Western Bluebirds Provide Pest Control
The Bluebird of Happiness has a new gig. Now it's the Bluebird of Ecosystem Services.
That would be the western bluebird, a widespread California native. This colorful little thrush nests in tree cavities, often moving in after the original property developer, a woodpecker, has moved out. New research by UC Berkeley postdoctoral scholar Julie Jedlicka suggests that setting up bluebird nest boxes - surrogate cavities - in vineyards can help control insect pests. As the birds colonize urban areas in the East Bay and South Bay, they may be a boon to home gardeners, too.
In nesting season, a western bluebird pair stays busy catching insects - grasshoppers, caterpillars, beetles, bugs, ants, wasps, flies, termites and scale insects - for their voracious offspring. "Bluebirds have a wide general foraging style: on the ground, on vegetable matter, on leaves, in the air," Jedlicka explained. "They're targeting a lot of different spaces that insects inhabit." In a good year, the parents can rear two broods; with four to six eggs per clutch, that's a lot of hungry mouths to feed.
To test bluebirds' pest control talents, Jedlicka set up redwood nest boxes in four Northern California organic vineyards managed by Fetzer in 2008. UC Santa Cruz interns built the boxes to North American Bluebird Society specs, with 1 1/2-inch entrances to exclude European starlings, notorious takeover artists. Western bluebirds quickly moved into about a third of the boxes.
Then she mimicked a pest outbreak, staking out beet armyworms near the bluebird nests. These caterpillars are known vineyard marauders and are about the size of the larvae of European grapevine moths, a new pest of concern. Eighty-three percent of the larvae near active nests were removed - presumably by the bluebirds, as only 24 percent disappeared at more distant control sites.
Prey besides the sacrificial caterpillars were too small for watchers to identify. Technology to the rescue! At Spring Mountain Vineyard near St. Helena, "We collect bird poop and scan it to see what they're eating," Jedlicka said, "comparing genetic bar-code sequences from the poop to known insect sequences in our database." This will show whether the bluebirds are feeding primarily in the vineyard or in native vegetation nearby.
Help on the vines
Spring Mountain owner Ron Rosenbrand says the vineyard has seen a dramatic drop in the number of blue-green sharpshooters, a vector for Pierce's disease, since the boxes were installed in 2007. Other growers are jumping on the bluebird bandwagon. John Schuster's Wild Wing Co. in Cotati sells nest boxes for bluebirds, barn owls and other beneficial birds to vineyards. A grower himself, Schuster is unconcerned about bluebirds' snacking on his grapes: "My vineyard crews eat more grapes than the birds do."
Jedlicka, who's studied bird-friendly coffee plantations in the Chiapas, Mexico, is brainstorming a bird-friendly certification for California wineries: "It would be difficult to find an agricultural system where this wouldn't work. We could expect they might be consuming pests in urban gardens."
That's good news for the birds' neighbors in places like south Berkeley, where western bluebirds have been nesting since 2008. Birder Rusty Scalf found a pair using the hollow limb of an old sycamore at San Pablo Park that spring. When a city crew removed the limb, Scalf mounted a nest box on the tree; the birds adopted it right away. This year two more pairs nested in ash-tree cavities on nearby Parker Street. Scalf saw a Parker Street male with "a great big juicy lime green caterpillar as big as its head," probably a tomato hornworm.
San Francisco had its first bluebird nesting record in decades when a pair set up housekeeping in a cypress tree cavity in the Presidio in 2005 and fledged a brood in their second attempt. Bluebirds are now regular nesters there. Birder Josiah Clark says another pair recently investigated a nest box in Golden Gate Park but did not move in. They're also pioneering in urban Santa Clara County and in San Diego, where a range expansion by Nuttall's woodpeckers has created new nest sites.
For bluebird hosts, nest box placement is critical. Both Jedlicka and Schuster warn against south-facing entrances: too likely to overheat. "You should have the entrance hole looking right out into the garden," Jedlicka added. Schuster recommends an eastern exposure. Landlords should also be willing to clean out the box after nesting: a small price to pay for such entertaining pest-control services.
North American Bluebird Society, www.nabluebirdsociety.org
Wild Wing Company, www.wildwingco.com
Fetzer Vineyards, www.fetzer.com
Spring Mountain Vineyard, www.springmountainvineyard.com
Joe Eaton and Ron Sullivan are naturalists and writers in Berkeley. E-mail comments to email@example.com.
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This article appeared on page M - 1 of the San Francisco Chronicle