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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 native to the . It is an extremely adaptable bird with a vast range and is the most widely distributed in the Americas. Its primary diet is , and , although it freely hunts any animal it can overtake, including and other small mammals, larger mid-sized , , , , and . In ornithological study, the great horned owl is often compared to the (Bubo bubo), a closely related species which, despite the latter's notably larger size, occupies the same ecological niche in , and the (Buteo jamaicensis), with which it often shares similar habitat, prey and nesting habits by day and is thus something of a diurnal ecological equivalent. The great horned owl is one of the earliest nesting birds in , often laying eggs weeks or even months before other raptorial birds.

Contents

Description[]

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.

Owl showing much of its camouflage pattern/color

There is a variable sized white patch 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. horned owls typically have a smaller white throat patch, often unseen unless actively displaying, and rarely display the white area on the chest. There are individual and regional variations in overall color, with birds from the showing a washed-out, light-buff color, while those from the 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 ). The bill is dark gunmetal-gray, as are the talons.

All great horned owls have a . 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 socio-sexual interactions with other owls is generally accepted.

Physiology and measurements[]

The eyes of great horned owls are amongst the largest of terrestrial vertebrates.

The great horned owl is the heaviest extant owl in and South America and is the second heaviest owl in North America, after the closely related but very different-looking . It is heavily built, with a barrel-shaped body, a large head and broad wings. Its size can vary considerably across its range, with interior and populations being largest and populations in and being smallest, though those from the and 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 length is 297–400 mm (11.7–15.7 in). The , the measured wing area in cm2 calculated against grams of 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 , being relatively short as is typical of most owls, is 175 to 252 mm (6.9 to 9.9 in) long.

Talons, legs and feet Great, Horned Owl (Canada)

The legs, feet and talons are large and powerful. 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 , 13 to 15 cm (5.1 to 5.9 in) in and 18 cm (7.1 in) in the . Great horned owls can apply at least 300 of crushing power in their talons, a PSI 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 .

The hard, inflexible of the great horned owl is 3.3–5.2 cm (1.3–2.0 in) long, although the , 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 that of the Eurasian eagle owl, being 2.3 cm (0.91 in) in vertical axis, with the left ear slightly larger than the right.

The great horned owl's eyes, just slightly smaller than the eyes of a , are large even for an owl and rank proportionately among the largest eyes of all terrestrial vertebrates. They are visually highly adapted for nocturnal hunting and provide a wide, almost completely binocular field of view, a large surface and a predominantly rod . Instead of turning its eyes, an owl must turn its whole head and the great horned owl is capable rotating its neck 270 degrees. The is yellow, except in the amber-eyed (B. v. nacurutu).

Calls[]

The great horned owl's 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 either last for five to four syllables. One 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 -like meee-owwwwww, a -like note of ke-yah, ke-yah and a -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.

Species identification[]

Illustrated comparison of a great horned owl, left, to its closest North American cousin, the snowy owl

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 (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 . 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 may superficially be somewhat similar, but the species is generically with the exception of wintering snowy owls. More tropical species with ear tufts, the (A. stygius) and (Pseudoscops clamator), are much smaller. Other large owls lack ear tufts.

Taxonomy[]

The great horned owl is part of the genus , which may include as many as 25 other extant taxa, predominantly distributed throughout Africa. The great horned owl represents one of the one to two radiations of this genus across the to the . Whereas the Magellanic 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 through northernmost North America separately from the radiation of the horned owl. It has been proposed that the great horned and Eurasian eagle owls may in fact be conspecifics, based on similarities in life history, geographic distribution and appearance. In one case, a zoo-kept male great horned owl and female Eurasian eagle owl produced an apparently healthy hybrid (although a case of a hybrid between the only distantly related great grey owl hybridized with a great horned owl is also on record). Genetic testing indicates that the snowy, not the Eurasian eagle owl, is the most closely related living species. fossils have been found of Bubo owls in North America, which may either be distinct species or , from as far east as but predominantly in the and to the west of them. Almost all fossils indicate these owls were larger than modern great horned owls.

Subspecies[]

Northern great horned owl (B. v. subarcticus) in Californian great horned owl (B. v. pacificus) stretching, Bernal Hill Park, Desert great horned owl, (wet feathered) Bubo virginianus pallescens waiting out a rainstorm in the Great Horned Owl, taking a midday siesta on top of a Joshua Tree in the Summer of 2018

A large number of , more than 20 altogether, have been named. However, many of these are not true races and only examples of individual or variation. Subspecies differences are mainly in color and size and generally follow and : The most conservative treatments of great horned owl races may describe as few as 10 subspecies, although an intermediate number is typical in most writings.

