We observed foraging behavior and success of gulls (Larus spp.) at spawning runs of eulachon (Thaleichthyes pacificus; Osmeridae) in Berners Bay, Alaska, in spring 1996–1998. Adults foraged more effectively (≥56% of dives were successful) than immatures (≤56%) when diving for fish, but there was little difference in the effectiveness of piracy (≤23% success for all gull species and age classes). The hypothesis that larger birds would be better pirates and less likely to lose prey to pirates was not well supported, although smaller species seldom attacked larger ones. Age classes less successful at foraging were more likely to act as pirates, but this relationship did not hold among species or between years. The frequency of piracy attempts was positively correlated with the availability of fish captured by other birds.
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Vol. 73 • No. 1