Predation on burrowing petrels by the brown skua (Catharacta skua loÈnnbergi) at Mayes Island, Kerguelen

The diet of the brown skua (Catharacta skua loÈnnbergi) was studied at Mayes Island, Kerguelen archipelago, by collection of prey remains on 11 territories during the complete breeding cycle of the species there. In nine territories, collections were daily or every two days for a quantitative investigation of the diet. The blue petrel (Halobaena caerulea) and thin-billed prion (Pachyptila belcheri ) accounted for, respectively, 72.9 and 19% of the remains. Differences in diet and prey capture rate were analysed according to territory size, local abundance of prey inferred from the vegetation cover of territories, colony attendance patterns of prey, and according to the breeding timing and success of skuas. Skuas holding large territories caught more prey, and especially more blue petrels than those with small territories. Diet re ̄ected local abundance of blue petrel and thin-billed prion but the blue petrel was apparently preferred to other available prey. Failure to breed was not signi®cantly related to hunting performance of the skua or to food availability, but sample size was small. Variations over time of prey capture rates re ̄ected the colony attendance patterns of the main prey. Captures of blue petrel and thin-billed prion were most numerous during their respective laying, incubation and hatching periods, decreasing during chick-rearing. Prey capture rates were greatest when brown skuas were hatching, and decreased during chick-rearing to a minimum when young skuas were ̄edged. Capture rate patterns differed according to laying date of skuas: early breeding skuas caught more prey and were well synchronized with the breeding cycle of the blue petrel and late breeders exhibited more dependence on the thin-billed prion for some periods. Finally, this study has allowed us to assess the impact of the brown skuas upon the burrowing petrels on this locality.