Predation by deer mice on Xantus' Murrelet eggs on Santa Barbara Island, California

Investigation of the breeding biology of Xantus’ Murrelet (Endomychura hypoleuca) on Santa Barbara Island revealed that 46% of all eggs laid in one area were eaten by deer mice (Peromyscus maniculatus), while in another only 4% were eaten. Population density of mice in the two areas was comparable, as were murrelet nesting density and total availability of eggs. Vegetational differences between the areas were pronounced, with grasses abundant at the site with low predation rate but nearly absent at the other. Grass seeds were utilized heavily by the mice when available. I concluded that vegetation differences between the two sites, hence the concomitant differences in abundance of alternate food items, were responsible for the differential predation rate on eggs; mice relied heavily on easily obtained, energy-rich grass seeds when available, taking less abundant items such as eggs mainly where grasses were scarce. Survival of dependent young mice was greater in the area where egg predation was severe. Increased survival may have been facilitated through the nutritional input from eggs. These findings were explained on the basis of predictions of current optimal foraging theory.