Diets and Foraging Effectiveness of Bald Eagles during Extreme Winter Weather in Nebraska

Few studies have focused on diets and foraging efficiency of bald eagles (Haliaeetus leucocephalus) during presumed energy stress in winter. Consequently, we studied these behavioral aspects of eagles on a system of reservoirs and canals adjacent to, and including a portion of, the Platte River System during extreme weather and extensive ice cover in southwestern Nebraska in 1989. Results of 3 methods used to determine, and 3 procedures to report, dietary composition were compared. Foraging opportunities and efficiency also were evaluated among 5 study sites. Numerical use of fish, birds, and mammals was 97, 3, and 0% by direct observation; 70, 29, and 1% by collection of prey remains; and 15, 57, and 27% by pellet analysis, respectively. Numerically, gizzard shad (Dorosoma cepedianum) were used most, but Canada geese (Branta canadensis), mallards (Anas platyrhynchos), American coots (Fulica americana), and common carp (Cyprinus carpio) yielded the most biomass and energy to eagles. Hunting, piracy, and scavenging comprised 87, 9, and 4% of 1,395 foraging attempts, respectively. Foraging opportunities and efficacy were enhanced by the maintenance of ice-free waters by hydroelectric and steam-plant operations, and the disabling of prey by hydroelectric facilities. Foraging efficiency generally was higher at the tailraces of hydros compared to a reservoir or the Platte River. Adults were more effective foragers than subadults. With proper management, power-generating facilities can benefit wintering eagles by providing foraging opportunities during periods of potential energy stress.