Gufo barrato

Identificazione

The barred owl is considered somewhat subdued and drab in coloration compared to the sometimes rich coloring of other sympatric owls. Overall, this owl is greyish-brown or brown. The brownish color extends from the head to the back. Barred owls are scalloped with white bars on the mantle and the back, bearing as well some whitish spotting on the wing coverts. The underside has a pale creamy gray-brown base color (ranging into dirty white in the palest individuals) overlaid about the throat and upper chest with horizontal, slightly crescent-shaped barring (hence its common name), while the belly is boldly streaked in a vertical pattern. The streaking is usually blackish, dusky brown, or sometimes rufescent- (reddish-) brown. The head is fairly large (although not especially so for a species in Strix) and rounded with no ear tufts. The facial disc is pale grayish-brown with darker yet subtle concentric lines. The bill is pale straw-yellow (occasionally showing a mild greenish tint) while the cere (a bare structure at the base of the beak) is "horn-colored". Its eyes are a dark brown color; the eyes may appear intensely black in the field and, although large, are fairly closely set.<p>The barred owl has well-developed eye anatomy. As is typical of owls, their ocular anatomy is quite distinct from diurnal raptors especially in terms of their photoreceptor cells, as they have a very large number of rod cells in their quite sensitive retina. However, their pecten oculi is smaller relative to the size of their large ocular globe (other large owls are known to have similar pecten proportions). The vision in limited or almost no light during a laboratory study of a barred owl was found to be similar to that of other owls, including the long-eared owl (Asio otus) and the barn owl (Tyto alba). The tarsi and toes are feathered up to the dark gray, black-tipped talons. These feathers are more sparse and bristled in the southern races. On individuals with bare sections of their toes, the toes are yellowish-gray in color. The flight feathers are barred with whitish buff and brown while the tail is brown or grayish-brown with 4–5 whitish bars. Young barred owls with their second set of down feathers are fluffy brownish-white, with indistinct darker barring on their head, back and mantle. They quickly become juveniles which resemble adults but have less distinct markings (especially about the head and neck), more buff coloring overall, often some remnant down, pinkish skin and a pale, blue-green cere. Also the tail at this age may have as many as seven bands (though sometimes have four like adults). Full adult plumage is obtained via molt after about a year as well as adult bare part characteristics. A study of tail molt in Washington showed that molt tends to occur relatively quickly, and that young individuals are difficult to age by state of molt alone. Southern barred owls tend to be darker and slightly smaller than northerly ones. Rare captive and wild barred owls with albinism have been described and are pure white but tend to retain their brown eyes.<p>The barred owl is a large species. The adult measures anywhere from 40 to 63 cm (16 to 25 in) in length while the wingspan may range from 96 to 125 cm (38 to 49 in). The wing area (measured by square centimeter relative to the body mass) is quite intermediate among American owls, with the wing loading being lower than larger, but proportionately small-winged larger owls and even than some smaller owls. The barred has high wing-loading. Wing-loading is related to hunting technique, with higher wing-loading owls typically hunting from a perch, with only a brief flight necessary to obtain food, whilst lower wing-loading owls often hunt their prey from active flight. As is the case in most owls, the various wing feathers of barred owls are uncharacteristically soft and bear a comb-like shape, which in turn renders their flight functionally silent during their hunts. Like most birds of prey, the female is larger than the male barred owl, sometimes described as reverse sexual dimorphism (due to the fact that males average larger than females in most non-raptorial birds).