  • Common/eastern great horned owl, Bubo virginianus virginianus (Gmelin, 1788)
eastwards from at least as far west as to ; northeastwards in distribution to southern , southern , and . Resident all-year. The race B. v. mesembrinus from Central America south of the , may be merely a southerly leg of this race, as its coloring is almost exactly the same as virginianus, although with rather smaller body size. However, mesembrinus type owls are discontinuous in range, with only paler owls from pallescens and mayensis found in the in-between range before the northern virginianus reappears back up in . The "nominate race" is a medium-hued form, neither darkly saturated nor strikingly pale. Darker gray or somewhat paler individuals are known. It tends to be richly tinged with rufous and barred distinctly blackish-brown below with rather soft contrast. The feet can range from tawny to buff to creamy and are the legs are typically barred dark to a moderate extent. The facial disc is often a solid cinnamon-red color. This is mid-to-large race, with a length of 319–371 mm (12.6–14.6 in), averaging 339 mm (13.3 in), in males and 343–388 mm (13.5–15.3 in), averaging 362.8 mm (14.28 in), in females. Unexpectedly, although it is not the longest-winged, the nominate is the heaviest known race as males weigh from 985 to 1,588 g (2.172 to 3.501 lb), averaging 1,318 g (2.906 lb), and females weigh from 1,417 to 2,503 g (3.124 to 5.518 lb), averaging 1,768.5 g (3.899 lb); the prior figures originally from a huge sample around . In comparison, B. v. subarcticus, though averaging longer in wing length averages somewhat less heavy. On the contrary, in , within the range of the possibly synonymous mesembrinus, great horned owls average approximately 1,000 g (2.2 lb), the lightest average mass reported anywhere for this species. Other standard measurements of this race are a tail length of 190 to 235 mm (7.5 to 9.3 in), a tarsus length of about 56 to 58 mm (2.2 to 2.3 in) and a bill length of 35 to 50 mm (1.4 to 2.0 in).B. v. virginianus also tends to have relatively the longest ear tufts on any race. A lowland form occurring in disjunct populations from eastern to the ; also from and south of the to , the in northern and western ; resident all-year. Includes the proposed subspecies scotinus, elutus, and deserti. 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 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 races. The length is 330–354 mm (13.0–13.9 in) in males and 340–376 mm (13.4–14.8 in) in females. The tail in both sexes can range from 184 to 217 mm (7.2 to 8.5 in). Only three birds have had published weights, two males scaling 1,011 and 1,132 g (2.229 and 2.496 lb) and one female weighing 1,050 g (2.31 lb). The most notable feature of this race is its large bill, at 43 to 52 mm (1.7 to 2.0 in), which is the biggest of any horned owl race despite the otherwise moderate size of B. v. nacurutu.
  • Northern/subarctic great horned owl, Bubo virginianus subarcticus (Hoy, 1852)
Breeding range from region east to the southern ; southern limit unclear but at least reaches to and . Non-breeding birds are regularly found south to , i.e. or , occasionally ranging beyond this limit especially in years with low prey in the north. This race includes the birds described as occidentalis (based on a wintering individual, as was the original subarcticus) and sclariventris. The older name wapacuthu was occasionally used for this subspecies, but it cannot with certainty be assigned to a recognizable and is thus considered a . 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 no little to no reddish coloration. B. v. subarcticus shows a very high degree of clinal variation, ranging from in the where owls are often medium-grayish and more heavily marked to the 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. In the west of Canada, subarcticus may hybridize with the dark saturatus and may do the same with this similarly dark heterocnemis in the east. In both cases, they may produce intermediate looking hybrids of reddish tone, like a virginianus but with sharper contrasting of colors. This is one of the largest-bodied subspecies. The length is 323–372 mm (12.7–14.6 in), averaging 346.7 mm (13.65 in), in males and 339–390 mm (13.3–15.4 in), averaging 362.5 mm (14.27 in), in females. Body mass ranges from 865 to 1,460 g (1.907 to 3.219 lb), averaging 1,196.5 g (2.638 lb), in males and from 1,112 to 2,046 g (2.452 to 4.511 lb), averaging 1,556 g (3.430 lb), in females. Tail length is 200 to 225 mm (7.9 to 8.9 in) and 220 to 240 mm (8.7 to 9.4 in) in males and females, respectively. Bill length is 35 to 43 mm (1.4 to 1.7 in) in both and one bird had a tarsus of 66 mm (2.6 in).
  • Californian great horned owl, Bubo virginianus pacificus (, 1854)
Central and southern California west of the except , south to Northwestern , . Intergrades with pallescens in , California (see also below). Resident all-year. Very rich brown, dark underside barring distinct but less pronounced than in saturatus but more pronounced than in pallescens. area is black. Feet are mottled dark. The facial disc is often even darkly mottled. This is a fairly small-bodied race, in fact including the lightest wild great horned owl adult ever weighed. The length is 305–362 mm (12.0–14.3 in), averaging 332.5 mm (13.09 in), in males and 335–375 mm (13.2–14.8 in), averaging 351.4 mm (13.83 in), in females. Body mass ranges from 680 to 1,272 g (1.499 to 2.804 lb), averaging 991.7 g (2.186 lb), in males and from 825 to 1,668 g (1.819 to 3.677 lb), averaging 1,312.7 g (2.894 lb), in females. Tail length is 175 to 218 mm (6.9 to 8.6 in) and 203 to 230 mm (8.0 to 9.1 in) in males and females, respectively. Bill length is 34 to 41 mm (1.3 to 1.6 in) and one bird had a tarsus of 57 mm (2.2 in).
  • Coastal great horned owl, Bubo virginianus saturatus (, 1877)
