Establishment of songbird nestboxes in vineyards increases insectivorous bluebird populations. Our study investigated the diets of vineyard-nesting Western Bluebirds (Sialia mexicana) to document whether bluebirds consume insect pest species and offer growers ecosystem services in the form of pest control. To evaluate the impact of avian predation on arthropods in vineyards, we sampled both birds and arthropods across three vineyards and adjacent native forest patches. Over 4500 arthropods were collected, sorted, and identified. For bird sampling, we used non-invasive methods by gathering fecal samples from adult and nestling bluebirds to evaluate what prey were consumed. We tested several DNA extraction kits before developing a novel methodology that provided better quality and quantities of DNA from bird fecal samples. We applied next-generation sequencing to obtain a list of diet contents in the form of DNA sequences. We compared these sequences to a reference database that we constructed from DNA sequences of our collected arthropods. We found a rich and diverse abundance of arthropods in both the vineyard and adjacent woodland habitats. This signifies that insectivorous birds nesting in vineyards had access to plenty of food resources. Bluebirds were consuming a diverse diet comprised of many different arthropod orders, from millipedes to butterflies. We found that adult bluebirds regularly feed their nestlings isopods (also called roly-polys or pill bugs). These were abundant in the vineyards and are known to offer one of the few sources of calcium available to insectivorous animals. Calcium can be a limiting nutrient, and it is likely that high densities of isopods in vineyards offer nesting bluebirds high-quality prey items for their growing young.
No significant vineyard pests (including blue-green sharpshooters) were found in vineyard and woodland traps. This meant that avian populations did not have access to these pest insects, so it is not surprising that we did not find evidence of bluebirds consuming vineyard pests in this study. We did find evidence of bluebirds consuming treehoppers and caterpillars, so in vineyards where these pests are present, bluebird boxes may invite predators that successfully lower pest populations. We did not find evidence that bluebirds were consuming parasitic wasps (beneficial insects that lower pests populations). Consequently the presence of bluebirds did not harm growers. Nestboxes can bolster declining bird populations, and increasing vineyard nest box presence can be an important sustainability practice for growers. Consequently we connected with growers and presented our findings in numerous venues, distributing informational pamphlets and 100 nest boxes to eager growers. Our goal is to provide growers with the resources they need to maintain healthy populations of birds in their vineyard for year.