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

Abstract

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.

Key words:

• Breeding stage
• carbon-13
• Cassin's auklet
• central-place foraging
• nitrogen-15
• Ptychoramphus aleuticus
• provisioning
• self-feeding
• stable isotope analysis

Acknowledgements

Many people assisted us with data collection, and we thank them all. For logistical support, we thank the Canadian Coast Guard and West Coast Helicopters for safe transport; BC Parks for permits to conduct research on the Anne Vallée Ecological Reserve; Monica Court, Judy Higham, Moira Lemon and Connie Smith for keeping us well organized; and Bob Elner, Elsie Krebs and Ron Ydenberg for support. Primary funding was provided by the Nestucca Oil Spill Trust Fund, The World Wildlife Fund Canada, Environment Canada, and the Centre for Wildlife Ecology at Simon Fraser University.

Editorial responsibility: Leandro Bugoni