coast from southeastern to northern California. Includes the previously described form of B. v. leucomelas. The often recognized race B. v. lagophonus, is often now considered a mere clinal variation of the same race from interior to , the , and northwestern . These forms may be reported in winter as far south as and , but these are in times of . A dark and overall brownish form with heavily barred and mottled underside, with a dull tawny base. Inland birds (lagophonus) tend to have a more grayish base, the coastal owls being more richly brown. Otherwise, inland and coastal owls are practically the same. The facial disc can range from gray to reddish-gray to dark rufous. The feet are fairly dusky gray typically, although some buff-footed individuals are known, and legs are more prominently barred with black than in other North American races. This is a large race going on linear measurements, which in owls outmatch those of all other races but for heterocnemis in (which may itself be a discontinuous eastern wing of this race). The length is 330–370 mm (13–15 in), averaging 348.3 mm (13.71 in), in males and 339–400 mm (13.3–15.7 in), averaging 374.7 mm (14.75 in), in females. Tail length is 191 to 245 mm (7.5 to 9.6 in) and 196 to 252 mm (7.7 to 9.9 in) in males and females. In both sexes, known bill and tarsal lengths are 35 to 44 mm (1.4 to 1.7 in) and 62 to 70 mm (2.4 to 2.8 in). No weights are known to have been published.
  • North Andean great horned owl, Bubo virginianus nigrescens (, 1884)
; arid and zones from to northwestern . Resident all-year round. Presumably synonymous with the described columbianus. A dark, cold gray-brown form with heavy fuscous blotching. Arguably this is the darkest colored race on average, though it could be rivaled by individuals from saturatus and elachistus. This race has only minimally the rufous tinge seen in other darkish races, although some nigrescens may have a cinnamon facial disc. The largest winged race of owl in South America, this owl has a length of 345–365 mm (13.6–14.4 in) in males and 350–382 mm (13.8–15.0 in) in females. The tail in both sexes can range from 185 to 217 mm (7.3 to 8.5 in). Bill length is 40 to 50 mm (1.6 to 2.0 in), again relatively long as in nacurutu and one bird had a tarsal length of 80 mm (3.1 in), indicating relatively long legs in the race. No published weights are known. Apparently, despite its sizeable wing area, nigrescens is notably smaller overall when specimens are compared side-by-side with those from saturatus.
  • Desert great horned owl, Bubo virginianus pallescens (Stone, 1897)
southeastwards through arid regions of southeastern California and southern eastwards to western and southwards to and western in ; intergrades with pacificus in , if not elsewhere; vagrant individuals of saturatus and the population, which look similar to intergrades, also seem to occur in its range. Resident all-year. This race is likely synonymous with melanocercus. A pale dusky buff form with indistinct barring, especially on the underside. Darker individuals tend to have a deeper base to their upper sides. Humeral area is umber in color and the feathers of the feet are white and usually unmarked. A small race, it averages slightly larger in wing length than pacificus but weighs less on average. The length is 318–367 mm (12.5–14.4 in), averaging 337.2 mm (13.28 in), in males and 332–381 mm (13.1–15.0 in), averaging 348.9 mm (13.74 in), in females. Body mass ranges from 724 to 1,257 g (1.596 to 2.771 lb), averaging 914.2 g (2.015 lb), in males and from 801 to 1,550 g (1.766 to 3.417 lb), averaging 1,142.2 g (2.518 lb), in females. In both sexes, tail length is 190 to 235 mm (7.5 to 9.3 in) and bill length is 33 to 43 mm (1.3 to 1.7 in).
  • Yucatán great horned owl, Bubo virginianus mayensis (, 1901)
Endemic to the southern two-thirds of the . Resident all-year. A medium-pale form, fairly similar to pallescens in both hue and ventral markings. Going on linear measurements, B. v. mayensis is smaller than all North American horned owls, even the smallish pallescens, and is only slightly larger at median than the following race. This race has and tail lengths of 297–340 mm (11.7–13.4 in) and 180 to 198 mm (7.1 to 7.8 in) in males and 303–357 mm (11.9–14.1 in) and 199 to 210 mm (7.8 to 8.3 in) in females. In both sexes, the bill length is 39 to 41 mm (1.5 to 1.6 in) and tarsal length is 54 to 65 mm (2.1 to 2.6 in).
  • Baja California great horned owl, Bubo virginianus elachistus (, 1902)
Southern , . Resident all-year. Similar in color to pacificus but even darker and more heavily barred, like a miniaturized saturatus. It is considerably (5–10%) smaller than pacificus linearly; some size overlap does occur though. On average, it is the smallest known subspecies. Males have a wing chord length of 305 to 335 mm (12.0 to 13.2 in), tail length of 175 to 206 mm (6.9 to 8.1 in) and bill length of 33 to 38 mm (1.3 to 1.5 in). A single female had a wing chord of 330 mm (13 in) and tail of 211 mm (8.3 in).
  • Northeastern great horned owl, Bubo virginianus heterocnemis (Oberholser, 1904)
Breeds in eastern Canada (northern , , ). It southern breeding range seems to be delineated by the . In winter, this race may disperses southwards throughout to as far as the . This subspecies may be synonymous with saturatus, although it is distributed far to the east of that race. B. v. heterocnemis is surrounded by the much paler subarcticus to its west and quite differently marked virginianus to its south, the latter two overlap and possibly hybridize in some of the northeast. A fairly dark and grey, heavily barred form. Feet pale with dusky mottling. Going on median reported linear measurements (since body mass is unknown), this is the largest bodied subspecies on average. Males have a wing chord length of 350 to 365 mm (13.8 to 14.4 in) and females range from 370 to 390 mm (15 to 15 in). In both sexes, the tail is 220 to 250 mm (8.7 to 9.8 in) and the bill is 38 to 48 mm (1.5 to 1.9 in).
  • Rocky Mountains great horned owl, Bubo virginianus pinorum (Dickerman & Johnson, 2008)
The population breeds south of the in south to , , and the of . Westwards, it is presumed to occur to the and of California. This race was included in the presumed subspecies occidentalis, but recently first described and named as distinct subspecies and make up the missing piece in the once muddled distribution of great horned owls in the West and Rockies. Downslope movements into valleys occupied by pallescens might occur, but this needs study. A medium gray form, intermediate in coloring between saturatus and pallescens. Moderately barred and tinged buff or ochraceous on the underside. Feet mottled. This is a largish race, wing chord lengths being inexplicably greater in males, at 350 to 397 mm (13.8 to 15.6 in), than in females, at 327 to 367 mm (12.9 to 14.4 in). Tail length can range from 190 to 233 mm (7.5 to 9.2 in) and a female weighed 1,246 g (2.747 lb).

