Twenty barn owl nest boxes were installed around the perimeter of this Vino Farms vineyard as part of a barn owl research project now under way.
—More than 100 new barn owl nest boxes will be placed in Lodi vineyards as a result of a $25,000 grant from Pacific Gas & Electric Co. (PG&E) to assist grapegrowers in removing existing boxes from utility poles. The new boxes are made from more durable plastic and designed by barn owl researcher Mark Browning, who gave a presentation and answered questions about using barn owls for natural rodent control and integrated pest management (IPM) at a Lodi Winegrape Commission
(LWC) grower education meeting Tuesday.
PG&E representative Mike Best, the utility’s avian protection program manager, said the LWC grant is designed to prevent avian mortality from birds that fly into power lines, where they may be electrocuted and can cause power failures and potentially wildfires by damaging overhead power lines. Wildlife-related outages in California are estimated to cost millions of dollars each year.
Best said, “This program is a way to educate growers on the proper placement of owl boxes away from power poles.” At the end of the meeting, Lodi grapegrowers lined up to receive vouchers covering the total purchase of one owl box, redeemable locally at San Joaquin Sulfur Co., which stocks and sells the boxes.
Each grower also received a handout detailing best practices for using barn owls for rodent control, with instructions for placement, installation and maintenance of vineyard owl boxes. The boxes normally retail for about $175. San Joaquin Sulfur also designed and sells a metal mounting pole with a telescoping, sliding design to easily raise and lower the owl boxes for cleaning.
Barn owl IPM
Browning has been an educator and researcher at the Pittsburgh Zoo in Pennsylvania for the past 20 years. He has studied barn owls for a number of years and began a barn owl breeding program in Pennsylvania to replenish wild populations in the state, which have been in severe decline. He also did a research project tracking barn owls, which showed they can migrate long distances under some circumstances. One barn owl he released in Pennsylvania was later found in New Orleans, La.
Barn owls are found throughout most of the United States, but populations are declining in eastern states. They inhabit every continent except Antarctica. Browning said barn owls have been used in Israel for IPM in ag operations since the 1960s; owl boxes are routinely used in Malaysia and Southeast Asia in palm plantations used for palm nut oil. IPM programs in the U.S., including the University of California Cooperative Extension
, also encourage the use of owl nest boxes on farms and have published instructions for building wooden owl boxes.
Browning listed the traits of barn owls that make them uniquely well-suited to rodent control IPM in vineyards, orchards and farms compared with other raptor/predator species:
• They tolerate other barn owls in their territory and can even be a colonial species with multiple nests in the same barn. They also occupy territory with great horned owls and other raptors, although occasional conflicts can occur with other raptors.
• They tolerate human activity. They will occupy tree cavities in a natural setting but will also occupy barns and human structures, so they are attracted to nest boxes. Browning said he’s observed barn owls nesting in a box only four feet off the ground in a farm equipment yard, where regular human activity occurs.
• They have high reproductive rates, producing from four to seven chicks per nest, with two or more broods per year. They will nest during almost any month of the year in any location in the U.S. However, they do have high mortality rates.
• They have voracious appetites. Adults will eat two rodents per night in warm weather and up to four rodents per night in cold weather. Young birds in the nest will eat up to six rodents per night during their rapid growth stage. A barn owl family will consume 2,000 to 3,000 rodents annually. Young will stay in the nest for eight weeks after hatch, until they are able to fly.
Browning summarized, “Barn owls have the characteristics to respond to high rodent populations. They are an opportunistic species that will expand their range and numbers where food supply is available.” He added, “A barn owl population can usually respond a lot more rapidly than the farmer can to increases in rodent populations, and they exert constant pressure on rodent populations.” Browning observed, “Barn owls will hunt the places that have the highest gopher density and that are the closest, first.” But they will hunt up to two miles from a nest site, if necessary.
Browning explained that growers should observe activity and change the number of boxes on their properties to match the site’s needs. Traditionally, the recommendation was one box per 40 acres. Browning recommends starting with one box per 20 acres. If all boxes are occupied, and gopher populations are still high, then more boxes should be added. He also explained, “If you start out with 20 boxes, and go from 18 occupied down to 12 occupied after a season or two, then you know it’s working, because the rodent population (food supply) is declining.” He added, “When you maintain box occupancy at around 60% to 80%, you’re at a good point.”
Vino Farms provides research site
Grapegrower and vineyard management company Vino Farms, based in Lodi, is a cooperative partner in a research project with Browning in a 100-acre vineyard block owned and managed by Vino Farms in Sacramento County in the Sloughouse AVA, a sub-appellation of the Lodi AVA. In January, 20 pole-mounted owl nest boxes were installed 200 feet apart surrounding the perimeter of this Chardonnay vineyard with 2-year old vines.
Vino Farms viticulturist Chris Storm, who is working with Browning, said the vineyard occupies sandy soils in the Cosumnes River floodplain and has high pressure from pocket gopher populations. Student interns studying population biology from the University of California, Davis
, are assisting with monitoring and data collection, and the PG&E grant funded the cost of the owl boxes used for the study. To date 11 owls have been identified living on the vineyard property; that includes seven living in an old barn at the site.
The study is planned to last five years. “We want to monitor and see the long-term fluctuations in the populations of gophers and barn owls at this site,” Browning said. He said this will be the first time ever a comprehensive study has been done to measure how barn owls control rodents.
Browning said that just two weeks after the owl boxes were installed, 19 of the 20 boxes showed signs of visitation. Most barn owl mating in California takes place from January to June. Browning expects some boxes to be occupied soon, noting that barn owl sounds heard in recent days indicate that mates are beginning to pair up. Browning predicted, “In the first year, we should see some significant numbers, and by the second spring we hope to start seeing some patterns.”
Owl box design, installation
The boxes designed by Browning (patent pending) are lightweight and longer lasting compared to traditional wooden boxes. They are made of white plastic with a special chemical formulation in the molding to prevent excess heat accumulation from direct sunlight on hot summer days and multiple ventilation holes located on each side of the box.
Each box has a 5.5-inch diameter entrance hole on the front, a landing ledge, a rain guard and a front panel that is easily removable for cleaning. Required cleaning intervals vary based on occupancy, although some organic certification programs require annual cleaning. Boxes should be installed with a 2- to 3-inch substrate of mulch inside the box for the nesting birds; shredded bark or wood are recommended. The mulch should be replaced every one to two years.
Some information sources have stated that boxes should be 12 feet or more above the ground, but barn owls have been known to nest on the ground. Browning recommends installing boxes 8- to 10-feet high. The box entrance can face any direction but should be away from the direct prevailing wind. There are no firm rules about the distance between boxes. It can be as little as 50 feet in an open field or vineyard, but boxes can potentially be clustered in a suitable location, and the owls will fan out to hunt.
The required number and distribution of boxes will vary. Browning said, “A single box on a 50-acre vineyard will result in a very well-fed barn owl family, but will not likely reduce rodent activity.” A nest box for every 10 to 20 acres is a good place to start and can be adjusted depending on results.
Browning is currently on sabbatical from his job at the Pittsburg Zoo and is in California to establish the research project at Vino Farms, and to promote and find additional distribution for his barn owl nest boxes through ag supply companies. More information is available on Browning’s website about barn owl biology and IPM, and nest boxes can be ordered directly from the site at barnowlbox.com