Numerical and functional responses of migrant Bald Eagles exploiting a seasonally concentrated food source

We determined the numerical and functional responses of migrant Bald Eagles (Haliaeetus leucocephalus) feeding on spawning kokanee salmon (Oncorhynchu s nerka) at Hauser Reservoir, Montana from 1991-1995. Number of Bald Eagles was positively correlated with the fluctuating number of salmon carcasses during four of five years. Immigration and emigration rates were similar ac ross years and were facilitated by several behavioral and physical characte ristics of eagles: group foraging, communal roosting, and keen eyesight. Nu mber of subadult eagles showed closer synchrony with density of salmon carc asses than did adult eagles. Eagles scavenging for salmon exhibited a Type II functional response. Handling times of scavenging eagles remained consta nt across the range of salmon carcass densities, whereas daily attack rates increased. Functional responses of scavenging eagles differed between age groups; adults exhibited a Type I response, whereas that of subadults could not be characterized. Handling times of scavenging adults were constant, b ut those of subadults increased with salmon density. Attack rates of scaven ging adults increased with salmon carcass density. Consumption rates differ ed between age groups and among eagles using foraging modes of scavenging, stooping, and pirating, which suggests that eagles viewed live and dead sal mon as alternative prey types. Bald Eagle scavenging of kokanee salmon was inversely density dependent.