FORAGING-RELATED ACTIVITY OF BALD EAGLES AT A WASHINGTON SEABIRD COLONY AND SEAL ROOKERY

From 1980 to 1998, Washington's Bald Eagle (Haliaeetus leitcocephalus) population increased at an annual rate of 10%. Over the same time period, foraging activity of Bald Eagles at marine bird breeding colonies also increased. From 1993 to 2008, we observed foraging-related behavior of Bald Eagles on Violet Point, Protection Island. This island hosts more than 70% of the breeding seabirds in Washington's inner seaways and serves as all important rookery for harbor seals (Phoca vitulina). We found that (1) eagles landed more frequently in seal haul-out (beach) areas than in gull-nesting (non-beach) areas of Violet Point, and that subadult eagles were more likely to land in gull-nesting areas than were adult, eagles; (2) the presence of eagles on the beach was positively related to the presence of harbor seals oil the beach; (3) a greater-than-expected number of adult eagles as compared with subadult eagles preyed on gull chicks; (4) subadult and adult eagles that attempted prey capture were equally successful at snatching gull chicks from the gull colony; (5) eagles were more likely to prey oil gull eggs in tall grass than oil gull eggs in sparse vegetation. Prey remains beneath one eagle nest oil the island did not accurately reflect the range and relative frequencies of observed eagle predation events. Although seal afterbirths and dead pups constitute a major component of the diet of Bald Eagles on the island, the effect of eagles oil live seals is probably negligible. In contrast, direct predation and indirect effects of eagle activity on Glaucous-winged Gull (Larus glaurescens) reproductive success may be substantial and may have been partly responsible for a 44% decrease in the number of gull nests in the colony from 1993-2008.