Table 2. Numbers of waterfowl and fish carcasses counted 
along sections of shoreline, Swan Lake National Wild- 
life Refuge, 1975-77. 




Date No. of Date No. of 
(1975-76) carcass (1976-77) carcass 
Waterfowl Waterfowl 
20 Oct ll 3-23 Oct (3)8 0 
4 Nov 89 6 Nov 6 
2 Dec 48 20 Nov 6 
9 Dec 40 28 Nov 14 
18 Dec 26 14 Dec 2 
1 Jan 15 18 Dec 28 
11-31 Jan (4) 0 7 Jan 0 
14 Jan 9 
23 Jan 8 
29 Jan-19 Feb (4) 0 
Fish Fish 
9 Feb 33 27 Feb 1,057 
10 Feb 850 6 Mar 1,100 
16 Feb 4,100 10 Mar 826 
1 Mar 1,680 24 Mar 384 
8 Mar 1,250 27 Mar 596 
15 Mar 875 



*Numbers of counts during the period between the inclusive dates 
are in parentheses. 
severe cold weather and snow occurred, and about 60 
Canada geese on Silver Lake died, apparently from winter 
stress. When the next carcass count was conducted on 
14 January, only 9 of the carcasses had not been eaten. In 
surveys on 29 January and thereafter, no carcasses were 
seen. 
Waterfowl carcasses were counted on a 48-ha field 
along the west border of the refuge in January 1976, when 
refuge personnel mowed standing corn and milo. About 
100 carcasses were seen, suggesting that large numbers of 
waterfowl carcasses were probably available in refuge 
fields. 
After increases early in the hunting season, our counts of 
carcasses declined or fluctuated erratically (Table 2) be- 
cause the numbers were highly dependent on waterfowl 
use of the refuge, which in turn depended primarily on 
weather. 
Severe winter weather and poor water conditions (low 
water level, high turbidity, and low dissolved oxygen) 
caused large fish kills on the refuge in 1975-76 and 
1976-77. When exposed by thaws, fish that had been 
frozen in the ice were eaten by eagles. Our transects 
yielded counts of relative numbers of fish carcasses avail- 
able to eagles. Most fish counted were 15 cm or longer. Six 
counts were made in 1975-76 and five in 1976-77 (Table 
2). Thirteen species of fish were identified among the car- 
casses: paddlefish (Polyodon spathula), shortnose gar 
(Lepisosteus platostomus), gizzard shad (Dorosoma ceped- 
ianum), mooneye (Hiodon alosoides), common carp 
(Cyprinus carpio), bigmouth buffalo (Ictiobus 
cyprinellus), river carpsucker (Carpiodes carpio), channel 
catfish (Ictalurus punctatus), builhead (Ictalurus sp.), 
largemouth bass (Micropterus salmoides), white crappie 
(Pomoxis annularis), bluegill (Lepomis macrochirus), and 
drum (Aplodinotus grunniens). 
In 1975-76 the spring thaw began in early February, 
and 33 fish carcasses were counted along transects on 
9 February. Numbers increased rapidly as the ice broke up 
and peaked at 4,100 on 16 February. They then declined 
steadily through early spring. In 1976-77 the spring thaw 
began in late February. Fish carcasses along transects 
numbered about 1,100 on 27 February and 6 March and 
then declined steadily through March. Refuge personnel 
estimated the fish kill at the refuge at 200,000 fish in 
1975-76 and 100,000 in 1976-77. Optimal water levels on 
the refuge before freeze-up probably prevented a fish kill 
in 1977-78. 
Before our study, there were only two records of winter 
fish kills on the refuge. One occurred in the winter of 
1968-69, and the other in the winter of 1973-74 (M. F. 
Lentz, personal communication). The increasing shallow- 
ness of the impoundments from siltation, combined with 
low water conditions in fall contributed to these kills 
(B. M. Hull, personal communication). 
Feeding Habits of Eagles 
Waterfowl as a Food Resource 
From October into January during the three seasons of 
our study, dead waterfowl constituted the primary food of 
bald eagles. Eagles fed primarily on Canada goose car- 
casses; fewer carcasses of snow geese (Chen caerulescens), 
mallards (Anas platyrhynchos), green-winged teal (A. 
crecca), and pintails (A. acuta) were available on the 
refuge. Eagles fed on carcasses along refuge impoundment 
shorelines or frozen in the ice. After most of these were 
eaten, the birds relied heavily on carcasses in refuge fields; 
however, these did not become accessible to eagles until 
the fields were mowed in mid-January. 
Crippled and diseased waterfowl, especially Canada 
geese, were also important sources of food. Weakened 
waterfowl were readily attacked and eaten. Although 
many attempts by eagles to kill apparently healthy water- 
fowl were noted each winter, we saw only five that were 
successful: twice an immature eagle captured a duck in 
flight soon after flushing a flock from the lake; an adult 
eagle caught one duck as it flushed from the ice; an 
immature eagle flew into a flock of mixed waterfowl on 
the ice and captured a duck; and an immature eagle 
forced a snow goose down into a field and fed on it. Al- 
though these observations are few, we believe that eagles 
prey on healthy waterfowl regularly at Swan Lake. 
Defensive tactics that waterfowl employ when an eagle 
comes near further suggest that they are attacked by 
eagles. 
