Barn Owls

February 19, 2005

By Jiordana Stark, Master Gardener

Q: I live near some vineyards and have noticed wooden boxes atop tall poles scattered amongst the vineyards. I have wondered what kind of bird they are trying to attract and why. Can you enlighten me? Thank you.

Becky K.
Paso Robles

A: Many vineyards today are incorporating owl boxes into their fields to fight rodent populations. This is due, in part, to pressure from environmental and consumer groups to reduce chemical use in the field. The boxes encourage the presence specifically of barn owls by providing nesting sites of snug, safe enclosures ten feet or more off the ground.

As hunters, barn owls are highly adapted creatures. The design of their wings renders them almost silent in flight, and their highly developed sense of hearing enables them to hunt in total darkness. They will fly as far as three and a half miles in search of food. Some growers have commented that while the owls do an admirable job controlling gophers and other rodents across their fields, the pests seem to thrive in the area immediately below the nesting box. One explanation for this could be that animals instinctively protect their young by not drawing attention to themselves at the nesting site. To the owl, this means not hunting in the immediate area around the nest box, since predators may observe the activity and follow the parent’s return flight home.

The barn owl on average is a short-lived creature. Studies indicate that about sixty percent of them die before completing their first year. Causes of death include accidental pesticide poisoning, starvation, human predation, accidents with moving vehicles, fences and power lines, and the most common cause, attacks by the Great Horned Owl. Of the remaining forty percent, the average life span is eighteen months to two years.

Barn owls are not known to have strong territorial instincts, and will even nest in colonies where food supplies are abundant. Pocket gophers and voles are the most frequent prey in barn owl diets. Deer mice and house mice are also common in their diets. Ground squirrels, not being nocturnal, are unfortunately not controlled. An adult barn owl may eat one rodent a night. Each chick, typically numbering from three to six in a clutch, may eat from two to five rodents a night. As many as three thousand rodents may be consumed by the parents and their young during the course of the breeding season.

Barn owls will not reduce a rodent population to zero. However, they can make a dramatic difference in an area with a rodent problem and keep it under control. In a mature vineyard, gophers can be reduced to a tolerable level without the use of baiting. This, in turn, might help to protect from poisoning other animals further up the food chain.

University of California Cooperative Extension Master Gardener Volunteers can provide additional gardening information upon request .Call the San Luis Obispo office at 781-5939 on Mondays and Thursdays from 1 to 5 PM. You may also call the Paso Robles office at 237-3100 on Wednesdays from 9 AM to 12 PM. The San Luis Obispo Master Gardeners website is at http://groups.ucanr.org/slomg/. Questions can be e-mailed to: mgsanluisobispo@ucdavis.edu.