Distribution and habitat[]

A great horned owl with its eyes closed

The breeding habitat of the great horned owl extends high into the of North America, where they are found up to the northwestern and southern , , northern , in , , and . They are distributed throughout most of North and very spottily in Central America and then down into South America south to upland regions of , and , before they give way to the Magellanic horned owl, which thence ranges all the way to , the southern tip of the continent. It is absent or rare from southern , , , and to (where only two records) in Central America and the mangrove forests of northwestern South America. The species is also absent from the , the and almost all off-shore islands in the Americas, its ability to colonize islands apparently being considerably less than those of barn owls and . 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 , , and forests, , , , mountainous areas, , subarctic , rocky coasts, forests, and some . It is less common in the more extreme areas of the Americas. In the and , 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 forest of the Rockies. They have only been recorded a handful of times in true such as the . In the , they appear to use old-growth forest but in are actually often found near temporary agricultural openings in the midst of large areas of woodland. Similarly in south-central , 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 , 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 zones at 4,100 to 4,500 m (13,500 to 14,800 ft) in and . They are generally rare in non-tidal wetland habitat, and are replaced in the high 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 and (Megascops asio & M. kennicottii) which may regularly occur in busy suburban settings. On the contrary, though, one nest (a large nest) was reported in a tree in the middle of a campus, under which no fewer than 3,000-4,000 people passed directly each day. 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[]

Composite photo of great horned owl flight phases Great horned owls are typically sluggish and passive but aware during daytime.

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 , 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 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..., 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. and other 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 (Corvus brachyrhynchos). Since owls are, next to , 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.

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, , 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 to an average of 2.1 km2 (0.81 sq mi) in .

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 in , 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 behavior[]

Closeup of great horned owl toes and talons

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 (Sciurus niger) while they're building their leaf nests and (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. and 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 , 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 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 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.

Prey and trophic ecology[]

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". In fact, the great horned owl has the most diverse prey profile of any raptor in the Americas. Over 500 species have been identified as great horned owl prey, with dozens more identified only to genus or general type (especially numerous ) and presumably several more unknown from their relatively little-studied populations in the . (more than 200 species) and (nearly 300 species) make up the majority of their diet. Their diet in North America is made up of 87.6% mammals, 6.1% birds, 1.6% and with the remaining 4.7% being made up by insects, other assorted and . Estimated mass of individual prey for the owls has ranged from as little as 0.4 g (0.014 oz) to as much as 6.8 kg (15 lb) Most prey is the range of 4 g (0.14 oz) () to 2,300 g (5.1 lb) (). A single owl requires about 50 to 100 g (1.8 to 3.5 oz) 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 and , great horned owls largely live off just a handful of prey species: three species of : the (Sylvilagus floridanus), the (Lepus americanus) and the (Lepus californicus); two species of : the and the (Peromyscus leucopus & maniculatus), approximately three species of : the , and (Microtus pennsylvanicus, ochrogaster & pinetorum) and one introduced pest, the brown rat.

Rodents[]

A surprisingly large portion of the great horned owl's food consists of small rodents, such as .

Small form the great majority of great horned owl prey by number. Weighing a mere 14 to 31.5 g (0.49 to 1.11 oz) and 20 to 58 g (0.71 to 2.05 oz) on average, the nine species of New World mice in Peromyscus and eight species voles in Microtus recorded in the diet would appear to be overly small to be as important as they are to a predatory bird of this size. The prominence of these genera is undoubtedly due to the abundance of both genera in the wooded edge habitats frequented by great horned owls. It is estimated that a family of owls with two offspring would need to take about a half dozen (voles) to a dozen (mice) of these rodents every night to satisfy their dietary requirements but apparently the accessibility and abundance of these foods is irresistible as their numeric dominance is indisputable. By winter in areas that hold heavy snow, mice often come to outnumber the voles in the diet since the mice tend to travel over the surface of the snow while the voles make tunnels underneath the snow. In fact, a healthy family of great horned owls can decimate a colony of field rats, thus potentially performing a key role in controlling a highly destructive pest. Great horned owls living in the timbered fringes of garbage or refuse dumps may subsist mostly on rats.

In the , California, the and northern , the diversity of this species' diet rises, in sync with the diversity of rodents. Especially important, from to is the (Thomomys talpoides), although assorted other pocket gophers (, , , and other ssp.) are readily taken. While the northern weighs from 90 to 120 g (3.2 to 4.2 oz), other pocket gophers hunted average from 95 to 545 g (0.209 to 1.202 lb) in mass. From to a very important food is the , primarily the (Perognathus parvus). While the Great Basin species is a relative giant at 22 g (0.78 oz), other hunted pocket mice (which may include both and ssp.) can average nearly as light as 8 g (0.28 oz). In , the 159 g (5.6 oz) (Sigmodon hispidus) is the most commonly recorded prey species. The same species constituted 75% by number of a small sampling in . In semi-desert and other arid habitats, become increasingly important prey, ten species have been reported in the diet but most prominently the and (Dipodomys ordii & merriami), both being widespread, numerous and relatively diminutive (at 42 and 48 g (1.5 and 1.7 oz). Eight known larger species of kangaroo rats, including the (Dipodomys ingens) averaging at 152 g (5.4 oz), are also taken.

The , including , (Marmota), (Cynomys), and , 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 62 g (2.2 oz) (Tamias cinereicollis) to the 5,775 g (12.732 lb) (Marmota caligata); thus, squirrels can be provide a very fulfilling meal. An even larger rodent is sometimes attacked as prey by great horned owls, the (Erethizon dorsatum), in which average adults range from 4,500 to 9,000 g (9.9 to 19.8 lb). 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 (Glaucomys ssp.), the (Ochrotomys nuttalli), & (Myodes & Synaptomys ssp.), the (Ondatra zibethicus), the (Onychomys leucogaster), the (Baiomys taylori) and (Zapus & Napaeozapus ssp.).

