Migration Ecology of Bald Eagles from Autumn Concentrations in Glacier National Park, Montana

During 1977-93, we studied the migration of bald eagles (Haliaeetus leucocephalus) from autumn concentrations in Glacier National Park, Montana (GNP). Nonnative kokanee salmon (Oncorhynchus nerka) had attracted migrating eagles to lower McDonald Creek (LMC) in GNP since 1939. In 1987 the number of spawning kokanee declined precipitously from previous peaks >100,000 and by 1991 no spawning salmon were observed in LMC. The annual peak number of eagles, which had reached 639 in 1981, fell to 25 in 1989. During 1977-88, we captured and leg banded 303 eagles; of these, 66 also were radio tagged and 121 received patagial markers. Patterns of migration, seasonal distribution, habitat fidelity, marker effects, mortalities, and foraging activities were documented. Code sightings (marker code read, n = 85) and marker sightings (orange marker reported, n = 195) came from 9 western states and Canada. Thirty-eight of 40 radio-tagged eagles wintered within the Intermountain Region (east of the Cascade and Sierra Nevada mountain ranges and west of the Continental Divide): 14 in Montana, 12 in Utah, 7 in Idaho, 2 in Oregon, 1 in California, 1 in Nevada, and 1 in Wyoming. Wintering sites of individual eagles shifted as far as 660 km in different years. Winter ranges varied from 102 km2 to nearly 4,000 km2. During spring migration, radio-tagged eagles followed converging routes to the GNP vicinity. Adult eagles followed a narrow corridor north through southern Alberta and then gradually diverged to summering areas; paths of immatures were more erratic. Thirty of 31 radio-tagged eagles summered within the Mackenzie River Basin, Canada: 21 in the Northwest Territories (NWT), 5 in Alberta, and 4 in Saskatchewan. Distances between wintering and summering areas were significantly different between juveniles (median = 1,480 km) and subadults (median = 2,268 km) or adults (median = 2,056 km). During a 10-year period, we documented a nesting territory shift from Alberta to Montana (910 km) and then back to Alberta for 1 eagle. In GNP, 27% of wing-marked eagles were resighted in years after marking. Twenty-three banded eagles were reported dead (1 in Alaska). We demonstrated an eagle migration flyway connecting northern Canada's Mackenzie River Basin and the Intermountain Region of the western United States. All sites used by eagles throughout the flyway should be highly valued and managed as part of a dynamic landscape mosaic, rather than being considered isolated habitat fragments