Great horned owl

The great horned owl was formally described in 1788 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae. He placed it with the other owls in the genus Strix and coined the binomial name Strix virginia. Gmelin based his description on that of English naturalist George Edwards who had described and illustrated the great horned owl in 1747 in the second volume of his A Natural History of Uncommon Birds. Edwards had seen a live specimen from Virginia at the house of the Earl of Burlington in Chiswick. Edwards also owned a preserved specimen, and another specimen formed part of the Leverian collection. The great horned owl is now placed in the genus Bubo that was introduced in 1805 by André Duméril. The great horned owl represents one of the one or two radiations of the genus across the Bering land bridge to the Americas. Whereas the lesser horned owl clearly divided once the owl had spread through the Americas, the consensus seems to be that the snowy owl and the great horned owl divided back in Eurasia and the snowy then spread back over the Arctic through northernmost North America separately from the radiation of the greater and lesser horned owls. One older study suggested that great horned and Eurasian eagle-owls may be conspecific, based on similarities in life history, geographic distribution, and appearance, Genetic testing indicates that the lesser horned owl, and then the snowy owl, not the Eurasian eagle-owl, are the most closely related living species. Almost all fossils indicate these owls were larger than modern great horned owls. A large number of subspecies, more than 20 altogether, have been named. However, many of these are not true subspecies and only examples of individual or clinal variation. Subspecies differences are mainly in color and size and generally follow Gloger's and Bergmann's rules: The most conservative treatments of great horned owl subspecies may describe as few as 10, although an intermediate number is typical in most writings. Fifteen subspecies are currently recognised: • B. v. algistus (Oberholser, 1904) – west Alaska • B. v. lagophonus (Oberholser, 1904) – central Alaska to northeast Oregon, Idaho and northwest Montana (US) • B. v. saturatus Ridgway, 1877 – coastal southeast Alaska to coastal north California (US) • B. v. pacificus Cassin, 1854 – coastal central California (US) to northwest Baja California (Mexico) • B. v. subarcticus Hoy, PR, 1853 – central west Canada to north Idaho (US) • B. v. pallescens Stone, 1897 – southwest US to south Mexico • B. v. pinorum Dickerman & Johnson, AB, 2008 – south Idaho to north Arizona and north New Mexico (US) • B. v. heterocnemis (Oberholser, 1904) – northeast Canada to the Great Lakes region • B. v. virginianus (Gmelin, JF, 1788) – southeast Canada to central, east US • B. v. elachistus Brewster, 1902 – south Baja California (Mexico) • B. v. mayensis Nelson, 1901 – Yucatán Peninsula (southeast Mexico) • B. v. mesembrinus (Oberholser, 1904) – south Mexico to west Panama • B. v. nigrescens Berlepsch, 1884 – Colombia to northwest Peru • B. v. nacurutu (Vieillot, 1817) – east Colombia through the Guianas to north, east Brazil, Argentina, Bolivia and central Peru • B. v. deserti Reiser, 1905 – northeast Brazil (central north Bahia) ==Description==

The great horned owl is generally colored for camouflage. All great horned owls have a facial disc. This can be reddish, brown, or gray in color (depending on geographical and racial variation) and is demarked by a dark rim culminating in bold, blackish side brackets. This species' eyebrow-like "horns" are tufts of feathers, called plumicorns. The purpose of plumicorns is not fully understood, but the hypothesis that they serve as a visual cue in territorial and sociosexual interactions with other owls is generally accepted. Adult great horned owls range in length from , with an average of , and possess a wingspan of , with an average of . Females are somewhat larger than males. Mean body weight is for females and for males. Depending on subspecies, maximum weight can reach . The wing chord length is . The wing loading, the measured wing area compared to weight, is high, meaning the wings are relatively small in surface area for the bird's weight; the species' wing loading has been described as proportionately the highest among raptors. The tail, being relatively short as is typical of most owls, is long. Like other owl species, the great horned owl is capable of "silent flight", which is the way owls fly while making almost no discernable noise, despite their large size. This is made possible thanks to three main components of the owl's wing structure. The leading edge of their feathers have serrations that help to disrupt the turbulence generated by wing flapping, then the softer feathers help deaden the sound, and finally the trailing fringe of the feathers works to finish cutting the sounds made by flight. The structure of the great horned owl wing also allows it to fly at a very low speed for the size of the species, as slow as 2 miles per hour when they are gliding on breezes. The legs, feet, and talons are large and powerful. Tarsal length is . Great horned owls can apply at least of crushing power in their talons, a pressure considerably greater than the human hand is capable of exerting. In some big females, the gripping power of the great horned owl may be comparable to much larger raptor