Barn Owls as a means of rodent control
Approximately 26 million years ago the first Barn Owl appeared on earth. Since then Barn Owls have reigned supreme as one of the most efficient hunters on wings. Before the advent of modern agricultural methods and the overuse of pesticides, Barn Owls were the farmer's best friend when it came to rodent control. A single Barn Owl can consume 53 pounds of gophers in a year or 3,000 rodents per family in a four month breeding cycle. They prey mainly on meadow voles and gophers as well as rats, mice, moles and the occasional rabbit, bat, and songbird.
Because Barn Owl populations have declined in recent history today's farmer does not enjoy the same widespread benefit from Barn Owls as did their predecessors. One significant factor that partially accounts for this decline is the existence of fewer mature trees with large enough cavities for the Barn Owls to nest in. Fortunately, however, the birds can be easily attracted to an artificial cavity such as a nest box. This means that the Barn Owl population of certain areas can be significantly increased by putting up suitable nest boxes. And because of the Barn Owl's unique non-territorial behavior, there can be as many owls in an area of high pest infestation as there are boxes available for them to use.
Although Barn Owls will not completely eradicate the rodent population they can certainly reduce the number of rodents to a manageable level. Along with other nonpoisonous methods of pest control, using owls is a long term, successful, and safe approach to rodent control. However, NO rodent poisons can be used while encouraging owls to your property. Rodent poison placed inside a building or anywhere outside can secondarily kill an owl if the bird eats a poisoned rodent. It can also kill other beneficial predators such as hawks, foxes, bobcats, coyotes and snakes. Without these predators the rodent problem can become ten-fold. Working with nature, not against it, can achieve a balance such that we all win.
Note: Ground Squirrels are active during the day and the nocturnal Barn Owl will not help with a squirrel problem. However, in areas of infestation, you can erect a substantial post of 20-25 feet in height to provide a perch from which hawks will hunt during the day. Red-Tailed Hawks in particular will hunt ground squirrels. If your vineyard is enclosed in deer fencing, you may wish to cut several coyote sized holes in the bottom of your fence to allow easy access for coyote, bobcat and fox. These animals are very good at hunting ground squirrels and rabbits. If you are concerned about these predators chewing drip lines, place a few pans underneath your drip lines to collect water for their use during the dry months.
Barn Owls live and hunt in open habitat—fields, meadows, marshlands, and agricultural land. As a rule of thumb, 5-6 boxes per 50 acres are recommended. According to the results of a Barn Owl prey study in the Lodi grape district by Chuck Ingels, Barn Owl densities of one pair per acre in non-irrigated fields would be necessary to remove the annual reproductive output of gophers. In irrigated fields, other non-poisonous methods of rodent control might be needed along with the owls. There are many methods of rodent control available that do not involve poisons.
Barn Owls do not hunt in close proximity to their nest. Therefore, nest boxes should be placed some distance away from heavily infested areas. Strategically place the boxes in different areas so their hunting areas overlap to provide complete coverage. And, by placing a perch in an area of high rodent infestation, you can encourage the owls to hunt in that area. In vineyards these perches can be placed at the end of a row of vines. The perch should be 10-12 feet high. Barn Owls will hunt from perches by locating their prey and pouncing upon it; but their main method of hunting is to fly low over the ground and listen for their prey. Out of any animal tested, the Barn Owl has the most acute hearing of all and can locate its prey by sound alone.
In any given area, there should always be some unoccupied boxes to allow for the natural fluctuation of both prey and Barn Owls. Once most of the boxes are occupied, more can be added. Boxes can be quite close together. Where there is a lack of nest sites but abundant prey, Barn Owls have been known to nest in colonies. Two families of Barn Owls have been observed nesting in a single tree. Placing the boxes one hundred yards apart (or a little less) is quite adequate.
Do not place boxes in the center of vineyards or at the end of vineyard rows. Place boxes on vineyard lanes taking care that there is open space in a five foot radius around the post. This allows easy access for maintenance. If you have trees on the perimeter of your vineyards, install post boxes on the outside of the foliage drip line.
Nest boxes can be attached to a tree, a building such as a barn, or a post away from intense human activity. They should be placed 12-15 feet off the ground and should face into an open area away from prevailing winds. There is no need to put any nest material inside the box. The owls will regurgitate pellets, consisting of undigested bones and fur into the box. The female will carefully break the pellets apart to create a soft nest for the young. Install the boxes in an open habitat away from heavily wooded areas. Great Horned Owls, which prefer wooded habitat, will prey on Barn Owls. Avoid eucalyptus trees which are favored nesting trees for Great Horned owls. Before installing the box, check for sharp edges or nail points sticking out inside the box and file them down. The edges of the box should be flush and watertight. We use a water based deck stain to protect the wood on the outside only. DO NOT PAINT ANY SURFACES INSIDE THE BOX.
The Hungry Owl Project's boxes are designed for installing in trees, but they can also be used on posts. Where there is no shade, a sunroof should be added. This is simply a piece of plywood painted white and secured to the roof with a 1 to 2 inch gap between the roof and the sunroof for air circulation. Side panels can be added if needed to maintain a comfortable temperature inside.
When installing a box on a post it is worth taking the time to make sure that the post is well secured in the ground. We recommend digging large, deep holes and using concrete to secure the post in place. Remember that these boxes will need to be maintained once a year and it is important to be able to put a ladder up safely against the box. High winds can cause top-heavy boxes that are installed on flimsy posts to collapse.
