Polar bear predation on barrier island bird colonies in Arctic Alaska increases with sea-ice decline

Abstract

Anthropogenic carbon emissions are warming the Arctic, disrupting ecosystems, and changing the behaviour and interactions of their component species. As summer sea ice diminishes, polar bears (Ursus maritimus) are losing access to traditional seal prey and are increasingly forced onto land, where bears can consume eggs, nestlings and sometimes adults of coastal-nesting marine birds. This behaviour has been poorly documented in northern Alaska. We present observations of polar bears in the Alaskan Beaufort Sea preying on Mandt’s black guillemot (Cepphus grylle mandtii) and Pacific common eider (Somateria mollissima v-nigrum). At Cooper Island, predation on guillemots was episodic (27% of years) and negligible from 1975 to 2007, but increased during 2008–2010, when bears caused 43% nestling mortality. Bear-resistant nest cases deployed in 2011 initially reduced losses until 2024 and 2025, when bear predation led to complete reproductive failure. In 2024, at a common eider colony on Spy Island, a single polar bear destroyed 89% of nests in less than a day. Despite extensive surveys of common eider colonies between the 1970s and early 2000s, reports of polar bear predation at these colonies were isolated and rare. These observations reveal a rapid shift in predation pressure, adding to existing climate-related impacts on these birds, including less prey and flooded or eroded nesting habitat. Our observations demonstrate how sea-ice loss is transforming Arctic ecosystems, creating unsustainable pressures on vulnerable bird populations and underscoring the urgency for solutions that will need to address the root causes of climate warming.