We compared results from aerial and road surveys of bald eagles (Haliaeetus leucocephalus) conducted over 2 winters in a 4000-km2 area of southwest Idaho. Road surveys were scheduled within 1 day of bi-monthly aerial surveys. Aerial surveys consistently underestimated numbers of bald eagles relative to road surveys. Detectability differed between age classes. Adults were underestimated by 31% during aerial surveys, and immatures were underestimated by 49%. Immatures were undercounted more along reservoirs than along rivers: aerial counts were 60% of road counts along rivers and 37% of road counts along reservoirs. Though results from aerial surveys are biased, they can be precise. Thus, their utility for assessing long-term trends in populations is valid. As a monitoring tool, aerial surveys are useful to managers, but do not necessarily yield true population numbers.
We studied bald eagle (Haliaeetus leucocephalus) foraging ecology on the South Fork Boise River, Idaho, during the winters of 1990-92. We compared habitat variables at 29 foraging sites, 93 perch sites, and 131 random sites. Habitat variables included river habitat type (pool, riffle, run, pocket water), distance to the nearest river habitat change, distance to nearest available perch trees, number and species of surrounding perches, and average river depth and velocity. Eagles foraged more at pools than expected and closer (within 15 m) to river habitat changes than expected. Eagles perched less at riffles and more at sites where trees were available than expected. Eagles foraged at riffles that were slower than riffles where they perched or were available at random. Eagles foraged at runs that were shallower and faster than runs at perch or random sites. Low surface turbulence may increase vulnerability of fish to eagle predation.
We studied bald eagle distribution and wintering ecology within the upper Boise River drainage, Idaho, during the winters of 1990-92. Our study area contained uncontrolled and controlled river reaches, as well as reservoirs. Counts of bald eagles were made during aerial surveys, and ice cover and water temperatures were recorded during road surveys. We also analyzed results from annual Midwinter Eagle Counts conducted since 1981. Both data from our study and from Midwinter Eagle Counts yielded the same results. Within our study area, eagles were most numerous along river reaches controlled by dams, and least common along uncontrolled river reaches. Less ice cover, higher water temperatures, and more consistent flows contributed to greater fish densities and more consistent foraging opportunities for eagles along the controlled river reaches. Big game carrion was an important but less consistent food source for eagles on reservoirs. Uncontrolled rivers were colder, had more ice cover, fewer fish, and fish were likely less available to eagle predation than in the controlled rivers within our study area. Elevation likely contributed to colder temperatures and greater ice cover in uncontrolled rivers.