Volume 44, No. 1
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Key words: Athene cunicularia, invasive species, Mus musculus, Oceanodroma homochroa, Oceanodroma leucorhoa, Ptychoramphus aleuticus, Southeast Farallon Islands, Tyto alba
I investigated diet of migratory owls on Southeast Farallon Island (SEFI), California, United States, including how diet changed in response to time of year and prey availability. I analyzed 523 pellets collected at SEFI from at least four different owl species during 2000-2003 (as well as 99 pellets for other time periods) and quantified the proportion of mice, insects and birds within pellets. The non-native House Mouse Mus musculus was the most abundant diet item across all four years, followed by birds and then insects. Examination of diet composition within each year revealed that between November and February owls primarily consumed mice, while they ate more birds between March and June. Between July and October, consumption of mice and birds was about equal. Previous mouse-trapping studies have shown an abundant mouse population on SEFI during autumn, when owls arrive, while winter mouse populations decrease in response to rains that flood burrows and lower food supplies. My study indicates that, as wintering owls lose a primary food source, they shift their diet from mice to other prey. Included in the shift were storm-petrels (Ashy Oceanodroma homochroa and Leach's O. leucorhoa), mostly consumed by Burrowing Owls Athene cunicularia, as well as Cassin's Auklets Ptychoramphus aleuticus, mostly consumed by Barn Owls Tyto alba. The presence of mice on SEFI may be indirectly affecting seabird populations by keeping migrating owls on the islands longer than they would stay if mice were absent.
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CHANDLER, S. 2015. Burrowing Owl (Athene cunicularia) diet and abundance at a stopover and wintering ground on Southeast Farallon Island, California. MSc thesis. San Jose, CA: San Jose State University.
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GROUT, D. & GRIFFITHS, R. 2013. Farallon Islands Restoration Project: A report on trials undertaken to inform project feasibility and non-target risk assessments. In: United States Fish and Wildlife Service, South Farallon Islands Invasive House Mouse Eradication Project: Draft Environmental Impact Statement. Federal register #FWS-R8-NWRS-2013-0036. Fremont, CA: Farallon National Wildlife Refuge, US Fish and Wildlife Service.
HAGEN, A.T. 2003. An evaluation of House Mouse diet on Southeast Farallon Island. Fremont, CA: Farallon National Wildlife Refuge, US Fish and Wildlife Service.
IRWIN, J. 2006. The impact of House Mice on Ashy Storm-petrel on Southeast Farallon Island. Fremont, CA: Farallon National Wildlife Refuge,US Fish and Wildlife Service.
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SYDEMAN, W.J., NUR, N., McLAREN E.B. & McCHESNEY, G.J. 1998. Status and trends of the Ashy Storm-petrel on Southeast Farallon Island, CA, based upon capture-recapture analyses. Condor 100: 438-447.
SYDEMAN, W.J., BRADLEY, R.W., WARZYBOK, P., ABRAHAM, C.L., JAHNCKE, J., HYRENBACH, K.D., KOUSKY, V., HIPFNER, J.M. & OHMAN, M.D. 2006. Planktivorous auklet Ptychoramphus aleuticus responses to ocean climate, 2005: Unusual atmospheric blocking? Geophysical Research Letters 33, L22S09. doi:10.1029/2006GL026736.
UNITED STATES FISH AND WILDLIFE SERVICE (USFWS). 2013. South Farallon Islands Invasive House Mouse Eradication Project: Draft Environmental Impact Statement. Federal register #FWS-R8-NWRS-2013-0036. Fremont, CA: Farallon National Wildlife Refuge, US Fish and Wildlife Service.
WARZYBOK, P.M. & BRADLEY, R.W. 2009. Breeding population estimates for burrow and crevice nesting seabirds at the South Farallon Islands: results of the 2009 all island nest site survey. Petaluma, CA: PRBO Conservation Science,.
WARZYBOK, P.M., JOHNS, M. & BRADLEY, R.W. 2014. Status of seabirds on Southeast Farallon Islands during the 2014 breeding season. Petaluma, CA: Point Blue Conservation Science.
WOLF, S.G., SNYDER, M.A., SYDEMAN, W.J., DOAK, D.F. & CROLL, D.A. 2010. Predicting population consequences of ocean climate change for an ecosystem sentinel, the seabird Cassin's Auklet. Global Change Biology 16: 1923-1935.