Strong breeding colony fidelity in northern gannets following high pathogenicity avian influenza virus (HPAIV) outbreak

High pathogenicity avian influenza virus (HPAIV) caused the worst seabird mass-mortalities in Europe across 2021-2022. The northern gannet (Morus bassanus) was one of the most affected species, with tens of thousands of casualties in the northeast Atlantic between April-September 2022. Disease outbreaks can modify the movement ecology of animals by diminishing spatial consistency, thereby increasing the potential for disease transmission. To detect potential changes in movement behaviour, we GPS-tracked breeding adults following the initial HPAIV outbreak, at three of the largest northern gannet breeding colonies where major mortality of adults and chicks occurred (Bass Rock, Scotland, UK; Grassholm, Wales, UK; Rouzic, Brittany, France). We also gathered back-ground epidemiological information and northern gannet colony dynamics during the outbreak. Our data indicate that HPAIV killed at least 50 % of northern gannets, and suggest the presence of HPAIV H5N1 antibodies in juveniles. GPS-tracked adult northern gannets remained faithful to their breeding sites despite the HPAIV outbreak and did not prospect other breeding colonies. They performed regular foraging trips at sea, similar to their behaviour before the outbreak. Comparison with GPS-tracking data gathered in 2019, i.e. before the HPAIV outbreak, suggested lower foraging effort in birds which survived HPAIV in 2022, potentially as a consequence of reduced intra-and interspecific food competition. Breeding colony fidelity of surviving adult northern gannets following HPAIV mass-mortalities indicates limited capacity for viral spread during our study. This may contrast with the behaviour of adults during the initial disease outbreak, and with that of younger individuals.