Determining Timing and Magnitude of Dietary Shifts in Black Guillemots (Cepphus grylle) Using Stable Isotope Analysis of Flight Feathers

Recent decreases in Artic summer sea and increases in SST have caused changes in the marine ecosystems associated with ice-covered waters. A four-decade study of Black Guillemots (Cepphus grylle) on Cooper Island near Point Barrow, AK has shown that nestling quality and survival is linked to proximity to sea ice and SST. In recent years when sea ice retreats from the foraging area and SST increases to >4°C during the nestling period, parent guillemots have switched from their preferred prey, Arctic cod (Boreogadus saida), to less desirable fourhorn sculpin (Myoxocephalus quadricornis). The guillemots’ shift from a cryopelagic fish to a nearshore demersal is hard to monitor through direct observation and we suggest a new method to assess the timing and magnitude of this dietary shift using stable isotope analysis of nestling flight feathers. Flight feathers (primaries, secondaries and tail feathers) provide a linear record of material incorporated into the body of the chick from the start of feather growth to fledging, a period of approximately 30 days. Arctic cod and fourhorn sculpin have distinctive diets and are reliant on different basal resources and should produce distinct isotopic signatures in guillemot tissues. We extracted vane tissues from sections of 30 black guillemot feathers grown in a breeding season that experienced an observed prey shift, and conducted stable carbon and nitrogen isotope analyses to determine whether such analyses could be used to assess the dietary shift linked with loss of sea ice and increasing SST. Most seabird populations are not as easily accessible as the guillemots of Cooper Island and thus observations of dietary composition are difficult to obtain. Development of a technique using nestling feathers to examine timing and magnitude of seasonal shifts in prey would preclude the need for daily observations and have great utility for monitoring the ecological effects of the continuing annual decrease in Arctic summer ice.