 1962 and of the Nest B pair on May 10, 1963, took place while the nest contained young four to five weeks old. Herrick ( 932) also observed matings after the eggs were layed and regards this phenomenon as "an instance of redundant activity". Duration of the observed attempts at matings,
5 to 1 5 seconds is very short in light of the matings observed by Herrick
(1932) in Ohio, which lasted for 1 to 2 minutes. The matings took place on top branches of trees in close vicinity to the nest tree in all observed Table 9- Observed copulations of bald eagles on San Juan Island, 1962 and 1963
Duration Date Time Nest Place * (seconds) Remarks
May 26, 1962 1204 A top of b tree 15 Copulation after adults perched side by side for 30 minutes. Both leave toward sea afterward.
March 24, 1963 1205 A top of n tree 10 Male chases a flock of crows away from nest area, on return alights on back of female. Male leaves afterward.
April 14, 1963 1000 A top of b tree 5 Copulation after adults perched side by side. Both stay on same perch after .
May 10, 1963 0812 B ._ top of p tree 5 Male comes over to p tree, perches just above female. Male alights on female's back. Both birds start fishing in bay afterward.
* see Figure 7. 88 instances, supporting the doubt of Herrick concerning Everett Smith's
supposition (in Herrick, 1932) that it is accomplished high in the air.
10.1.3. Parental Attention to the Eggs
The Turn Island nest contained one egg on March 9, 1963.
No information is available concerning the exact laying date for Nest B at
Rocky Bay. However, on March 14 and March 23, 1963, the adult bird was seen sitting in the nest. The single egg disappeared from Nest A before March 20, 1963 but both adults remained by the nest site and defended the nest from crows and gulls.
The following notes were made at Nest A during the incubation of the egg:
March 13, 1963: 1510 Male is on b tree; female is sitting in nest. Male is quiet and looks settled; female is more active, her head is always on the move and calls often in a faint "gull.-call" fashion.
1545 Male takes off toward sea; female also stands up, then resettles looks down, puts her legs apart and shakes them as she slowly settles.
1625 Female stands up, turns around and resettles in the same fashion as above, spends considerable time looking down.
1630 Male arrives back, lands on b tree, utters "annoyance-call" at a cormorant which flies by.
March 14, 1963 905 A change-over:, male takes off from b tree, circles around the nest with strong, wing beats and legs extended downward, alights on edge of nest. Female takes off right away, lands on b tree. Male circles around nest once more in the same fashion, then returns and settles. 89
March 14, 1963 (cont'd.) 919 Both adults take off, nest is left alone. Birds engage in chase- flight just over the nest.
921 Male leaves toward south; female comes back to nest and settles.
Similar behavior was observed at Nest B. At change-over
the nest was left unguarded for three minutes while the male bird was
escorting the female as she was leaving the nest. Eggs left unguarded
for short periods were never covered with nest material, as observed by
Herrick (1932).
10.1.4. Hatching
The length of the incubation period has been established by
former investigators to be 34 to 35 days (Herrick, 1932, Nicholson, 1952).
The clutch of Nest A was destroyed by unknown means, therefore hatching never occurred in 1963. Nest B , however, presumably contained on
eaglet on April 14, 1963.
The behavior of the parents as observed on above date:
1345 Female sits in the nest in a manner that suggests more than eggs. She is active, turns her head often. No sight of the male.
1415 Female stands up, places twigs around her self, then resettles very slowly.
1450 Female stands up after twisting her head, walks around the nest edge, jumps off and returns after a short circle around the nest. Settles very slowly.
1505' Male arrives at nest tree, lands on top. Female calls "gull-call", stands up and moves backward very slowly, then takes off and glides over to p tree and lands. Male jumps down to nest, moves twigs around himself then settles slowly. 90
A week later while flying over Nest B with an aircraft, the sitting bird refused to leave the nest in spite of the repeated close passes of the airplane .
10.2. Nestling Period
The nestling period includes the activities of the parents
i and the young from the time the eggs are hatched up to the time the young leaves the nest.
Continuous observations were started on May 14, 1962 at
Nest A on Turn Island and on May 8, 1963 at Nest B at Rocky Bay. In both instances, at the start, the nests contained young three to five weeks old.
10.2.1. Parental Attention to the Young
10.2.1.1. Care of the Young
The newly hatched young are helpless and require the close attention of the parents. During the early nest life of the young, one or both of the parents are always on guard by the nest, usually perched on top of the nest tree, or on some equally tall tree in the immediate vicinity.
The parental attendance at Nest A during the 1962 breeding season is presented in Table 10. Parental attendance is shown as the parents' relative presence by the nest, expressed as percentage of the observation time.
The parents' presence by the nest steadily decreases as 91
Table 10. The female and male parent eagle's participation in guarding the young during 1962 at Nest A on Turn Island.
Period Length of Parent present observation Nest * Month Days (minute s) Female Male Guar ded
May 14,15, 16, 466 82.2 64.5 93.4 17,18,19, 562 85.2 29. 5 94. 2 20,21,22, 797 94.9 20.5 94.9 23,24,25, 761 56.8 21.4 78.3 26,27,28, 489 95.6 15.3 95.6 29,30,31, 1047 80.0 16.7 87.8
June 1, 2, 3, 237 76.0 29.2 90.3 4, 5, 6, 1066 58.6 32. 9 92. 2 7, 8, 9, - - - - 10,11,12, - - - - 13, 14, 15, 2035 69.8 12.2 71 .7 16,17,18, 1950 47 .7 15.2 48.4 19,20,21, 1486 47 .7 1 . 5 47 .7 22,23,24, - - - - 25,26,27, 170 100.0 0.0 100.0 28,29,30, 1712 42.8 8.0 44.6 .
July 1, 2, 3, 1806 39. 5 4. 3 41 . 4 4, 5, 6, 1886 41 .7 6.6 45.7 7, 8, 9, 1349 31.8 13.5 42.4 10,11,12, 1660 49. 5 9.0 51 .7 13, 14, 15, 1466 17.8 12.1 29.9 16,17,18 1321 10.0 0.9 10.9
!c Percentage of observation time the young grows older . The female parent spends considerably more time at the nest site than the male parent but her attendance decreases at
a faster rate during the nestling period. The observation at Nest A
started on May 14, 1962 and there is no data available prior to this date.
However, it was assumed that during the first four weeks of the young's life one or both of the parents were constantly present by the nest. This
assumption is supported by the findings of Herrick (1933) and by the data presented in Table 11 which was obtained at Nest B at Rocky Bay during
May, 1963.
The data of Tables 10 and 11 are plotted in Figures 11 and
12 respectively to provide a graphic representation of the seasonal
decline in parental attendance. The total of percentages representing the sexes' participation in guarding the nest is usually larger than the percentage value representing the time during which the nest was attended by one of the parents, indicating that often both parents were present.
However, especially during the early nestling period, the parents took turns on guard. The daily changes in parental attendance at Nest B,
during May, 1963 are shown in Figure 12. The absence of one parent is compensated by the presence of the other, resulting in a high attendance
During the first eight weeks of the young's life, the nest was guarded 80 percent of the time of observation but during the following two weeks parental attendance rapidly decreased below 50 percent and
stayed at this level until the young flew from the nest. Table 11 . The male and female parent eagle's participation in guarding the young during May, 1963 at Nest B at Rocky Bay
Length of Parent present observation — : Nest * Days (minutes) Female Male Guarded
8 -124 100.0 70. 5 100.0 9 617 • 61.6 62.0 100. 0 10 536 88.6 67.0 100.0 11 430 59-7 47.6 100. 0 12 177 100.0 75.2 100.0 13 715 97. 5 21 .7 100.0 14 786 93.6 34.5 100.0 15 285 77. 5 26.8 77.5 16 556 88. 5 44. 6 100.0 17 175 50.0 100. 0 100. 0 18 - - - - 19 283 70. 6 7 . 3 73.0 20 540 94.9 23.4 94.9 21 750 43.0 49.8 92.4 22 380 100.0 10.7 100.0 23 244 32.0 24.6 56.6 24 - - - - 25 366 89.0 58.2 89. 6 26 - - - - 27 433 60.2 51 . 5 100.0 28 - - - 29 382 32.8 51 .1 69.2 30 - - - - 31 258 69.7 12.5 69-7
* Percentage of observation time 100
5 6 7 8 9 10 11 12 13 14
Approximate age of young (in weeks)
Fig. 11 Seasonal decrease in parental attendance by Nest A at Turn Island, 1962.
4^ Nest guarded 100 Female present —. Male present
Pi o •H +J > u cu CO & O O a cu CO cu u ft cu i—i DO H
6 7 Age of young (in weeks ) Fig. 12 The parent eagles' attendance at Nest B during May, 1963, at Rocky Bay. 96
In adverse weather conditions the young were brooded until the age of five weeks. The female parent spent the night in the nest with the young while the male was usually perched on top of the nest tree, during most of the nest life of the young.
The curve in Figure 11, representing the attendance of the female parent, closely follows the course of the curve for total attendance, especially during the latter part of the nestling period, indicating that the nest was guarded by the female parent for the majority of the time. In general, the female parent spent three times as much time by the nest as did the male parent.
10.2.1.2. Feeding of the Young
The female and male parent brought food items to the nest in equal numbers during the observation period, but since no data is
available for the first four weeks of the young's life, a general picture for the entire breeding season cannot be obtained in this respect. The numbers and dates of food items which were brought to Nest A in 1962
and to Nest B in May, 1963 by the female and male parent eagles are presented in Table 12.
Until the age of seven weeks, the young were unable to feed themselves and depended entirely on their parents. Attempts by the young to feed themselves were observed as early as May 24 in 1962 at
Nest A and May 17 in 1963 at Nest B, while only five weeks old, but these attempts consisted only of pecks at the food, succeeding only in Table 12. Number and date of food items brought to the nest on San Juan Island by the female and male parent eagles during the observation period in 1962 and 1963. (1) - length of observation (hours) (3) - food item brought in by female (2) - total number of food items (4) - food item brought in by male 1962; Turn Island Nest 1963: Rocky Bay Nest Day May June July (1) (2) (3) (4) (1) (2) (3) (4) (1) (2) (3) (4) (1) (2) (3) (4) 2 ------14.3 1 1 - - - - 3 ------4.5 2 2 - - - - 4 - - - - 1 .4 2 - 2 7.9 3 - 3 - - - 5 - - - - 7.8 2 2 ------6 - - - - 8. 5 1 - 1 ------7 ------7.4 1 - 1 - - - 8 ------2. 3 1 1 - 9 ------10. 3 1 1 - 10 ------9- 0 3 2 1 11 ------8.9 4 2 2 - - - 13 ------11 . 9 1 - 1 14 - - - - 13.1 1 1 - - - - - 13 . 1 2 - 2 15 5.2 2 1 1 10.6 1 1 - _ - - - - 16 - - - - 10.8 4. 2 2 - - - - 9- 3 3 - 3 17 5.1 2 1 1 ------18 4.2 1 1 - 14.2 2 1 1 _ - - - - - 19 ------_ - 4. 7 1 1 - 20 - - - - 12.1 1 1 ------• - - 21 6.0 1 1 ------12. 5 2 1 1 23 5.4 1 - 1 ------4. 6 1 - 1 24 2,. 8 1 1 ------25 5.3 1 1 ------5. 6 1 1 - 27 ------7. 2 2 - 2 28 4.2 1 1 - 8.1 2 1 1 ------29 3.2 1 1 - 11.4 2 - 2 - - - - 6. 4 1 1 - 30 7.1 1 1 - 9-1 1 - 1 ------31 7.1 2 1 1 ------Total 55. '3* 14 10 4 107.1* :19 9 10 44.0*11 5 6 99- 9* 19 8 11 * Totals do not include days when no food was brought in. 98
picking up morsels from the previous meal. While food was brought to the nest evenly by the female and male parent, the feeding of young was done almost entirely by the female. The male parent was observed to feed the young late in the nestling period, on occasions when he was having a meal and one of the young joined him.
