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.

Predation of artificial Xantus's murrelet (Synthliboramphus hypoleucus scrippsi) nests before and after black rat (Rattus rattus) eradication

Introduced rats depredate every life stage of island nesting seabirds, but the extent of predation is rarely quantified. Introduced black rat (Rattus rattus) and native deer mouse (Peromyscus maniculatus anacapae) predation on Xantus's murrelet (Synthliboramphus hypoleucus scrippsi) nests was experimentally quantified using artificial nests before and after rat eradication on Anacapa Island (California). The staged rat eradication programme provided experimental treatments: in 2002 rats were eradicated on one island (East Anacapa Islet) and remained on two islands (Middle and West Anacapa Islets), providing a control comparison, and, in 2003, rats were eradicated from the remaining islands (Middle and West Anacapa Islets). In 2002, 96% of artificial nests were depredated on control islands (rats present) with rats accounting for most predation. Nest predation on the treatment island (rats eradicated) in 2002 was significantly lower: 8% of artificial nests were depredated, mostly by endemic deer mice. In 2003, following rat eradication on the remaining islands (Middle and West Anacapa Islets), nest predation was reduced from 96% in 2002 to 3% of total nests in 2003. Predation of nests on East Anacapa Islet (rats eradicated in 2002) increased significantly due to reintroduction and recovery of native deer mouse populations, with 23% of artificial nests depredated. The inference is that rat predation on real Xantus's murrelet nests was responsible for the historically low nesting success and small population sizes of breeding murrelets on Anacapa Island. With rats removed, the hatching success of Xantus's murrelet chicks and the number of individuals nesting on Anacapa Island will increase dramatically. Artificial nest studies are particularly well suited to quantifying introduced rat impacts on hole and crevice nesting seabirds and can simultaneously serve as an effective monitoring tool to detect the presence of rats and the recovery of native nest predators.