Spatiotemporal Patterns in Diet and Habitat Use of Black-legged Kittiwakes (Rissa tridactyla) and Thick-billed Murres (Uria lomvia) in the Bering Sea

This study quantified the multi-trophic influence and spatial structure of a cross-shelf δ15N gradient in the Bering Sea and used it to contrast foraging patterns and seasonal changes in niche width between thick-billed murres and black-legged kittiwakes during July and August, 2008 and 2009. Gut contents of collected seabirds were characterized and stable isotope analyses (δ13C, δ15N) were conducted on whole prey tissues and seabird cheek feathers, pectoral muscle, and liver tissues. Gut contents of both seabird species were spatially auto correlated and varied with habitat type. Feather and muscle tissues indicated that murres underwent a trophic niche contraction with the onset of the breeding season, whereas kittiwakes did not. Significant geospatial structure was detected between stomach contents and isotope values from liver and muscle in murres, whereas little was observed in kittiwakes. A geospatial analysis of δ15N values in prey items identified a cross-continental shelf gradient in baseline isotopic enrichment that was clearly propagated to seabird tissues. Patterns in stomach contents and isotope values indicated that murres foraged locally with more consistency than did kittiwakes and that foraging habitat partitioning was greater among murres during the breeding season in the southern Bering Sea