Many viticulturists have installed nest boxes for Western Bluebirds and Tree Swallows in the hope they might help control insect pests, but little ecological research has investigated this idea. Working with collaborators at UC Davis (Danny Karp & Houston Wilson) and UC Riverside (Erin Wilson-Rankin), we began a project in 2023 in Napa Valley. We added 200 new nest boxes to 10 vineyards that previously didn't have them, and found that after just one year, the abundance of both bluebirds and swallows increased to the level of vineyards that already had nest boxes for years. One of our questions explores a core theme of our lab -- habitat selection. Specifically, how does the design and placement of nest boxes affect their occupancy by bluebirds and swallows? Additionally, we're testing the hypothesis that bluebirds use 'social information' when prospecting future nesting sites. 

We are also examining the role of local native habitats -- especially oak woodlands, grasslands, and riparian strips -- on the whole bird community in vineyards, including bluebirds and swallows. All evidence points toward the importance of wooded habitats. The abundance and diversity of insectivorous birds increases with increasing local and landscape habitat heterogeneity. Moreover, nest boxes are most likely to be used by bluebirds and swallows when placed in relatively open habitats, but with some tree cover nearby.

We're conducting a before-after-control-impact experiment (BACI) in Napa Valley, working on 10 vineyards to which we added new nest boxes and comparing them to another 10 vineyards with existing boxes. We use sticky cards to monitor pests and other insects, we collect fecal samples from the bluebirds and swallows for genetic analysis of diet, and we monitor nest boxes and conduct point counts for birds. Analysis is ongoing, but so far we've learned that both bluebirds and swallows are eating all of the major insect pests in winegrape vineyards, including the blue-green sharpshooter (Graphocephala atropunctata) pictured here, which is an important vector of Pierce's Disease that is deadly to winegrape vines!