species such as the golden eagle. The hard, inflexible bill of the great horned owl is long, although the culmen, the exposed bill portion as measured along the top of the beak, is only . The outer ear openings, which are concealed by feathers on the sides of the head, are relatively smaller than those of the Eurasian eagle owl, being in vertical axis, with the left ear slightly larger than the right. Like most exclusively (or near exclusively) nocturnal species, the great horned owl has asymmetrical ear holes that allow for the triangulation of sounds when hunting in the dark. The different-height holes, while still close together, are differentiated enough that the owl is able to use the timing and direction of the sound waves hitting each hole to precisely locate prey even if the prey is located under cover such as snow. The disc-like shape of their faces also helps to direct the sounds they hear toward their ears. While the true nature/purpose of the ear tufts that are present on the great horned owl is unknown, researchers agree that the tufts do not play any role in the hearing ability of the owl. It is estimated that their hearing is up to ten times that of a human being. The great horned owl's eyes, just slightly smaller than the eyes of a human being, are large even for an owl and rank proportionately among the largest eyes of all terrestrial vertebrates. The great horned owl has cylindrical eyes which creates more distance from the lens of the eye to the retina, which allows it to act more like a telephoto lens for farther distance sight compared to that possible from round eyes. They are visually highly adapted for nocturnal hunting and provide a wide, almost completely binocular field of view, a large corneal surface and a predominantly rod retina. The great horned owl's eye contains both rods and cones like most species that see in color, but the vision of a great horned owl closely resembles that of many other nocturnal species. The peak wavelengths that are observed by the cones is 555 nm, and the research suggests that the great horned owl has relatively weak color vision, especially compared to other bird species. Despite (or perhaps as a result of) the poorer sense of color vision, the owl manages to have excellent night vision. Instead of turning its eyes, an owl must turn its whole head, and the great horned owl can rotate its neck 270°. The iris is yellow, except in the amber-eyed South American great horned owl (B. v. nacurutu). Calls The great horned owl's song is normally a low-pitched but loud (or also transcribed as , or , , ) and can last for four or five syllables. The call is resonant and has warranted descriptions as varied as "solemn" and "terrifying". Calling seems to peak after rather than before midnight. Usually, territorial hooting decreases in February or March at the onset of egg laying. On occasion, this species exhibits "an indescribable assemblage of hoots, chuckles, screeches, and squawks, given so rapidly and disconnectedly that the effect is both startling and amusing". Descriptions of some of these odd sounds including a growling note pair, a laughing , a high-pitched ; a weak, soft , a cat-like , a hawk-like note of , and a nighthawk-like . These vocalizations may be variously uttered when the birds are disturbed and angered at the nest (frequently preceding an attack on an interloping human or other animal), represent the vocal development of young owls, or are given during courtship and during territorial disputes with other owls. Young owls still in the care of their parents make loud, persistent hissing or loud, piercing screeching sounds that are often confused with the calls of the barn owl. Unexpectedly, although it is not the longest-winged, the nominate is the heaviest known race as males weigh from , averaging , and females weigh from , averaging ; the prior figures originally from a huge sample around Michigan. In comparison, B. v. subarcticus, though averaging longer in wing length averages somewhat less heavy. Other standard measurements of this race are a tail length of , a tarsus length of about and a bill length of . The status of this form, especially the relationships between the scattered subpopulations and with ssp. nigrescens and the Magellanic horned owl, deserves more study. :Dull, earthy brownish color is typical; birds from the semiarid interior of Brazil often have much white on uppertail- and ear-coverts against a dull gray background (sometimes separated as deserti). This race is less fuscous than nigrescens. It is the only subspecies where the iris is amber, not yellow. The Magellanic horned owl, while somewhat similar in coloring, has yellow eyes like other horned owls, not amber eyes. B. v. nacurutu is a medium-sized race, smaller than most in North America but not as small as some of the Mexican races. The wing chord length is in males and in females. The tail in both sexes can range from . Only three birds have had published weights, two males scaling and one female weighing . The most notable feature of this race is its large bill, at , which is the biggest of any horned owl race despite the otherwise moderate size of B. v. nacurutu. The older name wapacuthu was occasionally used for this subspecies, but it cannot with certainty be assigned to a recognizable taxon and is thus considered a nomen dubium. The population described as algistus is probably based