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A Vineyard's Feathered Friend

Bluebirds, voracious insect predators, ignore ripening fruit

by Paul Franson
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Julie A. Jedlicka holds up a bluebird house, which could be installed in a vineyard and provide a home for the beneficial songbird.
Napa, Calif.—Almost every grower loves to see hawks soaring over vineyards and owls roosting in owl boxes because that means the predatory birds are hunting for pests. But recent studies have shown that songbirds may be important grower partners, too.

Songbirds such as western bluebirds, tree swallows, and violet-green swallows are voracious consumers of insects, especially while feeding their young. And unlike European starlings, a common pest, they don’t eat grapes and seeds, just bugs. “They rely solely on insects,” said Julie A. Jedlicka of the University of California, Berkeley, who has studied the impact of the cheery birds.

Their time of greatest feeding is March to July, ideal for controlling insects in vineyards.

Popular topic at Viticulture Fair
While research is still partly anecdotal about their importance in controlling pests, an eager crowd attended a talk at the Napa Valley Grapegrowers' 22nd annual Viticulture Fair Wednesday to learn more about attracting beneficial birds like bluebirds and owls. After listening to Jedlicka’s presentation the crowd swarmed to grab free bluebird houses offered at the end of the session.

Nearly 1,500 people attended this year’s Viticulture Fair at the Napa Valley Exposition Fairgrounds to peruse displays by 119 exhibitors and 20 community organizations.

Jedlicka noted that bluebirds are generalized insect feeders, who snack on the bugs on vines, on the ground and elsewhere. They prefer large, juicy insects, though she admits not much is yet known about what they eat; they might also eat beneficial insects.

Jedlicka pointed out that no unwanted birds could nest in a bird box designed for bluebirds, as its 1.5 in. diameter entrance hole is too small for starlings and other pests.

She referred to the use of bluebirds by vineyard manager Ron Rosenbrand at Spring Mountain Vineyard near St. Helena, Calif. The 845 hillside acres of Spring Mountain were invaded by the blue-green sharpshooter, which transmits Pierce’s disease. Jedlicka recommended trying bluebirds, and he asked, “Where do you buy them?”

You can’t buy native songbirds, but the UC researcher told him, “If you build them, they will come. There are a lot of bluebirds looking for homes.”

Rosenbrand began installing 700 bluebird houses throughout the vineyards in 2007. By 2010, Spring Mountain Vineyard was virtually sharpshooter free.

Studies have found 10 times as many bluebirds in areas with bluebird boxes, and 3.5 times as many sharpshooters where there weren’t nesting boxes.

Building bluebird homes
Jedlicka offered some guidelines on establishing bluebird boxes:
• Cedar or redwood is good for long life.

• Don’t paint them a dark color; in fact, no painting or sealing is needed and could introduce undesired chemicals. If you do paint, it should be a light color.

• Don’t paint the insides; they should be rough so the chicks can climb out.

• The entrance should be 1.5 inches. Woodpeckers sometimes enlarge the holes for nesting, and that could admit starlings. In that case, reinforce the opening with metal.

• Don’t add a perch. It’s unnecessary and could aid predators.
• The boxes should be 3.5 to 12 feet high and not facing south to keep from overheating them.

• Clean out nests at least once a year to provide space for a new nest.

• Protect from predators, especially domestic cats and raccoons. Place the bird box atop a slippery pole such as one covered with PVC.

• Keep bluebird boxes about 80 feet apart from each other.

Jedlicka added that it’s not true that mothers will abandon birds touched by humans. “Most birds can’t smell. Turkey vultures are an exception since they search by smell.”

Attracting owls
Barn owls are well-known predators of small rodents active at night.
Their favorite prey is voles, then pocket gophers and mice.

Owl boxes shouldn’t face south or even west. The openings should be 3-5 inches in diameter (or upright ovals) for barn owls, smaller for screech owls.

Owls don’t build nests, but just lay eggs, so the bottoms of the boxes should be built in a way to keep the eggs from rolling. Boxes should be 12 to 22 feet high, preferably facing vine rows, to allow them to swoop down on prey. The birds can’t fit between narrow rows with their generous wingspan of 3.5 feet.

It’s best to have owl box openings facing trees so the owlets can fly to a nearby perch and return. But boxes should not be placed in dense woodland or riparian areas favored by their predator, the great horned owl.

November to January is the ideal time to put up boxes or clean them out.

Jedlicka recommended putting up two or three owl boxes and waiting to see if owls move in. The optimum density depends of the amount of prey available.

One problem with owls as predators is that they’re nocturnal. Hawks hunt during the day, however, when ground squirrels and chipmunks are active, but they don’t inhabit boxes.

At any rate, whether they are huge predators of undesirable insects or not, installing the boxes helps a threatened species – and who doesn’t love bluebirds anyway?

Find a lengthy discussion of Jedlicka’s research with birds at She recommends Songbird, Bat and Owl Boxes, a publication of UC Agriculture and Natural Resources for information on building nesting boxes.

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