What Barn Owls Hear Over California Vineyards

After the sun sets and darkness falls over the vineyards near Lodi, American barn owls start to glide above the grapevines in search of rodents below. As the birds hunt, researchers at the University of California, Davis, and California State Polytechnic University, Humboldt, and San Luis Obispo, are paying attention to how noises echoing through the fields influence these nocturnal raptors.

Karen Gallardo Cruz, a Ph.D. candidate in ecology at UC Davis, is leading a research team studying whether nighttime farm noise, particularly tractor activity, affects where barn owls hunt and how their prey behave at vineyards and orchards across San Joaquin County.

“I’ve always been fascinated by noise effects on animals,” Gallardo Cruz said. “Now we’re working with barn owls to test how predator-prey interactions could potentially be affected by noise.”

For growers, the research could have practical benefits, since barn owls have become recognized as a highly effective, chemical-free form of pest control. Nest boxes are mounted throughout fields to encourage the birds to stay and hunt rodents that can damage crops and irrigation systems. Matt Johnson, a professor at Cal Poly Humboldt and collaborator on the project, has been studying barn owls in vineyards for over a decade, including the cost effectiveness of using nest boxes.

“Gophers, voles and mice are a problem in vineyards because they damage the vines and irrigation lines,” Johnson said. “Our previous results show that a single family of barn owls can kill 3-4,000 rodents in a single year, which can meaningfully reduce the activity of the pests in a vineyard. And since the owls do most of the work, they’re cheap; maintaining owl boxes is over eight times less expensive than labor-intensive rodent trapping.”

A late night in the vineyard

The project spans more than 200 nest boxes scattered across fields growing winegrapes and other crops. Across these sites, the researchers are trapping owls from nest boxes and fitting them with temporary, lightweight GPS devices that the birds wear as “backpacks” that track their nighttime movements. On a recent night in a vineyard owned by operators of Jessie’s Grove Winery, the team prepared to trap owls at active nest boxes.

“We have three nest boxes fitted with traps right now, so our plan is to catch the male and female, at each of them, if possible,” Gallardo Cruz said. “It might be a late night.”

Working alongside Gallardo Cruz are biological field technicians Tara Robey and Kateri Bilay, and UC Davis undergraduate student Salma Madueno, a wildlife, fish and conservation biology major.

“It’s definitely exciting,” Robey said after capturing a bird from a nest box.

Once an owl is gently removed from a trap placed on the nest box, researchers work quickly and carefully. The birds are fitted with two leg bands: one metal band linked to a U.S. Geological Survey bird numbering system and another brightly colored band visible from the ground. Wearing a headlamp to see in the dark, Bilay measured one of the owl’s legs before attaching a band.

“You want to make sure that you get the best fit to make sure that you’re not going to impede its movement and harm the owl in any way,” Bilay said.

The team also records the owl’s weight and other measurements before attaching the GPS device. “It’s an elastic harness that goes on like a backpack and it’s flexible, so the owl can move around freely,” Gallardo Cruz said.

The GPS devices collect about three hours of movement data each night for five nights before researchers recapture the birds and remove the backpacks.

Tractor noise and rodent rustling

Growers sometimes run loud machinery at night during the same hours barn owls are hunting. The problem is that barn owls rely on their hearing to find prey, so the researchers are testing whether noise affects where they hunt.

“Barn owls’ ears are not symmetrical; they have one ear that’s a little higher than the other so they can pinpoint exactly where their prey is,” Gallardo Cruz said. “So even if it’s pitch black and they cannot see anything, they can still find the rodent just through hearing.”

Not far from the nest boxes, large speakers sit among the rows of grapevines. The high-powered system, the kind used in hockey arenas and capable of playing across a large range of sound frequencies, broadcasts either tractor noise, rodent rustling sounds or silence depending on the night.

“With the tractor noise, I want to know if barn owls are avoiding noisy areas because they require their hearing to be able to capture their prey,” Gallardo Cruz said.

The team is also testing whether recordings of rodents rustling through leaves can lure owls into areas where pests are a problem. 

“The downside of owls is that you can’t tell them, ‘Go hunt here,’” said Daniel Karp, professor in the UC Davis Department of Wildlife, Fish and Conservation Biology. “But maybe growers could play rodent sounds to lure them to problem areas.”

The project is also tracking what the rodents are doing when the noise starts. Researchers placed rodent monitoring stations throughout the vineyard stocked with sunflower seeds and equipped with motion-sensitive cameras that record activity overnight. Audio recorders track sound levels at the same locations so the team can compare whether rodents act bolder in noisier conditions.

Connecting with communities

The researchers have been running experiments since March and will continue through July. During that time, the team also checks on the nest boxes about once a month using a GoPro camera attached to a long pole. The visits often come with cute surprises, including owlets just a few days old. 

Part of the project also focuses on outreach in the Lodi community. The team is working with Lauren Jackson, a graduate student at Cal Poly Humboldt, and the Lodi Wine Visitor Center, where a pair of barn owls are raising four babies inside a nest box with a livestream online. Gallardo Cruz, Karp, Johnson and others are giving a research demonstration during a public meet and greet (click upcoming events) at the visitor center on May 21 that includes owl banding and a chance to submit ideas to name the baby owls. In June, the team will be back at Jessie’s Grove to participate in a barn owl field day hosted by the Wild Farm Alliance and Salmon Safe.

Through the summer, researchers will keep tracking owl flights and testing sounds in the vineyard, hoping to help growers better understand what helps barn owls hunt and stay nearby.

“Everybody’s really excited about the barn owls,” Gallardo Cruz said. “A lot of these vineyards have at least 10 nest boxes on their properties, so they want to support barn owls, and they want to know what they’re actually doing.”

Other project collaborators are Gail Patricelli, professor of evolution and ecology at UC Davis, and Clint Francis and Tim Bean at Cal Poly San Luis Obispo. This research is also supported by grants from the UC Davis Academic Senate, the Agriculture Research Institute, the National Geographic Society and the California Department of Food and Agriculture.