on wandering individuals and/or various intergrades of subarcticus with other races. :This is the palest form of horned owl, with the ground color essentially whitish with a faint buff tinge above; black underside barring variable from indistinct to pronounced, being most often prominent on the upper chest amongst otherwise pale plumage. This race shows little to no reddish coloration. B. v. subarcticus shows a very high degree of clinal variation, ranging from in the Contiguous United States where owls are often medium-grayish and more heavily marked to the subarctic zone in Canada where very pale birds with almost non-existent markings are prominent. Very pale birds are similar to a young female snowy owl from a distance. In this race, the feet range from immaculate white to buff, with little or no mottling. This is one of the largest-bodied subspecies. The wing chord length is , averaging , in males and , averaging , in females. Body mass ranges from , averaging , in males and from , averaging , in females. Tail length is and in males and females, respectively. Bill length is in both and one bird had a tarsus of . California great horned owl (Bubo virginianus pacificus) :Central and southern California west of the Sierra Nevada except for the San Joaquin Valley, south to Northwestern Baja California, Mexico. Intergrades with pallescens in San Diego County, California (see also below). Resident all-year. Species identification The combination of the species' bulk, prominent ear tufts and barred plumage distinguishes it through much of the range, but it may be easily confused with the lesser horned owl (B. magellanicus), which may overlap in range. The Magellanic horned owl was once considered a subspecies of the great horned, but is now almost universally considered a distinct species, as is supported by genetic materials, with the great horned being the paraspecies. Overall coloration is similar, but the Magellanic is markedly smaller with smaller feet and a smaller head, with finer, but more numerous brownish bars on the underside, rather than the blotchy, irregular barring typical of great horned owls. Other eagle-owls may superficially be somewhat similar, but the species is generically allopatric with the exception of wintering snowy owls. More tropical species with ear tufts, the stygian owl (A. stygius) and striped owl (A. clamator), are much smaller. Other large owls lack ear tufts. ==Distribution and habitat==

The breeding habitat of the great horned owl extends high into the subarctic of North America, where they are found up to the northwestern and southern Mackenzie Mountains, Keewatin, Ontario, northern Manitoba, Fort Chimo in Ungava, Okak, Newfoundland and Labrador, Anticosti Island and Prince Edward Island. They are distributed throughout most of North America and very spottily in Central America and then down into South America south to upland regions of Argentina, Bolivia and Peru, before they give way to the Magellanic horned owl, which thence ranges all the way to Tierra del Fuego, the southern tip of the continent. It is absent or rare from southern Guatemala, El Salvador, Nicaragua, and Costa Rica to Panama (where only two records) in Central America and the mangrove forests of northwestern South America. The species is also absent from the West Indies, the Haida Gwaii and almost all off-shore islands in the Americas, its ability to colonize islands apparently being considerably less than those of barn owls and short-eared owls. Since the division into two species, the great horned owl is the second most widely distributed owl in the Americas, just after the barn owl. They have only been recorded a handful of times in true rainforests such as the Amazon rainforest. but in Arkansas are often found near temporary agricultural openings in the midst of large areas of woodland. Similarly in south-central Pennsylvania, the owls use cropland and pasture more than deciduous and total forest cover, indicating preference for fragmented landscapes. In prairies, grasslands and deserts, they can successfully live year-round as long as there are rocky canyons, steep gullies and/or wooded coulees with shade-giving trees to provide shelter and nesting sites. In mountainous areas of North America, they are usually absent above the tree line, but great horned owls can be found up to in California and in the Rockies. In the Andean Mountains, on the other hand, they have adapted to being a true montane species, often found at least above sea level and are regularly recorded in treeless Puna grassland zones at in Ecuador and Peru. They are generally rare in non-tidal wetland habitat and are replaced in the high Arctic tundra by snowy owls. Thus lightly populated rural regions can be ideal. This species can occasionally be found in urban or suburban areas. However, they seem to prefer areas with less human activity and are most likely to be found in park-like settings in such developed areas, unlike eastern and western screech owls (Megascops asio & M. kennicottii) which may regularly occur in busy suburban settings. All mated great horned owls are permanent residents of their territories, but unmated and younger birds move freely in search of company and a territory and leave regions with little food in winter. ==Behavior==