<p>Among standard measurements, the wing chord of grown males varies from 303 to 340 mm (11.9 to 13.4 in), with an average from three sources of 326.2 mm (12.84 in), the tail may measure from 182 to 250 mm (7.2 to 9.8 in), with an average of 219.6 mm (8.65 in) and the culmen from the cere may measure from 22 to 27 mm (0.87 to 1.06 in), with an average of 24.3 mm (0.96 in). Meanwhile, for the female, the wing chord may range from 318 to 357 mm (12.5 to 14.1 in), averaging 335.8 mm (13.22 in), the tail from 204 to 257 mm (8.0 to 10.1 in), averaging 223.3 mm (8.79 in) and the culmen from the cere 20 to 30.7 mm (0.79 to 1.21 in), averaging 25.1 mm (0.99 in). Sexual dimorphism is particularly pronounced in barred owls by body mass as males within a population are sometimes a third lighter in weight. In the nominate subspecies (S. v. varia), average weights for males have been reported as 621.9 g (1.371 lb) (sample size 12), 632 g (1.393 lb) (sample size 20) and 681 g (1.501 lb) (sample size unknown) in three samples. The weight range for adult males is known to vary from 468 to 812 g (1.032 to 1.790 lb). The considerably larger female of the nominate subspecies has been reported to average 801 g (1.766 lb) (sample size 24), 872.6 g (1.924 lb) (sample size 14) and 909.5 g (2.005 lb) (sample size unknown). Altogether, fully-grown female barred owls may weigh from 610 to 1,150 g (1.34 to 2.54 lb).<p>The barred owl is a powerful vocalist, with an array of calls that are considered "spectacular, loud and emphatic". Calls probably carry well over 0.8 km (0.50 mi). Its usual call is a series of eight accented hoots ok-ok-ok-ok ok-ok-buhooh, or the "typical two-phrase hoot" with a downward pitch at the end. The most common mnemonic device for remembering the call is "Who cooks for you, who cooks for you all." Due to its best known call, the barred owl is sometimes colloquially referred to as Old Eight-Hooter. At 80% of study posts in Virginia, barred owls responded to playback of this call. A further call is the "ascending type" or the "legato" call, a series of variable notes ending in oo-aw or hoo-aah. At least two other variations on the legato/ascending call are known. 56% of studied owls in Virginia engaged in the ascending type call but 36% uttered only the closing notes. The isolated hoo-aah, sometimes called the "inspection call", was the most common song type in north Florida and the most likely to be heard during daylight. Several other calls, although some are not dissimilar variations on the main calls, are known. Some of these vary into cackles, hoots, caws and gurgles, at times described as "sudden demonic laughter", "cat-like screams" and "prolonged outbursts of cackling" and seem to be, among Strix species, an idiosyncrasy endemic to the barred owl.<p>Another call type is the "mumble", a grumbling, slurred and subtle err-ERR-err, also an up-and-down "twitter" call at a high pitch. When agitated, this species will make a buzzy, rasping hiss about three times in three seconds, repeating every 10–30 seconds, and will click its beak together forcefully. Females and juveniles beg with high scratching skreeechch notes. The voice of the two sexes is similar, but the female has a higher-pitched voice with longer terminal notes. Of calls, 87 to 94% are identifiable to sex per one study. While calls are most common at night, the birds do call during the day as well, especially when provoked by human playback or imitation. They are more responsive than any hawk in the east to playback of calls of their own species. The barred owl is noisy in most seasons but peak vocalization times for barred owls tend to be between late January (in Florida) and early April (in Canada). Two seasonal peaks in vocalizations, one right before breeding and another after the young have dispersed, was detected in Connecticut, with peak vocalizations on nights with extensive cloud cover. Peak times for vocalizations are between 6:00 pm and 6:00 am, with the least frequent vocalizations around mid-afternoon.

