INFLUENCE OF AGE AND PREY AVAILABILITY ON BALD EAGLE FORAGING BEHAVIOR AT GLACIER NATIONAL PARK, MONTANA

摘要

It has been hypothesized that foraging tactics and ability of Bald Eagles (Haliaeetus leucocephalus) are influenced by age, phenotype, and prey availability. We studied the influence of eagle age class and prey availability of kokanee salmon (Onco-rhynchus nerka) on foraging behavior of Bald Eagles during autumns of 1983 and 1984 at Glacier National Park, Montana. The relative use of foraging tactics differed among four age classes of eagles during both years. Stooping was the most successful tactic and was most frequently used by older birds. The relative use of stooping increased with age and the use of ground piracy tended to decrease with age. The relative use of different foraging tactics also reflected changing prey availability. During 1983, when lower numbers of salmon precluded accumulation of carcasses, eagles rarely used ground tactics (i.e., scavenging and ground piracy). In 1984, when salmon carcasses accumulated in large numbers, all age classes used ground tactics, which became the predominant foraging method of younger eagles. Our results support the hypotheses that the ability to obtain food increases with age and that eagles forage by methods for which their age class is most suited based on morphology (e.g., size and wing loading) and experience.