Divergent migratory strategies of black guillemots from two colonies in the Norwegian Sea

Abstract Coastal zones are heavily impacted by anthropogenic development. However, they are also important habitats for seabirds, some of which are coastal year-round. We investigated the non-breeding season distribution and activity patterns of black guillemots ( Cepphus grylle ), a year-round coastal seabird species, from two colonies in central/northern Norway over multiple years using geolocator loggers equipped with wet-dry sensors. Migratory behaviour differed among birds from the two colonies. Birds from Røst (67.5°N, 12.0°E) travelled on average further (mean 283 km; max 1008 km) and to more southerly wintering areas than those from Sklinna (65.2°N, 11.0°E), which remained closer to the colony (mean 97 km; max 118 km). Birds from both colonies wintered along coastal stretches of Norway that are characterized by a high islet density and kelp forests, which appear to be important habitat characteristics for black guillemots year-round. Individuals tracked over multiple years showed an intermediate to high repeatability in migration movements. Saltwater immersion data suggested that birds used twilight periods to forage during the winter months, especially at higher latitudes. This suggests that reduced daylength and low temperatures constrained black guillemots to use suboptimal light conditions for foraging. Our study adds important and management-relevant knowledge about the non-breeding season movements and activity patterns of black guillemots.