Tassonomia

Nome scientifico

Strix varia

Autorità

Barton, 1799

Ordine

Strigiformi

Famiglia

Strigidae

Genere

Strix

Areale e habitat

The barred owl is distributed throughout most of the eastern United States, as well as much of southern Canada. They are found as far northeast as much of Nova Scotia (western two-thirds), New Brunswick and Sept-Îles, in much of Quebec, up to Lake Mistassini, and Ontario, up to Moosonee. The barred owl has been recorded as ended up as far north as central Labrador, though the species is not yet confirmed to breed in the province. The barred owl ranges in every part of the eastern United States continuously from northernmost Maine down throughout New England, the Mid-Atlantic states, much of the Midwest, the Southeast United States and all of Florida. They are found to as far west without substantial gaps to the limits of western Minnesota, easternmost South Dakota, the southeastern corner of Nebraska, the eastern half roughly of Kansas, most of Oklahoma and east Texas to as far west as Cisco and Burnet. Arguably and discontinuously from Texas, the species may range into central and southern Mexico but these populations are now often considered a separate species. These initial parts of the range in the eastern and central stretches would be considered as where the species is "native". A wandering barred owl was once seen flying over Lake Michigan 48 km (30 mi) from the nearest land.<p>The remaining parts of the range are considered where the barred owl introduced itself in the last century or so. The historical lack of trees in the Great Plains presumably acted as a barrier to the range expansion, and recent increases in forests broke down this barrier. Increases in forest distribution along the Missouri River and its tributaries provided barred owls with sufficient foraging habitat, protection from the weather, and concealment from avian predators. This allowed barred owls to move westward, initially solely along other forested river corridors (e.g. the Yellowstone and Musselshell), but increases in forests in the northern Great Plains decades later would allow them to connect their eastern and western distributions across southern Canada. These increases in forests were caused by European-American settlers via wildfire suppression and ceasing the fires historically set by Native Americans, as well as by increased tree-planting.<p>In Canada, the barred owls with range expansion now range through southern Manitoba (excluding the southwest corner), a broad section of south-central Saskatchewan, and east-central and nearly all of western Alberta, now up to High Level. The barred owl has been present in Manitoba at least since 1886, Alberta since 1932 and Saskatchewan since 1948. However, a study in Alberta has shown that barred owls have likely been present for no less than 100 years. This owl species currently ranges through much of British Columbia, where they have been expanding their range since at least 1943, including Vancouver Island and as far north as Fort St. John. The barred owl has had a further northward expansion in the west to southeastern Alaska (Skagway to Ketchikan) and extreme southwestern Yukon. In the western United States they also range into northwestern Montana and northern Idaho. Barred owls were first verified in southwest Montana in 1909 and in northwest Montana in 1921 (although reports of the species may date back to the 1870s) but not in Idaho until 1968. Of unknown origin, barred owls have been seen in Colorado since around the turn of the 20th century. They range broadly in eastern and western Washington (reached by 1965) and western Oregon (reached by 1972) (mostly along the forested corridors hugging the montane areas of the Cascade, Olympic and Blue ranges), and northern California (reached by 1976), now down to the Redwood National Forest, the Sierra Nevada and outer San Francisco.<p>The habitat used by barred owl is largely old deciduous, mixed forests and, occasionally, coniferous forests. Old growth forests are preferred due to more extensive potential nest sites, less lower-branch density to impede hunting (and perhaps superior structural complexity to aid hunting), greater security from mobbing and perhaps greater thermoregulation. They are often found in bottomland hardwood forests in the largest swath of the native breeding range, often (particularly from Virginia south and west) with deep, dark stands of oak, gum and cypress. Secondary habitat, often used during foraging forays in the south, are often oak savanna or cabbage-palm areas. Regardless of area, some variety of water is frequently present, including riparian areas or swampy ground. Closed canopy forests were preferred in a study from Oklahoma (62.8% of habitat used), followed by fallow agricultural fields (10.6%), wetlands (8.1%) and open terrain (6.2%). The latter three were visited during hunting forays, and wetlands and open terrain areas were not used outside of the breeding season. Along the Atlantic coast area from New England to New Jersey, barred owls are often found in mixed swamps areas with cedars, seldom wandering to adjacent wetlands or farmland. The species may be at home in wooded areas in mountainous regions. More upland wooded habitats, often in mixed woods containing hemlock, alder, poplar, pine and oak, are typical in the northern part of the range. A study in northern New Jersey found at least 15 species of both conifer and deciduous trees were routinely used for differing purposes. In Michigan, barred owl habitat usually consists largely of some combination of hemlock and maple trees, with mixed forest usage being use disproportionately to its prevalence in the environment. Large oak stands were preferred in Minnesota, a bit ahead of mixed forest and far ahead of white cedar swamps and other habitat types, which were either too dense, too open or had too few attractive nesting sites.<p>Barred owls are not confined to extensive forest, also dwelling extensively in semi-open wooded areas, locally in large parks with mature trees, and in forest adjacent regions recently logged. Recent studies show suburban neighborhoods can be ideal habitat for barred owls, and the species may be considered a local synanthrope. Using transmitters, scientists found that some regional populations, such as in Charlotte, North Carolina, increased faster in the suburban settings than in old growth forest. A factor of this suburban success may be easily accessible rodent prey in such settings. However, for breeding and roosting needs, this species needs at least some large trees and can be locally absent in some urban areas for this reason. The increased offspring offset the death rate due to impacts from cars, other types of collisions and disease. Similarly, in Piedmont, South Carolina, productivity of the owls was higher in suburban areas and they comprised 41% of the territories of the local owls but various sorts of anthropogenic mortality were seemingly higher. In suburban areas of Ohio, 41.4% of barred owl range was forested, 29.8% was low-density residential areas and less than 15% was pasture. On the other hand, studies from the Northeastern United States, such as in New Jersey, found barred owls breeding mainly in plots of old-growth woodlands, and rarely successfully breeding in peri-urban areas, in part because of competitive and predatory displacement by great horned owls. Furthermore, a study in North Carolina showed most barred owls appear to favor areas with at least 86 to 370 ha (210 to 910 acres) of woods but did not seem to be affected by the presence of roadways.<p>In the Pacific Northwest, they can be quite adaptive to secondary forests. On the other hand, in Ontario where the barred owl is native, secondary forest seemed to be largely avoided per a study. In the recent western part of the range, barred owls often dwell in mixed wood areas, often where there are lowland stands of balsam poplar, trembling aspen and white spruce, occasionally but not commonly in pure conifer boreal stands. Barred owls in California preferred stands of red alder. More so confined to inland areas, as in eastern Washington, Idaho, Manitoba and Montana, they prefer Douglas fir, ponderosa pine, paper birch, burr oak and western larch. In north-central Alberta, the use of old growth forest was far more prevalent than its occurrence in the wild. In the Cascades Range of Washington, barred owls usually dwell in areas with more grand firs, taller and more diverse tree heights, more enclosed canopies, higher numbers of trees per acre and less ground cover. In Saskatchewan, barred owls preferred areas with a minimum of 66% forest cover.

