Great Horned Owl (The Flora and Fauna of Palo Duro Canyon)

Great Horned Owl

Bubo virginianus

Summary ⁵

The great horned owl (Bubo virginianus), also known as the tiger owl (originally derived from early naturalists' description as the "winged tiger" or "tiger of the air"), or the hoot owl, is a large owl native to the Americas. It is an extremely adaptable bird with a vast range and is the most widely distributed true owl in the Americas. Its primary diet is rabbits and hares, rats and mice, and voles, although it freely hunts any animal it can overtake, including rodents and other small mammals, larger mid-sized mammals, birds, reptiles, amphibians, and invertebrates. In ornithological study, the great horned owl is often compared to the Eurasian eagle-owl (Bubo bubo), a closely related species, which despite the latter's notably larger size, occupies the same ecological niche in Eurasia, and the red-tailed hawk (Buteo jamaicensis), with which it often shares similar habitat, prey, and nesting habits by day, thus is something of a diurnal ecological equivalent. The great horned owl is one of the earliest nesting birds in North America, often laying eggs weeks or even months before other raptorial birds.

Species identification ⁶

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 or Magellanic 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 (Pseudoscops clamator), are much smaller. Other large owls lack ear tufts.

Description ⁶

The great horned owl is generally colored for camouflage. The underparts of the species are usually light with some brown horizontal barring; the upper parts and upper wings are generally a mottled brown usually bearing heavy, complex, darker markings. All subspecies are darkly barred to some extent along the sides, as well.

A variable-sized white patch is seen on the throat. The white throat may continue as a streak running down the middle of the breast even when the birds are not displaying, which in particularly pale individuals can widen at the belly into a large white area. South American great horned owls typically have a smaller white throat patch, often unseen unless actively displaying, and rarely display the white area on the chest. Individual and regional variations in overall color occur, with birds from the subarctic showing a washed-out, light-buff color, while those from the Pacific Coast of North America, Central America, and much of South America can be a dark brownish color overlaid with blackish blotching. The skin of the feet and legs, though almost entirely obscured by feathers, is black. Even tropical great horned owls have feathered legs and feet. The feathers on the feet of the great horned owl are the second-longest known in any owl (after the snowy owl). The bill is dark gunmetal-gray, as are the talons.

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' "horns" are tufts of feathers, called plumicorns. The purpose of plumicorns is not fully understood, but the theory that they serve as a visual cue in territorial and sociosexual interactions with other owls is generally accepted.

The great horned owl is the heaviest extant owl in Central and South America and is the second-heaviest owl in North America, after the closely related, but very different-looking snowy owl. It is heavily built, with a barrel-shaped body, a large head, and broad wings. Its size can vary considerably across its range, with populations in interior Alaska and Ontario being largest and populations in California and Texas being smallest, though those from the Yucatán Peninsula and Baja California appear to be even smaller. Adult great horned owls range in length from 43 to 64 cm (17 to 25 in), with an average of 55 cm (22 in), and possess a wingspan of 91 to 153 cm (3 ft 0 in to 5 ft 0 in), with an average of 122 cm (48 in). Females are somewhat larger than males. Mean body weight is 1,608 g (3.545 lb) for females and 1,224 g (2.698 lb) for males. Depending on subspecies, maximum weight can reach 2,503 g (5.518 lb).

The wing chord length is 297–400 mm (11.7–15.7 in). 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 175 to 252 mm (6.9 to 9.9 in) 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 that 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 speeds 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 54–80 mm (2.1–3.1 in). The average foot span of a fully spread foot, from talon to talon, is around 20 cm (7.9 in), as compared to 8 cm (3.1 in) in long-eared owls, 13 to 15 cm (5.1 to 5.9 in) in barn owls, and 18 cm (7.1 in) in the great grey owl. Great horned owls can apply at least 300 lb/in² 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 3.3–5.2 cm (1.3–2.0 in) long, although the culmen, the exposed bill portion as measured along the top of the beak, is only 2.1 to 3.3 cm (0.83 to 1.30 in).

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 2.3 cm (0.91 in) 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).

