Gene's Green Scene: A Little Night Magic

After hearing a screech owl outside my house a week or so ago and a story on NPR about building a screech owl house, I thought I would provide some information to you about it. I think I’m going to build one and see if I have any success on have some move in! -Gene

Want to improve the after-hours scene in your neighborhood? Try hanging a screech owl box.

By Kenn Kaufman

Night creatures go largely unseen, so people are often surprised to learn that they might have owls as neighbors. But it's true. Take screech owls, for example. Over much of this continent, from southern Canada south into Mexico, they can be common; in fact, studies have shown that screech owls can thrive in small towns, suburbs, even city parks.

Still, in America's tidy neighborhoods, as homeowners and townships trim away dead trees and large limbs and fill in natural cavities, good screech-owl nesting sites can be hard to find. Human-made nest boxes can make up for any shortage of natural nesting cavities, enabling these petite owls to live in places where they might otherwise be absent. This can help stabilize their populations.

There are two widespread species of screech owls, eastern and western, differing most obviously in voice. Neither actually screeches, except when agitated; most of their calls consist of mellow whistles and trills. Screech owls may spend their days sleeping in dense foliage or sitting next to tree trunks (where their mottled pattern provides camouflage), but more often they will be inside a cavity of some sort--a hollow limb, for instance, or a large woodpecker hole--hidden away from the sharp eyes of small songbirds, which will fuss about in a most irritating way when they find an owl sitting on its day roost. While a good cavity can help the owl avoid the annoying attentions of songbirds, it becomes essential during the nesting season.

Screech owl boxes have not inspired as much design experimentation as those for bluebirds or purple martins, but we do have some idea of what works. Fred Gehlbach of Baylor University, the world's leading authority on screech owl behavior, tried some variations on nest boxes as part of his studies on nesting habits. Gehlbach started by measuring natural cavities favored by the owls. He found the birds tended to use deeper cavities, more than 10 inches deep, with entrance holes not much larger than the minimum needed for their own entry. He also found that while the owls would use cavities with floors only six inches across, young owls were apt to fledge too early from such crowded nests. He had better success using boxes with floors that were eight inches square.

Although many experts recommend cleaning out nest boxes yearly, preferably in early spring, there's some disagreement on this point when it comes to screech owls. One reason for this is that the debris you'll find in screech owl boxes may have a life of its own.

Perhaps the oddest part of screech owls' behavior is their association with blind snakes, which have been found in the owls' nests. These odd snakes, which resemble large earthworms, normally appear only at night. Gehlbach and others have observed that the owls bring these small snakes to their nests and release them. The snakes feed on the larval and pupal stages of ants and flies that live in the nest debris, reducing the number of insects competing for the headless mice, dead beetles, and other tidbits cached by the owls. Gehlbach's studies suggest that the snakes actually contribute to the owls' breeding success.

It's important to place the box correctly. The natural cavities that screech owls choose are typically 12 to 20 feet above the ground and in deep shade. As the female incubates the eggs, the male is apt to spend the day roosting in dense foliage within about 20 feet of the nest. After you've placed an inch or so of dried leaves in the bottom of the nest, hang it at least 10 feet up on the trunk of a large tree. Try to place it away from sidewalks or doors, since the owls vigorously defend their nests against perceived threats. In fact, when the young are near fledging, some unusually spunky adults may swoop down at people or pets who wander too close, even raking them with their claws. So while placing the box in as secluded a spot as possible is good for the owls, it might be better for us, too.

Kenn Kaufman is the author of Birds of North America (Houghton Mifflin).