In most aspects of their behavior, great horned owls are typical of owls and most birds of prey. Like most owls, the great horned owl makes great use of secrecy and stealth. Due to its natural-colored plumage, it is well camouflaged both while active at night and while roosting during the day. During the daytime it roosts usually in large trees (including snags and large hollows but usually thick branches) but may occasionally be in crevices or small caves in rocks or in dense shrubbery. Pine and other coniferous trees may be preferred where available since they are particularly dense and provide cover throughout the year. Typically, males have a favorite roosting site not far from the nest, sometimes used over successive years. Outside of the nesting season, great horned owls may roost wherever their foraging path ends at dawn. When the owls try to fly off to avoid this harassment, they are often followed by the corvids. Territoriality and movements Typically, great horned owls are highly sedentary, often capable of utilizing a single territory throughout their mature lives. Although some species such as snowy owls, northern saw-whet owls, long-eared and short-eared owls are true migrants, most North American owls are not migratory and will generally show fidelity to a single territory year around. Northern populations occasionally irrupt south during times of food shortage, but there is no annual migration even at the northern limits of the great horned owl's range. Hunting tends to be most prolonged during winter by virtue of prey being more scarce. However, great horned owls can learn to target certain prey during daylight in the afternoon when it is more vulnerable, such as eastern fox squirrels (Sciurus niger) while they are building their leaf nests and chuckawallas (Sauromalus ater) sunning themselves on desert rocks. Owls hunt mainly by watching from a snag, pole or other high perch. During hunting forays, they often fly about from perch to perch, stopping to survey for food at each, until they sense a prey item below. From such vantage points, owls dive down to the ground, often with wings folded, to ambush their prey. they also sometimes hunt by flying low over openings on the ground, scanning below for prey activity. On occasion owls may actually walk on the ground in pursuit of small prey or, rarely, inside a chicken coop to prey on the fowl within. Many large prey items are dismembered. Great horned owls may behead large prey before taking it to its nest or eating perch. The legs may also be removed, as may (in some bird prey) the wings. The great horned owl will also crush the bones of its prey to make it more compact for carrying. On occasion, the owls may return to the kill site to continue eating if the prey is too heavy to fly with after dismemberment. ==Prey and trophic ecology==

depicting a great horned owl with one of its primary prey species, a snowshoe hare Prey can vary greatly based on opportunity. According to one author, "Almost any living creature that walks, crawls, flies, or swims, except the large mammals, is the great horned owl's legitimate prey". Their diet in North America is made up of 87.6% mammals, 6.1% birds, 1.6% reptiles and amphibians with the remaining 4.7% being made up by insects, other assorted invertebrates and fish. Most prey is in the range of (shrews) to (jackrabbits). A single owl requires about of food per day and can subsist on a large kill over several days. Despite the great diversity of prey taken by these predators, in most of the continental United States from the East to the Midwest as well as Canada and Alaska, great horned owls largely live off just a handful of prey species: three species of lagomorph: the eastern cottontail (Sylvilagus floridanus), the snowshoe hare (Lepus americanus) and the black-tailed jackrabbit (Lepus californicus); two species of New World mice: the white-footed mouse and the North American deermouse (Peromyscus leucopus & maniculatus); approximately three species of vole: the meadow, prairie and woodland voles (Microtus pennsylvanicus, ochrogaster & pinetorum); and one introduced pest, the brown rat. The same species constituted 75% by number of a small sampling in Oklahoma. In semi-desert and other arid habitats, kangaroo rats become increasingly important prey, ten species have been reported in the diet but most prominently the Ord's and Merriam's kangaroo rats (Dipodomys ordii & merriami), both being widespread, numerous and relatively diminutive (at ). Eight known larger species of kangaroo rats, including the giant kangaroo rat (Dipodomys ingens) averaging at , are also taken. The squirrels, including ground squirrels, marmots (Marmota), prairie dogs (Cynomys), chipmunks and tree squirrels, are diurnal and so are largely unavailable to great horned owls as prey. Occasionally though, one will be caught from their leaf nest, nest hole or burrow entrance first thing in the morning or in the late afternoon and approximately 35 species have been successfully predated by these owls. In general larger sized than other rodent families, the species hunting range from the gray-collared chipmunk (Tamias cinereicollis) to the hoary marmot (Marmota caligata); thus, squirrels can provide a very fulfilling meal. An even larger rodent is sometimes attacked as prey by great horned owls, the North American porcupine (Erethizon dorsatum), in which average adults range from . This has been determined from owls who have porcupine quills imbedded in them, sometimes resulting