10 
Discussion 
Factors Influencing Chronological Patterns 
in Eagle Numbers 
Bald eagles began arriving at Swan Lake NWR in early 
October and numbers increased as fall progressed. Num- 
bers were greatest in early to middle December, but 
smaller peaks occurred throughout the rest of winter. 
Once migration was complete, the numbers of eagles on 
the refuge were related to availability of food. However, 
numbers of waterfowl (primarily geese), waterfowl 
hunting pressure, time of year, and weather also affected 
eagle numbers, often by influencing food availability. 
Figure 4 shows the relation of eagle numbers to Canada 
goose numbers on the refuge to time of year and, for 
1975-76 and 1976-77, to numbers of carcasses of water- 
fowl and fish. 
Although there were large numbers of Canada geese on 
the refuge by mid-October in each field season, numbers 
of bald eagles did not increase substantially until middle to 
late November. This lag in buildup of eagle numbers sug- 
gested that the number of Canada geese on the refuge had 
little effect on eagle numbers during fall and early winter. 
Eagle abundance was probably more dependent on proxi- 
mate factors affecting the southward seasonal migration 
(discussed later). However, beginning in late November 
and continuing until the spring thaw, the occurrence of 
waterfowl at the refuge influenced eagle numbers. This 
was especially evident when there was an ice-free opening 
on otherwise frozen impoundments. Large numbers of 
waterfowl were attracted to the openings. During severe 
weather, many geese in open-water areas died, providing 
an important food source and attracting many eagles. 
Once the eagle migration was complete, eagle numbers in 
the study area were closely linked with numbers of water- 
fowl and fish carcasses. 
In 1975-76, numbers of eagles on the refuge peaked 4 
weeks after peak availability of waterfowl carcasses, but 
in 1976-77 the two peaks were roughly coincident. This 
apparent difference in chronologies probably reflects inac- 
curacies in our data on waterfowl carcass numbers. The 
important point is not the temporal relation between these 
two peaks, but the fact that after the fall influx, eagles 
were abundant whenever carcasses were available. 
Secondary peaks following the major peak of eagle 
numbers in each field season were associated with thaws 
or other reasons for increased food availability. Eagle 
numbers increased after impoundments began to thaw 
and snow-covered waterfowl or fish carcasses were ex- 
posed, Mowing of refuge corn and milo fields in January 
of all three field seasons also increased eagle numbers by 
providing access to waterfowl carcasses. Eagle numbers 
declined when cold weather returned, freezing impound- 
ments and covering carcasses with snow. Radiotelemetry 
results suggested that these peaks reflected movement from 










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DECEMBER 
JANUARY 
OCTOBER NOVEMBER FEBRUARY MARCH 
Fig. 4. Numbers of bald eagles wintering at Swan Lake National 
Wildlife Refuge related to abundance of Canada geese (1975- 
78) and waterfowl and fish carcasses (1975-77). (No eagle 
counts made 24 October to 19 December 1977.) 
nearby areas rather than large-scale ingress or egress for 
the entire Swan Lake vicinity. 
The average number of eagles on the refuge in the 
winter of 1977-78 was greater than the averages for the 
first two field seasons. The reasons for this increase are un- 
known but may be related to one or more of four factors: 
(1) increased severity of winter weather in 1977-78, 
forcing a larger percentage of the continental eagle 
population farther south; (2) increased continental num- 
bers of bald eagles; (3) concentration of more eagles into 
fewer areas by destruction of winter eagle habitat; or (4) 
increased waterfowl use of the refuge and an extended 
goose hunting season in the Zone which probably increased 
the food available. 
Factors Influencing Migration and 
Local Distribution of Eagles 
Migrational Movements 
Peak numbers of eagles at Swan Lake NWR occurred 
after large numbers of waterfowl had arrived on the 
