1975 - 1976 

o-r —— > Tt +; -- 
\976-\977 
8 Oe ee eee 
NUMBER OF BALD EAGLES 
1977-1978 / 
OCTOBER NOVEMBER DECEMBER JANUARY FEBRUARY MARCH 
Fig, 2. Number of bald eagles counted on Swan Lake National 
Wildlife Refuge on different dates, October-March 1975-78 
(no counts 24 October to 19 December 1977). 
December, dropped to 25 on 1 January, and then quickly 
increased to 51 on 6 January. After this third peak, eagle 
numbers declined almost steadily to 10 by 7 February, but 
then again increased to the fourth major peak (58) on 
19 February. Thereafter the population decreased rapidly 
through late February and March. 
In 1976-77, the first eagle was seen on the refuge on 
3 October, and three major peaks followed (Fig. 2). The 
first and largest was 73 birds on 14 December: numbers 
then declined abruptly to 26 on 18 December, rose to 41 
on 7 January, and then again declined. The final peak (56 
birds) occurred on 27 February. The population then fell 
steadily through March. 
In 1977-78, the first eagle was seen on the refuge on 
5 October, and three major population peaks followed 
(Fig. 2). The first and largest peak was 100 birds on 
19 December. Numbers then declined to 58 on 
11 January, climbed to 87 on 17 January, declined to 43 
on 27 January, increased to 83 on 9 February, and then 
fell steadily through March. 
1975-1976 

60 
50 
IS76-I977 
40 
30 

NUMBER OF BALD EAGLES 
70 
IMMATURE 
\977-I97B ADULT = 

OCTOBER 
NOVEMBER 
Fig. 3. Age ratio patterns of wintering bald eagles, 1975-78, 
Swan Lake National Wildlife Refuge. 
Although these population figures are minimums be- 
cause not all areas of the refuge were accessible for cen- 
susing, we believe they closely approximate the actual 
eagle numbers on the refuge. 
The average numbers of eagles on the refuge were 
similar in the first two field seasons—27.3 eagles per 
survey in 1975-76 and 29.8 in 1976-77—but differed 
greatly (45.1 per count) in 1977-78, when the peaks and 
the lows were both substantially higher than in the first 
two seasons. 
Age Ratio Patterns 
Age ratios of bald eagles on the refuge varied through- 
out each fall and winter. Age ratio patterns were similar 
in the early season during the three field seasons, but after 
December the patterns in 1975-76 and 1976-77 differed 
from that in 1977-78 (Fig. 3). During all three seasons, 
immature eagles arrived first in the fall and rapidly in- 
creased in numbers. Adults were less abundant in fall and 
early winter. In December, when eagle numbers peaked, 
67 to 77% were immatures. 
In 1975-76 and 1976-77, numbers of immatures and 
adults were roughly equal in late January. In February 
the numbers of adults increased rapidly and then quickly 
declined through March. Numbers of immature eagles 
also increased rapidly in late February and declined less 
rapidly than adults. Excluding the time of spring migra- 

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