Red-legged kittiwake feathers link food availability to environmental changes in the Bering Sea

The largest breeding population of red-legged kittiwakes Rissa brevirostris has undergone a dramatic decline and subsequent recovery since 1975. The causal mechanism is hypo thesized to be climate-induced food shortages during reproduction, yet little is known about how the basin ecosystem, where breeding red-legged kittiwakes primarily forage, is affected by climate variability. We examined when and under what conditions red-legged kittiwakes experienced food shortages. Head feathers (winter) and first primary feathers (summer; n = 24 and 27 yr, and 194 and 122 individuals, respectively) were collected between 1913 and 2016 on St. George Island and were analyzed for stable isotope signatures of carbon (delta C-13), nitrogen (delta N-15), and sulfur (delta S-34). Feathers were also analyzed for corticosterone concentration (fCORT), which indicates a bird's exposure to nutritional stress during feather growth. Summer fCORT concentrations were lower when values of the annual Pacific Decadal Oscillation index were positive. Winter fCORT concentrations and delta N-15 signatures increased when February sea-ice coverage was extensive. We conclude that, since 1913, warm conditions in the Bering Sea ecosystem have been associated with low nutritional stress in adult red-legged kittiwakes breeding on St. George Island. However, we found no strong evidence for isotopic shifts over this same period. A better understanding of the factors governing the abundance and availability of red-legged kittiwakes' prey is needed to predict the impact of anticipated warming on this species and its foraging habitat.