in death. On occasion, they are successful in killing porcupine, even adults as determined by the size of the quills left behind and prey remains at bloodied kill sites. Other rodents recorded as secondary prey in North America include flying squirrels (Glaucomys ssp.), the golden mouse (Ochrotomys nuttalli), red-backed voles & bog lemmings (Myodes & Synaptomys ssp.), the muskrat (Ondatra zibethicus), the northern grasshopper mouse (Onychomys leucogaster), the northern pygmy mouse (Baiomys taylori) and jumping mice (Zapus & Napaeozapus ssp.). These species are overall the largest regular prey for this species. In the short-grass prairie of Colorado, mountain cottontail (Sylvilagus nuttallii) and black-tailed jackrabbits predominated in October to December, making up 42.9% by number (and nearly all the biomass), thence dropping to 9.3% by number in April, while voles rose to 32.2% peak in May, down to a minimum of 10.2% by number in June. In central Utah, the lagomorphs (black-tailed jackrabbit/desert cottontail) and Ord's kangaroo rat each made up 39% of the food by number, respectively. The dependence on lagomorphs also extends into Mexico, as in Baja California about a quarter of identified prey was black-tailed jackrabbit and either desert or the larger Mexican cottontail (Sylvilagus cunicularius). In the northern boreal forest, great horned owls are even more dependent on the snowshoe hare. At the peak of the 10-year hare cycle, snowshoe hares were by far the largest component of both summer and winter diets (77–81% and 90–99%, respectively, in Alberta; 83–86% and 75–98%, respectively, in Yukon). At the lowest point of the hare's cycle, summer diets consisted of only 0–16% snowshoe hare in Alberta and 12.7% in Yukon. When hares were scarce, great horned owls in these regions fed mostly on large rodents, mice and voles, grouse and ducks. Because fewer of these alternative prey species are available in boreal forests during winter, owls had to emigrate or suffer high mortalities if they stayed. In Alberta, the local population of great horned owls can increase threefold from hare population lows to peaks. The dependency on the snowshoe hare by the great horned owl extends into Alaska as well. Other mammals Other mammals are taken readily as well. Several species of mammalian carnivore, such as ringtails (Bassariscus astutus), American minks (Neogale vison), American martens (Martes americana), black-footed ferret (Mustela nigripes) and various other small mustelids (Mustela ssp.), are sometimes taken as prey. In one case, a great horned owl was the likely killer of an adult female fisher (Martes pennanti), though young ones are typically taken. Prey in the form of canids, like foxes or coyotes (Canis latrans) are often juveniles presumably snatched from the mouths of dens by night. Kit and swift foxes of up to adult size may be taken. The most regular predatory association amongst relatively larger carnivores is that with skunks. Due to their poor sense of smell, great horned owls are the only predators to routinely attack these bold mammals with impunity. All six skunk species found in North America are reported as prey, including full-grown striped skunks (Mephitis mephitis), which can be three times as heavy as the attacking owl. In one single nest, the remains of 57 striped skunks were found. Due to the proclivity of skunk predation, great horned owls nests frequently smell strongly of skunk and occasionally stink so powerfully of skunk that they leave the smell at kill sites or on prey remains. Surprisingly, at least two cases of a great horned owl preying on an adult raccoon (Procyon lotor) have been reported. One instance of an owl taking a bobcat (Lynx rufus) as prey was also reportedly observed. and several of juvenile and adult cats (Felis silvestris catus) being killed by great horned owls have been reported. Aside from carnivorans, various other mammals are taken as supplemental prey. At least eight species of shrews are taken by opportunity and make up the smallest mammalian prey taken by great horned owls, as specimens of least shrew (Cryptotis parva) or masked shrew (Sorex cinereus) have had an estimated weight of only . In Brazil, juvenile white-eared opossum (Didelphis albiventris) weighing around were found in 12% of pellets weighing about. North American subspecies can prey on larger Virginia opossum (Didelphis virginianus) readily, and can be a major predator of this species. Remnants of armadillo, presumably nine-banded armadillo (Dasypus novemcinctus), have been found around owl nests in the south. One pellet in Texas was found to be composed entirely of Mexican free-tailed bats (Tadarida brasiliensis). Birds s are often a favored food source for great horned owls living near wetlands. After mammals, birds rank as the next most important general prey group. Birds are usually considerably secondary in the diet but outnumber the mammals in the diet by diversity, as more than 250 species have been killed in North America alone. Statistically, the most significant avian prey seems to be galliforms, of which they are known to have preyed on 23 species, basically consisting of all of the native species found in the United States. Usually coveys of quail are partially protected by spending the night roosting communally in dense thickets but should a hunting owl be able to track down the communal roost, losses can be fairly