Barn owls exert top-down effects on the abundance and behavior of rodent pests.

Predators can exert top-down effects on their prey by direct consumption, by imposing risk of predation, or a combination of both. However, empirical evidence for these effects is mixed and inconsistent. Barn owls in agriculture present a system with attributes that should, according to theory, lead to strong top-down effects, but research is limited and the distinction between effects on prey abundance and behavior remains unresolved. Understanding the top-down effect of predators on pests is a cornerstone of integrated pest management and important where barn owls may contribute to the control of rodents on farms. Vegetation can mediate the top-down effects of predators on prey, but this has received little attention for barn owls and rodent pests, limiting the information farmers can use to manage their fields to harness the effect of barn owls. To fill these gaps, we surveyed aboveground foraging rodents and monitored barn owl nest boxes at six winegrape vineyards in Napa Valley, California, from February to July in 2023. We deployed chew block grids to determine the effect of barn owls on rodent abundance and used giving-up density (GUD) trays and camera traps to determine the effect of barn owls on rodent perceived predation risk and activity, respectively. We found that, over the range of hunting pressures observed (from nearly no owls to about three occupied boxes with six adults and eight chicks within a 1-km radius), barn owls reduced rodent (predominantly deer mouse, Peromyscus spp.) abundance by 38%-52% and increased perceived predation risk by 16%-38%. Results also suggest that vegetation cover favors the acoustically attuned barn owls over the visually oriented rodent pests. These results contribute to growing evidence that barn owls can be used as biological control in agricultural systems and that vegetation can be managed to optimize their impact. We advise that growers maintain the vine canopy of leaves, pruned twigs on the ground, and vegetation cover in a cover crop row grown between vine rows to elevate perceived predation risk in mice. We also recommend that, to the extent feasible, farmers time mowing to reduce mouse activity when crops are vulnerable.