By Anita B. Stone
Using barn owls for natural rodent control is gaining traction among farmers and those in other agricultural sectors. This has come about from increasingly critical environmental issues regarding chemical use in the field for rodent population control. To reduce poisons and other invasive methods of pest management, one of the most beneficial owls on the planet is being called upon as an expert rodent assailant.
Barn owls are noted for being fond of nesting but the lack of nest sites, including the loss of tree habitats, has become a major reason for the decline and non-productivity of this owl species.
When hole-nesters cannot find suitable places to breed, the population decreases notably.
Farmers have been putting barn owls on patrol for prey, including moles and gophers, as part of an Integrated Pest Management (IPM) alternative.
Barn owls exhibit some of the best hearing among birds of prey. They have a white heart-shaped, monkey-faced appearance, and are distinguished by whitish or pale cinnamon underparts and rust-colored upper plumage. Velvety feathers allow them to approach their prey silently in darkness.
Recognizable barn owl sounds range from soft chirps to a long raspy screech. If alarmed, they emit a violent hiss and are even known to hoot every now and then.
The average barn owl weighs about one pound and is about 15 inches long with a wingspan of approximately 40 inches.
Home Sweet Home
Because barn owls don’t build nests of their own, artificial nest sites are a positive way to increase their population. Ideally, if everyone assists the accelerated loss by building owl boxes, the raptors will have a safe place to nest.
Barn owl nest boxes are relatively easy to construct. Except for hole size and placement, the dimensions of the owl house are not critical. There is no limit to the number of boxes that can be constructed and hung across the field or landscape, which is preferable.
Most barn owls are migratory, but some remain year-round. Known as cavity dwellers, barn owls will inhabit tree cavities, crevices between palm tree fronds or small caves in cliffs or banks. So long as they have a cavity which is snug and quiet, 10 feet or more off the ground, these raptors will be satisfied. Some have even been found in barrels, steel drums, cat litter boxes or between bales of hay. With a shortage of nesting locations available, they will be happy with as much assistance as farmers can offer. Owls will move into any cozy place they think is right for them. But the best home is the nesting box. If you find owls nesting in your hayloft or rafter, chances are they will probably stay there.
These owls will tolerate noise and commotion around their nest, as long as they are not threatened and the food supply remains constant. They have been known to return to their nesting boxes season after season.
The North American Barn Owl species inhabits open areas and is considered rare in several states, endangered in other states and requires new areas due to rapid urbanization and the technological development of agriculture.
Owls may have different mates during subsequent mating seasons. The female lays from one to 11 white eggs between November and July, and incubation is about 30 days. One to two broods are reared during the season with the young leaving the nest about eight weeks after birth.
Because barn owls are not known to have major territorial instincts, and will nest just about anywhere, the placement of owl boxes should serve the farmer’s need as well as the needs of the owl.
It is best to place the boxes in an area with little human activity, facing the box opening away from any prevailing winds. If a box is set on a post, it is best to be within 100 yards of a large tree to provide safety for the young.
One can build boxes that fit into existing barns or other farm buildings. Owls may already be living there and may be attracted to additional nesting sites. In a nest box, the babies require protection from falling and remain out of sight and therefore will be less likely to be fearful when anyone enters the building.
Another approach is to erect owl boxes either in or under trees adjacent to the rodent infested fields. While the owls are close enough to take advantage of the rodents in the fields, the young owls will be able to enjoy the safety net from the hot sun provided by the surrounding woodsy area, using the trees as refuge once they leave the nest.
Cats, opossums, raccoons, squirrels and great horned owls may prey upon both the young and adult barn owls if they are in the immediate area. A third method is to install the boxes in the fields where the food supply is found.
For example, you can install boxes at the end of produce rows on 16-foot 4 x 4s, buried 3. feet in the ground. The top of the box is set at the top of the post, leaving the bottom of the box at almost 11 feet.
There is no guideline for farmers and growers as to how many boxes per acre will be required. The bottom line is that it depends on how many gophers, rats and raccoons you want to get rid of.
It is recommended to space the boxes around any rodent infested areas, figuring about four to six for every 50 acres.
If rodents are not devastating a field or crop the same number of boxes will work for 100 acres.
Simply make sure to provide adequate sites. We need to remind ourselves that, unlike many species, barn owls do not build nests — they simply lay eggs in holes, including rotted trees, rocky cliffs or bluffs. But it’s always positive to build a nesting box and establish perching sites for them. Good locations are wooded areas or in open fields and meadows with a few trees.
Oak and sycamore are ideal tree selections. The box can face any direction and may be hung 3 feet below a stout tree limb suspended by cables or mounted on poles which are anywhere from 15 to 30 feet above the ground. Place about six boxes per square mile for a wide field. If you decide to place the box on a post, it is best to wrap the post with a metal, conical predator guard. The success rate of box inhabitation is about 50 percent.
Summer is the best time to erect a barn owl nest box. Boxes should not be disturbed during the nesting season or owls may desert them. Ideally, about 85 percent of barn owl nesting boxes produce fledging young. Reasons for mortality among this raptor include human interference, limb breakage and a variety of attacks by raccoons, bobcats, skunks and opossums.
Basic requirements for a man-made nest box include minimum dimensions of 12 inches x 12 inches for the first floor and a cavity depth of 16 inches. To keep out predators, including the great horned owls, the entrance should be approximately
5 to 6 inches in diameter and located near the floor of the box to provide availability for the young. Make vent holes to allow air circulation near the roof. Water drainage can be made by making holes in the floor, near the corners of the box.
Make sure you include a means of cleaning and inspecting the box regularly. Inspections should be done twice each year, in June and November. Once the last chick leaves in spring, remove the remains of any dead animals and old wood shavings. Disinfect the inside of the box with a solution of 2 percent household bleach sprayed into the box. In the fall, prior to the return of the adult owls for the breeding season, make sure that paper wasps or honeybees have not moved into the box since the owls left.
Within the design, you can also establish partitions, separating the entrance from the nesting area and protecting the eggs and babies from any predators.
If you want additional protection, a roosting room for the parents to perch during the day has about the same size opening as the main cavity. If you offer more room, the hen will lay more eggs. Nest box designs range from elaborate arrangements, complete with perches and insulation to simple one-room constructions. You can use scrap exterior grade three-eighths inch or one-half inch plywood and #4 or #5 galvanized hot dipped box nails. By painting the boxes a dull green, black or brown color, you will be helping to camouflage the owls’ home. Use a marine grade plastic resin or exterior wood glue to assemble the box. Make the roof 16 inches by 26 inches to give a 1-inch overhang all around. Painting also helps prevent wood warp. Place a 2-inch layer of sawdust or wood chips in the bottom of the box and replace this “flooring” each year.
When you are ready to hang the box, use at least a 9-foot piece of wire and raise the box from 15 to 30 feet above the ground. Perches for landing outside and roosting inside are optional but they will enable the young to stretch their wings and exercise before their first flight.
This article appeared in the December 2012 issue of Acres U.S.A.