Pest-removal services provided by songbirds on small organic farms in Humboldt County, CA

Land conversion for agriculture is a leading threat to bird biodiversity. However, wildlife-friendly practices such as organic farming and the use of hedgerows have been found to provide needed habitat for birds in agricultural landscapes. Many studies have also established that birds may provide a pest removal service, although few studies have taken place on row crop farms. In this study, I aimed to determine whether birds provide a significant pest removal service in organic row-crop agriculture in Humboldt County, California, and how this service was distributed within and between farms. I hypothesized that birds would remove insect pests, and that these pest-removal services would have a higher probability of occurring closer to uncultivated habitat, and on farms with higher avian biodiversity. In a sentinel pest experiment which simulated a pest outbreak, 0 - 80% of caterpillar presentation stations were depredated by birds within seven hours, with a mean of 24% depredation per farm; the probability of pest removal was higher in areas close to uncultivated shrub habitat. There was only weak evidence that the probability of pest removal was higher on farms with higher avian insectivore richness, and no evidence that pest removal varied with species diversity or abundance. Exclosure experiments on kale crops showed no significant effects of bird exclosure treatment on arthropod abundance or crop yield. These results suggest that birds may be more helpful in responding to changes in pest density than in controlling pests at non-irruptive densities on organic row crop farms in my study area. The prevention of pest outbreaks is an essential ecosystem service on any farm, and the rapid response of birds to pest outbreak conditions is an indicator of resiliency in the agroecosystem. Therefore, the retention of uncultivated shrub habitat in agricultural landscapes could benefit both birds and farmers.