Atlantic populations of a declining oceanic seabird have complex migrations and weak migratory connectivity to staging areas

Anthropogenic change is impacting ecosystems globally, causing declines in biodiversity. Long-distance migrants are particularly susceptible, as they depend on conditions over large geographical scales and are 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 routes or strategies. We tracked 131 Arctic skuas from Scotland, the Faroe Islands, Norway and Svalbard between 2009 and 2019 using geolocators. To investigate migration strategies, we applied a hidden Markov model, using saltwater immersion data to infer stopovers and transit flights. Skuas used several discrete staging areas during migration, with an area of high marine productivity in the mid-North Atlantic being of high importance. Individuals from the different breeding populations overlapped extensively in staging areas, resulting in weak spatial connectivity between breeding and staging areas during southbound (r(M) = 0.25, 95% CI = 0.09-0.42; 0 = weak connectivity, 1 = strong connectivity) and northbound (r(M) = 0.16, 95% CI = -0.02 to 0.33) migration. Variation in migration strategies was driven by individuals from Svalbard, which belong to a population that is declining less than the other populations tracked. The relative location of wintering areas also influenced migration strategies. Individuals migrating further spent a smaller proportion of their migration at stopovers than those wintering closer. Identifying the non-breeding distribution, migration strategies and weak migratory connectivity of Arctic skuas provides a vital step towards linking conditions during migration to population dynamics and prioritising future research and conservation actions.