Variation in diet composition during the breeding cycle of an Antarctic seabird in relation to its breeding chronology and that of its main food resource

This work had two aims related to the diet of brown skuas (Stercorarius antarcticus lonnbergi) breeding at Laurie Island (South Orkney Islands, Antarctic). The first aim was to explore whether there are changes throughout the breeding season. The second aim was to determine whether those changes relate to differences in food resource availability of their main prey, penguins, at different time periods of the penguins’ breeding cycles, or to different moments of the skuas breeding cycle, which may variably restrict the foraging activities of parents. Diet was analyzed from pellet samples grouped in two different ways. They were grouped in three periods defined for the skuas breeding cycle (laying and incubation; early parental care; later parental care), or the pellets were assigned to five periods based on the type of food resources available at the penguin colonies (eggs; eggs and small chicks; small and large chicks; large and fledged chicks; fledged chicks). A temporal variation in diet composition was evident from the analysis of contingency tables for both sample grouping methods. The more represented item in every period for both analyses was adult penguins, which may be related to the proposed cleaning function of the gut of penguin feathers. Both ways of grouping the samples suggest a relationship between the kind of resources available at the penguin colonies and the easiness of delivering them to the skuas chicks, reflected in a successive predominance of use of penguin eggs first and of penguin chicks and other birds later.