During the early nestling period, tearing of the prey followed soon after it was brought to the nest. At Nest A in 1962 the two young took positions at the sides of the parent and grabbed the piece offered by the adult bird. It was interesting to note that the two young very seldom grabbed at the same piece offered by the parent. The parent bird was steady at tearing the prey, and ten to fifteen bits of food were offered to the young per minute. The young which secured the first bit of food offered was usually busy swallowing while the parent offered food to the next one, which was then taken without any interference. Taking alternate turns was the general feeding picture during the early nest life.
Later only one young attended the parent, the other was usually struggling by itself on another piece of the food.
Later in the nestling period feeding did not always follow the arrival of new food items and at times hours passed before the fresh prey was touched. The young usually indicated their hunger by pecking at the food themselves, after which the parent bird on guard promptly descended to the nest from its perch and started tearing the prey.
Food was present in the nest at all times, and no regularity was observed in bringing it in. The number of food items 99 brought to the nest during the morning, mid-day and afternoon hours are
presented in Table 13. The data of this table was analyzed with an
"analysis of variance" and no significant difference was found between
the number of food items brought by the birds during the three different periods of the day.
While there were usually two or three feeding periods per
day, with an average length of 1 5 to 20 minutes during the early nest
life, feeding was done more irregularly after the young were able to tear the prey themselves. However, this procedure extended hours in length, interspersed with occasional rest periods. Often when;the young were
crouched down after an extensive meal and one of the parents came to the nest to feed itself, the eaglets jumped up and were attended by the parent. The feeding instinct followed the parents all through the summer
and the young were observed being fed as late as July 14, 1962 at Nest A,
almost a week after they were able to fly.
The relationship between the young's age and the number
of food items brought to the nest is shown by the data presented in Table
14. The number of food items brought to the nest per one hour of observation time varies slightly as the young grows older, however, no
significant difference was found between the number of food items brought to the nest during the weekly periods of the young's age when tested with
an "analysis of variance". The average number of food items per one hour observation is 0.136 for the period of observation, which gives, with an average length of daily observation of 7 . 3 hours, slightly less Table 13. Food brought to the nest by the parent eagles during the morning, mid-day and evening hours on San Juan Island, in 1962 and 1963.
(1) - length of observation (hours) (2) - number of observed food items (3) - number of observed food items per one hour of observation
1962 1963
Between hours May June July May Total
(1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3)
Prior to 1115 13.8 6 0.43 38.0 5 0.13 42.5 3 0.07 39-9 5 0.12 134.2 19 0.14
1115 to 1545 42.5 3 0.07 36.0 4 0.11 63.4 - 0.00 53.9 8 0.15 195.8 15 0.08
After 1545 13.2 4 0.30 67.3 10 0.14 113.1 8 0.07 44.5 6 0.13 238.1 28 0.12
Total 79-5 13 141.3 19 219.0 11 138.3 19 568.1 62 101
Table 14. Number of food items brought to the nest by the female and male parent bald eagle in relation to the young's age during the breeding season on San Juan Island in-1962 and 1963
Approximate Length of No. of Items No. item/Hr. observ. age of young observation (in weeks) (in hours) Female Male Total Female Male Total
5 63. 3 7 4 11 0.11 0.06 0.17
6 67.0 4 6 10 0.06 0.09 0.15
7 51 . 5 6 5 11 0.12 0.10 0.22
8 24.1 3 3 0.12 0.12 0.24
9 39 -1 2 2 0.05 0.05
10 60.1 4 3 7 0.07 0.05 0.12
11 23.0 1 3 4 0.05 0.12 0.17
12 74. 5 3 4 7 0.04 0.05 0.09
13 61 .0 2 3 5 0.03 0.05 0.08
Total 463.6 32 31 63 0.069 0.067 0.136
Average number of hours of observation per day -7.3 Average number of food items per day - 0.99 102
than one (0.99) food item brought to the nest per day. However, food was brought irregularly to the nest. On two occasions, on June 16 and
July 11 , 1962, four food items were observed being brought to the nest, while frequently no new food item was taken to the nest for three-days
long periods.
There was no exact way to measure the amounts of food
consumed by the eaglets. Rowe (1947) estimated the daily food
consumption of the young Verreaux eagle (Aquila verreauxi), a bird
similar in size to the bald eagle. He found that the eaglet's food
consumption grows from 3/4 ounce (21 .26 grams) per day, when newly hatched to 3 3/4 pounds (1 . 38 kg) per day when it is about to leave the nest. This amount of food is comparable to the.food consumption of the young of bald eagles on San Juan Island as treated above.
10.2.2. Development of the Young
The average size of a northern bald eagle egg.is 74.4 mm
by 57.1 mm (Bent, 1937) and the newly hatched young weighs 85 grams
(Herrick, 1932)^ During the 11 to 12 weeks which the eaglets spend in
the nest, they attain the size of their parents (weight of juvenile bald
eagle, captured on August 8, 1963, was 8.125 lbs-'3.675 kg), acquire
the ability to eat by themselves, and the ability to fly.
10.2.2.1. Changes in Size
Having no other way to, measure the size of the young,
sketches were made of its size as seen through the telescope. Figure 13 103
approximate age: 4 weeks
approximate age: 5 1 /2 weeks
May 27
approximate age: 8 weeks
Fig. 13 Diagrams showing the size of the eaglet in comparison to the size of the female parent at Nest B in 1963. 104 shows the size of the young eaglet of Nest B at Rocky Bay in comparison to the size of the female parent on four different dates in May and June,
1963. The growth of the young is fast during this period. By the end of
June the young's size approaches that of the female and exceeds the size of. the male parent. Fully fledged juvenile birds reveal average measurements with the exception of the bill which is.greater than those of the mature birds of the same sex, as found by Imler and Kalmbach
(1955) in Alaska.
10.2.2.2. Changes in Plumage
The newly hatched eaglet is covered with light grey down on its upper parts and white down on the head and underpafts. In about four weeks the young changes into a thicker dark grey down stage
(Herrick, 1932).
The two young of Nest A appeared light grey on May 6,
1962, when seen from an aircraft. Two weeks later > on May 21, 1962, black upper wing coverts were observed as the young used its wings to keep balance. The black feathers rapidly appeared and by the end of
May, both young had a dark appearance, showing light grey only in the under parts of the wing.and body. In 1962, the ROcky Bay nest contained young two to three weeks older than did the Turn Island.nest, and the former already appeared black on the May 6 aerial survey.
In 1963, the young of Nest B was in grey down on May 8, dark on May 21 and by the end of May showed grey only in the underparts. 105
The primaries and tail feathers appeared early but their growth was
slower, being only half grown at the start of the wing exercises.
10. 2.2.3 Wing Exercises
The young use their wings extensively from the moment
of their hatching (Herrick, 1932) but during the first four weeks of
their life they use the wings only as a support on which to lean, when they
are crawling about the nest.
Wing-flappings were observed as early as May 1 5 at
Nest B, May 29 at Nest A in 1962 and May 16 at Nest B in 1963, but the
extensive exercises so characteristic of eaglets did not start until two
weeks later. At the beginning, the length of these wing-flappings was
short and the exercises were performed only in the afternoons. They
were more often performed from the middle of June on, when the young
were nine weeks old, with an intensity that overshadowed all other i •
activities of the young, including feeding.
When exercising, the young usually held on to a branch
imbedded in the nest and with strong wingbeats performed the motions
of flying. Later the young were able to control their balance while wing
flapping and no longer held on to branches but hopped about in the nest.
Often the young grabbed a loose stick or remnants of previous meals and
lifted them as they jumped on every wingbeat. From the middle of June
the young were able to stay in the air for several seconds, performing
two, and later three, wingbeats on every jump. 106
When the young were in control of their large wings to the extent that the strong winds were utilized to stay longer in the air, they started taking excursions to perches around and above the nest. Figure
14a indicates the perches the young of Nest A climbed before their first departure from the nest. The young of Nest A in 1962 were first observed to jump out on June 29 to step No. 1; one of them later that day proceeded to step No. 2. On July 4, one of the young jumped to step No. 3, and on July 6 to step No. .4. The young of Nest Bfollowed a similar routine in 1962 and the steps involved are indicated in Figure 14b.
Once the young ventured out of the nest, the only time they climbed back was during the night and when new food was brought in. Often the young stayed on their perches in spite of the parents' feeding activities in the nest. Wing exercises continued on extreme ends of limbs and often the young were nearly blown off by the wind.
On July 7, one of the young flew from Nest A ; the other followed it on July 10, 1962. Both of the young in Nest B were flying by July 2, 1962. The single young of Nest B in 1963 departed from the nest for the first time on July 9-
The young's acquisition of flying ability, is a gradual process which reaches a climax in their first departure from the nest.
However, it is believed that the young were capable of flying two to three days prior to their actual departure. The young's first departure
0 was not induced by any perceivable actions of the parent birds. Herrick
(1924b) believes that in one instance the parents starved the young to Figure 14a Perches used by the eaglets of Nest A on Turn Island during 1962.
Figure 14b Perches used by the eaglets of Nest B at Rocky Bay during 1962 and 1963. 108 lure them into flight. On San Juan Island, food was present in the nest and feeding of the young was observed prior and after the date of the young's departure. Consequently, starving as a means to induce the young into flight cannot be considered.
10.2.3. Relationship between Young in the Same Nest
It is a well substantiated fact that among the birds of prey, and especially among eagles, the advantage of the first hatched young over its nest mate is so great that it often results in the elimination of the latter . Such was the case with a bald eagle pair in Ohio (Herrick, 1932) and similar behavior was observed with the golden eagle (Gordon, 1955) with the Verreaux eagle (Rowe, 1947) and with the African Fish eagle
(Haliaetus vocifer EJaudin) (Brown, I960).
Both Nest A and Nest B contained two young each in 1962.
Although the early nest life of the young was not witnessed, it appeared from the similarity in their sizes and development, that neither of the young suffered maltreatment earlier. Occasional fights between the nest mates were observed at Nest A in 1962. These fights appeared more playful than serious and never amounted to more than two or three pecks at each other. When the extensive wing exercises started, the nest proved to be too small for two birds practicing at the same time. The wing of the exercising bird often hit the other young accidentally, which then, in retaliation, grabbed the wing and held it for several seconds.
Rarely were pecks made at ithce other's head, but these pecks were judged 109
not to be damaging since they were done with little force. \
10.3 Post-nestling Period
This period closes the breeding season. It starts with the acquisition of flying ability by the young and ends in the disintegration of the eagle family. The young rapidly become independent; and. disappear from the area in three to four weeks after they first flew, from the nest.
The young bird's first departure from the nest never results in a long excursion, rather the young flies around the nest and shortly after lands on one of the perches usually used by the parents.
While taking off and flying presented no problem to the young, landing was a difficult task which was not mastered until days after the young first took wing. Perfect cofeoildination is required to bring a body of close to ten pounds, travelling at high speed, to a stall at the same time when the perch is in reach of the talons. This co-ordination is learned through practice and consequently the first landing efforts of the young are awkward. When entering the nest, the young approaches from above and tries to break its speed with furious wingbeats, which only help to lessen the impact the young makes with the nest. The young birds approached landing spots, other than the nest, in a similar,manner.
Branches did not provide the large surface on which to land as did the nest and were often missed on the first try which resulted in the young's falling downward. These landing attempts ended with the open-winged young held up by the dense foliage several yards below the intended 110
landing spot. Landing on tree-tops with open wings was often observed during the first five days of this period. The young were able to land on branches after the first five days of this period; however, still with some difficulty. The young, when in the vicinity of the nest, used the same perches which were used by the parents during the breeding season. The parents often accompanied the young on these perches.
During this period, the young are helpless against the attacks of a formidable avian enemy. Such helplessness was shown by the young of Nest B when attacked by an osprey on July 10, 1963. On this occasion, the young was flying toward O'Neil Island when the osprey started its attacks. The young eagle, flying only for the second day, performed the flip on the close approach of the osprey but lost balance and started falling toward the water . The eagle quickly regained its balance but lost it every time the flip was performed to ward off the consistent attacks of the osprey. As a result of the repeated falls, the young bird was close to the water surface when it escaped by heading for shore and landing with wings wide open on top of a short tree. It was interesting.to note that the female parent who was perched on the nest tree did not go to the young's aid. Similar awkwardness of the young eagle was observed on July 23, 1962 close to Nest A when the parent eagle's swoops made the young flip on its back for defense.
