| Summary: | The critical endangered Common and Brünnich guillemot are exhibiting contrasting population trends in the Barents Sea region, where the Common guillemots are increasing, while Brünnich guillemots are declining. The non-breeding season is regarded as the main mortality period among seabirds, especially in winter, as climate conditions may be harsh and prey availability rather limited. The contrasting trends seen in the Barents Sea Common and Brünnich guillemot populations are believed to be related to the two species different non-breeding ecology, although little is known about the bird’s ecology in this particular period, as the species are out at sea, unavailable for scientists. However, the development of light-weighted tracking equipment has made it possible following small migrating species, such as birds, throughout the entire year-cycle. In combination with stable isotopes, such methods can be used to investigate species ecology non-invasively. This study used combined feather stable isotopes and light-based geolocation data of the two sympatric seabird species to investigate intra- and interspecific dynamics in their non-breeding strategies (i.e., changes in foraging distribution and isotopic niche) in the Barents Sea. Differential responses were found between the species foraging distribution and isotopic niches, as they showed no overlap in isotopic niches when their distribution overlapped, and vice versa, which might indicate an avoidance mechanism for equal resource utilization during the non-breeding season. The species responded differently to limitations in food conditions in the Barents Sea during winter, as Common guillemot exhibited smaller degree of change in isotopic niches compared to the Brünnich guillemot, indicating that the Brünnich guillemot change from having a generalized foraging strategy in winter, to become more specialized during autumn when food is more abundant.
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