Troyer and Hensel 1965; Grier 1974, Whitfield et al. 
1974; and others). Many northern forests are managed for 
nesting bald eagles (Hager 1974, 1976). However, only re- 
cently has attention been focused on the winter ecology of 
the species, and there has not yet been a major attempt to 
improve winter habitat. Because habitat destruction and 
alteration have been more extensive in the wintering 
range, preservation and sound management of winter 
habitat may be the most critical factor affecting northern 
bald eagle populations (Steenhof 1976, 1978). 
In studies of the winter ecology of bald eagles, some in- 
vestigators found that wintering eagles fed primarily on 
fish, and to a much lesser degree (if at all) on waterfowl: 
Southern (1963, 1964) and Jonen (1973) in Illinois; Grewe 
(1966), Steenhof (1976), and Steenhof et al. (1980) in 
South Dakota; Ingram (1965) in Wisconsin; Shea (1973) in 
Montana; Lint (1975) in Idaho; Servheen (1975), Stal- 
master (1976), and Stalmaster and Newman (1978, 1979) 
in Washington; and Cooksey (1962) and Lish and Lewis 
(1975) in Oklahoma. Edwards (1969) and Platt (1976) re- 
ported that in Utah the birds fed mostly on jackrabbits 
(Lepus californicus), Other investigators, however, have 
shown that waterfowl are the major food of bald eagles at 
some wintering sites. Ducks were the principal food of 
bald eagles wintering near Bear River Migratory Bird 
Refuge in Utah (Swisher 1964). In New Brunswick, a pop- 
ulation of bald eagles changed from their primary diet of 
fish and carrion in summer to waterfowl in winter 
(Wright 1953). In British Columbia, bald eagles preyed on 
crippled ducks (Munro 1938). Many Federal and State 
waterfowl areas attract bald eagles, particularly after the 
hunting season, when crippled ducks and geese are avail- 
able as food (Snow 1973). Biologists and refuge managers 
throughout the United States have reported bald eagles 
associating with wintering waterfowl (Spencer 1976). Yet, 
little detailed information is available about eagle—water- 
fowl relations. 
Swan Lake National Wildlife Refuge, in north-central 
Missouri, harbors a large wintering population of Canada 
geese (Branta canadensis) and a large winter concentra- 
tion of bald eagles. Eagles are apparently attracted to the 
refuge by mild winter weather and large numbers of win- 
tering waterfowl. Historically, as the number of water- 
fowl wintering in the area increased, so did the number of 
eagles. Similar trends in numbers of waterfowl and eagles 
have been reported at three other wildlife refuges in the 
Midwest (Griffin et al. 1980). 
The present study was designed to investigate the 
ecology of bald eagles wintering in the vicinity of Swan 
Lake Refuge, and especially the factors that influence the 
association of bald eagles with wintering waterfowl. 
Study Area 
The study area encompasses about 260 km? in Chariton, 
Carroll, Linn, and Livingston counties in north-central 

Missouri. It includes Swan Lake National Wildlife 
Refuge, Swan Lake Wildlife Management Area, Fountain 
Grove Wildlife Management Area, and the Grand River 
between Swan Lake and Fountain Grove (Fig. 1). The 
entire study area lies within Swan Lake Zone, described 
below. 
Swan Lake National Wildlife Refuge (NWR) is 3 km 
east of the Grand River, and 23 km north of the con- 
fluence of the Grand and Missouri rivers. It is a 4,455-ha 
area developed and managed by the U.S. Fish and Wild- 
life Service. About 2,000 ha of the refuge consist of open 
water in three major impoundments. At the time of our 
study (1975-78), 1,215 ha were cultivated for wildlife 
food. Major crops were corn, milo, and winter wheat, An 
additional 405 ha of the refuge were in grass-legume 
fields and up to 325 ha of naturally produced moist-soil 
plants were available. Much of the rest of the refuge is 
oak-hickory and mixed timber along the streams and 
drainage ditches (Babcock et al. 1978). 
Swan Lake Wildlife Management Area is a 0,4-km- 
wide strip of land lying within the perimeter of Swan Lake 
NWR. It has been leased and managed for goose hunting 
by the Missouri Department of Conservation in coopera- 
tion with the U.S. Fish and Wildlife Service since 1955. 
Fountain Grove Wildlife Management Area, 11.5 km 
northwest of Swan Lake NWR, is a 2,185-ha tract of 
State-owned land managed intensively for waterfowl by 
the Missouri Department of Conservation and is a public 
hunting area. It includes 810 ha of shallow water in three 
major pools. Crop production for wildlife consists of about 
535 ha of corn, wheat, and milo. Bottomland forest 
borders much of the river and streams on the area (Bab- 
cock et al. 1978). 
Swan Lake Zone, a 3,625-km? Canada goose manage- 
ment unit, surrounds Swan Lake NWR and Fountain 
Grove Wildlife Management Area. The Zone attracts the 
largest concentration of the Eastern Prairie Population of 
Canada geese in the Mississippi Flyway. Swan Lake NWR 
and Fountain Grove Wildlife Management Area provide 
habitat for these geese during the fall hunting season, and 
supply about 50% of their winter food requirements (Bab- 
cock et al. 1978). 
Materials and Methods 
Field work extended from October through March dur- 
ing three field seasons: 1975-76, 1976-77, and 1977-78. 
In 1975-76 and 1977-78, observations were made daily 
when work schedules and weather permitted, but in 
1976-77 observations were limited to 2 days per week. 
Counts of Eagles, Geese, and Carcasses 
Weekly counts of eagles were made on the Swan Lake 
NWR from October through March when weather and 
