A new program has been launched to help Napa Valley winegrape growers and homeowners decrease rodent populations with a pesticide-free approach.
Napa Wildlife Rescue, a nonprofit wildlife rescue center, will be installing and cleaning barn owl boxes as an “organic way” to control rodents that infest homes and attack vine trunks, said John Comisky, president.
“Barn owls eat between 1,000 and 3,000 rodents in a season. When you install a barn owl box, that barn owl finds a mate. Together they raise six to seven fledglings who grow up to eat more rodents,” he said.
Comisky said the organization will charge $450 for installation and purchase of a box and $110 for each visit involving consultation and cleaning. Owl boxes must be cleaned out at the end of nesting season, which runs from March to August.
The owl boxes are mounted in the center on top of a tall steel pole. They are made of heavy-grade plywood and coated with several coats of light green paint. This color of paint repels sunlight, which helps keep the box cool in the hot sun. The color also blends in with the environment.
Comisky said the barn owl box program will present a source of steady funding for Napa Wildlife Rescue.
“We are adapting this program from a similar one at Sonoma County Wildlife Rescue. We believe it will generate support for our center as well,” he said.
Chris Pedemonte, vice president and vineyard manager at Colinas Farming Company, a vineyard management company based in Rutherford, plans to use Napa Wildlife Rescue’s cleaning service this spring.
“There are over 100 owl boxes already installed on our properties. Our staff had been cleaning them out. Now that Napa Wildlife Rescue’s offering the service, we’ll use them. We want to support them,” he said.
Pedemonte currently oversees 650 acres of winegrapes, spread out over Rutherford, Yountville, Carneros and American Canyon.
“Pocket gophers are always a challenge. That’s especially true in young vineyards established in the first four to five years. They’re not as much of a problem in established vineyards. Moles and voles are always a concern,” said Pedemonte.
Burrowing rodents ruin vines by gnawing on the crown of the girdle. The girdle is a circle of removed bark around the base of a vine’s trunk. The crown is the bottom layer of the trunk just above the soil.
“It depends on the age of a vine, but a vole can kill a vine in a few hours. When a trunk is collapsed, you can reach out of the ground and pull it out. It will separate from its roots,” said Pedemonte.
Jennifer Putnam, chief executive director of Napa Valley Grapegrowers, said Napa growers are always looking for more efficient and sustainable ways to manage all kinds of vineyard pests.
“Integrated pest management is a foundational approach. We know from experience that predatory birds such as owls are a successful way to help manage vertebrates. The Napa Valley Grapegrowers has held several seminars and field days that explore the options when it comes to introducing predatory species into the vineyard environment. They are always sold out,” said Putnam.
Putnam said barn owls will have enough prey because most of Napa’s vineyards are a “healthy mix” of vineyard rows and natural habitat.
“Trees lining the vineyards provide a natural setting for owls. Growers want to be sure native species of owls have ample places to nest. Taking the additional step and installing owl boxes can greatly increase the likelihood of the birds nesting and staying on site, which is of interest to everyone,” said Putnam.
Too many barn owls?
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Robin Leong, a member of the Napa-Solano chapter of the Audubon Society who focuses on owls, said he is concerned about increasing the valley’s barn owl population.
“It looks to me that there might be just the right number of barn owl boxes in the valley. There shouldn’t be any more installed,” said Leong.
Leong said it is important to keep in mind that great horned owls prey upon barn owls. A higher barn owl population may result in more barn owls being eaten.
“When you increase one species in particular, you change the balance for the whole environment,” said Leong.
Tracking the owls
Dr. Matt Johnson, a professor of wildlife ecology at Humboldt State University, said the school has developed a partnership with Napa Wildlife Rescue, to collect data about Napa Valley’s barn owl population.
Johnson, who began the study in 2015, said he and a team of Humboldt State students are monitoring owl boxes from American Canyon up the Napa Valley.
“We’re looking at how the owls are doing, how farmers can attract owls to their property, and how many rodents the owls are killing. We put Global Positioning System (GPS) tags on the birds and track them. We also install remote infrared security cameras in the owl boxes to see how many rodents the owl parents are delivering to the box,” said Johnson.
Johnson said there is no concern with attracting more barn owls to Napa Valley.
“Barn owls are well-known to respond to prey. If there is not enough prey for them, they’re not going to stick around. What we see in Napa Valley is an artificially elevated rodent population. There are more rodents because of the vineyards. More barn owls help balance this situation,” said Johnson.
Johnson said barn owls’ impact on other species is minimal.
“Great horned owls and screech owls will not nest where a barn owl prefers to nest. The bigger owls prefer a more forested environment. Foxes and coyotes overlap with barn owls in terms of what prey they consume. Barn owls focus much more on rodents,” said Johnson.
Johnson said barn owls are a much more ecologically sound way to lower rodent populations than rodenticide. Many rodenticides are anticoagulants. They do not allow blood to clot. Often, rodents consume only a little bit of poison. They are still active before they die. When predators like hawks and coyotes eat poisoned rodents, they also get sick. In addition, rodenticide can negatively affect insects and songbirds and wash into the water supply to harm fish and wading birds.
Johnson said he believes barn owl boxes will offset what may have been a decrease in the population of this species in the last century. Although there are no data on barn owl populations in the past century in Napa Valley, Johnson believes the population of the species decreased as growers and farmers replaced old wooden barns with non-wooden, more tightly sealed structures.
Murray Berner, co-author of “Breeding Birds of Napa County” and a birding tour leader in the North Bay, said it is preferable that new barn owl boxes be placed near traditional barn owl foraging habitats, grassland and oak savannah. Good spots include areas close to the Napa River, the St. Helena wastewater treatment plant, and open spaces on the edge of towns and cities.
“Starvation is the primary cause of owl mortality. It is believed that without adequate foraging habitats in an agricultural landscape, owls cannot support themselves or their nestlings. These factors should be considered when placing nest boxes in and around vineyards,” said Berner.
Napa Wildlife Rescue Center can be reached at email@example.com. or (707) 685-5411.