The great horned owl's song is normally a low-pitched but loud ho-ho-hoo hoo hoo (or also transcribed as bu-bubu booh, who-hoo-ho-oo or who-ho-o-o, whoo-hoo-o-o, whoo) and can last for four or five syllables. One transliteration is You still up? Me too. The call is resonant and has warranted descriptions as varied as "solemn" and "terrifying". The female's call is higher and rises in pitch at the end of the call. Female vocalizations are higher in pitch because of a smaller syrinx in the larger sex. 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 krrooo-ooo note pair, a laughing whar, whah, wha-a-a-a-ah, a high-pitched ank, ank, ank; a weak, soft erk, erk, a cat-like meee-owwwwww, a hawk-like note of ke-yah, ke-yah, and a nighthawk-like peent. 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.

Physiology and measurements ⁶

Behavior ⁶

In most aspects of their behavior, great horned owls are typical of owls and most birds of prey. From experimentally raising young owls in captivity, Paul L. Errington felt that they were a bird of "essentially low intelligence" who 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 his captive raised behaviorally 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". Arthur C. Bent also noted the variability in temperaments of great horned owls to their handlers, some generally pleasant, though most are eventually aggressive. Most captive specimens, once mature, seem to resent attempts at contact and are often given to attacking their keepers. They will only follow cues when conditioned from an early age but rarely with the same level of success seen in some diurnal birds of prey trained for falconry or entertainment, although this does not necessarily correlate with intelligence as posited by Errington. Carl D. Marti also disagrees with Errington's assessments, noting that their prey selection is not as "completely random as Errington suggested"; while "Great Horned Owls appeared to select their mammalian prey in general relation to the prey populations...Cottontails, appeared to be selected as prey out of relation to their population status."

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 & 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. While roosting, great horned owls may rest in the "tall-thin" position, where they sit as erect and hold themselves as slim as is possible. The kind of posture is well known as a further method of camouflage for other owls, like long-eared owls or great grey owls, especially if humans or other potential mammalian carnivores approach them. The Eurasian eagle owl rarely, if ever, assumes the tall-thin position. Outside of the nesting season, great horned owls may roost wherever their foraging path ends at dawn. Generally great horned owls are active at night, although in some areas may be active in the late afternoon or early morning. At dusk, the owl utters a few calls before flying to a more open sing-post, i.e. large bare branch or large rocks to deliver song. Normally several perches are used to mark occupied territory or to attract a female. Despite its camouflage and cryptic locations, this species can still sometimes be spotted on its daytime roosts, especially by American crows (Corvus brachyrhynchos). Since owls are, next to red-tailed hawks, perhaps the main predator of crows and their young, crows sometimes congregate from considerable distances to mob owls and caw angrily at them for hours on end. When the owls try to fly off to avoid this harassment, they are often followed by the corvids.

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. In great horned owls, mated pairs occupy territories year-round and long-term. Territories are established and maintained through hooting, with highest activity before egg-laying and second peak in autumn when juveniles disperse, and can range from an average of 16 km² (6.2 sq mi) in Yukon to an average of 2.1 km² (0.81 sq mi) in Wyoming.

Most territorial defense is performed by males, but females frequently assist their partners in hooting contests with neighbors or intruders, even during incubation. On occasion, although territory borders may be successfully maintained via vocalizations alone without even seeing the competing owl, such confrontations may turn physical, with various levels of threats distinguished. The highest threat level involves the spreading of wings, bill-clapping, hissing, higher-pitched screams of longer duration, with general body poised to strike with its feet at intruder. If the intruder continues to press the confrontation, the defending owl will "hop" forward and strike it with feet, attempting to grasp and rake with claws.

Territoriality appears to place a limit on the number of breeding pairs in a given area. Individuals prevented from establishing a territory live a silent existence as "floaters". Radio-telemetry revealed that such floaters concentrate along boundaries of established territories. At Kluane Lake in Yukon, incursions into neighboring territories were observed only twice—by females when neighboring female had died or emigrated, suggesting that territorial defense may be sex specific. At least four dead great horned owls in Kluane were apparently killed by others of their own species in territorial conflicts. Owls killed by other horned owls are sometimes cannibalized, although the origin of the killing may have been territorial aggression. Northern populations occasionally erupt 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 activity tends to peak between 8:30 pm and 12:00 am at night and then can pick back up from 4:30 am to sunrise. Hunting activity 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're 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 50 to 100 m (160 to 330 ft) 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. Effective maximum hunting distance of an owl from an elevated perch is 90 m (300 ft). Due to their short but broad wings, great horned owls are ideally suited for low speed and maneuverability. Despite reports that they do not hunt on the wing, they also sometimes hunt by flying low over openings on the ground, scanning below for prey activity. Great horned owls can fly at speeds of more than 65 km/h (40 mph) in level flight. Hunting flights are slow, often quartering low above the ground where prey is likely to occur in open country or open woodland. Brief hovering flight (for about 6–18 seconds) have been described, especially in windy areas. 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. Rodents and invertebrates may be caught on foot around the base of bushes, through grassy areas, and near culverts and other human structures in range and farm habitat. The great horned owl is generally a poor walker on the ground; it walks like a starling, with a pronounced side-to-side gait. They have been known to wade into shallow water for aquatic prey, although this has been only rarely reported. Owls can snatch birds and some arboreal mammals directly from tree branches in a glide as well. The stiff feathering of their wings allows owls to produce minimal sound in flight while hunting.

