Barn owls as agents of functional biodiversity: vegetation and landscape effects on pest control in vineyards

Abstract: Predators in agroecosystems can provide valuable ecosystem services by exerting
top-down control on pests through direct consumption, perceived predation risk, or both.
Harnessing these services for pest management requires understanding how top-down effects
are mediated by vegetation structure and landscape complexity, which remain poorly
understood, especially for vertebrate pests and their predators. Globally, farmers install nest
boxes to attract barn owls (Tyto spp.) for rodent control, yet evidence for their effectiveness
remains limited and mechanisms largely unresolved. We studied American barn owls (Tyto
furcata) in winegrape landscapes of Napa Valley, California, USA to quantify (1) rodent
removal rates, (2) owl habitat use between vineyards and adjacent habitats, (3) whether owl
hunting reduces rodent activity and abundance, and (4) how these effects vary with vegetation, landscape composition, wind, and moonlight, which influence prey detection. On average, each owl family removed 3,466 rodents per year and spent 35-50% of their hunting time in vineyards. As the proportion of natural habitat in the surrounding landscape increased, nest box occupancy also rose, but owls hunted proportionally less in vineyards. Owl hunting reduced gopher (Thomomys bottae) and mouse/vole (Peromyscus and Microtus) activity by 14 % and 36 % in vineyards, respectively, and increased perceived predation risk for mice/voles by 27 %. Results also suggest that dense vegetation favored acoustically oriented barn owls over visually oriented rodent pests. Together, these findings provide strong evidence that barn owls contribute to biological control in agricultural landscapes and that vegetation and landscape composition can be managed to enhance their impact.