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

Arctic Skua with ring and geolocator on leg, by Sarah Harris / BTO

Author(s): O'Hanlon, N.J., van Bemmelen, R.S.A., Snell, K.R.S., Conway, G.J, Thaxter, C.B., Aiton, H., Aiton, D., Balmer, D.E., Are Hanssen, S., Calladine, J.R., Hammer, S., Harris, S.J., Moe, B., Schekkerman, H., Tulp, I. & Humphreys, E.M.

Published: March 2024  

Journal: Marine Ecology Progress Series Volume: 730

Digital Identifier No. (DOI): 10.3354/meps14533

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Tracking the long-distance migrations of Arctic Skuas from their north-east Atlantic breeding grounds revealed complex migration strategies, with mixing of individuals from different populations at important staging areas before the birds reached their southern wintering grounds.  

Arctic Skuas are long-distance migrant seabirds that have seen large declines in breeding numbers across areas of the north-east Atlantic. Part of these declines has been attributed to poor food availability during the breeding season, exacerbated by predation from Great Skuas, particularly in years where food availability is low. However, Arctic Skuas only spend around a third of the year at their breeding grounds. Therefore, they likely also face a range of threats during the non-breeding season.

To shed light on the migration routes and strategies of Arctic Skuas, researchers, including BTO scientists, tracked 131 individuals with small tracking devices called geolocators between 2009 and 2019, collecting information from four breeding populations: Scotland, the Faroe Islands, Norway and Svalbard. This collaboration revealed extensive mixing of Arctic Skuas from different breeding populations during migration in several discrete staging areas. An area of high marine productivity, part of which has recently been designated as a high seas Marine Protected Area, was particularly important to the skuas during both their south-bound (autumn) and north-bound (spring) migrations. Because of their predictable food sources, such staging areas are vital, fuelling long flights to the wintering areas and, during spring, enabling individuals to build up reserves for the upcoming breeding season.

This considerable mixing of individuals means that if adverse conditions affect the skuas in these important staging areas, then it has the potential to negatively impact multiple breeding populations through reduced survival or productivity.

However, the data also revealed some differences in the migration routes and staging areas of individuals from the different breeding populations. Specifically, during southbound migration, skuas from Scotland largely migrated south through the North Sea and along the Iberian Peninsula, whilst those from the other more northerly populations tended to head west towards the mid-Atlantic staging area. Individuals from Svalbard staged much further west in the Atlantic during both migrations, where they may have encountered different, potentially more favourable, conditions given that the Svalbard population appears to be declining less severely than other populations in the north-east Atlantic.

Understanding where long-distance migrants, such as Arctic Skuas, are distributed during migration and the strategies they use is a vital first step in identifying threats that individuals may encounter en route, and how this may affect their survival, productivity and therefore population trends. This new knowledge will help us prioritise future research and conservation actions for this declining charismatic seabird.


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, 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 (RMantel = 0.25, 95% CI: 0.09 - 0.42; 0 = weak connectivity, 1 = strong connectivity) and northbound (RMantel = 0.16, -0.02 - 0.33) migration. 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. 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 to link conditions during migration to population dynamics and prioritise future research and conservation actions.


British Trust for Ornithology (BTO) research was funded by several generous individual donors. The work at Slettnes was financed by the Netherlands Organisation for Scientific Research (NWO project number 866.13.005) and supported by The Norwegian Institute for Nature Research (NINA). Thanks to the Faroese Research Council which supported the Faroese fieldwork financially. Catching and deploying geolocators was approved in Scotland by the Special Method Technical Panel, part of the BTO–Joint Nature Conservation Committee (JNCC) Avian Demographic Scheme; in the Faroe Islands by the Copenhagen Bird Ringing Centre and The National Museum of the Faroe Islands; and in Svalbard and Norway by the Governor of Svalbard and Norwegian Food Safety Authority (FOTS ID 2086, 3817, 6329, 8538, 15726).
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