Proceedings of the Vertebrate Pest Conference

Attracting natural enemies to farms to reduce pests has long been a part of integrated pest management for insects, but knowledge of the impact of raptors on rodent and other vertebrate pests is comparatively sparse. Using wooden nest boxes to attract rodent-eating barn owls (Tyto alba and Tyto furcata) to farms has been practiced in many regions for decades, but to date there have only been a handful of studies comparing rodent numbers in the presence and absence of barn owl nest boxes, and none done within the Western United States. In this study, we surveyed rodents on winegrape vineyards in Napa California with and without occupied barn owl nest boxes by live-trapping for rodents and using the open-hole method for gophers. We collected data before the owl breeding season, when hunting pressure should be light, and again when adult owls were hunting actively to feed their chicks. We found that gopher activity declined from before to peak hunting pressure on the vineyard with barn owl nest boxes, whereas it slightly increased on the vineyard without nest boxes. Live trapping revealed that the abundance of mice declined from before to peak hunting pressure, but this decline was not significantly affected by the presence of nest boxes. Results were inconclusive for voles because they not well-sampled by our live trapping method, even though analysis of owl pellets confirmed they are an important source of prey for barn owls.