
Ecology of Bald Eagles Wintering Near a 
Waterfowl Concentration 
by 
Curtice R. Griffin 
School of Forestry, Fisheries and Wildlife 
University of Missouri— Columbia 
Columbia, Missouri 65211 
Thomas S. Baskett 
U.S. Fish and Wildlife Service 
Missouri Cooperative Wildlife Research Unit 
University of Missouri 
Columbia, Missouri 65211 
Rollin W. Sparrowe 
U.S. Fish and Wildlife Service 
Office of Cooperative Units 
Washington, D.C. 20240 
Abstract 
Bald eagles (Haliaeetus leucocephalus) associate with wintering waterfowl concentrations throughout 
the United States, but little detailed information is available on eagle-waterfow] relations. This report 
concerns the ecology of bald eagles wintering in the vicinity of Swan Lake National Wildlife Refuge in 
north-central Missouri and, more specifically, examines the association of the eagles with waterfowl 
during the winters of 1975-76, 1976-77, and 1977-78. Bald eagles were on the study area from October 
through March; populations peaked in early to middle December at 66 birds in 1975-76, 73 in 1976-77, 
and 100 in 1977-78. Two or three secondary peaks in eagle numbers occurred during each winter. 
Waterfowl were on the study area from September to March. Populations of Canada geese (Branta 
canadensis) in the Swan Lake Zone, a 3,625-km? goose management unit, reached peaks of 196,000 
birds in 1975-76, 211,400 in 1976-77, and 224,000 in 1977-78. Eagles were closely associated with 
waterfowl concentrations. Goose hunting in the Swan Lake Zone resulted in large numbers of crippled 
and dead waterfowl on and near the refuge, and crippled and dead geese were a primary food of win- 
tering eagles. Severe winter weather and low dissolved oxygen in water in refuge impoundments caused 
large winter kills of fish in 1975-76 and 1976-77. During these two years, eagles ignored waterfowl con- 
centrations after the impoundments thawed, and fed almost exclusively on dead fish. Wintering bald 
eagles also occasionally preyed on healthy waterfowl and on mammals. In 600 bald eagle regurgitated 
pellets, Canada geese were by far the most common prey species, and mallards (Anas platyrhynchos) 
ranked second, Among the few mammal species represented in the pellets, cottontail rabbits (Sylvilagus 
floridanus) were the most common. Among 18 radio-tagged bald eagles monitored on the study area for 
5 to 75 days, local movements and minimum winter ranges varied widely. Local movements and distri- 
bution of eagles near Swan Lake were closely related to those of waterfowl, but the availability of car- 
casses of waterfowl and fish on or near the refuge was also influential. Each fall, eagles arrived at Swan 
Lake well after live geese and carcasses became abundant, suggesting that eagles do not pursue the geese 
southward. Rather, the eagles’ southward migration may be occasioned by other phenological events in 
northern portions of their range, and ultimately, perhaps, by food shortages there. 
Numbers of bald eagles (Haliaeetus leucocephalus) have (Sprunt 1968; Snow 1973). Much information has been 
slowly declined since the advent of the white man on the — collected on food, habitat requirements, and population 
North American continent; however, the decline has be- dynamics of northern bald eagles on their breeding 
come more pronounced and rapid since World War II grounds (Herrick 1934; Munro 1938; Retfalvi 1965, 1970; 
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