Migrazione

Resident species

Comportamento

The barred owl, like most owls, is largely adapted to nocturnality. Between 5:00 am and 8:00  pm, juvenile barred owls were recorded to sleep an average of 28% of each hour. Peak times in Minnesota were found to be right after sunset and just before dawn. Nonetheless, they are not as fully nocturnal as many owls and rank around 6th amongst 19 regular North American owl species for the regularity of their activity outside of nightfall, especially in particular circumstances such as when a rival or a human impersonator is emitting barred owls calls or whilst hunting. Often daytime activity tends to be early in the morning or around dusk but potentially at any time (overcast days being preferred). This species often spends the daytime hidden away in the dense foliage of a tree, often at minimum 5 m (16 ft) above the ground, but sometimes also roosts in a branch close to a broad trunk or in a natural tree hollow. Roost tree heights in Minnesota was typically 8 to 12 m (26 to 39 ft) while, in Illinois, they were up to 9 m (30 ft). Recently fledged owls sometimes roost in tall grass, usually after falling from the nest tree.<p>Roost site selection may be partially dictated by thermoregulation, as in spotted owls, with shadier roosts likely to mitigate heat stress. They seldom rely on camouflage, instead often flying at the least disturbance and not allowing close approaches, making them potentially difficult to observe. Yet, on the other hand, they can be surprisingly tame and seemingly curious of people in the wild; further they are considered "as mild and engaging" as a predator can be. Barred owls are regularly subject to mobbing by small birds, from several small passerines to corvids and woodpeckers, and mammals when discovered by them during the daytime, and such situations may lead to them being attacked by diurnal birds of prey. There are some records of barred owls engaging in allopreening, presumably between pairs, with each other in the wild. They are skilled and silent fliers and frequently use routine forest flyways with open understory and low branch densities.<p>Like most species of owl in the Strix genus, the barred owl tends to be highly territorial regardless of the time of year. The territories are claimed by singing from different perches, often near the perimeter of its perceived home range. The boundaries are almost always well-maintained by barred owls and are generally stable from year to year and even generation to generation. Territory sizes have been determined via radio telemetry. The average territory size of 13 in Minnesota was 273 ha (670 acres), of 7 in Michigan it was 282 ha (700 acres) and of 10 in Wisconsin was 337.9 ha (835 acres). Another Minnesota study found pairs to occupy a mean of about 226 ha (560 acres) in mixed hardwood-conifer woods. Pairs in an Oklahoma study were reportedly found every 110 to 165 ha (270 to 410 acres). The mean territory size from 10 studies was estimated from throughout the range. In this study, the breeding season mean area was calculated at 256.7 ha (634 acres) in males and 297.8 ha (736 acres) in females; whereas the means in nonbreeding season were 900.4 ha (2,225 acres) in males and 536.2 ha (1,325 acres) in females. Overall, the annual mean home range for males was 782 ha (1,930 acres) and for females was 538.7 ha (1,331 acres). The breeding range's mean home size in Washington was 321 ha (790 acres), while it was 971 ha (2,400 acres) in the non-breeding season.<p>The ranges of pairs overlaps in the breeding season, at 87–95% range overlap, but decreases down to 45% after hatching. In a Florida study of barred owls, territorial responses, including several duets, by a pair were found to be provoked by researchers playing calls of both "stranger" owls and recordings of owls that were neighbors to the pair being tested. The aggressive response even to known neighbors in this study is unusual. In tawny owls in a study from Italy, for instance, they responded mildly or not at all to the calls of neighboring tawny owls known to them but with great aggression to the calls of "stranger" owls. Due to its rather stolidly territorial nature, the barred owl is not normally a migratory species. Claims in the past of "rather impressive" flights in New England in the past were lacking in verifiable details. Of 158 banded recoveries in the northern part of the range, movements during winter were found to cover no further than 10 km (6.2 mi), while all those recovered in Saskatchewan and Alberta scarcely moved at all. There is little to no evidence of nomadic behavior as has been recorded in several other owls in the north.

Conservazione

Tendenza della popolazione

rising