Once the young learns to fly, the nest rapidly becomes abandoned. Table 15 presents data on the young's and adult's presence in or around Nest A during the post-nestling period in 1962. During the Table 15. Family disintegration and nest abandonment at Nest A on Turn Island in July, 1962.
Observation Birds present at nest site
Period Length young parent young and parent either young together or parent
(day s) (minutes) (percent^ (percent)* (percent) (per cen
4, 5, 6, 1886 100.0 45.7 45.7 100. 0 7, 8, 9, 1349 100.0 42.2 42.2 100.0 10, 11, 12, 1660 80. 5 51 .7 51 .7 80.5 13, 14, 15, 1466 31 .2 29.9 7.2 51 .9 16, 17, 18, 1321 6.4 10.9 5.2 12.1 19, 20, 21, 912 30.2 30. 2 12.1 48.3 22, 23, 24, 1372 16.7 17.3 3.6 30.4 25, 26, 27, 1176 0.0 10.0 0.0 10.0 28, 29, 30, 537 0.0 1.2 0.0 1.2
* Percentage of observation time 112
first ten days of this period, the young's presence in the nest's vicinity decreased at a fast rate. In the first five days, the young returned to the nest to spend the night and to eat. Later they only flew by or accompanied one of the parents on their rare visits to the nest. The presence of the parents decreased at a slower rate. From the fir st week of the period, the adult birds visited the nest site more frequently,than did the juveniles. There was an apparent increase in the presence of both adults and juveniles during the second week of the period but from then on eagles were rarely sighted in the nest's vicinity. The data of
Table 15 is plotted in Figure 15 to give a graphic representation of the nest-abandonment and family disintegration at Nest A in 1962*. The fast rate of nest abandonment in the first week of the period is due to the increasing absence of the young from the nest area to a greater degree than to the adult birds'absence, whose attendance by the nest site decreases at a slower rate. Family disintegration, represented by the curve which indicates the decrease in time that adults and young spend together, is a slower process than is the abandonment of the nest.
Food was observed to be brought to Nest A for the last time on July 11, 1962. On July 16, one of the young exhibited a behavior which was interpreted as begging for food. The young was observed to crawl close to the female parent who was perched at the end of a limb on b tree. The young bird moved up and down on the limb with head
* - nest abandonment is shown by the curve for "eagle (either adult or young) present" - family disintegration is shown by the curve for "young and adult present together" 113
Fig. 15 The presence of adult and juvenile eagles at Nest A on Turn Island during July, 1962.
bird present by nest (either adult or young) adult and young present at same time 114
held low and wings slightly dropped and endlessly uttered the "gull-call".
There was no response from the parent bird. Similar behavior was observed by Nest A on July 22 and August 1, 1962.
One of the young of Nest A was shot around July 22, the other was seen in the area on August 2, and for the last time in 1962.
Adult birds were sighted by the nest site occasionally thereafter; the last sighting of an adult bald eagle was on August 12, 1962. .
10.4. Summary and Conclusions
Nest-repair activities of the eagles were observed at the end of February, more than two weeks before the laying of the eggs. No new material was observed being brought to the nest, the repair activities consisted of the rearranging of small twigs within the nest by the female and male birds. Dried grass was seen brought to the nest by the birds during May, at which time the nest contained young 4 to 7 weeks old.
Copulation of the birds has been observed up till the end of May, long after the eggs were hatched.
During the incubation of the eggs, the nest was constantly guarded by one of the adults. Both sexes took turns in incubating the eggs. The bird intending to relieve its mate exhibited a flight which resembled the nuptial display before it entered the nest. The eggs were left uncovered for short periods; however, one of the adults always remained in the proximity of the nest. The incubating bird appeared active at all times and was often observed to rearrange twigs around.itself. 115
After hatching the attention of the adults slowly decreased.
The decrease in parental attention is gradual throughout the breeding season. From the time the eggs are layed until the young.are four to five weeks old, the nest is constantly guarded by one of the parents, which usually is the female, whose share in guarding the nest is three times as large as that of the male. During this period, the young are in the downy stage and are unable to feed themselves. The parents' close attendance provides a security against possible predators as well as adverse weather conditions. Parental attention sharply decreases when the young's wing exercises start at the age of eight to nine weeks.
During this time the parent birds spend more time away from the nest than they spend by it and usually visit the nest only when food is brought.
The young have attained the size of the parents and learned to feed themselves during this period. The major part of the young's time was spent flapping their wings. There was no appreciable change in the amount of food supplied to the young during the entire season. Parental attention slightly increased during the period the young started flying. The fast rate of nest abandonment during the post-nestling period was caused by the young's increased absence from the nest site. The parent eagles still visited the neat after the young had left the area. 116
11 . BREEDING SUCCESS
Breeding activities were observed at 8 nests out of the
13 bald eagle nests on San Juan Island. Eagles used Nests A, B, C_,
D and Nests J, K, L for various lengths of time during 1962 and 1963.
Nest M was in use during the spring of 1964 when observations stopped.
Nests E, F, G, H and I appeared abandoned. Table 16 presents a
summary of the eagle's breeding activities at nests in use during 1962 and 1963. The breeding activities at Nest A and B are discussed in detail on the preceding pages. Information regarding breeding activities at the other nests was obtained by means of aerial surveys and by occasional ground checks at their sites.
Nest A was used during both years of observation, however, the eagles managed to raise a brood only in 1962. Nest A is located on Turn Island. The nearest house* is on San Juan Island more than 700 yards away. Turn Island itself is a Wildlife Refuge but its shore is a public picnic ground. Great numbers of tourists visit the San Juan
Island for camping, setting up their tents less.than 200 yards from the nest tree. The nest is not visible from the shores of Turn Island, however, it is easily spotted either from the water or from the opposing
shores of San Juan Island. The presence of humans did; not seem to disturb the eagle's breeding activities in 1962 when two young were reared in the nest. In 1963, however, the eagles failed to continue their initial
* by house, an inhabited human dwelling is meant. 117
Table 16. Breeding success at active bald eagle nests on San Juan Island in 1962 and 1963.
Nest Success of breeding activities in: in use 1962 1963
2 young fledged 1 egg laid, later disappeared. No further attempts to breed.
B 2 young fledged 1 young fledged. Original clutch size not known.
Initial breeding No breeding activities activities. Nest abandoned due to human interference.
D 1 young fledged 1 young fledged
J Nest-building activities Initial breeding activities in June. Nest later in early spring. Nest abandoned. later abandoned.
K Not known 1 young produced
L Not known 1 young produced
M Not known Not in use
Total number of 5 4 young produced: 118
breeding activities presumably due to human interference. Two new houses were being built in the spring of 1963 only 800 yards away from the nest.
The noise and commotion which accompanied the construction could have been the primary cause for the failure of the eagle's breeding activities.
Possibly an additional cause of this failure is the more frequent human visits to the base of the nest tree, resulting from greater public awareness
of the Turn Island nest due to the publicity given to the present study.
Nest B was the most productive nest on San Juan Island producing two eaglets in 1962 and one eaglet in 1963. The nest is located
on the southern shore of Rocky Bay in an area of little real estate
development. The birds were easily disturbed by humans and showed hostility toward intruder s approaching the vicinity of their nesting site.
Summer cottages are located 500 - 700 yards away from the nest tree but
are separated from the nest by dense vegetation, thus the nest is relatively isolated from human habitations.
Initial nesting activities were observed at Nest C in May,
1962. The nest was discovered on May 6, 1962, and on that day one adult
bird was sitting in the nest and another was perched near by on a tall tree.
On May 7, 1962, the birds abandoned the nest after they were disturbed by
the author while attempting to look inside the nest by climbing a tree only
30 yards away. The birds stayed by,the nest for the next three days during which time they chased intruding juveniles from the vicinity of the nest on
two occasions. No adult birds were observed at this nest during the
remaining part of the following summer and during the entire summer of 1963. The nest tree is located at the southern foothills of Mount Dallas, at
an elevation of 3000 feet, 1600 yards away from the sea shore. An area
densely vegetated with alder lies between the nest tree and the nearest farm
building located over 1000 yards away. The nest is not readily apparent; it is situated on a tall tree in a bowl of thick branches 75 feet from the ground.
Its existence was not known to local residents prior to its discovery on May
6, 1962. Open fields, 600 to 700 yards from the nest , are used for rabbit hunting. It is likely that eagles cannot endure this disturbance. However, it is believed that the disturbance caused by the author on May 7, 1962 made
the birds abandon the nest in 1962 and cease to use it in 1963.
One eaglet was reared in Nest D during both the 1962 and
1963 breeding seasons. This was the most productive nest after Nest B
since it produced young in both years. The original clutch size, however, was not known. The nest is well isolated from human habitations, the nearest house being over 500 yards away. The nest tree is 250 yards from
the shore and it stands on a hillside at an elevation of 1 20 feet. The area
surrounding the nest site belongs to the University of Washington.
Nest building activities of an eagle pair were observed at
Nest J in early June, 1962. Later that month an eagle was seen sitting in
the nest but no egg was produced. The adult birds were not seen by this
nest from July on. The proximity of Nest J to Nest C (1500 yards) suggests
that both nests belong to the same pair of eagles. It is also likely that the
eagles observed at Nest J were the same birds whose nesting activities
had been disturbed at Nest C during early May,> 1962. An adult pair of 120
eagles were seen perched on the nest tree again on February 7 and 23, 1963 but no breeding activities were observed from thereon. The nest tree is located on the shore in an area frequented by humans. The nearest house is only 100 yards away. A road passes 150 yards from the nest and provides a clear view into the nest. It was interesting to find that in spite of so much human activity in the neighbourhood of the nest, the birds made attempts to breed.
The existence of Nest K was not known in 1962. It was found on April 21,. 1963 during an aerial survey, at which time a bird was seen on the nest. During that year one eaglet was reared in the nest. The original clutch size was not known. The nest tree is located on the tip of a peninsula between False and Kanaka bays, 650 yards away from the nearest house and is well isolated from human activities.
Nest L was discovered on April 21, 1963. It is located on the north-west part of Henry Island, 180 yards away from the shore. There are only two houses built on this island and both are more, than one.mile away from the nest. It is believed that the birds are subjected to little disturbance by humans. The nest was inspected occasionally by means of air surveys. It is not known whether it was used in 1962; however, one eaglet was reared in the nest during the 1963 season.
Nest M was discovered on April 14, 1964, during the last visit to San Juan Island. Two adult birds were seen by the nest at this date; one bird was sitting in the nest. The nest is located on the northern shore of
False Bay, 800 yards away from Nest K. The distance from the nest to the 121
nearest house is 850 yards, however, a road runs only 600 yards from the nest. It is assumed that Nests K and M belong to the same pair of eagles
and are used by them in alternate years. Nest K was used in 1963 and
initial breeding activities were observed at Nest M in April, 1964. Either
of the two nests could have been used during 1962 as an adult pair of eagles
were frequently sighted over False Bay during the summer of that year.
Five pairs of eagles attempted breeding in 1962 and only 3
pairs succeeded in raising.a brood. Nests A and B produced two young
each while only one young fledged in Nest D which gave an average of 1.66
young per successful nest. Nesting success, an indication of the success
of active nests, was 60%. In 1963 the nesting success was higher (66.6%);
however, the productivity was lower, only one young per successful active
nest. The comparison of nesting success between 1962 and 1963 is provided
in Table-17. Table 17. Nesting success of bald eagles on San Juan Island in 1962 and 1963
No. young Percentage No. No. nests No. young per successful of pairs Year active nests successful fledged nest successful
1962 5 3 5 1.66 60.0
1963 6 4 4 1.00 66.6 Chapter IV
FEEDING HABITS 124
12. INTRODUCTION
The feeding habits of the bald eagle are well documented in
literature and perhaps belong to the best illustrated phases of its biology.
Recent studies have centered on the variety of its diet to reveal its
economic status and to provide scientific proof for the justification of the protection by American and Canadian legi slations.
