Schekkerman, H. | Search bioRxiv

bioRxiv2023

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

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.

ecology↗

bioRxiv2023

Ocean-scale variation in migration schedules of a long-distance migratory seabird is fully compensated upon return to the breeding site

BackgroundMigratory birds generally have tightly scheduled annual cycles, in which delays can have carry-over effects on the timing of later events, ultimately impacting reproductive output. Whether temporal carry-over effects are more pronounced among migrations over larger distances, with tighter schedules, is a largely unexplored question. MethodsWe tracked individual Arctic Skuas Stercorarius parasiticus, a long-distance migratory seabird, from eight breeding populations between Greenland and Siberia using light-level geolocators. We tested whether migration schedules among breeding populations differ as a function of their use of seven widely divergent wintering areas across the Atlantic Ocean, Mediterranean Sea and Indian Ocean. ResultsBreeding at higher latitudes led not only to later reproduction and migration, but also faster spring migration and shorter time between return to the breeding area and clutch initiation. Wintering area was consistent within individuals among years; and more distant areas were associated with more time spent on migration and less time in the wintering areas. Skuas adjusted the period spent in the wintering area, regardless of migration distance, which buffered the variation in timing of autumn migration. Choice of wintering area had only minor effects on timing of return at the breeding area and timing of breeding and these effects were not consistent between breeding populations. ConclusionThe lack of a consistent effect of wintering area on timing of return between breeding areas indicates that individuals synchronize their arrival with others in their population despite extensive individual differences in migration strategies.

ecology↗