Scientific Name: Otus asio
Common Screech Owl: Eastern Screech Owl
(Information for this specie page was gathered in part by Ms. Kacey Dananay for an assignment in Biology 220W, Spring 2007)
The eastern screech owl (Otus asio) is a small (7 to 10 inches long) owl with prominent, feathered ear tufts and large, yellow eyes. It has two color phases: a bright, red-brown and a relatively drab gray. Both color types have lighter colored, black streaked, breast feathers. The red-brown type is more common here in the eastern sections of the species range. Eastern screech owls are among the most common owls in North America. Their name is derived from their distinctive, warbling, wailing night call that they sing either alone or in mating duets.
The eastern screech owl can live in a wide range of both natural and human-modified habitats. They are found in hardwood and coniferous forests, in meadow and field edge ecotones, in parks, and in rural and suburban woodlots and yards throughout the United States east of the Rocky Mountains. Their northern range extends into southern Ontario and their southern range drops into Mexico. In winter, this broad range may contract into its milder, southern areas, but no true seasonal migration occurs. Home ranges of 14 to 200 acres are established by each owl. These home ranges include suitable hunting areas and also protected, daytime roosting sites (dense vegetation, tree holes (often old woodpecker holes), or human structures).
Eastern screech owls are active, nocturnal hunters. Their primary prey are shrews and deer mice, but they also opportunistically take insects (especially large moths, beetles, and grasshoppers), earthworms, snails, spiders, crayfish, other owls, chipmunks, squirrels, song birds, snakes, frogs, toads, salamanders, rats, and rabbits. Small prey are swallowed whole while large prey may be torn into bite-sized pieces. In a single night these 121 to 244 gram (4 to 8 ounces) birds may eat up to one third of their body weights. Prey is located both by vision and by the owl’s excellent sense of hearing.
Predators and Other Threats
Eastern screech owls are, in turn, preyed upon by a wide range of other species including larger owls and hawks. Nestlings and eggs may be eaten by weasels, mink, raccoons, skunks, snakes, opossums, crows, and blue jays. Humans are also significant causes of eastern screech owl mortality. Nest cavities may become infested with ants and other insects. Eastern screech owls have been observed bring live Texas blind snakes to their nest cavities where the snakes will live in a mutualistic symbiosis with the owl (snakes eat ants and other insects, owls provide the snakes a protected habitat). The owls are also subject to a variety of parasites and diseases including malaria and avian pox.
The position of these owls at or near the top of their trophic webs makes them quite vulnerable to the bioaccumulation of environmental toxins. Pesticides and vermicides used in the agricultural control of insects and rodents inevitably become concentrated in the tissues of higher order consumers. Birth defects in owlets, egg shell thinning, and direct adult mortaility from these toxins have all been observed in the eastern screech owl.
Mating and Reproduction
Eastern screech owls mate between mid-March and late May. Owls tend to form monogamous pairs which may, if the pairing successfully produces offspring, be lifelong. In years of rich prey availability a male may mate with two females. Success of these two matings, though, depends upon the energy of the male who must simultaneously feed both females throughout their incubation periods, and the ease of prey acquisition. A brood nest is basically an unadorned tree cavity or nest box. The owls utilize whatever materials were in the cavity for nesting substrate and add little or no nest materials of their own. The mated female will lay 2 to 7 eggs (4 is the most common), one egg per day. If there is a delay in laying the clutch the first laid egg may hatch well before the later eggs. This “first born” owlet, then, may develop sufficiently to be able to kill later hatching siblings. Overall success of the clutch, then, depends on precisely timed egg laying and development.
Eggs are incubated entirely by the female for 30 days. During this period the female is fed entirely by the male (again, if a male is keeping “two households” he will be a very busy owl!). Nestlings are initially fed only by the male, but eventually, as the owlets become larger, more feathered and, thus, better able to thermoregulate, the female is able to leave the nest and begin hunting for both herself and the clutch.
Owlets fledge after 28 days, and the fledges may stay with the parents for the next 8 to 10 weeks.
Mortality in owlets is very high. Abundant nest predators (listed above), injury due to accidents (in one study 33% of a screech owl cohort died due to collisions with cars and trucks and a smaller, but quite significant number, died from injuries sustained after striking walls and windows), poisoning (discussed above), and active human predation (hunting and trapping). Only half of a season’s owlets survive their first year of life. After this first year, though, an eastern screech owl may live anywhere from 14 (the wild owl record) to 20 years (the captivity record).