Climate-driven changes in the marine environment may affect inter-and intraspecific resource partitioning by marine organisms. When and how resources are partitioned may depend on access to diverse foraging habitats. Here we examined the variability in the isotopic niche of breeding seabirds with respect to trophic level and habitat use in years with cold and warm sea temperatures in the Bering Sea. Between 1999 and 2015 (n = 12 yr), we collected blood from blacklegged kittiwakes Rissa

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... a, and common Uria aalge and thick-billed murres U. lomvia breeding on St. George and St. Paul Islands on the southeastern Bering Sea continental shelf. We examined isotopic niche dynamics at the group and species levels. Stable isotope values of blood tissues corroborated published observations of seabird distributions in the region. All 3 species increased foraging on shelf-based prey during warm oceanographic conditions, in contrast to a higher reliance on oceanic-based prey during cold conditions. Under warm ocean conditions, the isotopic niche of the seabird group with access to only shelf habitat (St. Paul Island) contracted, whereas the isotopic niche of the seabird group with access to shelf, slope, and basin habitats (St. George Island) expanded. These group-level responses were associated with increased food availability. We conclude that habitat heterogeneity in the vicinity of breeding colonies may mediate how predators partition food resources in response to changes in climate-driven food availability. KEY WORDS: Black-legged kittiwake • Murre • Stable Isotope Bayesian Ellipses in R • SIBER • Layman's metrics • Stable isotope analysis • Habitat heterogeneity • Resource partitioning Contribution to the Theme Section 'Seabird responses to a changing Bering Sea'