Screech owls (Otus sp.) are very important predators of insects and rodents, though they also hunt birds, reptiles, crayfish and amphibians. Accordingly, it's beneficial for home owners to encourage the owls' presence to control pest populations. Screech owls require woodpecker nests, natural tree cavities or similar sites for nesting, but the owls may accept and use a properly conceived and placed nesting box.
Screech Owl Basics
Two screech owl species are found in the United States: the eastern screech owl (Otus asio) and the western screech owl (Otus kennicottii). Though the western species grows slightly larger, both are small owls rarely exceeding 10 inches in length. Screech owls are usually monogamous and will exhibit site fidelity, using the same nest for years. Screech owls are known to live 14 years or more, though young screech owls are preyed upon by a number of predators and experience heavy mortality.
The Nest Box
Nest boxes for screech owls can be made or purchased, but should be constructed of a non-toxic, durable material. Cedar is the most commonly used material, though pine and engineered wood are also used in some models and plans. Screech owl boxes should be approximately 16 inches tall and 8 inches square, with a 3- to 4-inch entrance hole. The roof should be slanted to better shed rain, and it should overhang the opening slightly to keep water out of the box. Two or three ventilation holes should also be present to keep the air quality inside the box adequate.
Site Selection and Mounting
Screech owls vary in their personalities and tastes, and some boxes may be inhabited immediately, while other seemingly identical nests and locations will never be accepted by an owl. Maximize your chances of success by mimicking the natural nest sites of the species. Select a pine or hardwood tree that provides the owls with deep afternoon shade and a clear path into and out of the box. Natural sites chosen by owls are between 12 and 20 feet off the ground, so place the nest box at least 10 feet high. Place the nest box where it is easily visible from a window or favorite bird-watching spot, but far enough away that the owls feel comfortable -- 20 feet is usually sufficient. Nest boxes can face any direction except north; cold north winter winds may dissuade the owls from accepting the site.
Once the box is installed on the tree, drop a few handfuls of dead leaves inside. Once this is done, little maintenance is needed for the boxes. Though ants, flies and other insects will eventually inhabit the nest -- irritating the owls and possibly endangering the eggs or young -- screech owls have evolved an interesting behavior to cope with this. First reported in a 1987 issue of “Oecologica,” F.R. Gehlbach, et al., observed screech owls bringing live blind snakes (Leptotyphlops dulcis) back to the nest to consume these invertebrate pests.