heavy until the roost relocates. In the boreal forest, especially in years where the snowshoe hare experiences population decreases, great horned owls prey fairly heavily (approximately 25% of biomass) on ruffed grouse (Bonasa umbellus) and spruce grouse (Falcipennis canadensis), enough so in the earlier bird to possibly contribute to population reductions. Larger species of galliform are not immune to predation either. On Protection Island in Washington state, introduced common peafowl (Pavo cristatus) are an important prey item. The wild turkey (Meleagris gallopavo), on average between the sexes, is probably the largest bird the great horned owl hunts in which they kill adults. Both full-grown wild turkeys and adult domestic turkeys 77% of the ducks in that study were juveniles, the largest duck being a male mallard (Anas platyrhnychos) weighing approximately , but nearly all the coots were adults. The nestlings of even larger species like trumpeter swans (Cygnus buccinator), American white pelicans (Pelecanus eryhtrorhynchos), brown pelicans (Pelecanus occidentalis) and sandhill cranes (Grus canadensis) have also been killed by these owls. Other assorted birds are taken seemingly at random opportunity. The predatory effect of this species on other raptorial birds, which is often considerable, is explored in the following section. In Brazil, it was found in a small study that birds overall outnumbered mammals in pellets, although most were not determined to species and the ones that were shown a tremendously diverse assemblage of birds with no obvious dietary preference. Although not usually numerically significant, 86 species of passerine have been taken by great horned owls. Members from most North American families are known as prey, although among smaller types such as chickadees, warblers, sparrows, cardinalids, wrens and most tyrant flycatchers only a few species from each have been recorded. Nonetheless, an occasionally unlucky migrant or local breeder is sometimes snatched. On rare occasion, salamanders, frogs and toads are reported as prey. On rare occasions, fish are taken including goldfish (Carassius auratus), bluegill (Lepomis macrochirus), bullheads (Ameiurus ssp.), other catfish, suckers, sunfish, eels and dace and chub. In some cases, the content of insects in great horned owl pellets may actually be due to the owls eating other birds which have freshly eaten insects in their own stomachs. Road kills are sometimes opportunistically eaten. A case of an owl scavenging a white-tailed deer (Odocoileus virginianus) carcass, ultimately tearing off the deer's leg, was captured on a motion capture video camera set out to film wildlife. Urban vs rural diet Studies comparing the diets of rural and urban great horned owls have identified that the most abundant rodent prey in their environment fulfils the majority of their diet. A study of food niche overlap between closely nested barn and great horned owls living in rural north-eastern Oregon identified voles as by far the most common prey. Although a stable and highly abundant food source, a diet consisting of primarily rats can be harmful to urban great horned owls due to bioaccumulation of rodenticides. Interspecific predatory relationships Due to their very broad dietary habits, the great horned owls share their prey with many other predators, including avian, mammalian and reptilian ones. Almost every study comparing the diets of North American owls illustrates the considerable overlap in the dietary selection of these species, as all species, besides the primarily insectivorous varieties, rely on many of the same small rodent species for most of their diet, extending from the small northern saw-whet owl and eastern screech owl to the great horned and great grey owls. The relationship between great horned owls and other raptorial birds in its range is usually decidedly one-sided. While certain species, such as the red-tailed hawk and northern goshawk, might be seen as potential competition for the owls, most others seem to be regarded merely as prey by great horned owls. The great horned owl is both the most prolific and diverse predator in America of other birds of prey, with other accomplished raptor-hunters such as the goshawk and the golden eagle being more restricted in range, habitat and number in North America and thus having a more minor impact. All studies have found raptors are a small portion of this owl's diet but predation can be seriously detrimental for such prey, as raptors tend to be territorial and sparsely distributed as a rule and thus can be effectively decimated by a small number of losses. Raptorial birds in general tend to have large, conspicuous nests which may make them easier for a hunting owl to locate. The great horned owl gains an advantage by nesting earlier than any other raptor in its range (indeed any bird), as it is able to exploit the other raptors as food while in a more vulnerable state as their own nestlings have become well developed. More so than diurnal varieties of raptor, fairly significant numbers of owls are hunted, as all species are to some extent nocturnal and thus their corresponding activity can attract the horned owl's unwanted attention. The extent of predation on other owls depends on the habitat preferences of the other species. Eastern and western screech owls may be most vulnerable since they prefer similar wooded edge