Lagomorphs[]

Although generally no match for rodents in sheer quantity of individuals, in terms of prey biomass, the most significant prey of North American great horned owls are and . About a dozen lagomorphs species are known to be hunted by the owl, from the relatively tiny 420 g (0.93 lb) to several weighing more than 2,000 g (4.4 lb). Two hare species, the and snowshoe hare, are so important to the owls as a food source that the local owl populations sharply rise and fall in sync with the hares' cyclical population trends. With adult weights of 800 to 1,900 g (1.8 to 4.2 lb) in adult cottontails, 900 to 2,000 g (2.0 to 4.4 lb) in snowshoe hares and 1,400 to 2,700 g (3.1 to 6.0 lb) in black-tailed jackrabbits, these species are overall the largest regular prey for this species. In , where great horned owls are dependent on the jackrabbits, average brood size rose from 2 at jackrabbit population lows to 3.3 when the jackrabbits were at their peak. At the peak of population cycle, jackrabbits accounted for 90.2% and (Sylvilagus audubonii) for another 8.7% of prey biomass. In the short-grass prairie of , (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. Further north in Colorado, in the absence of jackrabbits, the mountain cottontails falls to third place by number (12.9%) behind the northern pocket gopher (36.5%) and prairie vole (24.7%) but still dominates the biomass, making up about half. In central , the lagomorphs (black-tailed jackrabbit/desert cottontail) and Ord's kangaroo rat each made up 39% of the food by number, respectively. The mountain cottontail dominates the biomass of prey in the of California, making up 61.1% of the biomass, although are numerically secondary to desert woodrat. Remarkably, in the of , individual rodents (1159 counted) were more than 10 times more numerous than lagomorphs (114 counted) by quantity and yet the jackrabbit and mountain cottontail still made up approximately half of the biomass. The dependence on lagomorphs also extends into , as in about a quarter of identified prey was black-tailed jackrabbit and either desert or the larger (Sylvilagus cunicularius). In the northern , 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 ; 83–86% and 75–98%, respectively, in ). 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 , mice and voles, and . Because fewer of these alternative prey species are available in boreal forest during winter, owls had to emigrate or suffer high mortalities if they stayed. In , 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 as well.

Other mammals[]

Other mammals are taken readily as well. From both the tropics and the United States, several species of opossum may be taken, down to the size of the tiny (Thylamys velutinus). In , (Didelphis albiventris) were found in 12% of pellets, but all specimens appeared to be juveniles each weighing about 1,000 g (2.2 lb). Quite differently, in , (Didelphis virginianus) made up 6% by number of prey but due to their large size (approximately 2,500 g (5.5 lb)) and that all specimens were adults, they occupied the highest percentage of biomass of any species in a wide study from that state. At least eight species of are taken by opportunity and make up the smallest mammalian prey taken by great horned owls, as specimens of (Cryptotis parva) or (Sorex cinereus) have had an estimated weight of only 2 g (0.071 oz). One of the more regularly taken shrews, though, is the larger 19.5 g (0.69 oz) (Blarina brevicauda), which was represented in more than 2% of pellets in the ., of at least four or five species, are also widely but lightly reported as prey. Remnants of , presumably (Dasypus novemcinctus), have been found around owl nests in the south. 11 species of bat are known to be hunted by great horned owls. One pellet in was found to be composed entirely of (Tadarida brasiliensis). Smaller species of , such as (Bassariscus astutus), (Neovison vison), (Mustela nigripes) and various other small (Mustela ssp.), are sometimes taken as prey. Prey in the form of , like or (Canis latrans) are often juveniles presumably snatched from the mouths of dens by night. and of up to adult size may taken. Surprisingly, at least two cases of a great horned owl preying on an adult (Procyon lotor) have been reported. One instance of an owl taking a (Lynx rufus) as prey was also reportedly observed. In one case, a great horned owl was the likely killer of an adult female (Martes pennanti), though young ones are typically taken. Occasionally, domestic carnivores are also prey. A few cases of young or small (Canis lupus familiaris) and several of juvenile and adult (Felis silvestris catus) being killed by great horned owls have been reported. The most infamous 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 adult (Mephitis mephitis), which is some case at least three times as heavy as the attacking owl. In one single nest, the remains of 57 different striped skunks were found. Due to the proclivity of skunk predation, great horned owls nests frequently smell strongly of skunk and occasionally will even stink so powerfully of skunk that they leave the smell at kill sites or on prey remains.

Birds[]

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 , of which they are known to have preyed on 23 species, basically consisting of all of the native species found in the . In the , the (Phasianus colchicus) and (Colinus virginianus) were the fifth and sixth (out of 124 identified species) most significant prey species in 4838 pellets. Errington characterized the predatory pressure exerted on bobwhites by great horned owls as "light but continuous pressure", which may be considered characteristic of the species' hunting of all galliforms. 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. Similarly, owls may track down sleeping , which also roost in vegetation but more openly than quail. Some grouse, such as (Tympanuchus cupido), (Tympanuchus phasianellus) and (Centrocercus urophasianus), may also been vulnerable to great horned owls while displaying conspicuously in openings on a first thing in the morning. 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 (Bonasa umbellus) and (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 in , introduced (Pavo cristatus) are an important prey item. The (Meleagris gallopavo), 4 to 8 kg (8.8 to 17.6 lb) 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 have been hunted and killed. Under normal circumstances, (Gallus gallus domesticus) will be ignored in favor of wild prey. On occasion, individual owls, especially inexperienced juveniles, will become habitual fowl killers. These errant owls mainly hunt chickens, though will also take domestic , turkeys and anything else available. In general, chickens kept in locked cages with enclosed tops overnight are safe from great horned owls; not so chickens left free range or in open enclosures.

