One of the most common owls in low elevation woodlands and deserts from central Mexico and the western United States north along the Pacific Coast, the Western Screech-Owl exhibits considerable geographic variation both in size and coloration, and its subspecific taxonomy is complicated.
This owl occurs in a wide variety of wooded habitats ranging from arid woodland in southern areas to confer-dominated forests in the Pacific Northwest. The species reaches highest densities in riparian deciduous woodlands at low elevations. It is tolerant of humans and often nests and hunts in residential areas and suburban parks if suitable trees for nesting and roosting are available. It eats a diverse array of small animals—primarily small rodents, but also birds, amphibians, reptiles, fish, insects, crayfish, slugs, snails, and worms. It is sexually dimorphic in size, females averaging about 4% larger than males in linear dimensions.
Like many species of small owls, the Western Screech-Owl nests in tree cavities and is easily attracted to nest boxes. Nesting duties are strongly divided, with the male providing almost all of the food for the female and young, while the female incubates the eggs and broods the young. The young leave the nest while still in a fluffy juvenile plumage and before they can fly well, but are independent by midsummer and disperse from their natal territories at that time. There is no evidence of migration in this species; pairs are generally resident on territories year-round.
Populations are generally thought to be stable throughout much of the range, but habitat loss due to high-density housing developments and forest clear-cutting have a localized negative impact. In the Pacific Northwest, there is correlative evidence that predation pressure from expanding Barred Owl (Strix varia) populations is causing serious population reductions.
Despite its abundance, the Western Screech-Owl has not been extensively studied, perhaps because it was long considered conspecific with the better-known Eastern Screech-Owl (Megascops asio); most published studies are short works on diet and distribution. Much of the early work on the Western Screech-Owl was carried out by Joe Marshall in the southwestern United States. More recently, John Doremus, Jeff Marks, and others have studied the breeding biology of the species along the Snake River in southern Idaho, and Jim Belthoff and colleagues continue work in that state ( Ellsworth and Belthoff 1997 , Ellsworth and Belthoff 1999 , Belthoff and Dufty 1998 ), concentrating on the endocrine control of juvenile dispersal. Basic research on breeding ecology has been done by Fred Gehlbach in southeastern Arizona.