The bald eagle is primarily a carrion feeder. Direct
observations carried out by various investigator s indicate that the food item obtained is more often a dead or dying animal, or one disabled by
injury, sickness or food deficiency (Munro, 1938, Murie, 1940, Oberholser,
1906, Smith, 1936). The bird will feed readily on carrion even when the
carrion is well advanced in decomposition. On the other hand, the eagle
is quite capable of killing live animals and will do so during the winter when there is no easier way of securing food (Bent, 1937).
Fish constitute the largest portion of the bald eagles' diet;
mammals, birds and reptiles are eaten in proportion to their availability.
The various methods of collecting the feeding habit data also yield different
results.
12.1 Methods of Collecting Food Data
The most accurate method of collecting data is by direct
observation of the food items consumed in the nest; the difficulty, however,
of keeping a close watch on the nest from dawn to dusk makes this method
hard to employ. Also, correct identification of food items requires that 125
the observer is in close vicinity of the nest. This is hindered;by the height factor on the Pacific Northwest where nests generally are built in trees 100 feet and taller. In order to provide an eye-level view of the nest, the observer has to remain a greater distance from the nest, which distance in turn has to be overcome by the use of a powerful telescope. By observing the nest for several hours each day throughout the breeding season, an accurate picture of the bird's diet can be obtained.
A relatively easy, and often practised, method is the analysis of debris collected underneath the eagle's nests. Proportions of items can be calculated but not their weights because of the bird's tendency to tear their larger prey and take only small portions to the nest. A huge fish skull underneath the nest would only indicate that a fish head was consumed, which the birds could have picked up from a nearby cannery. The method, however, is advantageous in providing a qualitative picture of the eagle's diet throughout the years of the nest's use.
A third and accurate method is the analysis of the stomach contents. However, the large quantity of stomach contents required to yield an accurate result makes this method not applicable in light of the great efforts made to preserve the bird's existence. The only study using stomach contents of bald eagles was made in Alaska. The specimens were collected from 1940 to 195 3 during which period the bounty law in Alaska was still in effect (Imler and Kalmbach, 1955).
The problem of killing the birds to collect stomach contents can be eliminated by analyzing eagle "pellets". Pellets are the undigestible 126
and regurgitated parts of consumed food items. This method tends to minimize the representation of items which are mainly composed of digestible materials and overestimate items that are hard to digest.
12.2. Former Food Habit Studies
Food habit studies of the bald eagle employing these techniques are presented in Table 18.
In column 1 of Table 18, the analysis of 435 bald eagle
stomach contents is presented (after Imler and Kalmbach, 1955). The larger part of this sample was collected from the coastal area south and
southeast of Juneau, Alaska, between 1940 and 1952, thus the figures provide a picture of the bald eagle's diet in the area of its greatest abundance. Items of the stomach contents definitely identified as carrion, are dealt with separately. The authors (op. cit.) believe that more than half of the food items found by the birds was carrion.
Columns 2 and 3 present data collected by Murie (1940) on the Aleutian Islands, Alaska, in 1936 and 1937 respectively. The figures for 1936 were obtained to a greater degree from pellet analysis. The representation of invertebrates in this analysis is exaggerated due to their undigestible exoskeletons. The 1937 data, obtained from a larger sample, is based on food remains found within the nest, consequently it.gives a more exact figure of the bird's diet on the Aleutian Islands.
Food remains collected by W. B. Tyler (in Imler and Kalmbach,
1955) are from underneath nest trees in Maryland and Virginia between 1936 and 127
Table 18. Diet of the bald eagle in different localities of its range. (Numbers represent percentages.)
(1) stomach analysis of 435 stomachs (Imler and Kalmbach, 1955) (2) analysis of remains collected from 19 nests (Murie, 1940) (3) analysis of remains collected from 18 nests (Murie,. 1940) (4) analysis of remains collected from underneath nests in Maryland and Virginia by W. B. Tyler (in Imler and Kalmbach, 1955) (5) analysis of 59 pellets collected by R. D. Smith at the Blackwater Wildlife Refuge (in Imler and Kalmbach, 1955) (6) analysis of 630 pellets collected by R. H. Imler in Kansas (in Imler and Kalmbach, 1955)
Items (1) (2) (3) (4) (5) (6)
Fish 65.7 18.8 6.1 52.4 4.2 -
Birds 18.8 58.9 86.0 33. 3 50.4 1.1
Mammals 1.2 5.3 7.6 4.7 29.8 98.9
Reptiles - - -• 2.6 2.1 -
Invertebrates 2.0 16.1 - 0.7 -
Vegetable matter - - -• 3 3 12.1 -
Carrion 12.3 _ 128
1937 are the basis for the data presented in column 4, Table 18. Fifty-nine pellets collected by R. D. Smith (in Imler and Kalmbach, 1955) in the
Blackwater Wildlife Refuge between 1933 and 1934 were analyzed and the figures arrived at are presented in column 5, Table 18. According to
Imler and Kalmbach (1955) the pellet material tends to minimize the proportion of fish eaten because many of the small fish bones are either digested or scattered after regurgitation. No evidence of fish remains were found in the 630 pellets collected by R. H. Imler (in Imler and
Kalmbach, 1955) between 1935 and 1941 near Stockton, Kansas. The data presented in column 6, Table 18 shows that the eagles were subsisting almost entirely on mammalian food: jackrabbits (Lepus calif or nicus Grey) and cottontail rabbits (Sylvilagus floridanus Allen) were eaten almost exclusively. 129
13. FEEDING HABITS OF BALD EAGLES ON SAN JUAN ISLAND
13.1. Sources of Information
The methods used to study the feeding habits of the bald eagles on San Juan Island were direct observations of food items brought to the nest and analysis of debris collected underneath the nest trees.
13.1.1. Direct Observations
Observations of fooditems brought to the nest during the nesting seasons of 1962 and 1963, combined with information on feeding activities gathered after the young left the nest but still associated with the parents, are presented in Table^ 19. The observations were carried out at two separate locations. During, the summer of 1962, Nest A on
Turn Island was observed and food consumption recorded, whereas in 1963 the information on food was gathered at Nest B at Rocky Bay, due to the failure of the A Nest's pair to raise a brood that year.
The percentages presented in Table 19 are calculated on the basis of volumetric amounts consumed. Since exact measurements could not be taken, the size of the food item was estimated into categories of small, medium and large. Volumetric factor s of 1, 2 and 3 were applied to them respectively and the percentages were calculated on this basis.
Rabbit (Oryctolagus cuniculus L.) is consumed to the largest extent, then fish (mainly Ophiodon and Sebastodes species) to a lesser degree. Portions of a harbour seal (Phoca vitulina L.) were brought to the nest in July, 1962, obtained from a carrion that was washed ashore 130
Table 19- Diet of bald eagles on San Juan Island, based on observations of food items brought to the nest during 1962 and 1963.
No. of Observations Estimated Volume Consumption*
Items:
1962 1963 1962 1963 1962 1963
Fish 19 20 34 27 43.0 36. 5
Rabbit 20 22 38 43 48.3 58.1
Seal 3 - 4 - •5.1 -
Birds 1 2 1 4 I-2 5.4
Frog 1 - 1 1.2 -
Crab 1 1 - 1.2 -
Total 45 44 79 74 100.0 100. 0
* (in percentages) 131
near Turn Island. A bufflehead (Bucephala albeola) was seen to be captured in May 1963 and on two other occasions unidentified birds were seen being brought to the nest. On another occasion, a large crab from the beach was taken up to the nest. A food item picked up by the female adult of Nest A from a nearby pond in 1962 was identified as. a frog.
Since the data presented in Table 19 was collected during the breeding seasons, the figures only show the bird's diet during,the spring and the summer and in particular the kind of food that was fed to the young.
The birds' year around feeding habits are not provided by these figures.
The constitution of the eagles' diet changes as the young grow older. The number and kind of food items brought to the nest during the summer months of 1962 and 1963 are tabulated in Table 20. The monthly change in fish and mammalian food consumption during the month of May,
June, July and August, 1962 and 1963 has been analyzed with an "analysis of variance". "F" was found to be significant at the 0.05 probability level;
Source Df SS MS F
Month 3 214 .71.3- 1.64 N.S.
Type 1 20 20.0 1.00 N.S.
M x T 3 558 186.0 4.28* at 0.05
Error 8 347 43.4
Total 15 1139 Table 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.
(1) Number of observations (2) Amount of food (no unit) (3) Percentage value of items in diet
May June July Aug. Sept. Items (1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3)
Fish 24 38 74.5 7 15 36.6 3 3 6.4 5 5 45.5 39 61 40.6
Rabbit 5 9 17.5 14 25 61.0 20 40 85.0 2 5 45.5 41 79 52.7
Seal - - - - - 3 4 8.6 - 3 4 2.7
Bird 1 3 6.0 1 1 2.4 - - - " ~ - 2 4 2.7
Frog ------11 9-0 1 1 0.6
Crab' 1 1 2.0 ------11 0.6
Total 31 51 22 41 26 47 8 11 87 150 133
The consumption of fish predominates during the earlier part of the season; in the later phase mainly rabbit is consumed. Figure
16 is the graphical representation of the above trend and is based on the data of Table 19. While the birds of Nest A fed on mammalian food during the early nestling stage to a larger extent than did the birds of Nest B, the trend in changing.from a predominantly fish diet to a predominantly mammalian diet was observed in both instances. A similar tendency, although less pronounced, can be seen in the data of Imler and Kalmbach
(1955) when transformed into a graph (Figure 17); the decrease in fish consumption is pronounced as the young nears the end of its nest life while the consumption of other food items suddenly increases.
13.1.2. Debris Analysis
Food remains collected underneath Nest A on Turn Island in 1962 are analyzed in Table 21 . The method of collecting the debris was such that bones lying in close vicinity of each other were collected in separate bags to avoid duplication of the same item in the calculation of the diet. The percentages expressed therefore indicate numbers rather than volumes.
The most abundant items in the debris were the remains of rockfish (Sebastodes species) and ling cod (Ophiodon elpngatus Girard).
Skulls of giant sculpin (Scorpaenichthys marmoratus Ayees), arrowtooth flounder (Atherestes stomias Jordan and Gilbert) and Irish lord
(Hemilepidotus hemilepidotus Tilesius) were also found. 134
Fig. 16 The monthly change in fish and rabbit consumption during the years of 1962 and 1963 on San Juan Island. 100
90
Fig. 17 The monthly change in diet of bald eagles in Alaska. (After Imler and Kalmbach, 1955)
Ul 136
Table 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.
Occurrence Food Items Number Percent
Fish rockfish (Sebastodes species) 17
ling cod (Ophiodon elongatus Girard) 8
giant sculpin
(Scorpaenichthys marmoratus Ayres) 3
arrowtooth flounder
(Atheresthes stomias Jordan and Gilbert) 2
red Irish lord (Hemilepidotus hemilepidotus Tilesius) 1 Total 31 50.8
Birds Gull (Larus species) 2
surf scoter
(Melanitta per spicillata) 2
California murre (Uria aalgae) .2
Unidentified 11
Total 17 27.9
Mammals Rabbit (Oryctolagus cuniculus L.) 13 21 . 3 137
The bird remains included wings, legs and skulls of gulls
(Larus species), surf scoters (Melanitta per spicillata) and California murre
(Uria aalgae). Skeleton remains of several other birds were also found;
however, thesebirds could not be identified.
Rabbit (Oryctolagus cuniculus) remains were abundant.
Several skulls, parts of the skeleton and torn pieces of hide were present.
The skins, however , were, excluded, from the counts as.their condition
indicated that they were picked up as remains of rabbits skinned-by hunters.
The base of Nest A tree on Turn Island was visited again
in 1963; however, no new food remains were found. No food remains were
found in the immediate area surrounding the base of Nest B tree in 1962 and
1963. The debris collected underneath the Turn Island nest is likely an
accumulation of several years' food remains.
The percentages figures of Table 19 and 20 are plotted
in Figure 18, indicating the possible results in calculation of the diet, which
varies according to the method by which the data is obtained. Whereas the
data of the two tables cannot be compared since Table 19 te representative
only for the food consumed during the breeding season and Table 21 is an
indication of the year around diet; however, they serve as a means to
-demonstrate the variety of results possible when calculating the diet by /
employing different methods of collecting data.
