Print Page Insectivorous Western Bluebirds (Sialia mexicana) are known to occupy vineyard nest boxes established by California winegrape growers who want to encourage avian conservation. Local concentration of these foraging predators with nest boxes provides an alternative to avian exclosure methods for experimentally testing the potential for these birds to provide growers with ecosystem services in the form of pest control. Using California vineyards experimentally split into treatment and control sides, we compared the abundance of foraging birds, avian species richness, and removal rates of sentinel prey in plots with and without songbird nest boxes. To estimate the maximum foraging services of insectivorous birds, we also measured larval removal rates immediately adjacent to occupied nest boxes.
Results/Conclusions
Species richness of avian insectivores nearly doubled and insectivorous bird density nearly quadrupled (led by the tenfold increase in Western Bluebird abundance) within the nest box treatment. Nest boxes were occupied by three other species of insectivorous birds, but no omnivorous or granivorous species; there was no significant difference in the abundance of fruit-eating species between treatments. Removal rates of sentinel prey were significantly higher in the nest box treatment and rates near active nest boxes averaged nearly 3.5 times greater than in control plots. Consequently the presence of Western Bluebirds in vineyard nest boxes strengthened ecosystem services to winegrape growers, illustrating a benefit of agroecological conservation practices. Both predator addition and sentinel prey experiments provide robust, easily replicated methodologies for quantifying regulating services. Given the overlap between the growing and bird breeding seasons and the fact that insectivorous birds consume large amounts of prey when compared with arthropod predators, more studies investigating the top-down trophic effects of avian predators are warranted.
