Breeding together, wintering an ocean apart: Foraging ecology of the northern Bering Sea thick-billed and common murres in years of contrasting sea-ice conditions

Abstract

Assessing impacts of environmental change on Arctic-breeding seabirds requires a better understanding of their year-round movement and foraging ecology. Here we examined the post-breeding movements and diving behavior of thick-billed (Uria lomvia) and common murres (U. aalge) breeding on St. Lawrence Island, northern Bering Sea, by using geolocators deployed in 2016 (n = 3, per species). During 2016–2019, we examined foraging niches and exposure to nutritional stress by using stable isotope signatures and corticosterone titers of blood and feather tissues (n = 60–96, per species). We found that thick-billed murres migrated to the Chukchi Sea in the fall and wintered in the western North Pacific, whereas common murres stayed in the eastern Bering Sea in the fall and wintered in the eastern North Pacific. Nutritional stress levels of breeding common murres were higher in 2017–2019, the period of historic low winter sea-ice extent, than in 2016. Higher nutritional stress levels of post-breeding thick-billed murres were associated with lower fall sea-ice extent in the Chukchi Sea. These results indicate that the loss of sea-ice might negatively affect murres breeding in the Pacific Arctic. Divergent migratory connectivity between the two murre species might also lead to different conservation threats both inside and outside the Arctic.