Burrowing Owl diet on Southeast Farallon Island, California Jim Tietz [email protected]

On the Farallon Islands, 48 km Island from September 2010 to likely reduce the number of west of the coast of central May 2011 to quantify and Burrowing Owls wintering California, Burrowing Owls document seasonal changes in there, ultimately reducing (Athene cunicularia) are a Burrowing Owl diet. We did this impacts to the storm-petrel native migrant and a predator by collecting and assessing population. of the nonnative house mouse pellets of regurgitated (Mus musculus) and the native undigestible body parts of the Main Points Ashy Storm-Petrel owls’ prey. We estimate that (Oceanodroma homochroa). 23 owls over-wintered during Mice and storm-petrels make The Farallon Islands are our study, spending an average up most of the diet of Burrowing Owls on the believed to support of 118 days on the island. Farallon Islands. approximately half of the world’s breeding population of In the 679 pellets that we Owls switch from mice to collected, insects were the storm-petrels as their main Ashy Storm-Petrels. most numerous prey item food as mouse numbers naturally decline during Previous research found that detected, but mice and storm- winter. the Burrowing Owl predation is petrels were by far the most negatively impacting the storm- important components, A mouse eradication program on the Farallon petrel population, and comprising 98.5% of the total Islands could result in fewer speculated that the very high prey by mass. owls on the islands and thus abundance of house mice in the reduce impacts to the storm- Our results confirmed that fall, might encourage more owls petrel population. mouse consumption was to overwinter than would do so strongly correlated with mouse naturally. It was hypothesized Chandler, S.L., J.R. Tietz, R.W. that when the mouse abundance: Burrowing Owl diet Bradley, and L. Trulio. 2016. shifted noticeably from Burrowing owl diet at a migratory population crashes in winter as stopover site and wintering ground part of a natural annual cycle, primarily mice from October to on Southeast Farallon Island, mid-January to storm-petrels California. Journal of Raptor the owls switch to eating the Research 50:391-403. from February to May, when storm-petrels, a species of the numbers of mice on the conservation concern. island declined sharply. To test this hypothesis, we conducted daily Burrowing Owl These findings suggest that eliminating the mouse surveys on Southeast Farallon population on the islands would

December 2016, Point Blue Conservation Science visit www.pointblue.org/publication-briefs