Non-breeding spatial ecology of migratory Audouin’s Gulls from the westernmost populations

Understanding how seabirds use space during the non-breeding period is essential for advancing our knowledge of their ecology and informing conservation. We investigated migration phenology, wintering ecology and the relative importance of environmental conditions and industrial fishing activity in Audouin’s Gull ( Ichthyaetus audouinii ) breeding at two recently established colonies located at the Atlantic–Mediterranean transition, Ria Formosa (southern Portugal) and Ceuta (North Africa), representing the westernmost limit of the species’ breeding range. We characterised migration timing, identified non-breeding areas (wintering and secondary high-use areas), quantified home-range size and spatial fidelity, and evaluated the environmental and fisheries-related drivers of residence time within non-breeding areas. To do so, we analysed GPS tracking data collected over four non-breeding seasons (2022–2026) from 34 migratory adults. Migration strategies differed between colonies. GPS-tracked gulls from Ria Formosa were almost exclusively migratory (94.4%), whereas tracked individuals from Ceuta showed partial migration (41.4% resident), with resident and migratory individuals coexisting within the same population. Although migration phenology matched that previously described for traditional Mediterranean colonies, birds from Ria Formosa departed earlier and returned later than those from Ceuta, resulting in a longer non-breeding period. Non-breeding home-ranges from both colonies were concentrated along the continental shelf off southern Morocco and Western Sahara, confirming this region as the principal wintering area for Atlantic–Mediterranean populations of Audouin’s Gull. However, birds from Ria Formosa used more non-breeding areas per season and had larger non-breeding home-ranges than birds from Ceuta, indicating a broader spatial organisation of the non-breeding period. Spatial overlap among individuals was generally low, whereas interannual overlap within individuals was higher, indicating consistent fidelity to their wintering areas. Residence time within wintering areas was best explained by bathymetry and lagged sea-surface temperature and chlorophyll- a , whereas industrial fishing effort had little effect. Movement strategies and habitat use in Audouin’s Gull appear to be shaped by different ecological processes across the annual cycle: once released from central-place foraging constraints, birds appear to rely more on natural oceanographic productivity than on fisheries. Because wintering grounds are shared by birds from different breeding populations, conservation of this Vulnerable seabird, as of all threatened species, requires a full annual-cycle perspective, with measures extending beyond breeding colonies and national waters. Not applicable.