The Hungry Owl Project recommends using 14' long sustainable redwood posts (Nominal 6x6) that are set in concrete with 31⁄2 feet of the post below grade. The posts must sit on 10" of packed gravel and they must be plumb.
Attach the box to the post with lag screws, making sure that the box is level and centered on the post.
Locate the bottom of the box at a height of 9' above the ground (the upper 6" of the back of the box will extend above the top of the post.) Although this might seem low, the owls will accept this height. Note that in areas where there might be any human activity we would recommend that the box be higher, but for multiple boxes in vineyards and other agricultural settings in rural areas, the lower installation height will work fine for the birds while making box access more convenient and ensuring easier maintenance.
To deter predators from attempting to climb up to the nesting box, wood posts should have a 3-foot width of metal flashing wrapped around the base of the post 1 foot above the ground. In vineyard situations, it is very helpful to number the boxes before they are installed.
DO NOT place boxes on or near utility poles. Barn Owls, along with many other raptors and different species of birds, commonly die from interactions with power lines and this can cause wild fires. A recent study concludes that the estimated cost of wildfires caused by power outages on the California economy is $34 million per year. Any bird found dead from interaction with power lines should be reported to PG&E. The exact location will be needed. PG&E will retrofit 10 poles either side of the electrocution site, making it safer for the birds to land.
Other types of boxes
In areas with few trees and where the spring/summer temperatures remain consistently high, HOP recommends using a larger horizontal box with a partition inside and an open-bottomed area with a perch for the adults to roost in. Since the parent owls would normally roost in a tree, this kind of box with a perch inside will provide them with a safe place to roost away from predators. Install as many boxes as possible in this situation.
NO PERCHES should be put on the outside of any box, and the size of its hole should be no more than 5 inches in diameter. This provides protection against predation from Great Horned Owls. Make sure the box has plenty of ventilation and that it has drainage holes. All holes must be properly located and carefully sized to ensure the safety of the birds.
There are many different designs of boxes. Some are better than others. Please take time to research options in order to find a design that will keep the Barn Owls safe and productive.
Nesting season starts as early as December and can last until October. Clutch size can be anything from 3-11 chicks and Barn Owls often have two, sometimes three, clutches in a year. Although it might be tempting to check boxes during the breeding season, we don't recommend this. If disturbed, a female on eggs could abandon the box and her family. In addition, an adult owl flushed out during the day can become easy prey for a variety of hawks or can be dangerously harassed by crows and ravens.
If a box is occupied a white ring will often appear around the entry hole and scratch marks from the owls exiting and re-entering the box may be visible. Other possible signs of use are pellet debris hanging from the drainage holes and whitewash on and around the box. The sound of hissing is an indication that young owls are in the box—they will vocalize in this manner if they are disturbed. The presence of adult owls in the area around the box may also indicate that the box is inhabited. The adult owls can be heard shrieking over the fields after the sun has gone down and metallic clicking sounds can occasionally be heard when they fly in the vicinity of their nest. Occasionally there will be no signs of occupancy, but the box will indeed be in use.
Checking boxes, in order to maintain and clean them, should be done annually in late fall (preferably late October to mid-November) when the box is less likely to be occupied. A single adult might be found roosting in the box at this time, so caution should always be used when approaching the box. If this is the case, try not to disturb the bird and postpone checking that box until another day.
When cleaning the box, use a mask and gloves. Hanta virus, which is carried by rodents, can be a possible health risk. If the box has been occupied it will be filled with a thick carpet of pellet material. All that is necessary is to remove this debris and check to see if any repairs need to be made. For your own records you should make note of any dead owls that might be found in the box. Occasionally bees and other creatures such as opossums, squirrels, screech owls, wood ducks or kestrels (a small falcon) could be using the box. If you find honeybees, contact your local beekeepers for removal of the insects from the box WITHOUT the use of pesticides or poisons in order to ensure the safety of future owl families.
Other species nesting in Barn Owl boxes
Opossums found in barn owl boxes having a 5" or less diameter entry hole will be juveniles and can be left where they are. They will soon outgrow the box and will no longer be able to enter. Squirrels can be a problem in boxes installed in trees. Cleaning out the nests at the end of the season can help discourage reuse, but the best method is prevention. Do not place the box high up in the foliage of the tree. Attaching the box to the bole of the tree no more than 10' from the bottom of the box to the ground can help discourage squirrel use. Please leave Western Screech Owls, American Kestrels and Wood Ducks in the boxes. The first two species also provide beneficial rodent control and Wood Ducks need all the help they can get when it comes to nesting habitat. The nests of all native birds are protected against deliberate disturbance or destruction by state and federal laws. Another problem for nest boxes can be paper wasps and/or biting ants. Remove paper wasp nests at the end of fall after the wasp nest has been vacated. Never spray or use any type of pesticide inside the box. Ant bait can be nailed to the tree next to the box or if you can locate the ground nest place ant bait next to their exit hole.
Additional Ways to Encourage Beneficial Predators
Leaving old tree snags and keeping some areas wild will encourage beneficial predators. Planting hedges, creating hedgerows, and encouraging native plants and trees to grow will also help to support them.
The use of herbicides or pesticides will destroy the natural balance of our environment. The Hungry Owl Project is dedicated to conserving habitat and encouraging the concept of stewardship. Rather than exploiting nature, we are better served when we live with it in harmony.