Document Type

Article

Publication Date

3-1-2010

Keywords

Bald Eagle, Diet, Foraging, Glaucous-winged Gull, Haliaeetus leucocephalus, Harbor seals, Larus glaucescens, Protection Island, Washington

Abstract

From 1980 to 1998, Washington's Bald Eagle (Haliaeetus leucocephalus) 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 an 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 on 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 on gull eggs in tall grass than on gull eggs in sparse vegetation. Prey remains beneath one eagle nest on 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 on live seals is probably negligible. In contrast, direct predation and indirect effects of eagle activity on Glaucous-winged Gull (Larus glaucescens) 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 19932008. © 2010 The Raptor Research Foundation, Inc.

Journal Title

Journal of Raptor Research

Volume

44

Issue

1

First Page

19

Last Page

29

DOI

10.3356/JRR-08-107.1

First Department

Biology

Second Department

Mathematics

Acknowledgements

Retrieved February 22, 2021 from https://bioone.org/journals/journal-of-raptor-research/volume-44/issue-1/JRR-08-107.1/Foraging-Related-Activity-of-Bald-Eagles-at-a-Washington-Seabird/10.3356/JRR-08-107.1.full

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