Interactions between seabirds and endemic deer mouse populations on Santa Barbara Island, California

Nesting seabirds alter habitat and food availability for insular rodent populations; in turn, rodents can reduce seabird nest success by consuming eggs and chicks. Predation by deer mice ( Peromyscus maniculatus elusus Nelson and Goldman, 1931) is considered a significant threat to reproductive success of Xantus’ Murrelet ( Synthliboramphus hypoleucus (Xantus de Vesey, 1860)), a small, burrow-nesting seabird that breeds off the coast of southern California and Baja California. We live-trapped mice in and out of seabird colonies on Santa Barbara Island, California, USA, to determine the effects of seabirds on mouse populations. We used stable isotope analysis to determine if mice fed on murrelet eggs and chicks. Mouse densities increased significantly on all sites from winter to summer, but there were no significant differences in densities between areas with and without seabirds. Although mice were abundant in murrelet colonies, mouse populations appeared to be affected more by habitat factors than seabird populations: areas with greater rock cover supported higher densities, fewer juveniles, and larger adults in winter and spring, whereas grassland sites had high densities and more reproductive adults in summer. We found no evidence of consumption of murrelet chicks or eggs, suggesting that eggs are not a major component of the diet of most mice. However, mice can still have a significant impact on local murrelet productivity because few eggs are laid each season relative to the high numbers of mice present.