habitat. In a block of Wisconsin, great horned owls were responsible for the failure of 78% of eastern screech owl's nests. In a pair of studies from Colorado, the average weight of prey for long-eared owls was , for barn owl and for the great horned owl. Great horned owls were the leading cause of mortality in juvenile spotted owls (30% of losses) and juvenile great grey owls (65% of losses). Less is known about relations with the snowy owl, which may compete with great horned owls for food while invading south for the winter. Anecdotally, both snowy and great horned owls have rarely been reported to dominate or even kill one another depending on the size and disposition of the individual owls, although the snowy's preference for more open areas again acts as something of a buffer. The snowy may be the one North American owl too formidable for the great horned owl to consider as prey. In a study of red-shouldered hawk (Buteo lineatus) and broad-winged hawk (Buteo brachyurus) breeding in New York, despite their nesting in deeper woods than those that host these owls, the main cause of nest failure was great horned owl predation. The fact that many of the nests great horned owls use are constructed by accipitrids may lead to localized conflicts, almost always to the detriment of the hawks rather than the owls. While the young of larger diurnal raptors are typically stolen in the night, great horned owls also readily kill large adult raptors both in and out of breeding seasons, including osprey, northern goshawk and rough-legged buzzard. Great horned owls are frequently mobbed by other birds. Most accipitrids will readily mob them, as will falcons. Hen harriers, northern goshawks, Cooper's hawks, Harris's hawks, red-tailed hawks, Swainson's hawks, ferruginous hawks, red-shouldered hawks, American kestrels, peregrine falcons, prairie falcons (Falco mexicanus) and common ravens (Corvus corax) are among the reported species who have been recorded diving on great horned owls when they discover them. In Arizona and Texas, they may be mobbed by Mexican jays and western scrub jays (Aphelocoma wollweberi & californica) and western and Cassin's kingbirds (Tyrannus verticalis & vociferans). In addition, there are several documented incidences of American crows mobbing a great horned owl, in groups of dozens or even hundreds of crows. In response to mobbing, if the owl flies it alights to the nearest secluded spot. If an owl alights on ground or on exposed branch or ledge, it may respond to swooping and stooping flights of corvids and raptors with threat display and raising of its wings. ==Reproduction==

, Oregon, United States Great horned owls are some of the earliest-breeding birds in North America, seemingly in part because of the lengthy nightfall at this time of year and additionally the competitive advantage it gives the owl over other raptors. In most of North America, courtship is from October to December and mates are chosen by December to January. During courtship in late fall or early winter, the male attracts the attention of his mate by hooting emphatically while leaning over (with the tail folded or cocked) and puffing up his white throat to look like a ball. They nest in a wider variety of sites than any other North American bird. Most tree nests used by great horned owls are constructed by other animals, often from a height of about off the ground. They often take over a nest used by some other large bird, sometimes adding feathers to line the nest but usually not much more. Allegedly there have some cases where the owls have reinforced a nest structure or appeared to have reconstructed a nest, but as a rule no owl species has ever been known to actually build a nest. The nests they use are often made by most larger types of acciptrids, from species as small as Cooper's hawks to bald eagle and golden eagle, though perhaps most often those of red-tailed hawks and other buteonines. Secondly in popularity are crow and raven (Corvus ssp.) nests. Even Canada goose, black-crowned night heron (Nycticorax nycticorax) and great blue heron nests have been used, the latter sometimes right in the midst of an active heronry. The leaf nests of squirrels are also regularly used but in general great horned owls are partial to stick nests since they provide a much firmer, safer foundation. In northwestern Utah and north-central Alberta, egg-laying can be 3–4 weeks earlier than usual when food is abundant and weather is favorable. The average egg width is , the average length is and the average weight is , although mass could be slightly higher elsewhere because this figure is from Los Angeles County, CA where the owls are relatively small. The incubation period ranges from 28 to 37 days, averaging 33 days. The female alone usually does all the incubation and rarely moves from the nest, while the male owl captures food and brings it to her, with the first nightly food delivery typically occurring soon after dark. Young owls move onto nearby branches at 6 weeks and start to fly about a week later. However, the young are not usually competent fliers until they are about 10 to 12 weeks old. The offspring have been seen still begging for food in late October (5 months after leaving the nest) and most do not fully leave their parents territory until right before the parents start to reproduce for the next clutch (usually December to January). Birds may