The detailed analysis of debris collected underneath Nest
A is presented in Appendix IV. 138
100 r Direct observation
Debris analysis 80 -
.2 60 -
3.1
Items Fish Bird Mammal Other
Fig.'; 18 Food habits of bald eagles on San Juan Island. Comparison between data obtained by direct observation and by analysis of debris collected underneath the Turn Island Nest. 139
13.2 Sources of Food
13.2.1. Mammals
The reason for the eagles' extensive consumption of food
of mammalian origin on San Juan Island is the abundance of rabbits.
Rabbits are picked up as carrion from roadsides and.fields.
No rabbit kills by the eagles were ever observed. Rabbits showed no fear
towards the eagles even when the birds perched on fence posts 10 to 15
yards away. It appears that the availability of rabbit carrion accounts for
the eagle's lack of interest in the live rabbits.
The abundance of carcasses is due to the rabbits' high
morality through collision with automobiles and to the strong hunting
pressure that is exerted on the rabbit population throughout the year.
A larger number of rabbits are killed during the summer months due to the
increased tourist traffic and a population containing many young animals.
An estimated 450 rabbits are killed daily on the San Juan Island, roads
during the summer . (Appendix V).
Similarly, rabbit .carcas.ae.s-.are al so abundant in the
'fields, which are unretrieved animals killed or wounded by hunters. Many
rabbits are also killed by the hay-cutting machines in late May and early
June when the hay-fields are harvested. These freshly mowed fields
attract eagles as well as crows and red-tailed hawks. Up to four mature
eagles could be seen at times on a field 5 acres in area. Within one half
hour on one occasion, the female parent of Nest A brought two rabbits to 140
to the nest from a field mowed the day before some 900 yards away. In both instances the bird had to fight a flock of crows which previously had been feeding on the carrion (Figure 6b).
Most of the rabbits are killed during the night when blinded by the glare of the headlights; they are hit by automobiles. Consequently, rabbit carcasses are most abundant, on roads in the morning. The carcasses rapidly disappear from.the roads during the day. Only a few flattened remains which probably are no longer palatable to the birds are found on the pavement by the afternoon. The eagles are not the only birds feeding on car-killed rabbits. Crows, ravens, red^tailed hawks and turkey vultures
along with feral housecats take part in consuming the carcasses. Table 22
shows the disappearance of car-killed rabbits based on counts of carcasses
on the road made at the start and end of 12 hour periods during the summer months of 1963. Two routes of equal length were used: between Friday
Harbor and Roche Harbor, and between Friday Harbor and Limekiln
Lighthouse. The two routes were used to indicate differences between the northern forested region with low rabbit population and the souther open fields vi th high rabbit occurrence.
The rate of disappearance based on the counts was found to be 20 percent higher for the southern route: 10.8 cabbits disappeared in the south against 8.4 rabbits in the north during the 12 hour period. These two means were tested'with "t test" and "t" was significant only at 0.25 probability level. While the statistical test shows no significant difference between the two values at higher probability level, it is believed on the 141
Table 22. Rate of disappearance of car-killed rabbits on San Juan Island. Numbers of rabbits counted on roads at the beginning and end of twelve hour periods, during the summer of 1963. ("d" is number of disappeared carcasses.)
North Route South Route
6 AM 6 PM d 6:30 AM 6:30 PM d
May 25 15 3 12 11 1 10
May 29 8 7 1 7 1 6
June 4 7 7 0 9 2 7
June 18 23 3 20 18 3 15
June 22 18 5 13 17 2 15
June 26 12 2 10 : 5 0 5
June 28 5 1 4 n 1 10
July 11 5 1 4 13 2 11
July 29 14 6 8 16 1 15
Aug. 7 20 8 12 14 0 14
Aver age 12.7 4. 3 8.4 12.1 1. 3 10.8 142 basis of general observations that a difference between the disappearance rate of the two routes exists. A larger sample size possibly would have shown the difference more conclusively. These figures can provide only a rough comparison since there were feeding activities prior to the morning count which altered the count, also there were additional rabbits killed during the day before the afternoon count. The higher numbers of birds of prey, along with crows and ravens in the southern part of the island are responsible for the difference.
A congregation of eagles around the fields occurs during lambing season in late February and early March. Some of the sheep-raising farmers of San Juan Island believe that eagles kill new-born lambs; however, the majority of them do not subscribe to such a belief. Nevertheless, early accounts of the habits of the bald eagle give an ample supply of eyewitnesses observing feats of the eagle such as carrying a lamb of its own weight for several miles (Bent, 1937, Oberholser, 1906). Later investigator s, however, did not give credit to such accounts. An eagle is capable of killing a lamb; however, to carry it is beyond its power. An eagle's weight- lifting capability varies with circumstances. Altitude, wind conditions and the birds' individual characteristics contribute to the weight-lifting capabilities. Sperry (in Arnold, 1954) found that an 11 pound golden eagle is unable to lift 5 1/4 pounds, while Walker and Walker (in Huey, 1962) claim that 8 pounds is beyond eagles' weight-lifting capacity. Huey (1962) considers that birds can carry a weight up to 21 percent of their body weight. 143
According to local information, eagles on San Juan Island fed readily on carcasses of lambs. Feeding on placentas was also reported.
The cause of early mortality of lambs is most often exposure; however, at times birds, most often ravens, may be responsible for injuries that could result in the death of the young animal. In the early summer of 1962, a lamb was reported to be blinded by birds. Both eyes were pecked out at a very early age yet the lamb managed to feed normally and follow its mother by sound alone. Usually this kind of injury would have resulted in death.
It was presumed that ravens were responsible for the injury since, in the case of an eagle attack, the lamb would not have survived.
There was no instance of finding remains of sheep of any age underneath the nest trees, nor were sheep remains observed being brought into the nest at any time during the two summers the eagles were kept under observation.
The skeleton of a full grown deer was found on the shore of Rocky Bay in 1963, immediately below Nest B. Any association between the death of the deer and the eagles of Nest B is unlikely. The carcass was probably washed ashore close to Nest B by chance.
The carcass of a harbor seal was washed aihore not far from Turn Island in July, 1962. The parents and the already flying young of Nest A, along with two turkey vultures, attempted to feed on it. A considerable amount was consumed from the head region around the bullet hole, but the tough hide of the seal prevented the birds from tearing the rest of the body apart. After five days the carcass was washed away by the sea. 144
13.2.2. Fish
Fish constitute a large portion of the eagle's diet on San
Juan Island. Fish are picked up either alive ,or dead and floating from the waters around the nest. Fishing activities were often observed during 1963
at Rocky Bay. Most of the live fish caught were small, varying between
estimated weights of one to five pounds. Floating dead fish were undoubtedly picked up by the eagles if seen. When observation is made from a considerable
distance, there is difficulty in knovaaag whether the fish is dead or alive when
picked from the water.
The young eagles are already flying when salmon fishing
and canning operations commence. A great amount of waste is dumped into
the waters by the cannery in Griffin Bay and the eagles along with the ever
present gulls take their share in the feast. Eagles are often spotted on
Dinner Island, south of the cannery; however, no sightings were recorded
in the immediate vicinity of the cannery. Tides wash the fish wastes further
offshore which makes it likely that the eagles utilize them in the more open waters.
13.2.3. Birds
Birds make up a very small portion in the eagles' diet on
San Juan Island. Remains of only three birds were observed being brought
into the nest during 1962 and 1963 and of these only one capture was seen. i
In the debris under the nests, bird remains were represented to a larger
extent. It was presumed that more birds were caught during the winters
when direct observations were not carried out. During the early spring the 145
waters around the island abound in numbers of scoters, buffleheads and
scaups along with cormorants, yet none of these birds showed fear towards the eagles flying overhead. During the winter months, some of these birds would no doubt fall prey to the eagles.
Crows are the greatest antagonists of eagles, although their remains were not found in the debris. Crows caught in traps, intended for eagles, (see section 13.3) were left there as bait but were never molested by the eagles. Remains of crows and ravens were found in debris collected by Murie (1940) and Imler and Kalmbach (1955) which indicates their vulnerability, but the scarce record of their remains suggests that these species are most often able to elude the eagles' attack.
13.2.4. Other Items
Food items other than mammalian, fish and avian in origin seldom occurred in the bald eagles' diet on San Juan Island. An item identified as a frog was picked up by the female parent of Nest A. from the
shore of a small pon^twhich was often visited by the birds during 1962. The eagle's manner of catching the frog was spectacular; a sudden dive sideways from a low gliding flight after which the bird flew straight back to the nest with the prey. During 1962, a crab was picked up from thejf'shore of Turn
Island and taken to the nest. The eagle was first noticed while it was on the ground making several attempts to secure a hold on the crab with one foot. The bird had difficulty in taking off as only one foot was used in the kick-off process; the other holding the crab was held off the ground. 146
Dried grass was seen brought to the nest in 1962 and 1963.
Vegetable material was found by Imler and Kalmbach (1955) in the stomachs of Alaskan bald eagles and in pellets collected on the Atlantic Coast by
W. B. Tyler and F. R. Smith (in Imler and Kalmbach, 1955). It is believed, however, that the vegetable matter was ingested by chance while the eagles consumed stomachs of some vegetarian prey. Feeding on vegetable matter was not observed in cases of the San Juan Island birds.
13.3. Baiting Experiments
Experiments to trap and mark eagles were carried out during the latter half of the summer in 1962 and 1963.
Single spring, Victor No. 1 traps were used to capture birds by their toes. Rabbit carrion was used for bait. Three and four traps were placed around a rabbit carcass. Both the bait and traps were anchored by stakes driven into the ground. Trap-lines were set up at four different locations (No. 1 to No. 4 in Figure 19).
This method proved unsatisfactory in capturing eagles due to the extreme wariness of the birds. At the beginning.of the trials there were definite signs of eagles feeding on the baits. The jaws of the traps found snapped were probably released by the eagles but the tape- padded jaws were unable to hold them. With the removal of paddings and by camouflaging the traps, other species of birds as well as mammals were 147
Point
Fig. 19 The location of baits used to lure eagles and other carrion feeders. 148 easily captured; eagles no longer were using the baits.
The participation of eagles in consuming the baits started again when heaps of dead rabbits were placed put as free baits. The participation of several other carrion-feeders was also recorded. After it was realized that these records could indicate the relative share of these other species in the consumption of the rabbit carcasses, more free baits were placed out in 1 963 on further locations (No. 5 to No.7 in Figure 19).
Set number of rabbit carcasses were placed outand the participants' relative consumption was observed and recorded, the results of which are presented in Table 23.
In 1962 the baiting experiments were started in July and were carried through August. In 1963 baits were regularly placed out during June, July and August. Five and ten carcasses were placed in one spot which numbers were kept constant by replacing disappeared carcasses and strongly decomposed ones. Two to three checks were made daily at the location of baits and the species present were recorded. Local residents also aided in obtaining information on the "participation of different species. Each sighting of a species counted as a unit, thus the percentages presented in Table 23 are based on numbers rather than volumes. Consequently, the result is biased since-large birds consumed a larger part of the bait than did the smaller birds. 1. During 1962, four birds (1 recte-tailed hawk and 3 crows) and six mammals (2 minks and 4 feral housecats) were captured by the traps. One bald eagle, seven crows and one raccoon were caught in 1963, The eagle, a juvenile, was captured at Trout Lake with traps set under water, using salmon heads as baits. The young eagle died shortly after it was released. The raccoon was captured under similar conditions at Sportsmans Lake. 149
Table 23. ( - The participation of different species of birds and mammals in consuming the rabbit carcasses, used to bait eagles during the summer of 1962 and 1963.
Participation
Species Number Percentage
1962 1963 1962 1963
Bald eagle (Haliaeetus leucocephalus) 17 11.5 28.8
Red-tailed hawk (Buteo jamaicensis) 13.1 8.5
Turkey vulture (Cathartes aura) 11.5 11.8
Harrier (Circus cyaneus) 1.6
Northwestern crow (Corvus caurinus) 25 19 .41 . 0 32. 2
Raven (Corvus corax) 9.9 8.5
Mink 2 3.3 (Mustela vison) 5 8.2 10.2 Feral .housecat
Total 61 59 150
It was found that crows consumed most of the rabbit
carcasses. Eagles participated more in 1963 than in 1962 due to the placing of bait in view of Nest B (No. 5 in Figure 19)- Rabbits were always promptly consumed at this location. Possibly raccoons, red foxes and dogs participated in consuming the bait mainly at night; however, no records were made of them. In 1962 a trap line was set up at Location No. 3
(Figure 19) which was intended for the eagles of Nest A on Turn Island. By
coincidence, the trap was placed in front of a mink den where two young minks were caught. Afterwards this trap line was removed. Feral housecats are abundant on the island and it is believed that their percentage participation is somewhat higher than that shown in Table 23.
