North or south? Niche separation of endemic red‐legged kittiwakes and sympatric black‐legged kittiwakes during their non‐breeding migrations

Abstract

AbstractAimSpecies that breed sympatrically often occupy different foraging niches to mitigate competition for prey. When resource availability declines at the end of the breeding season, some animals migrate to regions with more favourable environmental conditions. When these life‐history traits combine, foraging habitat preferences may continue to influence migration patterns and habitat utilization. The Bering Sea is home to the red‐legged kittiwake (RLKI),Rissa brevirostris, which is endemic, and the black‐legged kittiwake (BLKI),Rissa tridactyla, which has a circumpolar breeding distribution. Since the 1970s, numbers ofRLKIs at the largest colony have declined and then recovered, whilst theBLKIpopulation has remained stable. Knowledge of the migration ecology of kittiwakes is key to understanding differences in population trajectories, and predicting possible future responses of these species to climate change.LocationPribilof Islands, Bering Sea, subarctic North Pacific.MethodsUsing geolocation loggers, we tracked adultRLKIs andBLKIs during their non‐breeding migrations. We used iterative methods to assess suitable sample sizes for determining space use. Kittiwakes are surface foragers; therefore we used wet–dry data to distinguish active foraging behaviour and to test the species' responses to environmental conditions. Stable isotope ratios of feathers grown during the non‐breeding period were used to assess dietary niche.ResultsRLKIs remained largely in the Bering Sea, where they experienced colder conditions and shorter days; individual birds used multiple habitats, including the continental shelves, the sea‐ice edge and pelagic waters. In contrast,BLKIs migrated to the subarctic North Pacific, where they dispersed laterally across the basin; the majority of birds travelled to the western subarctic.RLKIs spent less time actively foraging thanBLKIs, and consumed higher trophic‐level prey.Main conclusionsThe disparate wintering ranges and foraging behaviour ofBLKIs andRLKIs suggest distinct environmental factors drive variation in overwinter survival. A strong association with sea ice, and specialization both in diet and foraging behaviour, may makeRLKIs particularly vulnerable to climatic change.