While galliforms are widely reported, the few cases where great horned owls locally turn to birds as the primary food source over mammals, these may often be local responses to the abundance of breeding or concentrations of roosting water birds, since they tend to roost in relatively open spots. They have been known to predate more than 110 different species of assorted water bird. In prairie wetlands of , avian prey, primarily represented by and the (Fulica americana) came to represent 65% by number and 83% by biomass of the diet of the local owls, also including secondarily , smaller and as well as upland-based species like (Perdix perdix), and . 77% of the ducks in that study were juveniles, the largest duck being a male (Anas platyrhnychos) weighing approximately 1,250 g (2.76 lb), but nearly all the coots were adults. On , , where they are no native land mammals, (Cerorhinca monocerata), both adults and nestlings, were the most numerous prey, present in 93% of 120 pellets. Species as large as adult , and have been successfully killed. The nestlings of even larger species like (Cygnus buccinator), (Pelecanus eryhtrorhynchos), (Pelecanus occidentalis) and (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 , 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 have been taken by great horned owls. Members from most North American families are known as prey, although among smaller types such as , , , , and most only a few species from each have been recorded. Nonetheless, an occasionally unlucky migrant or local breeder is sometimes snatched. Fledgling songbirds are regularly taken in spring and summer. The smallest avian prey known for great horned owls are the 5.8 g (0.20 oz) (Polioptila caerulea) and the 6.2 g (0.22 oz) (Regulus calendula). Somewhat larger bodied families are more prominent, i.e. the (14 species) and (14 species) and, secondarily, the (Tyrannus ssp.), , and (Sturnus vulgaris). This is likely due to the fact that the larger passerines usually roost in relatively open spots and have larger, more conspicuous nests. and tend to be grabbed off of their communal roosts by night.

Other prey[]

The great horned owl rarely misses an opportunity to hunt and . However, are largely unavailable as prey due to their typically diurnal periods of activity. However, some snakes are partially or largely nocturnal, and more than a dozen species are hunted in North America. Snakes hunted range from small, innocuous (Thamnophis ssp.) and (Hypsiglena torquata) to venomous species like (Agkistrodon piscivorus) and (Crotalus virdis) and formidable, large species like (Lampropeltis getula) and (Pantherophis obsoletus), which in mature specimens can rival the owl in mass and sheer predatory power. The capture of the hatchlings of very large reptiles such as (Caretta caretta) and (Alligator mississippiensis) by great horned owls has been reported, in both cases likely when the baby reptiles are attempting to make their way to the security of water. On rare occasion, , and are reported as prey. On rare occasions, fish are taken including (Carassius auratus), (Lepomis macrochirus), (Ameiurus ssp.), other , , , and . Many types of invertebrates are recorded as prey. These include mainly insects, but also , , , and . The occasionally invertebrate prey taken largely consists of common, large insects such as various , , , and , some of which the great horned owl has even reportedly caught via "hawking", i.e. swooping at on the wing. 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. It is commonly believed that routine insectivory in great horned owls is mainly restricted to inexperienced young owls too unskilled to graduate to larger prey yet. It is clearly inefficient for owls of this size to attempt to raise young on a diet of foods as small as insects. Although rare, -feeding has been recorded in great horned owls, especially ones wintering in Canada and other northern areas during harsh weather. Road kills are sometimes opportunistically eaten. A case of an owl scavenging a (Odocoileus virginianus) carcass, ultimately tearing off the deer's leg, was captured on a motion capture video camera set out to film wildlife.

Interspecific predatory relationships[]

An immature red-tailed hawk eats a vole, one of the many prey items that feed both the competing hawks and great horned owls.

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 to the great horned and great grey owls. In a long-term study of a block of , all nine species of accipitrid, falcon and owl that stayed to breed there were found to be primarily dependent on the same two rodent genera, the and the two common species. In the , the owls share black-tailed jackrabbit and as the primary prey with golden eagles, red-tailed hawks and (Buteo regalis); all four species had diets with more than 90% of the biomass is made up of those lagomorphs. Of these, the great horned owl and golden eagle were able to nest most closely to one another because they had the most strongly dissimilar periods of activity. In California, when compared to the local red-tailed hawks and (Crotalus atrox), the diets were most similar in that by number about 15-20% of all three species' diets depended on , but the largest portion was made up of in the hawk and the rattlesnake and desert woodrats and other assorted in the great horned owl. In the , the great horned owl's prolificacy as a hunter places it second only to the (Lynx canadensis) among all predators. Although locally dependent on the hares as their main food, (Accipiter gentilis), red-tailed hawks and golden eagles apparently do not have as large of an impact on the hares. Nor do mammalian carnivore generalists that also kill many hares, like the , , (Gulo gulo), and larger varieties (i.e. (Canis lupus), (Puma concolor) and (Ursus ssp.)).

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 , 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. This is not to say that raptors ever comprise a significant portion in any known study 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. In the (Buteo plagiatus), for example, in a study of one breeding block of , the owls were observed to visit nests nightly until all the nestlings were gone. 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. On average, great horned owls begin nesting about three weeks before red-tailed hawks begin to build nests, although some raptors may locally breed as much as two months after the owls.

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 , great horned owls were responsible for the failure of 78% of eastern screech owl's nests. Long-eared owls and, to a lesser extent, barn owls tend to hunt in open, sparsely treed habitats more so than great horned owls, but since they may return to wooded spots for nesting purposes, they may be more vulnerable there. The long-eared owl and barn owls are often compared to the great horned owl as these medium-sized species often occur in abutting habitats and often hunt primarily the same vole and mice species, although the alternate prey of the great horned tends to be much larger, including the smaller owls themselves. In a pair of studies from , the average weight of prey for long-eared owls was 28 to 30 g (0.99 to 1.06 oz), 46 to 57.1 g (1.62 to 2.01 oz) for barn owl and 177 to 220 g (6.2 to 7.8 oz) for the great horned owl. Both young and adult great grey owls, despite their superficially superior size, are apparently hunted with impunity by great horned owls. In the boreal forests, both the northern hawk owl and great grey owl appear to be in greater danger of great horned owl predation in years where the snowshoe hare have low populations. 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.

