Winter Ecology of Bald Eagles in the Upper Boise River Drainage, Idaho

Publication Date


Type of Culminating Activity


Degree Title

Master of Science in Raptor Biology



Major Advisor

Marc J. Bechard


James C. Munger


Karen Steenhof


Chapter 1

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. Detectability differed between age classes: adults were underestimated by 31%, and immatures were underestimated by 49% during aerial surveys. 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, and as a monitoring tool, they are useful to managers, but may not yield true population numbers.

Chapter 2

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, 93 perch sites. Habitat variables at 29 foraging sites, 93 perch sites, and 131 random sites. Habitat variables included river habitat type (pool, riffle, run), 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 perched or random sites. Low surface turbulence may increase vulnerability of fish to eagle predation.

Chapter 3

We studies Bald Eagle distribution within the upper Boise River Drainage, Idaho, during the winters of 1990-92. Eagle distribution was influenced by a combination of factors including abundance and availability of prey, water temperatures and ice cover, elevation, and the presence of dams. Counts of Bald Eagles were made during aerial surveys, and ice cover and water temperature were recorded during road surveys. Within our study area, eagles were most numerous on the South Fork between Anderson Ranch and Arrowrock Reservoirs, and least common on the Middle Fork and South Fork between Featherville and Pine. Less ice cover, higher water temperatures, and more consistent flows on the South Fork between Anderson Rance and Arrowrock Reservoirs contributed to greater fish densities and more consistent foraging opportunities for eagles. Big game carrion was an important but less consistent food source for eagles on reservoirs. Carrion became more abundant during periods of harsh weather, but these foraging opportunities were scattered and inconsistent.

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