refuge, and sometimes long after waterfowl carcasses be- 
came available in substantial numbers. This late arrival of 
eagles on the refuge in relation to waterfowl and availa- 
bility of food indicates that eagles probably remained at 
more northern areas until they were forced farther south. 
As Brown and Amadon (1968) indicated for most migrant 
birds of prey, food shortage was probably the major factor 
forcing the eagles south. 
Numbers of Canada geese in the Swan Lake Zone 
peaked at about the same time during each year of the 
study, as did numbers of eagles on the refuge. The similar 
relation of goose and eagle peaks each year probably was 
not a cause-and-effect phenomenon, but reflected charac- 
teristic responses of each group of birds to factors in- 
fluencing migration. We have no evidence that eagles 
were migrating with the major goose flocks in fall. 
Complex food and habitat interactions occurred after 
goose and eagle numbers reached their peaks in December 
or early January. These interactions affected local move- 
ments and therefore numbers of geese and eagles on the 
study area throughout the rest of the winter. 
During the first 2 years of this study, northward eagle 
migration coincided with the spring thaw in late 
February. As refuge impoundments became ice-free, large 
numbers of winter-killed fish became available to eagles. 
Although fish carcasses were abundant for at least 4 weeks 
after the thaw, eagle numbers began declining rapidly the 
second week after the thaw. In the third year, migratory 
movements were noted in mid-March, but a thaw had not 
occurred and there was little food available at the refuge 
during this period. Thus, the onset of spring eagle migra- 
tion was not related to local food availability and did not 
always coincide with the local spring thaw. 
Highly variable age ratios of the bald eagle population 
on the refuge throughout winter indicated a difference in 
seasonal movements of immature and adult birds. In fall, 
the presence of more immature than adult eagles indicated 
that some immature birds migrated south earlier than 
adults. In spring, adults began moving northward earlier 
than immatures. A rapid buildup and decline of adult 
birds occurred in mid-February; in contrast, numbers of 
immature birds, which also increased rapidly in middle to 
late February, remained relatively stable and then 
declined slowly as the early spring progressed. The wide 
variability in age ratios in fall and early spring indicated 
that eagles using the refuge during these periods were 
highly transient. 
Local Movements 
Daily and local movements of eagles on and off the 
study area were influenced strongly by the three interre- 
lated factors of food availability, waterfowl distribution, 
and weather, among which food availability was the most 
important; however, availability of food often depended 
on locations of waterfowl concentrations, which in turn 
depended heavily on weather. Concentrations of eagles 
11 
were highest where waterfowl concentrations were 
highest. Often when waterfowl left the refuge, they 
moved to the Grand River or farther south to the Missouri 
River. Eagles often followed the waterfowl and changed 
their roosting or feeding areas to correspond with the new 
sites that waterfowl were using. Waterfowl concentrations 
were ignored by eagles if fish from winter kills were avail- 
able as in early spring 1975-76 and 1976-77. 
The high ratio of adult to immature eagles during 
periods of harshest weather and lessened food availability 
on the refuge suggested that immatures may be less adept 
than adults at securing food, and may seek other areas not 
used by adults during these periods (Griffin 1981). 
Acknowledgments 
We thank the many persons who assisted us in this 
study. Employees of Swan Lake National Wildlife Refuge, 
especially A. O. Manke, Refuge Manager, arranged for 
materials and equipment. Cooperating staff of the Mis- 
souri Department of Conservation included K. M. Bab- 
cock and D. D. Humburg, who conducted the aerial 
waterfowl inventories and reviewed the manuscript 
(Humburg); R. Dobbins, former Area Manager at Foun- 
tain Grove Wildlife Management Area, and J. E. Fries- 
ner, former Assistant Manager at Swan Lake Wildlife 
Management Area, who provided support and equipment; 
and L. J. Korschgen, who aided in identifying mammalian 
materials in cast pellets. R. C. Laybourne, National Fish 
and Wildlife Laboratory (U.S. Fish and Wildlife Service), 
at the U.S. National Museum, aided in identifying avian 
material in cast pellets. J. R. Acker, B. B. Griffin, B. S. 
Johnson, and M. Murphy assisted in the field work. L. H. 
Fredrickson, M. W. Sayre, D. J. Randleman, J. K. 
Hampy, D. A. Darrow, and S. S. Clark, School of 
Forestry, Fisheries and Wildlife, University of Missouri- 
Columbia, reviewed or otherwise assisted with the manu- 
script. 
Financial support was provided in part by a National 
Audubon Society Fellowship, and by the U.S. Fish and 
Wildlife Service Contract No. 14-16-0008-757, awarded 
to the University of Missouri. This report is a contribution 
from the Missouri Cooperative Wildlife Research Unit 
(University of Missouri-Columbia, Missouri Department 
of Conservation, U.S. Fish and Wildlife Service, and 
Wildlife Management Institute, cooperating), and from 
the Missouri Agricultural Experiment Station Project No. 
184, Journal Series No. 8883. Publication costs were paid 
by funds from the Office of Migratory Bird Management, 
U.S. Fish and Wildlife Service. 
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