Social Organization and Foraging Ecology in the Arctic Skua Stercorarius Parasiticus: A Test of the Food Defendability Hypothesis

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Abstract
Colonial and solitary breeding in birds have been hypothetically explained as adaptations to different types of food. Evenly distributed food with stable occurrence can be defended, which favors solitary nesting in feeding territories. Patchily distributed food at unpredictable locations is not economically defendable, which selects against feeding territoriality. Colonial breeding may then be adaptive. This hypothesis was tested in arctic skuas, which show both forms of social system in northeast Norway. As predicted, colonial pairs mainly forage far from the nest, at sea, whereas solitary pairs often hunt in large (.apprx. 1 km2) defended feeding territories around the nest. The diets differ accordingly, with the highest proportions of terrestrial food (birds, eggs, rodents, insects, berries), and the lowest proportions of marine food (fish, Polychaeta and Crustacea), in droppings from solitary pairs.