At Grgich Hills Estate in Napa Valley, a quiet revolution is unfolding among the grapevines. A family of barn owls living in a wooden nest box mounted on a tall pole can catch up to 4,000 rodents in a single year — all without a single drop of poison.
That number caught the attention of winemaker Ivo Jeramaz, whose family has farmed this land for nearly five decades. "Encouraging the presence and reproduction of top predators such as owls is crucial to maintaining this equilibrium," Jeramaz said. "It ensures that our vines are not overeaten by rodents."
The idea that owls could protect vineyards sounds simple, but scientists wanted hard evidence. Cal Poly Humboldt wildlife professor Matt Johnson and graduate student Katherine Larson, working with UC Davis professor Roger Baldwin, spent years studying whether barn owls truly help vineyards — and how.
The researchers monitored six Napa Valley vineyards in 2023, placing food trays to measure how boldly rodents foraged (more food left behind meant the rodents felt safer) while counting barn owls in nearby nest boxes. The results, published in the journal Ecological Applications, were striking: vineyards with more owls had 38 to 52 percent fewer rodents than those with few or no owls. Even more surprising, rodents in owl-friendly areas acted jumpier — they left 16 to 38 percent more food untouched, a sign that mere presence of the predators was enough to stress them out.
"For years, we've studied how wildlife and agriculture can work together," Johnson said. "Wine grape systems provide a unique opportunity to demonstrate that biodiversity and farming can not only coexist but can create mutual benefits."
Johnson's team has tracked more than 500 nest boxes installed in California vineyards since 2015. Between 30 and 75 percent of those boxes are home to breeding owl families in any given year. The owls seem to prefer tall wooden boxes — about 24 inches high — mounted on poles at least 12 feet off the ground, with open grassland nearby and minimal forest cover. The design matters because barn owls hunt mainly by listening, so they do best in areas where they can hear their prey clearly.
Grgich Hills Estate has partnered with Johnson's research since the project began. Jeramaz said the data has transformed how the winery manages pests. "It has allowed us to make informed decisions regarding our integrated pest management system, which has reduced rodent pressure significantly in our vineyards over the years," he said.
The approach sidesteps the need for harmful rodenticides, which can poison animals further up the food chain. For Johnson, the message is clear: nature already has workers willing to put in overtime. The question is whether farms are willing to make room for them.