Whereas owls of any age are freely attacked by great horned owls whether nesting or not, when it comes to diurnal raptors, great horned owls are mainly a danger around the nest. They often hunt diurnal raptors when they come across their often relatively conspicuous active platform nests during hunting forays in spring and summer, taking numbers of both nestlings and brooding adults. Again, like owls, diurnal raptors are attacked depending on the relative similarity of their habitat preferences to the owl. (Accipiter cooperii) and red-tailed hawks tend to be most vulnerable, as they prefer the same wooded edges frequented by great horned owls. Other diurnal raptors may be attracted to more enclosed wooded areas, such as (Accipiter striatus) or (Buteo albonotatus), or more open plain and meadow areas, such as (Circus hudsonius) and ferruginous hawks, but this is almost never a total insurance against predation as all of these are recorded prey. In a study of (Buteo lineatus) and (Buteo brachyurus)s breeding in , despite their nesting in deeper woods than those that host these owls, the main cause of nest failure was great horned owl predation. Similarly, the great horned owl was the primary cause of nesting failure for both desert-dwelling (Parabuteo unicinctus) and forest-dwelling in (39% and 40% of failures, respectively), wetland-inhabiting (Pandion haliaetus) in (21% of failures) and (Falco peregrinus) in the (27% of failures). 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 , and .

Great horned owls are frequently mobbed by other birds. Most accipitrids will readily mob them, as will falcons. , , , , red-tailed hawks, , , , , , (Falco mexicanus) and (Corvus corax) are among the reported species who have been recorded diving on great horned owls when they discover them. In and , they may be mobbed by and (Aphelocoma wollweberi & californica) and and (Tyrannus verticalis & vociferans). The most infamous nemesis of this owl, however, is the , who seems to regard it with great scorn. Several incidences have been recorded of dozens or even hundreds of crows appearing to mob a great horned owl. Such daytime flights normally cover short distances; if the owl flies, it alights to the nearest secluded spot to escape its tormentors. 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[]

Nestlings of the Rocky Mountains great horned owl (B. v. pinorum) in

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. This species was once thought to be strictly monogamous, but recent analysis indicates one male may mate with two females simultaneously, as was discovered for the first time in 2018 in Reno, Nevada. 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. The white throat may serve as a visual stimuli in the low light conditions typical of when this owl courts. He often flies up and down on a perch, while approaching the potential mate. Eventually, he comes to approach the female and tries to rub his bill against hers while repeatedly bowing. If receptive, the female hoots back when the pair meet but is more subdued in both her hoot and display. The male may convince the female by bringing her freshly caught prey, which they will share. While males often hoot emphatically for about a month or six weeks towards the end of the year, the period where females also hoot is usually only a week to ten days. Pairs typically breed together year after year and may mate for life, although they associate with each other more loosely when their young become mostly independent. Pairs rekindling their reproductive relationship in the winter may perform a milder courtship to strengthen pair bonds before producing young.

Males select nesting sites and bring the females' attention to them by flying to them and then stomping on them. Nests with open access, considering this birds large size, as opposed to enclosed with surrounding branches, are preferred. Like all owls, great horned owls do not build their own nest. They nest in a wider variety of nest sites than any other North American bird. Many nests are in cavernous hollows of dead trees or their branches, especially in southern states in large trees along the edge of old-growth lots. In mountainous or hilly areas, especially in canyons of the and , cliff ledges, small caves, and other sheltered depressions may be used. Owls living in prairie country, in the absence of other animals' nests, riparian trees or non-native trees or the bare ground of tree hollows or man-made structures, will use boulders, buttes, railroad cuts, low bushes and even the bare ground as nest sites. Ground nests have also been recorded in the midst of tall grasses in and in the midsts of brushy spots on the desert ground. Even the burrow entrances of and dens have reportedly been used as nests, in spite of the inherent risk of sharing space with such potentially dangerous co-inhabitants.

A brooding female great horned owl (B. v. virginianus) on her nest in

Most tree nests used by great horned owls are constructed by other animals, often from a height of about 4.5 to 22 m (15 to 72 ft) 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. Great horned owls in the southwest may also use nests in , built by and red-tailed hawk, as well as large hollows in cacti. The nests they use are often made by most larger types of acciptrids, from species as small as to and golden eagle, though perhaps most often those of red-tailed hawks and other . Secondly in popularity are and (Corvus ssp.) nests. Even , (Nycticorax nycticorax) and nests have been used, the latter sometimes right in the midst of an active . The leaf nests of are also regularly used but in general great horned owls are partial to stick nests since they provide a much firmer, safer foundation.

The stage at which eggs are laid is variable across North America. In , eggs may be laid as early as late November to as late as early January. In the , from south to , egg laying may begin from late December to early February. From to northern , egg laying is from early February to late March. The largest swath of egg-laying owls from central California to to as far north as and is from late February to early April. In the , , northern and eastern Canada, egg laying is from early March to late April. In the rest of Canada and Alaska, egg laying may be from late March to early May. The latest known date of egg laying was in mid-June in and the . In northwestern and north-central , egg-laying can be 3–4 weeks earlier than usual when food is abundant and weather is favorable. For owls found in more tropical climates, the dates of the breeding season are somewhat undefined. There are usually 2 eggs per clutch, but clutches range in size from 1 to 6 eggs (over 3 is uncommon, over 4 is very rare), depending on environmental conditions. The average egg width is 46.5 mm (1.83 in), the average length is 55 mm (2.2 in) and the average weight is 51 g (1.8 oz), although mass could be slightly higher elsewhere because this figure is from , 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.