not breed for another year or two, and are often vagrants ("floaters") until they establish their own territories. Urban vs rural nesting While urban and rural populations show little difference in productivity, there are differences in nest selection. Rural owls use old raptor nests more frequently than urban birds, who utilize crow or squirrel nests. Additionally, urban nesting individuals utilize trees that are taller/wider in diameter and nest much higher compared to rural nesting Great Horned Owls. The reason behind this increased tree height is due to the fact that urban areas have large trees used for ornamentation, shade and shelter. The higher nesting within the taller trees was attributed to human avoidance. Both rural and urban nesting sites were often within range of paved roads, likely a result of the great horned owl's tendency to hunt along roadways. Nesting owls at sites in Winnipeg, Manitoba began nesting five to six weeks earlier than those in rural parts of Manitoba, presumably due to experiencing an extremely warm winter by Winnipeg's standards, as well as benefitting from the local urban heat island. ==Mortality and longevity==

Longevity and natural mortality Great horned owls seem to be the most long-lived owl in North America. Among all owls, they may outrank even the larger Eurasian eagle owl in known longevity records from the wild, In captivity, the record for the longest lived great horned owl was 50 years. A more typical top lifespan of a great horned owl is approximately 13 years. Cases where the quills of porcupines have killed or functionally disabled them have been observed as well. When a peregrine falcon repeatedly attacked a great horned owl near its nest along the Hudson River, it was apparently unable to dispatch the larger raptor despite several powerful strikes. During their initial dispersal in fall, juvenile owls have a high mortality rate, frequently more than 50%. Among 209 banded nestlings in yet another study, 67% were found dead after independence: 56 were found shot, 41 were trapped, 15 hit by cars, 14 found dead on highways and 14 electrocuted by overhead power lines. Secondary poisoning from pest control efforts is widely reported variously due to anticoagulant rodenticides, strychnine, organophosphates (famphur applied topically to cattle (Bos primigenius taurus)), organochlorines, and PCBs. Frequently, the species were denominated a pest due to the perceived threat it posed to domestic fowl and potentially small game. The first genuine nature conservationists, while campaigning against the "Extermination Being Waged Against the Hawks and Owls", continued to advocate the destruction of great horned owls due to their predatory effect on other wildlife. Hunting and trapping of great horned owls may continue on a small scale but is now illegal in most countries. Effect on conservation-dependent species Occasionally, these owls may prey on threatened species. Following the devastation to its populations from DDT, the reintroduction of the peregrine falcon to the Mississippi and Hudson Rivers was hampered by great horned owls killing both young and adult peregrines at night. Similarly, as mainly recorded in New England, attempts to reintroduce ospreys, after they were also hit hard by DDT, were affected by heavy owl predation on nestlings, and the owls were also recorded to take a large toll locally on the threatened colonies of roseate terns. While at least the ospreys and peregrines have rebounded admirably nonetheless, bird and mammal species that are much rarer overall sometimes fall prey to great horned owls, many in which even sporadic losses can be devastating. Among the species considered threatened, endangered or critically endangered by the IUCN which are also known to be killed by great horned owls are Townsend's ground squirrels (Urocitellus townsendii), giant kangaroo rats, Stephens' kangaroo rat (Dipodomys stephensi), black-footed ferrets, greater and lesser prairie-chickens, marbled murrelets (Brachyramphus marmoratus), ivory-billed woodpeckers, Florida scrub jays (Aphelocoma coerulescens), pinyon jays, Kirtland's warblers (Setophaga kirtlandii) and rusty blackbirds (Euphagus carolinus). ==Captivity==

From experimentally raising young owls in captivity, Paul L. Errington felt that they were a bird of "essentially low intelligence" that could only hunt when partially wild and instinctually driven by hunger to hunt whatever they first encounter. He showed captive birds that were provided strips of meat from hatching, rather than having to hunt or to simulate hunting to obtain food, had no capacity to hunt. On the contrary, William J. Baerg compared behaviorally his captive-raised great horned owls to parrots, which are famously intelligent birds, although not as often playful: "It knows its keeper and usually accepts whatever he wishes to do with a good deal of tolerance." Carl D. Marti disagrees with Errington's assessments, noting that their prey selection is not as "completely random as Errington suggested". He further notes that though great horned owls appear to "select their mammalian prey in general relation to the prey populations", cottontails seem to be selected as prey "out of relation to their population status". ==Iconography and myth==

Many warrior-based tribes of Native Americans admired the great horned owl for their "strength, courage and beauty". ==References==