Winter ecology of bald eagles in the Nisqually River drainage, Washington
Stalmaster, Mark V.
Kaiser, James L.
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We studied the winter ecology of a population of bald eagles (Haliaeetus leucocephalus) on the Nisqually River and a tributary, Muck Creek, in Washington, for 3 years. Peak eagle numbers on the river and creek combined occurred in early February and were 168 in 1991-92, 128 in 1992-93, and 156 in 1993-94. For the 3 years, 79% of eagles used the Nisqually River and 21% used Muck Creek. Eagle use of the river was high between 8 and 18 km (5 and 11 miles) upstream of Puget Sound and at the mouth of Yelm Creek; use on Muck Creek was concentrated on the lower 4 km (2.5 miles). The subadult proportion in the study area was 46% in 1991-92, 41% in 1992-93, and 43% in 1993-94; it increased throughout each winter; and it was higher where most eagles congregated. Chum salmon (Oncorhynchus keta) was the primary food source, and this eagle population was predicted to consume 1,100 salmon carcasses each winter based on the yearly average of 6,952 eagle use days on the river and creek. Black cottonwood (Populus balsamifera) was the most widely used tree species by both perching (53% of 1,423 eagle sightings) and roosting (53% of 94 roost trees) eagles, and 8 of 9 communal roosts were located in old-growth forests. Management to enhance chum salmon runs, maintain forest habitat, and regulate human disturbance is needed to protect this eagle population