Bluebirds are one of several groups of birds that catch insects on the wing, but because they're constantly on the move and the animals they eat are tiny and often hard to identify, it's difficult to determine exactly what species make up their diet.
Researchers from the Missouri Western State University and the University of California, Berkeley tackled this question using molecular scatology, analyzing DNA barcodes found in the birds' feces to determine insect species the Western Bluebirds (Sialia mexicana) were eating. They collected 237 fecal samples from adult and nestling birds living on three vineyards in Napa County, California and did a metabarcoding analysis utilizing a short (250bp) COI fragment sequenced on the Illumina MiSeq platform. Identifications were done using BOLD's ID engine.
They found that the bluebirds in Napa Valley vineyards mostly ate mosquitos and herbivorous insects, likely having only negligible effects on the predaceous insects that benefit vineyard production by eating pests. Even though the authors did not find specific pest species in bluebird diets, they did confirm that bluebirds are mainly eating herbivorous insects, including those in the same families as major pests. This suggests bluebirds may contribute to ecosystem functioning in these systems. The bluebirds' presence is likely a net positive which is why the authors hope that these results encourage more vineyard owners to install bluebird boxes, thereby helping to replace natural tree cavities lost when land is cleared.