Abstract
On the Farallon Islands the burrowing owl (Athene cunicularia) is a native migrant and predator of the non-native house mouse (Mus musculus) and the native ashy storm-petrel (Oceanodroma homochroa). Burrowing owl predation on the ashy storm-petrel is 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 prey upon storm-petrels, a species of conservation concern. In 2009, the U.S. Fish and Wildlife Service proposed eradication of the house mouse from the Farallon Islands in order to reduce owl predation on and aid recovery of the storm-petrel. From September 2010 to May 2011, I conducted daily owl surveys and a pellet study of burrowing owls on Southeast Farallon Island, the largest island. The goal was to document burrowing owl abundance, overwintering duration, and seasonal changes in diet composition prior to any mouse eradication effort. During my study I counted 23 owl migrants; a subset of these owls overwintered an average of 118 days. Insects were most numerous, but mice and storm-petrels comprised 98% 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. It is likely that mouse eradication would result in fewer owls overwintering and subsequently reduce impacts to the storm-petrel population.
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