The eastern screech owl (Megascops asio) or eastern screech-owl is a stocky, short-tailed owl that is relatively common in Eastern North America, from Mexico to Canada. Mid-sized for a screech owl, the species resides in most types of woodland habitats across its range and is relatively adaptable to urban and developed areas compared to other owls. Although it often lives in close proximity to humans, the eastern screech owl frequently avoids detection due to its strictly nocturnal habits.
Taxonomy The eastern screech owl was formally described in 1758 by Swedish naturalist Carl Linnaeus in the 10th edition of his Systema Naturae. He placed it with all the other owls in the genus Strix and coined the binomial name Strix asio. Linnaeus based his account on the "little owl" that had been described and illustrated by English naturalist Mark Catesby in his book The Natural History of Carolina, Florida and the Bahama Islands. The specific epithet is the Latin word for a type of eared owl. The eastern screech owl is now one of 22 screech owls placed in the genus Megascops that was introduced in 1848 by the German naturalist Johann Kaup.
Five subspecies are typically recognised for the eastern screech owl, but the taxonomy in the species is considered "muddled". Much of the variation may be considered clinal, as predictably, the size tends to decrease from north to south and much of the color variation is explainable by adaptation to habitat.
M. a. asio (Linnaeus, 1758) includes previously described races no longer considered valid such as M. a. carolinensis, M. a. naevius and M. a. striatus. It is resident from eastern Minnesota to southwestern Quebec and southern New Hampshire south to Missouri, Tennessee, and northern South Carolina. Dorsal color is cold gray; the red morph is common (about 39% of overall population). The nominate's markings are coarse and sparse and its toes are densely feathered. Its primary song has a terminal, tremulous whinny. This is a medium- to large-sized race, measuring in wing chord length. The owls of southern Ontario are on the larger end of the scale, of similar size to the relatively big owls of Colorado and Wyoming. M. a. maxwelliae (Ridgway, 1877) includes M. a. swenki, it is resident from central Montana, southeastern Saskatchewan, and southern Manitoba south to western Kansas. This race is similar to M. a. asio, but its dorsal color tends to be a paler gray, the ventrum being whiter and less heavily marked and red morphs tending to paler and rarer (about 7% of population). With a wing chord length of , this is the largest race in average linear measurements. This subspecies was named in honor of Martha Maxwell by ornithologist Robert Ridgway of the Smithsonian Institution. M. a. hasbroucki (Ridgway, 1914)is the replacement name for the formerly described M. a. trichopsis. This subspecies is a resident from Oklahoma panhandle and southern Kansas south to Edwards Plateau of central Texas. This subspecies is also similar to M. a. asio, but its dorsal color is buffy gray, the red morph being rare (about 5% of the population), and markings are coarse and dense. This race averages at a similar size as the first two, at in wing chord length. M. a. mccallii (Cassin, 1854) includes previously described races such as M. a. enano and M. a. semplei. Resident from southern Texas (Big Bend to lower Rio Grande Valley) and northwestern Chihuahua and northern Coahuila southeast to eastern San Luis Potosí, this race is similar to M. a. hasbroucki, but its markings are fine and dense, so the dorsum looks heavily mottled, with red morphs being rare (apparently entirely absent in South Texas). Its body size is smaller to the northern races, with a wing chord length of . Unlike other subspecies, the primary song of M. a. mccallii lacks a terminal whinny. M. a. floridanus (Ridgway, 1873) is resident in Florida and southern Georgia west through Gulf Coast states to western Louisiana and north in the Mississippi River valley to southeastern Arkansas. This race's dorsal color is often rusty-brown (the red morph is equally common), with fine and dense markings. As described above, this subspecies may occur in a true "brown morph". It is the smallest race of eastern screech owls, ranging in wing chord length from .
Description
Adults range from in length and weigh . Among the differently sized races, length can average from . The wingspan can range from . In Ohio, male owls average and females while in central Texas, they average and , respectively. They have either rusty or dark gray intricately patterned plumage with streaking on the underparts. Midsized by screech owl standards, these birds are stocky, short-tailed (tail averages from in length) and broad-winged (wing chord averages from in length) as is typical of the genus. They have a large, round head with prominent ear tufts, yellow eyes, and a yellowish beak, which measures on average in length. The feet are relatively large and powerful compared to more southern screech owls and are typically feathered down to the toes, although the southernmost populations only have remnant bristles rather than full feathering on the legs and feet. The eastern screech owl (and its western counterpart) are actually some of the heaviest screech owls; the largest tropical screech owls do not exceed them in average or maximal weight, but (due to the eastern screech owls' relatively short tails), they are surpassed in length by Balsas (M. seductus), long-tufted (M. sanctaecatarinae), white-throated (M. albogularis), and rufescent owls (M. ingens), in roughly increasing order.CRC Handbook of Avian Body Masses, 2nd Edition by John B. Dunning Jr. (Editor). CRC Press (2008), .