13.4. Summary and Conclusions
Information on the feeding habits of bald eagles on San
Juan Island was obtained by means of direct observations of food items taken to the nest by the birds and by means of analyzing the debris collected underneath the nests.
The diet of the young changed from a predominantly fish
diet during the early stage of their nest life to a predominantly mammalian
diet during the later stage.
The diet of the bald eagles based on direct observation was found to be different than the diet calculated by means of debris analysis.
Rabbits were mainly consumed by the eagles. This food item was obtained as carrion from the roads and fields where many rabbits 151
were killed by cars and by farming equipment.
Local residents reported eagles feeding on carcasses of
lambs; however, no evidence of sheep remains were found in the debris
underneath the nests.
Eagles were observed to catch small fish. Remains of
large fish found in the debris were possibly picked up as carrion. No
evidence of salmon was found in the diet of the eagles on San Juan Island.
Birds constituted a small percentage of the eagles' diet.
The large amount of rabbit carrion available on the
island was consumed by several species of birds. Eagles utilized this
source of food to a large extent as indicated by their participation in
consuming rabbit carrions placed out as free baits.
While Table 21 is far from a true representation of the
year around diet of San Juan Island eagles, nevertheless, it provides information by presenting a sample of food items which were consumed
throughout the years the nest was in use. The tendency of the eagles to
tear their larger prey and take only parts into the nest is partly responsible
for the differences in the data of Table 19 and Table 21 . Furthermore, fish
caught in the water are always taken whole and torn up in the nest, leaving
all remains in the vicinity, thus largely exaggerating the proportion of fish
in the diet. Table 19 presents data on feeding habits during the breeding
season, consequently it presents the diet of the growing young.
Rabbit carrion is fed to the young during the second half
of their nest life; a kind of food" item that the young are most likely to 152
encounter once the parent-offspring relationship ceases. The alteration in feeding habits showmin Figure 16 cannot be explained by the seasonal change in food availability. While there is an apparent increase in the abundance of road-killed rabbits during the summer, such carrion is also available throughout the year . Fish is available to the same extent the year around; with a possible increase when salmon fishing starts in the second half of the summer. No remains of salmon, however, were found in the analysis of the debris collected from the base of the Turn Island nest tr ee.
A definite preference for fish is thought to exist. On several occasions during the summer of 1962, fish and rabbit were brought to the nest within one half hour . In every instance the young abandoned the rabbit for the fish. During the early part of the nestling period when the young were fed by the parents, the feeding adult behaved similarly: when fish was brought to the nest by the mate, the rabbit was abandoned and the tearing of the fish proceeded.
Outside the breeding season, the adult birds are likely to have feeding habits which resemble the data of Table 21 , while the young of the year feeds to a larger extent on rabbit carrion. The stomach contents of one captured juvenile bald eagle contained a great amount of rabbit hair, a few rabbit claws and some grass. The contents of this stomach and the numerous observations of the island's inhabitants of young eagles feeding on dead rabbits in the fall, suggest that the young eagles after breaking relationship with the parents, feed almost entirely on rabbit carrion. The 153
flying ability of these young birds is still limited, therefore the task of
securing a fish out of the water may present too difficult a problem for
them. Feeding on rabbit carcasses requires no great flying ability.
The bald eagle is primarily a fish eater in the greatest part of its range. Whereas the birds show a certain preference for fish,
they will nevertheless feed on the most available food items. Murie (1940)
found that eagles on the Aleutian Islands, Alaska, fed mainly on sea birds,
the most abundant members of the fauna. Similarly on San Juan Island,
the local eagles fed predominantly on rabbit carrion, the most abundant
food source of that area. Chapter V
GENERAL CONCLUSIONS AND SUMMARY 155
14. CONCLUSIONS
It is believed that bald eagles were more abundant on San
Juan Island during past years. Breeding activities were observed during
1962 and 1963 at 8 nests out of the 13 bald eagle nests on the island; the other 5 nests were not used. The number of unused nests indicates that more eagles were using San Juan Island as a breeding ground during the past.
The density of breeding eagles on San Juan Island is considered low in light of eagle densities reported in Alaska and Florida.
According to Lockie and Ratcliffe (1964), eagles tend to exhibit a territorial behavior when densities increase to a certain point. Territoriality prevents a further increase in numbers and thus acts as a safeguard to ensure food supplies (Amadon, 1 964). " The low densities on San Juan Island qannot be attributed to competition for nesting sites npr to food shortage. Breeding eagles showed hostility towards other birds, including their own kind in the vicinity of their nest; however , neighbouring active nests are spaced so far apart that the birds of these nests do not come in contact with each other .
In general, the areas available for the occupants of active nests without interference from the closest neighbours is over lTJjtirnes as large as the territory size requirement of ±>ald eagles, estimated by Broley (1947). The low densities cannot be attributed to food shortage either, in light of the ever-present food supply in the form of rabbit carrion. The high rabbit mortality on the island provides a virtually inexhaustible food supply for 156
the eagles as well as for other species of birds. Rabbit carrion is fed to the eaglets during the latter part of their nestrlife; likewise, the adults also feed on it extensively. Rabbit carrion is also the primary source of food for young eagles in their post-fledgling period, when their flying ability is still limited.
It appears that the only major factor responsible for the low densities of eagles on San Juan Island is human interference. The failure of the eagles' breeding activities at Nest A in 1963, Nest C in 1962 and Nest J in 1963 can be explained only by the disturbance caused by human activities. In addition to human interference which disturbs the eagle's breeding activities and therefore reduces the number of young produced during the breeding season, the indiscriminate killing of bald eagles by humans also contributes to the reduction of eagle numbers on San Juan Island. Rabbit hunters frequent the island throughout the year and eagles often fall victim to their shooting. Their action is difficult to explain since the shooting of a bird offers no material gains, nor does it appeal to the trophy-consciousness of some of the hunters. The bald eagle, being a national bird of the U.S.A. , approaches the status of a sacred.object and killing a bird is considered an unpatriotic act. Nevertheless, eagles are still shot at and frequently killed by hunters. Tourists who visit the island by boat often carry guns which they use for target shooting. During the summer of 1962, tourists using the shores of Turn Island for camping were often observed to do such shooting.
On one occasion the local police force had to be called to stop their actions when target practice evolved into a careless shooting at floating objects in 157
the water with the stray bullets approaching, residences on the opposing shore. Such tourists, no doubt, will shoot at eagles which come to their sight.
In addition to the eagle mortality occasioned by the indiscriminate shooting of certain rabbit hunter s and tourists, a number of eagles are killed yearly by local residents, mainly sheep-raising farmers who still hold the belief that eagles kill new-born lambs. The majority of them do not subscribe to such a belief, but some will not part from their conviction and intend to protect their stock by the shooting of eagles which come close to their farms. The 1940 Bald Eagle Act, forbidding the killing of eagles, leaves provisions for such action in situations when one's material concerns are in danger. Due to this fact, the local conservation officer is unable to enforce the-law since a farmer who shoots an eagle always can claim that he has done so in protection of his livestock.
During the period of the study, one juvenile eagle was found dead of a shot-gun wound and four other dead eagles (2,adults and 2 juveniles) were reported by various local residents. Several other birds may have been killed during this time which makes it safe to assume that at least 3 eagles are killed yearly by visiting hunters, tourists and.local residents. This number constitutes more than 50% of. the number of young produced yearly and is more than 20% of the average number of bald eagles which are present throughout the year on San Juan Island.
Possibly the greatest factor in the reduction of bald eagle numbers on San Juan Island is the fast rate of real estate developments 158
on the seashore. The San Juan Island group is an attractive area for
retired persons and due to sudden increases in real estate prices farther
south on the coast, this,region is becoming more popular. If the present trend in housing developments continues, the nesting sites of bald eagles
on San Juan.Island will be destroyed at a fast rate.
The nesting failure of the Turn Island eagles in 1963 and
the absence of the birds from the nest site in the spring of 1964 are believed
to be due to the upsurge in housing developments on the opposing shore of
San Juan Island. More new houses will likely be built in this location since
the area is not overly developed. Since Turn Island is federally owned, the
island itself is safe from real estate developments; however, the increased human activities and consequent noise on the opposing shore 600 to 700
yards away may cause the eagles to avoid the island. .The birds were found to be tolerant towards human disturbance during 1962, therefore it is possible that the eagles will resume using the nest when the excessive noise which accompanies the constructions ceases after all the intended houses are
built.
The future of Nest B at Rocky Bay, the most productive nest during 1962 and 1963, is less promising. The general area around
the nest site, presently underdeveloped, is considered to have a high real
estate value. The birds of this nest were intolerant towards the presence
of humans in the vicinity of the nest in 1962 and 1963. Any future activity in the area in the form of road and house construction will undoubtedly
result in the eagles' abandonment of the nest. To save the nest, the 159
surrounding area, measuring a minimum of 300 yards in all directions from the nest tree, should be left undisturbed. The possibility of such an endeavour, considering the costs involved, appears remote.
With the exception of Nest _J, the sites of other active nests are likely to remain undisturbed in the near future.
Nest C is located in an area where little human disturbance is likely to occur. Since the nest site is farther away from the shore, it is outside the sphere of real estate interests. The surrounding area.is not usable for farming because of the abundance of rocky outcrops.
The future of Nest D appears to be the most promising among all the active nests on the island. The nest is located in a state- owned area which belongs to the Friday Harbor Oceanographic Laboratories and is intended to be kept in its natural state.
Nests K and Mare situated on the shore of False Bay in an area which is made unattractive for real estate development by the unpleasant odor of the bay when it is totally exposed at low tides. However, a further agricultural development of the area is possible.
Nest L, located on the northwest side of Henry Island is likely to remain free from human disturbance. The west side of the island is steep and it faces the open waters of Haro Strait, which makes the island almost inaccessible to boats. Due to these factors and to the lack of roads, there is no real estate development on the western shore of the island.
The general conclusion is that due to the recent upsurge in real estate developments and to the expected continuation of such 160
developments, the bald eagle population of San Juan Island will probably be reduced in future years. Since most of the waterfront areas are privately owned, the preservation of nest sites as wildlife refuges is unlikely. This is an illustration of the economics of land use where real estate interest is competing against wildlife conservation and in the future the latter likely will lose. 161
15. SUMMARY
1 . The breeding biology and the feeding habits of the bald eagle
(Haliaeetus leucocephalus L.) were studied on San Juan Island,
Washington during 1962 and 1963.
2. The purpose of the study was to find facts which would relate to the
general decline of bald eagles throughout North America.
3. The method of study involved direct observations carried out from a
ground location near the nests and observations by means of aerial
surveys.
4. On the basis of the spacing of nests, it was concluded that the density
of breeding eagles on San Juan Island is low.
5. The number of eagles on San Juan Island changed throughout the year .
The highest numbers were present during February and the lowest
numbers during October. . The change in numbers of eagles was caused
by the fluctuating numbers of juveniles.
6. Breeding bald eagles showed hostility toward intruding adult and
juvenile bald eagles and toward man. The eagles were indifferent
toward harassment by crows and gulls.
7 . Breeding eagles appeared by their nest in January and nest-repair
activities were observed in February. The chronology of breeding
activities was as follows: egg-laying between March 4 and 10, hatching
between April 8 and 14, the young's first departure from the nest
between July 1 and 10. 162
8. The young were closely guarded by the parents during the first five
weeks of their life. Parental attention markedly decreased after the
start of the young's wing exercises at the age of eight to nine weeks.
The female parent guarded-the nest to a larger extent than did the
male parent.
