Contribution of allochthonous resources to breeding in a high-arctic avian predator

Allochthonous input of resources (i.e., originating from a place other than where they are found) can have a significant impact on food availability for consumers. We assessed the impact of an allochthonous source of food (the sewage outfall stream of a military base) on an avian predator breeding in a low-productivity, high-arctic site (Alert, 83°N, 62°W, Ellesmere Island), the long-tailed skua ( Stercorarius longicaudus ). We collected blood samples throughout the breeding season during two contrasting years of lemming abundance to characterize diet composition of skuas and evaluated the contribution of the anthropogenic and lemming food sources using stable isotopes (carbon 13 C and nitrogen 15 N). The isotopic signature of skuas changed seasonally because they switch from a marine to a terrestrial diet when they come ashore to breed but also differed between the 2 years of the study. Anthropogenic food source accounted for 33 % of the summer diet but this proportion varied between years, from 41 % (5–95 ‰: 13–59 %) in a year of low lemming abundance to 16 % (5–95 ‰: 10–21 %) in a high year. Skua nest density recorded in years of low lemming abundance at Alert (0.15 nests/km 2 ) was higher than at any other comparable arctic sites (0–0.02 nests/km 2 ). Overall, the use of an anthropogenic food sources apparently subsidizes skua reproduction at this site, which could affect the food web of this low-productivity ecosystem.