Barn owls are ‘killing machines’ and good for pest management in vineyards

Rodent damage to grape vines can be economically painful to vineyard growers, but poisons, traps and explosives don't always work to rid vineyards of the pests. A different lethal method that is sustainable and more environmentally friendly is to install barn owl boxes instead.

'They're killing machines,' said Nancy Lang about barn owls. 'They're voracious.'

Lang, co-owner and founder of Safari West in Santa Rosa is an expert on raptors. She has a PhD in zoological biology with a specialty in raptor studies. Prior to Safari West she was the curator of the avian collection at the San Francisco Zoo.

Barn owls are 'very good hunters,' she said. 'They will eat up to three mice and voles a day.'

Add that up and that's more than 1,000 rodents in a year – for one barn owl. Most vineyards have more than one owl box and barn owls are proficient maters.

'They're really prolific,' Lang said. 'They can have three clutches every season and can lay between two and 18 eggs. That's a lot of little mouse catchers.'

She said a typical clutch is three to six chicks. One owl box family can alleviate a vineyard of 25 to 30 gophers and other rodents a night, said Doris Duncan, executive director of Sonoma County Wildlife Rescue, which is headquartered in Petaluma.

Rodents such as gophers, moles and voles can damage grape vines by eating the bark and roots of the vines.

The Sonoma County Wildlife Rescue group now has a program and service that sells and maintains barn owl boxes for a fee. The Barn Owl Maintenance Program (BOMP) – of which the Napa Wildlife Rescue is also a member – installs boxes designed especially for barn owls, cleans and maintains the boxes and takes photos of who is living inside, Duncan said.

Over the seven years of development, Duncan said they've come up with a design that helps protect the barn owls from predators, and is sturdy enough that it will last 25 to 30 years.

'There's a pretty huge number of vineyards that use owl boxes,' Duncan said.

She said the nonprofit organization has about 250 clients and have installed anywhere from one to 100 boxes in vineyards and at other private properties.

They collected $6,000 the first year of the program and are about to hit $100,000 this year, she said.

'It helps keep our doors open,' Duncan said.

Barn owls hunt primarily at night and they look for cavity areas, or protected and secluded areas to nest, Lang said. Though barn owls themselves are predator birds, larger predator birds such as great horned owls hunt barn owls. Duncan said their barn owl boxes are designed to protect barn owls and their offspring.

A poorly designed box, or one that is too small, may still attract a barn owl out of instinct, but if the box is inadequate, it can be harmful to the birds.

The Wildlife Rescue boxes have an offset oval-shaped entry hole located in the upper corner of the box. The hole is not large enough or shaped for great horned owls – a natural predator of the smaller barn owl – to enter. Inside is a predator safety shield that Duncan said the birds and their hatchlings can hide behind should a raven or hawk peek in. It is roomy enough for a family, and the height of the box allows chicks to practice flying.

Boxes need to be mounted in areas where other predators, such as wild cats, will not easily access them, and they need to be mounted so other climbers such as raccoons and opossums can't get in either.

Using owls and other natural predators is part of an integrated pest management practice that works with the auspices of the Sonoma Winegrowers Association commitment to sustainability.

Controlling rodents with poison is ill advised because once a rodent eats the poison it can take a couple of days to kill the rodent. If a cat, owl or other animal finds the rodent and eats it, the cat or owl ingests the poison that was in the rodent.

Duncan said the region 'lost a lot of habitat due to the fires. There is a huge housing shortage,' so adding an owl box is providing a new home for an owl on the hunt.

The BOMP boxes also have an access panel to keep the boxes clean, something that must be done yearly to rid the boxes of the pellets, or castings, and excrement of the owls that take up residence inside.

The number of owl boxes in Sonoma Valley is unknown, but more and more vineyard managers are having them installed. Monte Rosso Vineyard on Moon Mountain has several, and Sam Sebastiani at La Chertosa Wines had five installed at his Le Gemelle vineyard last month.

Duncan can be reached at 992-0274 or office@scwildliferescue.org.

Contact Anne at anne.ernst@sonomanews.com.