Birdhouse Experiment: The Insectivorous Western Bluebird in two California Vineyards

This is the first time I have seen a study that demonstrated the benefit of having multiple birdhouses in your orchard, or vineyard, in this case, and it determined that the insectivorous western bluebird (Sialia mexicana), which largely inhabited the houses, consumed an average of 3.5 times as many armyworms near those that were occupied and 2.4 times within the entire testing area compared to that of the control. While I don’t particularly care for the method they used, since it does not reflect real world activity as much as I would like, I understand why they took this route, and it does give us an idea of what to expect.

Furthermore, they did remedy some of my concerns with a few other studies, one of which was done at a similar location three years later. Basically, they made evident, in a noninvasive manner, that herbivorous insects were the slight majority of what the western bluebird consumed, and while none of them were detrimental to grapevines, some were to other crops, or were closely related to those that are. Additionally, some of the insects they considered to be important were absent from their traps throughout the course of the study, which likely explains why their DNA was never found in any of the samples.

They also noticed that the western bluebird fed on mosquitoes, a previously unknown prey that was found at a higher frequency than anything else, and that less than three percent of their diet consisted of predatory insects. This is, of course, quite positive, but if they did consume more of the latter, the result may not have been as negative as one might expect. A meta-analysis determined that, while intraguild predation between different arthropod species weakened the suppression of those that were herbivorous (thus decreasing plant biomass), the opposite effect was likely to occur when insectivorous vertebrates became the dominant predator, even if they reduced the concentration of predaceous arthropods quite significantly.

Overall, this is something I have wondered about for awhile now, and I would like to find more studies like it, since one of the things I have noticed was that birds would protect gooseberry bushes from being defoliated to any noticeable degree by the larva of the gooseberry sawfly, and likely other species, when they were located near a wooded area, but the bushes that were over one hundred feet away from that point weren’t nearly as lucky. Unfortunately, they don’t seem to make a dent in the population of certain high-value pests in bird friendly environments, such as the plum curculio, but in this particular case, that might be because they are primarily active at night.