On the Farallon Islands, 48 km west of the coast of central California, Burrowing Owls (Athene cunicularia) are a native migrant and a predator of the nonnative house mouse (Mus musculus) and the native Ashy Storm-Petrel (Oceanodroma homochroa). Previous research showed that Burrowing Owl predation on Ashy Storm-Petrels is likely unnaturally exacerbated by the abundance of house mouse prey in the fall, which may encourage more owls to overwinter. When the cyclic mouse population crashes in winter, the owls switch to preying upon the storm-petrels, a species of conservation concern. From September 2010 to May 2011, we conducted daily owl surveys and a diet study of Burrowing Owls on Southeast Farallon Island. One goal was to document seasonal changes in Burrowing Owl diet composition through collection and assessment of pellets. During our study period, 12 banded owls overwintered for an average of 118 d and contributed 64% of the 679 analyzed owl pellets. Insects were the most numerous prey item detected in pellets, but mice and storm-petrels made up 98.5% of the total prey biomass in the diet. Mouse consumption correlated positively with mouse abundance, and owls exhibited seasonal prey-switching behavior from mice to storm-petrels, when mouse abundance declined during the winter. Our findings suggest that a mouse eradication program on the Farallon Islands would result in fewer owls overwintering and subsequently reduce negative effects on the storm-petrel population.