Seabird seasonal trophodynamics: isotopic patterns in a community of Pacific alcids

Abstract

We measured δ 15 N and δ 13 C values in the blood of breeding adults and nestlings of 5 species of alcids at Triangle Island, British Columbia, to estimate the extent to which these seabirds alter their foraging ecology across successive breeding stages. Considerable intraspecific (stage-to- stage) and interspecific variation was found. Two species—common murre Uria aalge and pigeon guillemot Cepphus columba—fed consistently at high trophic levels (i.e. diets of fish) in inshore or benthically linked habitats. The foraging ecology of 3 others—Cassin's auklet Ptychoramphus aleu- ticus, rhinoceros auklet Cerorhinca monocerata and tufted puffin Fratercula cirrhata —was more variable. Tufted puffins exhibited especially dramatic trophic and habitat shifts between early and late-season diets. With the exception of tufted puffin, the diet of provisioning adults differed from that fed to their nestlings. Trophic level of the community as a whole increased as the season progressed due to the combination of trophic shifting by rhinoceros auklets and tufted puffins, and earlier breed- ing by zooplanktivorous Cassin's auklets than by piscivorous murres and guillemots. Our results con- tribute to a growing body of evidence that marine bird species exhibit considerable flexibility in their foraging behaviour and also shed new light on seasonal patterns in the trophic relations within marine bird communities.