After placing the nest boxes in the vineyard, the team documented where the bluebirds spent most of their time in relation to these boxes. Predictably, they found that the bluebirds were concentrated near their nest boxes early in the season, which is when pairs establish ownership over their chosen nest cavity, construct their nest, lay their eggs and incubate them. As the season progressed, bluebirds were observed at greater distances away from their nests with increasing frequency when bluebird adults were often seen foraging with young birds in small flocks consisting of three to five individuals (figure 1):

But what impact were all these birds having on insect populations? To answer this question, the team set up sentinel prey studies.

“Sentinel prey studies, which monitor removal rates of immobilized, tethered, or frozen prey in the field are common in the entomology literature for comparing relative predation pressure under different conditions”, the authors explain in their paper.

Dr Jedlicka pinned larvae from a variety of insect species to pieces of cardboard and placed them 5 metres apart between the rows of grapes. All cardboard pieces were in place before 7:00 am. Dr Jedlicka returned approximately six hours later to collect the cardboard pieces and scored the pinned larvae as either present or missing — and likely consumed by predators (figure 2):

As you can see, sentinel insects near active bluebird nest boxes were 3.5 times more likely to disappear than those located randomly throughout the vineyard (controls), indicating a high level of predation by resident bluebirds (figure 2, larger view). Since a previous study found that each pair of bluebirds raising five nestlings requires 124 grams of “bugs” (arthropods) daily (doi:10.1890/1051–0761(2006)016[0696:AEOVPO]2.0.CO;2), this level of efficiency is hardly surprising.

A western bluebird nest containing six eggs.

Image: Julie Jedlicka (with permission).

A bluebird pair produces at least one and often two broods per year in California, and each clutch averages between four and six eggs. Young birds from the first clutch typically remain at the nest to help the parents rear the second brood. So using a little mathematics, we can see that one pair of western bluebirds will quickly becomes a group of 12 hungry birds that consumes 248 grams of insects — a very large number. These would include crop pests although the birds are probably also consuming neutral insects as well as arthropod predators of crop pests.

In this study, not only were the sites with nest boxes better protected from potential pest infestations, but even more important, there was no evidence that other bird species that may eat grapes were attracted to the nest boxes. Most birds that are considered “grape predators” don’t nest in cavities (nor in nest boxes).

“I think it is important for the public to know that agricultural systems can provide habitat for wildlife”, explains Dr Jedlicka in email. “There are ways to design and structure [agricultural areas] so they are highly productive and beneficial for wildlife.”

Providing nest boxes to bluebirds is a low-maintenance, low-cost and ecologically-friendly way to benefit from the significant “ecosystem services” that these birds provide by simply being alive. Further, these methods are regionally adaptable.

“Great tits (Parus major) are cavity nesting species that respond well to nest boxes in agricultural lands” in Europe, explains Dr Jedlicka in email. “They were found to reduce caterpillar damage in apple orchards in the Netherlands (doi:10.1046/j.1365–2664.2002.00761.x).” Mols and Visser later expanded their first study to commercial orchards and found similar effects [doi:10.1371/journal.pone.0000202].

The question now remains whether vineyards themselves are providing enough food for bluebird populations. Perhaps the bluebirds are relying on uncultivated natural patches in the vineyards to find food instead of only preying on insects in the grapes?

“I am trying to systematically analyze what role bluebirds are playing as predators in vineyards”, replies Dr Jedlicka. “My future work is analyzing what adult and nestling bluebirds have been eating by analyzing prey [DNA] from [the birds’] fecal matter.”

Knowing precisely which arthropods are being consumed by generalist insectivores, and in what quantities, is still not known.

“It has been a black hole in the community ecology literature for a long time because prey items are so small and birds are so highly mobile. We are just beginning to gain the tools we need to answer these questions.”

Sources:

Julie A. Jedlicka, Russell Greenberg, & Deborah K. Letourneau (2011). Avian Conservation Practices Strengthen Ecosystem Services in California Vineyards, PLoS ONE, 6(11):e27347 | doi:10.1371/journal.pone.002734

Other studies cited:

Mols, C., & Visser, M. (2002). Great tits can reduce caterpillar damage in apple orchards, Journal of Applied Ecology, 39 (6), 888–899 | doi:10.1046/j.1365–2664.2002.00761.x

Mols, C., & Visser, M. (2007). Great Tits (Parus major) Reduce Caterpillar Damage in Commercial Apple Orchards, PLoS ONE, 2 (2) | doi:10.1371/journal.pone.0000202

Germaine, Heather L. and Stephen S. Germaine (2002). Forest Restoration Treatment Effects on the Nesting Success of Western Bluebirds (Sialia mexicana), Restoration Ecology, 10 (2), 362–367 | doi:10.1046/j.1526–100X.2002.00129.x