Almost all prey are killed by crushing with the owl's feet or by incidentally stabbing of the talons, though some may be bitten about the face as well. Prey is swallowed whole when possible. When prey is swallowed whole, owls regurgitate pellets of bone and other non-digestible bits about 6 to 10 hours later, usually in the same location where the prey was consumed. Great horned owl pellets are dark gray or brown in color and very large, 7.6 to 10.2 cm (3.0 to 4.0 in) long and 3.8 cm (1.5 in) thick, and have been known to contain skulls up to 3 cm (1.2 in) in width inside them. However, not all prey can be swallowed at once, and owls will also fly with prey to a perch and tear off pieces with their bill. Most dietary studies focus on pellets found under perches and around nests, since they provide a more complete picture of the diversity of prey consumed, but prey remains outside of pellets may provide clues to prey excluded from the pellets and a combination of both is recommended. 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. Many owls will accrue a cache of prey, especially those who are nesting. Caches must be at a safe location, usually the crotch of a tall tree. In northern regions, where large prey is prevalent, an owl may let uneaten food freeze and then thaw it out later using its own body heat. Hunting success seems to require fairly open understory, and experimental testing of microhabitat proved that open areas provided more hunting success on five species of rodent, with cloudy nights and denser bush foliage both decreasing success.

Distribution and habitat ⁶

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 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 Queen Charlotte Islands 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.

The great horned owl is among the world's most adaptable owls or even bird species in terms of habitat. The great horned owl can take up residence in trees that border all manner of deciduous, coniferous, and mixed forests, tropical rainforests, pampas, prairie, mountainous areas, deserts, subarctic tundra, rocky coasts, mangrove swamp forests, and some urban areas. It is less common in the more extreme areas of the Americas. In the Mojave and Sonora Deserts, they are absent from the heart of the deserts and are only found on the vegetated or rocky fringes. Even in North America, they are rare in landscapes including more than 70% old-growth forest, such as the aspen forest of the Rockies. They have only been recorded a handful of times in true rainforests such as the Amazon rainforest. In the Appalachian Mountains, they appear to use old-growth forest but in Arkansas are actually often found near temporary agricultural openings in the midst of large areas of woodland. Similarly in south-central Pennsylvania, the owls uses 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 around as long as there are rocky canyon, steep gullies and/or wooded coulees with shade-giving trees to provide them 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 2,100 m (6,900 ft) in California and 3,300 m (10,800 ft) in the Rockies. In the Andean Mountains, on the other hand, they have adapted to being true montane species, often found at least 3,300 m (10,800 ft) above sea level and are regularly recorded in treeless Puna grassland zones at 4,100 to 4,500 m (13,500 to 14,800 ft) in Ecuador and Peru. They are generally rare in non-tidal wetland habitat, and are replaced in the high Arctic tundra by snowy owls. It prefers areas where open habitats, which it often hunts in, and woods, where it tends to roost and nest, are juxtaposed. Thus lightly populated rural regions can be ideal. This species can occasionally be found in urban or suburban areas. However, it seems to prefer areas with less human activity and is 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.

Sources and Credits

(c) Minette, some rights reserved (CC BY-NC), http://www.flickr.com/photos/7232133@N08/2925340745
no rights reserved, uploaded by Braden J. Judson
(c) Christian Grenier, some rights reserved (CC BY-NC), uploaded by Christian Grenier
(c) Sean Cozart, some rights reserved (CC BY-NC), uploaded by Sean Cozart
(c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Bubo_virginianus
(c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Great_horned_owl

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Great Horned Owl

Summary 5

Species identification 6

Description 6

Physiology and measurements 6

Behavior 6

Distribution and habitat 6