Cross population comparison of complex migration strategies in a declining oceanic seabird

Abstract Human-driven change is impacting ecosystems globally, with consequential declines in biodiversity. Long-distance migrants are particularly susceptible as they depend on conditions over large geographical scales and are more likely to experience a greater range of pressures. One long-distance migrant that has experienced substantial declines across the North-East Atlantic is the Arctic Skua Stercorarius parasiticus however, little is known about their migratory behaviour or the routes they undertake. We used geolocators with salt-water switches to track Arctic Skuas from four breeding populations. To investigate migration strategies, we developed a Hidden Markov Model approach that used saltwater immersion data to classify stopovers and transit flights. We found that the skuas used several discrete staging areas during their south and northbound migrations with an area of apparent high marine productivity in the mid-North Atlantic being of high importance. Extensive overlap of individuals from different breeding populations occurred in staging areas, resulting in weak spatial connectivity between breeding and staging areas, further emphasizing their importance. At the population-level, variation in migration strategies was driven by individuals from Svalbard, which is declining less than the other populations tracked. Relative location of wintering areas also influenced migration strategies, with individuals migrating further spending a smaller proportion of their migration at stopovers compared to those wintering closer, and instead employed a fly-and-forage strategy. Identifying the complex non-breeding distribution, weak migratory connectivity and highly conserved migration strategies of Arctic Skuas is a vital step to link how conditions experienced during migration influences population dynamics and to prioritise future research and conservation actions.