Diet and foraging areas of Southern Ocean seabirds and their prey inferred from stable isotopes: review and case study of Wilson’s storm-petrel

Abstract

Analysis of stable isotope ratios in animal tissues has emerged as a powerful tool for determining the trophic level and composition of prey and foraging location. We summarize here data on the stepwise trophic enrichment in δ 15 N and latitudinal gradient in δ 13 C in the Southern Ocean, and derive a regression equation to estimate latitudes from δ 13 C values. We analysed isotope ratios of feathers of the small, pelagic seabird Wilson's storm-petrel Oceanites oceanicus, in different breeding stages, in comparison to isotope ratios of 4 other seabird species breeding in close vicinity on King George Island, South Shetland Islands. δ 15 N analysis of feathers and albumen from Wilson's storm-petrels indicated a shift in diet from mainly crustaceans during egg formation to an increased proportion of fish during chick-feeding and moulting. δ 15 N values of Wilson's storm-petrels during the chick-rearing season were closer to the mainly piscivorous-carnivorous skuas than to krill-feed- ing penguins, confirming that fish is an important part of their diet. δ 13 C analysis of feathers identi- fied 4 distinct foraging areas: δ 13 C values in egg-white suggest that egg-forming females moved south to the sea ice edge. This coincides with the distribution of their main prey, Antarctic krill Euphausia superba, during this period. During the breeding season, Wilson's storm-petrels fed in the area around the colony, which is also used by penguins and skuas. δ 13 C of the feathers indicate that adults migrated to the Subtropical Front and beyond (north of 44°S) during the inter-breeding period. Feathers were also analysed from 10 Wilson's storm-petrels caught by mistnet and thought to be pre- breeders because they lacked foot markings; 8 of these had moulted in the same area as breeding birds, while 2 birds had moulted in an area further north (north of 30°S). Adelie penguins Pygoscelis adeliae and Gentoo penguins P. papua had significantly different δ 13 C, suggesting that the Adelie penguins foraged further south than the Gentoo penguins. The foraging areas of brown skuas Ster- corarius antarctica and south polar skuas S. maccormicki could not be separated by their isotope ratios.