Ducks and coots (Fulica americana) were pursued by 
eagles over water; Canada geese were pursued over both 
water and fields. When an eagle approached, snow geese, 
ducks, and coots appeared to be more restless and flushed 
sooner than Canada geese, possibly reflecting the greater 
vulnerability of the smaller birds, All waterfowl species 
appeared to flush sooner at the approach of an eagle when 
impoundments were frozen, possibly indicating increased 
vulnerability on ice. Eagles usually ignored waterfowl 
concentrations after the spring thaw if winter-killed fish 
were available; however, they were seen diving on ducks 
as late as 3 March, and harassing a dying goose on 
13 March (in 1976). In spring 1978, when no winter fish 
kill occurred, feeding on waterfowl continued until the 
eagles left the area. 
Fish as a Food Resource 
Fish replaced waterfowl as the principal food of eagles 
after winter-killed fish were exposed by thaws. We did not 
see eagles capture live fish, and few fish carcasses were 
found under eagle perches during fall and early winter, 
when waterfowl were the primary food. Thus, during the 
first two seasons of study, fish were not an important food 
at Swan Lake until several months after the eagles arrived. 
And in the third season when there was no winter kill, fish 
were never a major food of eagles at Swan Lake. Under 
the circumstances of our study, fish became important 
only when adverse water conditions caused massive fish 
kills, followed by thaws that made the carcasses available. 
When these prerequisites were met, eagles fed almost ex- 
clusively on fish, 
Feeding Behavior 
After leaving night roosts, most eagles arrived at feeding 
areas during the half hour before sunrise. Feeding sites 
were used consistently each day until food at the site was 
depleted or made unavailable by snow or freeze-up. 
Feeding began intensely as soon as light allowed and con- 
tinued until about 1000 hours. When prey was abundant, 
eagles sometimes began feeding again in late afternoon 
and continued until they returned to night roosts. After a 
day of such intense feeding, feeding on the following day 
seemed to be reduced. When prey was scarce, eagles were 
seen feeding and searching for food throughout the day. 
Blizzards during the day either forced eagles to stay in the 
immediate vicinity of the night roost or severely restricted 
their feeding. 
Feeding was highly communal. However, even though 
a few (2 or 3) to more than 30 eagles fed close together, 
they rarely shared the same food item. Because waterfowl 
and fish carcasses were too heavy for eagles to carry, they 
were eaten on the ground or on the ice. When food was in 
shallow water, eagles waded to carcasses and dragged 
them to shore or onto a low perch before feeding. The 
birds sometimes gathered small carcasses from the water, 
frozen impoundments, or shore by swooping upon them 
without landing. Eagles commonly displaced each other 
from carcasses and perches at feeding areas (Griffin 1981). 
Eagles often attacked crippled and diseased waterfowl. 
In one instance, as many as 32 eagles surrounded and 
harassed a crippled Canada goose, Individual eagles ap- 
proached the goose and attacked it until the goose drove 
them off. After several hours, however, the goose weak- 
ened and was eaten. When preying on crippled ducks, 
eagles flew low over prey on water or ice. As the eagle 
approached, healthy ducks flushed, but crippled ducks 
that were unable to fly remained to be attacked by the 
eagles. 
Pellet Analysis 
In 1975-76 and 1976-77, pellets regurgitated by bald 
eagles were collected beneath night roosts and diurnal 
perches on Swan Lake NWR. Of 1,206 pellets collected, 
600 (300 from each field season) were selected at random 
and analyzed for prey remains, 
Remains of birds were far more common in pellets than 
remains of mammals. Bird parts occurred in 599 pellets 
and mammal parts in only 22. Seven species of birds and 
six species of mammals were represented (Table 3), 
Canada goose remains were by far the most common 
avian species in pellets in both years, occurring in 527 of 
the 600 pellets. Remains of mallards ranked second (in 211 
pellets), snow geese third (23 pellets), and coots fourth (11 
pellets). Cottontail rabbits (Sylvilagus floridanus) were 
the most common mammal represented, occurring in 11 of 
22 pellets that contained evidence of mammalian species 
(Table 3). 
Fish remains occurred in only 35 pellets. However, few 
fish remains were expected, inasmuch as Steenhof (1976) 
reported that when eagles are feeding predominantly on 
fish, all remains are digested and there is no pellet cast. 
Pellet analysis clearly indicated that Canada geese were 
by far the most important food of eagles. The high fre- 
quency of occurrence of mallard remains suggested that 
mallards were fed upon more commonly than our field 
observations indicated, 
Of 1,206 pellets analyzed for lead shot, only 113 con- 
tained one or more shot (Griffin et al. 1980). This low 
frequency of occurrence implied that eagles were feeding 
mainly on healthy waterfowl, but field observations indi- 
cated that they fed most heavily on crippled and dead 
waterfowl that resulted from waterfowl hunting (Griffin 
et al. 1980). The true contribution of hunter-killed or crip- 
pled waterfowl as eagle food remains unresolved. Recent 
work by the Missouri Department of Conservation has 
shown that 35% of the Canada goose losses at Swan Lake 
NWR in 1980 were due to combinations of lead poisoning, 
fowl cholera, aspergillosis, and esophageal impaction 
(D. D. Humburg, personal communication), 
