In some seabirds sexually dimorphic in size, males and females segregate at sea or diverge in other aspects of foraging behavior. We examined factors influencing foraging strategies of Cory's Shearwater and compared the sexes' flight morphology and activity patterns. Trip duration, incubation-shift length, total mass gain, and rate of mass gain at sea of birds from our two study colonies differed. The colonies are situated in regions of contrasting oceanographic conditions: Selvagem Grande, a remote subtropical oceanic island, and Berlengas, an island on the Portuguese continental shelf. Although the wing loading and wing span of males and females breeding at Selvagem Grande differed significantly, sex did not consistently influence activity patterns of Cory's Shearwaters foraging at sea during the incubation period. Moreover, both sexes breeding at Selvagem Grande foraged in areas with similar sea-surface temperatures. Our study suggests that sexual differences in size and shape may be poor predictors of differentiation in the ways male and female pelagic seabirds use the marine environment.
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Vol. 111 • No. 3