NAPA VALLEY, Calif. — Researchers at Humboldt State University are studying how barn owls can provide natural rodent control in California vineyards.

Researchers, led by Matt Johnson, professor of wildlife habitat ecology at Humboldt, have set up about 300 owl nest boxes across 75 vineyards in the Napa Valley, a long-term experiment that started in 2015 and has recently been reinvigorated with USDA funding.

So far, researchers say, the owls are proving effective. Four-fifths of the vineyards have noticed a difference in rodent levels, and according to the latest data, an average family of barn owls in a single nest box — two adults and three or four chicks — eats about 3,400 rodents per year.

“It’s pretty amazing,” said Johnson. “Do the math. If you’ve got 10 occupied boxes on your property, that’s 10 times 3,400 — a lot of rodents killed.”

Left unchecked, gophers, voles and mice can wreak havoc in vineyards.

Most farmers have largely relied on rodenticides. Today, however, a growing number of farmers are exploring natural methods. Some are trying to get ahead of regulations, as the California Department of Pesticide Regulation placed stricter limits on rodenticide use last January.

Johnson said he became excited about “reciprocal relationships” between farmers and wildlife while doing research on shade-grown coffee farms in the jungles of Jamaica, where he saw songbirds controlling insect populations.

Back in the U.S., he explored the history of barn owls for pest control.

He discovered that the barn owl is a friend to growers around the world. Malaysian growers encourage barn owls to nest on palm oil-producing farms, Kenyan growers have relied on owls to keep rodents away from vegetables and Israel even has a national rodent control program using barn owls.

Farmers in the U.S. have a history of putting up nest boxes, too, but Johnson found little data had ever been collected — so he set out to learn more.

First, he and his team of graduate students did an occupancy study, calculating what percentage of nest boxes owls inhabited and trying to understand what makes a box more attractive. They found, for example, that Napa Valley barn owls prefer nest boxes facing away from the sun with oak savannah habitats nearby.

The next research question was whether the owls would actually hunt in the vineyards.

For this, Johnson’s team had to track the birds. They set up trap doors on nest boxes, then caught and tagged adults and chicks.

“It was fun,” Johnson said.

It was also challenging.

Samantha Chavez, one of the current cohort’s graduate students, said the barn owls were somewhat feisty, though surprisingly light and fluffy. To calm an owl, she said, she would put a “hood” — a baby beanie — over its head.

“It was very nerve-racking at first,” said Chavez. “You don’t want them to hurt you, but you also don’t want to hurt them.”

Tracking revealed that the owls spent about a third of their time hunting in the vineyards.

The researchers also put cameras inside nest boxes and dissected regurgitated pellets to track what the birds were eating. According to Johnson, the owls’ diets consistently almost entirely of rodents: 50% voles, 25% gophers, 15% mice and the remaining 10% miscellaneous.

Johnson said he believes the American barn owl can be an ally to farmers across diverse regions and crops.

Jaime Carlino, another of Johnson’s graduate students, agreed.

“I think the potential is there for other areas and other crops,” she said.

However, Carlino said she would like to see more region-specific and crop-specific research.