Winter Foraging Ecology of Bald Eagles on a Regulated River in Southwest Idaho

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We studied Bald Eagle foraging ecology on the South Fork Boise River, Idaho, during the winters of 1990-92. We compared habitat variables at 29 foraging sites, 94 perch sites, and 131 random sites. Habitat variables included river habitat (pool, riffle, run), distance to the nearest change in river habitat, distance to nearest available perch, number and species of surrounding perches, and average river depth and flow. Eagles foraged more at pools than expected, and closer ((15 m) to changes in river habitat than expected. Where eagles foraged at riffles, those riffles were slower than riffles where they perched or riffles that were available at random. Where eagles foraged at runs, those runs were shallower than runs at either perch or random sites. Eagles perched less at riffles and more at sites where trees were available than expected. Changes in river habitat represent habitat edges where river depth and flow change, making fish more vulnerable to eagle predation. Fish are more susceptible to predation at shallower river depths and slower flows. Slower river flows may be related to decreased surface turbulence, which also increases vulnerability of fish to aerial predation.

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