Four sympatrically nesting auks show clear resource segregation in their foraging environment

Auks share ecological similarities and are often found in sympatrically nesting assemblages at suitable nesting sites. It is expected that co-existing species would have evolved strategies to reduce niche overlap in response to resource competition. The presence of Atlantic puffins Fratercula arctica, razorbills Alca torda, thick-billed murres Uria lomvia and common murres U. aalge at the Gannet Islands, Labrador, is an opportunity to study interspecific interactions in a major seabird community. We compared the foraging movements of adult breeding birds and the diet of the birds inferred through (1) stable isotopes in the blood of adults, and (2) prey delivered to the chicks, as dimensions in the foraging niche among these 4 auks. Our results revealed that puffins and thick-billed murres headed offshore to forage, while razorbills and common murres were more coastal, heading towards mainland Labrador. The patterns of spatial segregation found among coastal and pelagic birds were mirrored by their stable isotope ratios, with one group of species foraging at a higher trophic level of an inshore food web and the other foraging on a lower trophic level of an offshore, pelagic food web. The ‘pelagic’ species segregated by prey choice for their chicks while differential alternate prey selection was the source of segregation between the ‘coastal’ species. Our research highlights the importance of segregation patterns in both spatial and diet dimensions of the foraging niche among the 4 auks—a potential mechanism for reducing interspecific competition among ecologically similar seabirds and explaining the persistence of such seabird assemblages through time.