Migration behaviour, wintering areas and conservation biology of brown skuas breeding in the subtropical Amsterdam Island

Invasive non-native species are a major threat to seabirds, leading to the implementation of numerous eradication campaigns. However, eradication can also affect non-targeted species. There are concerns over the fact that the invasive mammal eradication using poisonous bait planned on Amsterdam Island might affect negatively the local population of subtropical brown skuas Stercorarius antarcticus hamiltoni . Here, movements of 21 adult brown skuas breeding at Amsterdam Island, southern Indian Ocean, its most northerly breeding site were studied during the non-breeding period using geolocation, in order to provide relevant information for conservation prior to the eradication program. Post-breeding movements of brown skuas vary considerably, ranging from residency on the breeding grounds to long-range migrations to reach distant northern non-breeding zones in the Southern Hemisphere. Most individuals remained in the Indian Ocean (with the exception of one that wintered in the Tasman Sea), targeting areas along a continuum from the subantarctic to the tropics. Wintering grounds were generally situated in productive dynamic upwelling waters or frontal systems, with brown skuas avoiding the less productive area of the South Subtropical Gyre in the Central Indian Ocean. Inter-individual differences were not fully explained by sex: if males and females exhibited differences in activity metrics, they did not differ in duration or distance reached during the non-breeding period. Feather isotopic values confirmed that the birds mainly moulted their body feathers in the wintering area. The low δ ¹⁵ N values of feathers grown in mixed subtropical-subantarctic waters suggest that skuas feed on low trophic level prey in these areas. Overall, our results provided relevant information for conservation, and in particular helped identify the optimal period for scheduling land-based operations for eradicating introduced species on Amsterdam Island.