The Barn Owl (Tyto alba) is a year-round resident through much of San Diego County, California. And an increasing number of San Diegans are attracting Barn Owls as an agent of natural rodent control by hanging nestboxes in trees or mounting them on poles. An adult Barn Owl will typically eat 2-3 rodents a night. And a family of owls can increase that to about 10. So that equates to more than 2,000 rodents per year!
Barn Owls are nocturnal raptors and kill their prey with strong grasping talons. With their remarkable hearing and excellent low-light vision, they can accurately strike and capture prey at night in total darkness. They have a large 40-inch wingspan, and fly slowly and silently while hunting. Owls swallow their prey whole, in one piece. Later they regurgitate the left-overs in a "pellet" of fur, bones and teeth. These pellets are found under their nests and where they roost. The Barn Owl makes a loud scree-eech sound; a raspy hiss; or a series of clicks. The Great Horned Owl makes the more familiar "who hoo-ing" sound.
Barn Owls are generally monogamous and pairs often stay together for as long as both are alive. In San Diego County, Barn Owls often lay eggs and raise their young between January and May. Female Barn Owls lay an egg every other day, with clutch sizes usually ranging from 3 to 8 eggs. The female incubates the eggs for a month while the male feeds her. Right after hatching, the mother tears up pieces of food with her beak to feed the chicks. She will stay with the young owls until the youngest is about 12 days old and has a good coat of down. Once the owlets are able to swallow their food whole, then both parents bring rodent after rodent to them all night long for 2-3 months. At about 8 weeks, the owlets start flight practice for 2-3 weeks before fledging, when the nest is abandoned. Barn Owls are fun to observe and lights don't seem to bother them much. A pair of binoculars will allow you to watch their activities up close.
Barn Owls in San Diego County nest in buildings and among the bases of palm leaves more often than in cavities in native trees or on natural cliff ledges. They also readily use nestboxes designed for them. The best way to attract Barn Owls to your yard is to build a box and mount it 15 to 20 feet high on a pole or in a large tree. Trees are nice because they provide shade in warm weather and lots of perching branches for the fledglings. When installing the box in a tree, use metal cables or strong rope or chain covered with hose so that it won't cut in to the tree; and wedge the box close to the main trunk so there is no swaying in wind. The opening is best facing North-east or South-east, away from prevailing wind and weather. Afternoon shade on the box is a good idea in inland areas, as the box will be abandoned if it gets too hot inside. It is also a good idea to apply a thin layer of Ivory bar soap on the inside surface of the roof to prevent bees and wasps from colonizing the box. And avoid using rodent poison baits in the surrounding area, as they kill birds as well as other wildlife, pets and children.
A box configuration with a large doorway opening that extends down to within a few inches of the floor allows for good air circulation and observation of the owlets; and doesn't require cleaning, as the female can easily remove unwanted material through the large opening. If the box has a small opening, you can clean out the box in the fall. Be sure to wear a good dust-mask and latex gloves when cleaning out the box as a guard against diseases that affect humans but not birds. Barn Owls don't require nesting material in the box, as the female will create a bed of shredded pellets on the floor. It may take up to two years for Barn Owls to move in, but once they've discovered the box, it will often be used year after year. A Barn Owl box can be made from one sheet of 3/4-inch exterior plywood and some lumber stores will make all the cuts for you. To extend the life of the wood, paint the outside of the nestbox with water-based paint.