9. No appreciable change was observed in the amount of food supplied to
the young throughout their nest life. Both male and female parents
took an equal share in bringing food to the nest. The young of the same
nest did not appear to suffer maltreatment from each other or from the
parents.
10. Feeding habits were studied by means of direct observation of food
items brought to the nest and by analysis of debris found underneath
the nest.
11 . Rabbit was consumed to the largest extent. Rabbit was picked up by
the birds as carrion. Rabbit carrion was. the most abundant food item
on San Juan Island.
12. Feeding habits altered during the breeding season from a predominantly
fish diet during the first five weeks of the young's life, to predominantly
mammalian diet thereafter. Rabbit carrion was the primary source of
food for young eagles in their post-nestling period.
13. The number of bald eagles on San Juan Island can be expected to decrease
in future years due to the destruction of suitable breeding areas by real
estate, developments. APPENDICES The monthly maximum, minimum and average temperatures on San Juan Island for the years of 1962 and 1963 (in F°). (Obtained from F. Dearborn, Friday Harbor, Washington.)
Year Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.
Max. 56 58 66 71 68 80 85 80 83 64 58 54 1962 Min. 38 22 29 37 36 42 45 48 42 42 36 37 Avg. 47.4 43.4 44.7 50.0 53.5 57.3 60.2 60.4 59-0 52.9 48.1 42.2
Max. 52 62 60 70 90 84 83 85 81 66 55 48 1963 Min. 15 31 32 38 38 44 48 49 45 38 29 30 Avg. 36.6 47.7 45.5 51.1 56.9 59-2 60.9 61.8 60.6 52.8 46.2 41.1 The monthly variations in rainfall on San Juan Island for the years of 1962 and 1963 (in inches). Obtained from F . Dearborn, Friday Harbor , Washington.
Year Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Total
1962 1.07 0.88 1.51 0.85 0.86 0.45 0.17 1.64 0.44 1.34 4.33 2.78 16.44
1963 0.79 1.83 0.44 0.93 0.30 0.66 1.07 0.30 1.51 4.06 3.58 0.71 16.18 166
Appendix III
List of Birds of the San Juan Islands
Species of birds observed by Miller et. al. , (1935) * Species of birds observed by Goodge (1950) **• Species of birds observed by McMannama (1950) *** Species of birds observed by Retfalvi (1963)
1. Common Loon Gavia immer (Brunnich) 2. Arctic Loon Gavia arctica L. 3. Red-throated Loon Gavia stellata Pontoppidan 4. Red-necked Grebe Podiceps grisegena Boddaert 5. Horned Grebe Podiceps auritus L. 6. Western Grebe Aechmophorus occidentalis Lawrence 7. Pied-billed Grebe Podilymbus podiceps L. 8. Short-tailed Albatross Diomedea albatrus Pallas 9. Sooty Shearwater Puffinus griseus Gmelin 10. Fork-tailed Petrel * Oceanodroma furcata Gmelin 11 . Double-crested Cormorant Phalacrocorax auritus Lesson 12. Brandt's Cormorant Phalacrocorax penicillatus Brandt 13. Pelagic Cormorant Phalacrocor ex pelagicus Pallas 14. Great Blue Heron Ardea herodias L. 15, American Bittern ** Botaurus lentiginosus Rackett 16. Canada Goose Branta canadensis L. 17 . Brant Branta bernicla L. 18 . Black Brant Branta nigricans Lawrence 19. Snow Goose Chen hyperborea Pallas 20. Mallard Anas platyrhynchos L. 21 . Gadwall Anas strepera L. 22. Blue-winged Teal * Anas disc or s L . 23. Pintail Anas acuta L. 24. Green-winged Teal Anas carolinensis Gmelin 25. European Widgeon Mareca penelope L. 26. American Widgeon Mareca americana Gmelin 27. Canvasback Aythya valisineria Wilson 28 . Greater Scoup Aythya marila L. 29- Common Goldeneye Bucephala clangula L. 30. Barrow's Goldeneye » Bucephala islandica Gmelin 31 . Bufflehe.ad Bucephala albeola L. 32. Oldsquaw Clangula hyemalis L. 33. Harlequin Duck Histrionicus histrionicus L. 34. White-winged Scoter Melanitta deglandi Bonaparte 167
35. Surf Scoter Melanitta per spicillata L. 36. Common Scoter Oidemia nigra L. 37 . Hooded Merganser Lophodytes cucullatus L. 38. Common Merganser Mergus merganser L. 39. Red-breasted Merganser Mergus servator L. 40. Turkey Vulture Cathartes aura L. 41 . Goshawk Accipiter genfilis L. 42. Sharp shinned Hawk Accipiter striatus Vieillot 43. Cooper1s Hawk Accipiter cooperi Bonaparte 44. Red-tailed Hawk Buteo jamaicensis Gmelin 45. Rough-legged Hawk Buteo lagopus Pontoppidan 46. Golden Eagle Aquila chrysaetos L. 47. Bald Eagle Haliaeetus leucocephalus L. 48. Marsh Hawk Circus cyaneus L. 49- Osprey Pandion haligetus L. 50. Peregrine Falcon Falco peregrinus Tunstall 51V Pigeon Hawk Falco columbarius L. 52. Sparrow Hawk Falco sparverius L. 53. Blue Grouse Dendragapus obscurus Say 54. Ruffed Grouse Bonasa umbellus L. 55. Bobwhite Colinus virginianus L. 56. California Quail Lophortyx californicus Shaw 57 . Mountain Quail Oreortyx pictus Douglas 58. Ring-necked Pheasant Phasianus colchicus L. 59- Chukar Partridge*** Alectoris graeca Meisner 60. Grey Partridge Perdix perdix L. 61 . Sandhill Crane Grus canadensis L. 62. Virginia Rail Rallus limicola Vieillot 63. American Coot * Fulica americana Gmelin 64. Black Oyster catcher Haematopus bachmani Audubon 65. Semipalmated Plover Charadrius semipalmatus Bonaparte 66. Killdeer Charadrius vociferus L. 67. American Golden Plover Pluvialis dominie a Muller 68. Black-bellied Plover ** Squatarola squatarola L. 69- Surfbird Aphriza virgata Gmelin 70. Black Turnstone . Arenaria melanocephala Vigor s 71 . Ruddy Turnstone ** Arenaria inter pre s L. 72. Whimbr el Numanius phaeopus L. 73. Spotted Sandpiper Actitis macularia L. 74. Wandering Tattler ** Heteroscelus incanum Gmelin 75. Greater Yellow-legs * Totanus melanoleucus Gmelin 76. Least Sandpiper Erolia minutilla Vieillot 77 . Dunlin Erolia alpina L. 78 . Long-billed Dowitcher Limnodromus scolopaceus Say 79. Semipalmated Sandpiper Ereunetes pusillus L. 168
80. Western Sandpiper Ereunetes mauri Cabanis 81. Sander ling Crocethia alba Pallas 82. Northern Phalarope Lobipes lobatus L. 83. Parasitic Jaeger ** Stercorarius parasiticus L. 84. Glaucous-winged Gull Larus glaucescens TNTaumann 85. Western Gull Larus occidentalis Audubon 86. Herring Gull Larus argentatus Pontoppidan 87. California Gull Larus californicus Lawrence 88. Ring-billed Gull ** Larus delawarensis Ord 89. Mew Gull Larus. canus L. 90. Bonaparte's Gull Larus Philadelphia (Ord) 91 • Heermann's Gull Larus heermanni Cassin 92. Sabine's Gull * Xema sabini Sabine 93. Common Tern Sterna hirundo L. 94. Caspian Tern ** Hydroprogne caspia Pallas 95. Common Murre Uria aalge Pontoppidan 96. Pigeon Guillemot Cepphus columba Pallas 97. Marbled Murrelet Brachyramphus marmoratum Gmelin 98. Rhinoceros Auklet Cerorhinca monocerata Pallas 99. Tufted Puffin Lunda cirrhata Pallas 100. Band-tailed Pigeon Columba fasciata Say 101. Barn Owl *** Tyto alba (Scapoli) 102. Screech Owl Otus asio L. 103. Great Horned Owl Bubo virginianus Gmelin 104. Snowy Owl Nyctea scandiaca L. 105. Pigmy Owl Glaucidium gnoma Wagler 106. Speotyto cunicularia Molina 107 . Saw-whet Owl Aegolius acadicus Gmelin 108 . Common Nighthawk Chordeiles minor For ster 109- Rufous Hummingbird Selasphorus rufus Gmelin 110. Belted Kingfisher Megaceryle alcyon L. Ill . Yellow-shafted Flicker Colaptes auratus L. 112. Red-shafted Flicker Colaptes cafer Gmelin 113. Pileated Woodpecker Dryocopus pileatus L. 114. Lewis' Woodpecker Asyndesmus lewis Grey 115. Red-breasted Sapsucker Sphyrapicus varius L. 116. Hairy Woodpecker Dendrocopos villosus L. 117 . Downy Woodpecker Dendrocopos pubescens L. 118. Traill's Flycatcher Empidonax traillii Audubon 119- Western Flycatcher Empidonax difficilis Baird 120. Olive-sided Flycatcher Nuttallornis borealis Swain son 121 . Wood Pewee * Contopus sondidulus Sclater 122. Horned Lark * Eremophila alpestris L. 123. Tree Swallow ** Iridoprocne bicolor Vieillot 124. Violet-green Swallow Tachycineta thalassina Swain son 169
125. Rough-winged Swallow Stelgidopteryx ruficollis Audubon 126. Barn Swallow Hirundo rustica L. 127. Cliff Swallow Petrochelidon pyrrhonata Vieillot 128. Purple Martin *** Progne subis L. 129. Steller's Jay Cyanocitta stelleri Gmelin 130. American Raven Corvus corax L. 131. Northwestern Crow Corvus caurinus Baird 132. Chestnut-backed Chickadee Parus rufescens Townsend 133. Red-breasted Nuthatch Sitta canadensis L. 134. Brown Creeper Certhia familiaris L. 135. House Wren Troglodytes aedon Vieillot 136. Winter Wren Troglodytes troglodytes L. 137. Bewick's Wren Thryomanes bewicki Audubon 138. Long-billed Marsh Wren Telmatodytes palustris Wilson 139. Rock Wren Salpinctes obsoletus Say 140. Robin Turdus migratorius L. 141 . Varied Thrush Ixoreus haevius Gmelin 142. Hermit Thrush Hylocichla guttata Pallas 143. Swainson's Thrush Hylocichla ustulata Nuttall 144. Western Bluebird Si alia mexicana Swain son 145. Golden-crowned Kinglet Regulus satrapa Li cht en stein 146. Ruby-crowned Kinglet Regulus calendula L. 147. Water Pipit Anthus spinoletta L. 148. Cedar Waxwing Bombycilla cedrorum Vieillot 149- European Starling *** Sturnus vulgaris L. 150. Hutton's Vireo * Vireo huttoni Cassin 151. Solitary Vireo Vireo solitarius Wilson 152. Warbling Vireo Vireo gilvus Vieillot 153. Orange-crowned Warbler Vermivora celata Say 154. Yellow Warbler Dendroica petechia L. 155. Audubon's Warbler Dendroica auduboni Townsend 156. Townsend's Warbler Dendroica townsendi Townsend 157. MacGillivray's Warbler Oporornis tolmiei Townsend 158. Yellowthroat Geothlypic trichas L. 159- Wilson's Warbler Wilsonia pusilla Wilson 160. House Sparrow * Passer domesticus L. 161. Western Meadowlark * Sturnella neglecta Audubon 162. Red-winged Blackbird Agelaius phoeniceus L. 163. Brewer' s Blackbird Euphagus cyanocephalus Nagler 164. Brown-headed Cowbird *** Molothrus ater Boddaert 165. Western Tanager Piranga Ludoviciana Wilson 166. Black-headed Grosbeak *#* Pheuticus melanocephalus Swainson 167. Purple Finch Carpodacus purpureus Gmelin 1 68 . Pine Siskin Spinus pinus Wilson 169- American Goldfinch Spinus tristis L. 170
170. Red Crossbill Loxia curvirostra L. 171. Rufous-sided Towhee Pipilo erythrophthalmus L. 172. Savannah Sparrow Passerculus sandwichensis Gmelin 173. Vesper Sparrow Poorcetes gramineus Gmelin 174. Oregon Junco Junco oreganus Townsend 175. Chipping Sparrow Spizella passerina Bechstein 176. White-crowned Sparrow Zonothrichia leucophrys Forster 177. Golden-crowned Sparrow Zonothrichia atricapilla Gmelin 178. White-throated Sparrow Zonothrichia albicollis Gmelin 179. Fox Sparrow Passerella iliaca Merrem 180. Song Sparrow Melospizo melodia Wilson 171
Appendix IV
The detailed analysis of debris collected from the base of Nest A tree on Turn Island in 1962
Groups of food remains scattered underneath the nest were collected into separate paper bags. The contents of each bag were analyzed as to the number and kind of food items. The partial remains of a food item in each bag were counted as one unit.
