Seasonal variation in the foraging ecology of a zooplanktivorous seabird assessed with stable isotope analysis

We used δ15N and δ13C analyses on blood drawn from zooplanktivorous Cassin's auklet (Ptychoramphus aleuticus Pallas, 1811) on Triangle Island, British Columbia, over four years to address two questions: (1) do these seabirds systematically alter their trophic level and foraging habitat from one breeding stage to the next; and (2) do auklets select prey at the same trophic level and from the same foraging habitat to self-feed and provision offspring? Adult trophic level (inferred from δ15N values) tended to be higher prior to egg-laying than during incubation, perhaps reflecting an abundant zooplankton biomass in early spring. Season-long declines in δ13C values suggested that the auklets foraged on the continental shelf early in the season, but thereafter increasingly used habitats seaward of the continental shelf-break to obtain Neocalanus cristatus copepodites. Similarity in the δ15N and δ13C values of provisioning adults and nestlings suggested that adults consumed the same prey from those oceanic habitats to both self-feed and to provision offspring at the nest, a behaviour that may save time and energy. The results of this study show the dynamic nature of seabird foraging ecology and the utility of stable isotope analysis for assessing seasonal variation in the foraging ecology of oceanic species.