Two color variations are referred to as "red or rufous morphs" and "gray morphs" by bird watchers and ornithologists. Rusty birds are more common in the southern parts of the range; pairings of the two color variants do occur. While the gray morph provides remarkably effective camouflage against the bark of hardwood trees, red morphs may find security in certain pine trees and the colorful leaves of changing deciduous trees. The highest percentage of red morphs is known from Tennessee (79% of population) and Illinois (78% of population). A rarer "brown morph" is known, recorded exclusively in the south (i.e. Florida), which may be the occasional product of hybridation between the morphs. In Florida, brown morphs are typically reported in the more humid portions of the state, whereas they appear to be generally absent in the northern and northwestern parts of the state. A paler gray variation (sometimes bordering on a washed-out, whitish look) also exists in western Canada and the north-central United States.
Confusion with other species In the closely related western screech owl (Megascops kennicottii), no color morphs are known; all owls of the western species are gray. Besides coloration, the western screech owl is of almost exactly the same general appearance and size as the eastern. The only reliable distinguishing feature is the bill color, which is considerably darker (often a black-gray) in the western and olive-yellow in the eastern; their voices also differ. The eastern and western screech owls overlap in the range in the Rio Grande valley at the Texas–Mexico border and the riparian woods of the Cimarron tributary of the Arkansas River on the edge of southern Great Plains. Other somewhat similar species that may about the eastern screech owl's range in its western and southernmost distribution, like the Middle American screech owl (Megascops guatemalae; formerly called "vermiculated screech owl"), whiskered screech owl (Megascops trichopsis), and flammulated owl (Psiloscops flammeolus), are distinguished by their increasingly smaller body and foot sizes, different streaking pattern on breast (bolder on the whiskered, weaker on the others), different bare part coloration, and distinctive voices. Through much of the eastern United States, eastern screech owls are essentially physically unmistakable, because other owls with ear tufts are much larger and differently colored and the only other small owl, the northern saw-whet owl (Aegolius acadius), is even smaller, with no ear tufts, a more defined facial disc, and browner overall color.
Plumage polymorphism Eastern screech owls exhibit a similar polymorphism as tawny owls, whose plumage ranges from rufous to gray. The inheritance of morph in owls is likely complex, but rufous plumage may be controlled by a dominant allele, and gray plumage alleles are recessive. Latitudinal clines are seen in screech owl polymorphism, with northern latitudes containing mostly gray individuals and southern latitudes containing primarily rufous individuals. This cline may be driven by higher metabolic rates in rufous individuals compared to gray individuals. Evidence of higher metabolic rates was shown by a higher proportion of gray morphs in the rural areas surrounding Waco, TX compared to the warmer suburban areas. Rufous screech owls also had higher mortality during cold winters.
Distribution and habitat
Eastern screech owls inhabit open mixed woodlands, deciduous forests, parklands, wooded suburban areas, riparian woods along streams and wetlands (especially in drier areas), mature orchards, and woodlands near marshes, meadows, and fields. They try to avoid areas known to have regular activity of larger owls, especially great horned owls (Bubo virginianus). Their ability to live in heavily developed areas outranks even the great horned and certainly the barred owl (Strix varia); screech owls also are considerably more successful in the face of urbanization than American barn owls (Tyto furcata) following the conversion of what was once farmland. Due to the introduction of open woodland and cultivated strips in the Great Plains, the range of eastern screech owls there has expanded. Eastern screech owls have been reported living and nesting in spots such as along the border of a busy highway and on the top of a street light in the middle of a busy town square. They often nest in trees in neighborhoods and urban yards inhabited by humans. In such urban environments, they often meet their dietary needs via introduced species that live close to humans such as house sparrows (Passer domesticus) and house mice (Mus musculus). They also consume anole lizards and large insects such as cicadas. They occupy the greatest range of habitats of any owl east of the Rockies. Eastern screech owls roost mainly in natural cavities in large trees, including cavities open to the sky during dry weather. In suburban and rural areas, they may roost in manmade locations such as behind loose boards on buildings, in boxcars, or on water tanks. They also roost in dense foliage of trees, usually on a branch next to the trunk, or in dense, scrubby brush. The distribution of the species is largely concurrent with the distribution of eastern deciduous woodlands, probably discontinuing at the Rocky Mountains in the west and in northern Mexico in the south due to the occupation of similar niches by other screech owls and discontinuing at the start of true boreal forest because of the occupation of a similar niche by other small owls (especially boreal owls (Aegolius funereus). Eastern screech owls may be found from sea level up to in elevation in the eastern Rocky Mountains and up to in the eastern Sierra Madre Oriental Mountains, although their altitudinal limits in the Appalachian Mountains, near the heart of their distribution, is not currently known.
Behavior