Predation on seabird eggs by Keen's mice (Peromyscus keeni): using stable isotopes to decipher the diet of a terrestrial omnivore on a remote offshore island

We used stable isotope techniques to analyze tissues of Keen's mice (Peromy scus keeni) and Townsend's voles (Microtus townsendii cowani) and a subset of prey items at Triangle Island, British Columbia, western Canada's larges t seabird colony. Isotope analysis allowed us to investigate the importance of seabird prey in rodent diets in a system where seabirds and non-introdu ced rodents occur sympatrically. The delta N-15 values for terrestrial plan ts and terrestrial invertebrates on Triangle Island exceeded levels found i n many terrestrial biomes and are typical of localities with high inputs of marine-derived N. We used multiple-source mixing models to estimate the re lative inputs of potential prey items to vole and mouse diets. The delta C- 13 and delta N-15 values of liver and muscle tissues of voles indicate that voles on Triangle Island derived their protein primarily from terrestrial plants, with some contribution by terrestrial invertebrates. In contrast, i sotopic values of liver and muscle tissues of mice on Triangle Island indic ated that mice prey primarily on seabird eggs and terrestrial invertebrates . Our results show that egg predation on Triangle Island is a general pheno menon in the mouse population, rather than occurring in only a few speciali st feeders. Mice appear to feed on eggs once they become available and cont inue to utilize seabird prey, likely in the form of abandoned eggs or carca sses of chicks and adults, throughout the breeding season.