Age differences in kleptoparasitism by laughing gulls ( Larus atricilla) on adult and juvenile brown pelicans ( Pelecanus occidentalis)

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We recorded the choice of victims by a population of adult and first-winter laughing gulls ( Larus atricilla) making attempts to steal food from adult and juvenile brown pelicans ( Pelecanus occidentalis). Adult gulls made food theft attempts on juvenile pelicans with a disproportionately great frequency; first-winter gulls appeared to select pelican victims at random. Adult pelicans were more frequently successful in their foraging attempts than juvenile pelicans, suggesting that adults may be more reliable as potential food sources for gulls making theft attempts. However, juvenile pelicans attempted to evade kleptoparasitizing gulls less frequently than did adult pelicans, suggesting that the fish prey of juveniles may be more easily stolen. These patterns are discussed in relation to optimal victim choice by gulls, and to deferred maturity in both species.