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Ecology of Bald Eagles in the Greater Yellowstone Ecosystem

Jon E. Swenson, Kurt L. Alt and Robert L. Eng
Wildlife Monographs
No. 95, Ecology of Bald Eagles in the Greater Yellowstone Ecosystem (Apr., 1986), pp. 3-46
Published by: Wiley on behalf of the Wildlife Society
Stable URL: http://www.jstor.org/stable/3830668
Page Count: 44
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Ecology of Bald Eagles in the Greater Yellowstone Ecosystem
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Abstract

The ecology of the bald eagle (Haliaeetus leucocephalus) was studied from 1972 to 1982 in the Greater Yellowstone Ecosystem in northwestern Wyoming and adjacent Idaho and Montana. This population consisted of 3 ecologically, but not genetically, distinct units: the Yellowstone Unit (YU), the Continental Unit (CU), and the Snake Unit (SU). The population probably was stable at 30-31 breeding pairs from 1960 to 1970, but increased exponentially to 50 known breeding pairs from 1971 to 1982. There were 50-58 breeding pairs and a total of 180-210 bald eagles in this population in autumn 1982. Spraying of DDT for forest insect control between 1955 and 1963 apparently depressed reproduction, recruitment, and population levels in sprayed areas. Breeding bald eagles were adaptable in selecting breeding areas. A stable food source, which was available from early spring, appeared to be the most important factor in breeding area selection. Reproductive rates in the Greater Yellowstone Ecosystem (0.98 young/occupied nest and 60% success of occupied nests) were above that required for population stability. However, reproductive rates were low in the YU (0.53 and 34%, respectively) due to severe spring weather found at the high elevation of the YU. Recruitment from the SU and CU probably maintained population stability in the YU. Acceptance of young subadults into occupied territories by adults that had failed in their reproductive attempt suggested that a behavioral mechanism compensated for reduced natality. During the breeding season, diet varied in relation to food availability; 68% was birds in the YU, 47% was birds in the CU, and 67% was fish in the SU. Availability of food in winter was correlated with movement patterns. Limited availability of food in the YU resulted in movement of 82% of the adults and 99% of the subadults out of the area. More food (waterfowl and fish) was available in the CU, where adults were sedentary and about 86% of the subadults left the area. The abundance of ungulate carrion in the SU was probably the reason for the 45% influx of adults and less movement of the subadults (52%) out of the area. Subadults that leave the ecosystem in winter migrate westward to the Pacific Northwest. Bald eagles in the Greater Yellowstone Ecosystem exhibited a remarkable degree of behavioral plasticity. Differences in movements, breeding success, nest site selection, and nesting chronology among units were due primarily to differences in the amount and timing of food availability.

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