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Seabirds as part of migratory owl diet on Southeast Farallon Island, California
Citation
MILLS, K.L. 2016. Seabirds as part of migratory owl diet on Southeast Farallon Island, California. Marine Ornithology 44: 121 - 126
Received 21 September 2015, accepted 9 March 2016
Date Published: 2016/04/15
Date Online: 2017/02/28
Key words: Athene cunicularia, invasive species, Mus musculus, Oceanodroma homochroa, Oceanodroma leucorhoa, Ptychoramphus aleuticus, Southeast Farallon Islands, Tyto alba
Abstract
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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