The influence of salinity on the diet of nesting Bald Eagles

Although the breeding density of Bald Eagles (Haliaeetus leucocephalus) in the lower Chesapeake Bay is known to vary with salinity, the ecological factors that contribute to this distribution have not been explored. In an effort to examine whether variation in prey use is associated with nest density patterns, we investigated the influence of salinity (tidal-fresh vs. mesohaline zones) on Bald Eagle diet composition by using video-monitoring to observe food delivered to nests during the 2002-2003 breeding seasons. Delivered prey items were identified to the lowest taxonomic level possible and sizes were estimated relative to eagle bill length. We used species-specific length-weight relationships for prey to estimate biomass delivery. Overall, the diet included at least 12 species of fishes, three species of birds, four species of mammals, and four species of reptiles. Salinity had no significant influence on diet composition; Ictaluridae and Clupeidae species were the most frequent prey items in both salinity zones. We suggest that pairs nesting in both tidal-fresh and mesohaline zones have access to similar fish species. However, the length and biomass of fish prey varied with salinity such that larger prey on average were delivered to nests in the mesohaline reaches compared to fidal-fresh zones. Thus, we suggest that foraging eagles may be exploiting more energetically-favorable conditions in higher salinity waters. Temporally, diet composition varied between study years, potentially reflecting annual changes in the availability of prey species. We consider differences in weather patterns between study years as the most likely factor contributing to this interannual variation in diet.