Bag 1 . Fish 1 Hemilepidotu s hemilepidotu s - skull roof 1 Ophiodon elongatus - skull, jaws, vertebra, parts of shoulder girdle 2 Sebastodes species - jaws, preopercula, Hyoid bones, shoulder girdles
Bag 2. Fish 1 Ophiodon elongatus - jaws Bird 1 Larus species - synsacrum, femur, humerus, wing assembly and feather s Rabbit 1 Oryctolagus cuniculus - hind foot
Bag. 3. Fish. 1 Sebastodes species - articular bone Bird 1 Larus species - synsacrum, wing assembly - feathers brown with light tips
Bag 4. Fish 2 Sebastodes species - jaws and opercular series of one; lower jaw of other 1 Ophiodon elongatus - premaxillary and maxillary Birds 2 Unidentified - one very large synsacrum - one skull and part of keel, 2 leg bones Rabbit 1 Oryctolagus cuniculus - half of lower jaw, five vertebrae 172
Bag 5. Bird 1 Unidentified - wing assembly with long black feathers and part of keel; very large
Bag 6. Birds 2 Unidentified - synsacrum, vertebrae, leg bones, part of keel of small bird - keel, humeri of large bird
Bag 7. Fish 3 Sebastodes species - 2 complete sets of jaws, 1 dentary; 2 pair operculars, 2 pair articulars, and 1 extra; 1 pair maxillary, 2 individual preopercular s Birds 1 Melanitta perspicillata 1 Unidentified - skull, humerus, wing assembly of smaller bird, black feathers Rabbit 1 Oryctolagus cuniculus - skull
Bag 8. Fish 2 Sebastodes species - ribs, dorsal spines and interneurals - rear half skull, jaw and palatine series, operculars, preoperculars
Bag 9.- Bird 1 Larus species - large keel with white feathers; humerus black with white tips
Bag.10. Fish 1 Ophiodon elongatus - large jaws Rabbits 2 Oryctolagus cuniculus - skull, vertebrae, pelvic girdle of one - lower jaw of another
Bag 11. Fish 1 Sebastodes species Bird 1 Unidentified - synsacrum, humerus, leg bone, and large black feather s Rabbit 1 Oryctolagus cuniculus - lower jaw, vertebrae, pelvis, sacral vertebrae
Bag 12. Fish 4 Atherestes stomias (1) Ophiodon elongatus (1) Sebastodes species (2) Bag 12. Birds Unidentified (Cont'd.) - 1 large synsacrum, keel - smaller keel, synsacrum, skull, humerus Rabbit 1 Oryctolagus cuniculus - skull, leg bones
Bag 13. Fish Sebastodes species - skull, maxillaries, dentaries and articulars - dentary, operculars, and articular of another
Bag .14. Bird Unidentified
Bag .15. Fish Ophiodon elongatus - 2 sets of jaws Sebastodes species - 1 set of jaws; 2 unlike preoperculars, operculars 1 set maxillaries
Bag 16. Fish 1 Sebastodes species - 1 dentary, articular, maxillary, pr eoper cular Bird Uria aalge - skull with beak, synsacrum, .wing Rabbit 1 Oryctolagus cuniculus - skull "
Bag. 17. Bird 1 Uria aalge - skull with beak, synsacrum, wing
Bag .18. Fish 1 Ophiodon elongatus 1 Atherestes stomias 1 Scorpaenichthys marmoratus 1 Sebastodes species 1 Clam shell Bird 1 Melanitta per spicillata Rabbit 5 Oryctolagus cuniculus 174
Appendix V
Rabbit mortality through collision with automobiles
Approximately 450 rabbits are killed daily on the roads of San Juan Island by means of collision with automobiles. To arrive at this figure, the following calculation was made:
Number of rabbits killed per day =
(Number of rabbits killed per road mile per automobile = X)
times (Total length of roads = Y) times (Total number of
automobiles = Z).
A count was made to determine X. Routes on the northern and southern parts of the island were used to count the number of rabbits
• • . ! killed when driving with an automobile (See Figure 19 for routes). Both routes are paved and are 10 miles long. Since rabbits are killed more frequently during the night counts also were made after dark. The number of rabbits killed during the trials is as follows: In Daylight In Dark Date North South North South
May- 25 1 _ 3 3 May 29 - 3 2 2 June 4 2 1 1 3 June 18 - 1 2 1 June 22 - - 2 - June 26 2 2 - 2 June 28 1 2 1 2 July 11 2 1 1 3 July 29 1 2 3 1 Aug. 7 - 1 3 2
Average 0.9 1 . 3 1.8 1.9 175
The north and south average for the daylight hours is 1 . 1 rabbits and 1 .85 rabbits for dark. A total of 2.95 rabbits are killed by a single car over a 24 hour period on a 10 mile long route.
X i s then 0.3 r abbit s,
Y is 50 miles (an approximation of the total length of paved and unpaved roads, which permit speeds at which rabbits are hit accidentally
Z is 30 cars (estimated number of cars which travel the 50 miles of road daily),
then:
Number of rabbits killed per day =(0.3) (50) (30) = 450 LITERATURE CITED 177
Literature Cited
Amadon, D. 1964. The evolution of low reproductive rates in birds. Evolution 18: 105-110.
American Ornithologists' Union. 1957. Check-list of North American birds.
Lord Baltimore Press, Baltimore. 691 pp.
Armstrong, E. A. 1942. Bird Display. Cambridge, England.
Arnold, L. W. 1954. The golden eagle and its economic status. U.S. Fish and Wildlife Service, Circular 27. 35 pp. Bent, A. C. 1937. . Life Histories of North American Birds of Prey, Part I U. S. National Museum, Bull. 167. 409 pp.
Broley, C. L. 1947. Migration and nesting of Florida bald eagles. Wilson Bulletin 59: 3.23.
Brooks, A. C. 1922. Notes on the abundance and habits of the bald eagle in British Columbia. Auk 39: 556-559-
Brown, L. H. I960. The African fish eagle Haliaeetus vocifer especially in the Kavirondo Gulf. Ibi:s;i02: 285-297.
Dixon, J. P. 1909. The life history of the northern bald eagle. Condor 11: 187-193.
Fawks, E. 1961 . A survey of wintering bald eagles. Iowa Bird Life 30: 55-58.
Friedmann, H. 1950. The birds of North and Middle America, Part IX.
U.S. National Museum., Bull. 50. 793 pp.
Goodge, W. 1950. Some notes on the birds of San Juan Island. Murrelet 31:
Gordon, S. P. 1955. The Golden Eagle. King of Birds. Collins, London. 246 pp. Hensel, R. J. and W. A. Troyer. 1964. Nesting studies of the bald eagle
in Alaska. Condor 66: 282--286. . .
Herrick, F. H. 1924. An eagle observatory. Auk.41: 89-105.
Herrick, F. H. 1924b. Nests and nesting habits of the American eagle. Auk 41: 213-231. 178
Herrick, F. H. 1924c. The daily life of the American eagle. Late phase. Auk..4.1: 389-422, 517-541.
Herrick, F. H. 1929. The eagle in action. National Geographic Magazine 55: 635-660.
Herrick, ,F. H. 1932. The daily life of the American eagle. Early phase. Auk 49: 307-323,428-435.
Herrick, F. H. 1933. The daily life of the American eagle. Late phase (concluded). Auk 50: 35-53.
Hobie, J. E. and T. J. Cade. 1962. Observations on the breeding of golden eagles at Lake Peters in northern Alaska. Condor 64: 235-237.
Holloran, A. F. I960. Notes on the wintering populations of bald eagles on the Wichita Mountain Wildlife Refuge. Proc. Okl. Acad.Sci., 40: 120-121.
Howell, J. C. 1937. The nesting bald eagles of Southeastern Florida. Auk 54: 296-299.
Howell, J. C. 1949. Comparison of 1935, 1940, and 1946 populations of nesting bald eagles in east-central Florida. Auk 66: 84.
Howell, J. C. 1954. A history of some bald eagle nest sites in east-central
Florida. Auk 75: 96-98.
Hoxie, W. J. 191.0. Notes on the. bald eagle.in Georgia. Auk 27: 454.
Huey, L. M. 1962. Comparison of the weight lifting capacities of the house finch and the golden eagle. Auk 79: 485. Imler, R. H. and E. R. Kalmbach. 1955. The bald eagle and its economic status. U.S. Fish and Wildlife Service, Circular 30. 51 pp.
McLellan, R. D. 1927. The geology of the San Juan Islands. University of Washington Publications in Geology, Volume 2. 185 pp.
McMannama, Z. 1950. Additional notes on the birds of the San Juan Islands. Mur.relet 31: 29-
Miller, R. C, E. D. Lumley and F . S. Hall. 1935. Birds of the San Juan Islands, Washington. Murrelet 16: 51-65. 179
Meinertzhagen, R. 1959- Pirates and predators. Oliver and Boyd, Edinbur gh. 28 4 pp.
Munro, J. A. 1938. The northern bald eagle in British Columbia. Wilson Bulletin 50: 28-35.
Murie, O. J. 1940. Food habits of the northern bald eagle in the Aleutian Islands, Alaska. Condor 42: 198-202.
Murie, O. J. 1959- Fauna of the Aleutian Islands and Alaska Peninsula. U. S. Fish and Wildlife Service, North American Fauna series No. 61 . 406 pp.
Murphy, J. R. 1962. Aggressive behavior of a bald eagle. Auk 79: 712-713.
Musselman, T. E. 1950. Concentrations of bald eagles on the Mississippi River at Hamilton, Illinois. Auk 66: 8 3.
Nicholson, D. J. 1952. Little known facts about Florida bald eagle s. Florida Naturalist 25: 23-26.
Oberholsex, H. C. 1906. The North American eagles and their economic relations. U.S.D.A. Biological Survey, Bull. 27. 42 pp.
Peters, J.L. 1931. Check-list of the birds of the world. Volume I. Harvard Univer sity Press, Cambridge, Massachusetts. 258 pp.
Retfalvi, L. 1963. Notes on the birds of the San Juan Islands, Washington. Murrelet 44: 1-2.
Ripley, S. D. 1961. Aggressive neglect as a factor in inter specific competition in birds. Auk 78: 366-371.
Rowe, E. G. 1947. The breeding biology of Aquila verreauxi Lesson. Ibis 89: 387-410, 576-6olT
Sprunt, A.I. V. and R. L. Cunningham. 1961. Continental bald eagle project. National Audubon Society, New York, Progress Report No. 1 . 7 pp.
Smith, F. R. 1936. The food and nesting habits of the bald eagle. Auk 53: 301-305.
Southern, W. E. 1963. Winter population, behavior and seasonal dispersal of the bald eagles in Northeastern Illinois. Wilson Bulletin 75: 42-55. 180
Trautman, M. B. 1942. Ducks following bald eagles. Wilson Bulletin 54: 139.
Van den Akker, J. B. 1954. A wintering concentration of eagles in Oklahoma. Wilson Bulletin 66: 136.
Van Kammen, I. J. 1916. Relative to the bald eagles in Alaska. Oologist 33: 156-158.
Vermeer, K. 1963. The breeding ecology of the. glaucous-winged gull (Larus glaucescens). B. C. Provincial Museum, Occasional Papers No. 13. 104 pp.
Willet, G. 1927. Destruction of eagles in Alaska. Auk 44: 591-592.
Yeager, L. E. 1950. Bald eagles attack crippled gull. Wilson Bulletin 62: 210.