The young weigh around 34.7 g (1.22 oz) at birth on average and can gain an average of about 33.3 g (1.17 oz) a day for the first four weeks of life, with typical weights in the range of 800 or 1,000 g (1.8 or 2.2 lb) by 25–29 days for males and females, respectively. When first hatched the young are covered in whitish gray down, with some brownish about the wings. Gradually the soft juvenal downy plumage comes through the down, being typically a cinnamon-buff color, but with variable hues predicting the eventual color of the mature owls. The extent of down gradually diminishes, developing mature-looking plumage by late summer, although many first year birds still have scattered bits of down into autumn. By late autumn, first-year birds look similar to adults but with a slightly warmer, reddish tinge, less well developed ear tufts and a smaller white throat patch. The nestling owls develop mostly in behavior between two weeks and two months of age, in which time they adapt the ability to defend themselves, grasp foods and climb. Vocally, the young are able to exert weak chips while still in the egg, developing into a raspy chirp shortly after hatching. The calls of the young increase rapidly in intensity, pitch and character, some juvenile males mimicking their father's hooting in fall but usually they conclude with various odd gurgling notes. The earliest competent hooting by juvenile owls is not until January. 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 age at which the young leave the nest is variable based on the abundance of food.

The young birds stay in an area ranging from 13.1 to 52 ha from the nest into fall, but will usually disperse up to several thousand hectares by the end of fall. 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. Based on the development of the bursa, great horned owls reach sexual maturity at two years of age.

Great horned owls (B. v. virginianus) in nest near Madison, Wisconsin

Mortality and longevity[]

Longevity and natural mortality[]

Great horned owls seem to be the most long-living owl in North America. Among all owls, they may outrank even the larger Eurasian eagle owl in known longevity records from the wild, with almost 29 years being the highest age for an owl recorded in North America. A more typical top lifespan of a great horned owl is approximately 13 years old. In general, great horned owls are most vulnerable in the early stages of life, although few species press attacks on the owl's nests due to the ferocious defensive abilities of the parents. Occasionally, nestlings and fledglings will fall from the nest too early to escape or to competently defend themselves and have then fallen prey to , , , or wild or feral . Occasionally and consume eggs and nestlings from tree nests and may take the rare unguarded egg. and have been reported eating eggs and small nestlings. This can normally only happen when owls are driven from the nest by human activity or are forced to leave the nest to forage by low food resources but on occasion huge flocks of crows have been able to displace owls by harassing them endlessly. In general, great horned owls rarely engage in , unlike many other raptorial birds. Siblicide occurred at 9 of 2,711 nests in . Most cases where young owls are killed and/or consumed by their siblings or parents appear to occur when the nestling is diseased, impaired or starving or is inadvertently crushed. Adults generally have no natural predators, excepting both North American and other owls of their own species.

Occasionally, great horned owls may be killed by their own prey. Although typically able to kill skunks without ill effect, five owls were found blind after getting sprayed in their eyes by skunks. Cases where the quills of porcupines have killed or functionally disabled them have been observed as well. Violent fights have been observed between great horned owls after attempts to capture and . When a peregrine falcon repeatedly attacked a great horned owl near its nest along the , 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%. For owls in the , juvenile survival in the 9 weeks after dispersal has dropped from 80% to 23.2% in a span of three years in response of instability of food supply. In the Yukon, adults on territory had an average annual survival rate of 90.5%. Anemia, caused by and the drinking of blood by swarming, blood-drinking (Simulium ssp.), was a leading cause of juvenile mortality in the Yukon.

Human-related mortality[]

The great horned owl is not considered a globally threatened species by the . Including the Magellanic species, there are approximately 5.3 million wild horned owls in the Americas. Most mortality in modern times is human-related, caused by owls flying into man-made objects, including buildings, cars, power lines, or barbed wire. In one study, the leading cause of death for owls was collision with cars, entanglement with wires, flying into buildings and, lastly, electrocution. 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 (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. Thus, small bounties were offered in trade for owl bodies. Around the turn of the 20th century, the great horned owl was considered endangered in the state of because of a large quantity of poachers who were illegally hunting and collecting it. 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 , the reintroduction of the to the and was hampered by great horned owls killing both young and adult peregrines at night. Similarly, as mainly recorded in , attempts to reintroduce , after they were also hit hard by DDT, were effected by heavy owl predation on nestlings, and the owls were also recorded to take a large toll locally on the threatened colonies of . Where clear-cutting occurs in old-growth areas of the , spotted owls have been badly affected by considerable great horned owl predation. 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 (Urocitellus townsendii), (Perognathus pacificus),, (Dipodomys stephensi),, and , (Brachyramphus marmoratus),, (Aphelocoma coerulescens),, (Setophaga kirtlandii) and (Euphagus carolinus). The 's "green list" includes birds with considerable population declines (many classed as by the ) or other immediate threats and/or restricted populations. Altogether, great horned owls hunt 50 different species from that list.

Iconography and myth[]

Many warrior-based tribes of admired the great horned owl for their "strength, courage and beauty". The of the believed that owls were reincarnations of slain warriors who fly about by night. The of the had mystic owl societies in which initiates were made to adorn facial mask made of the wing and tail feathers of great horned owls. Some Indian nations regarded the great horned owl as a friendly spirit who could aid in matters of love, such as the of who felt the call of this species was a magical love flute designed to ignite human passions. The of the also associated this owl with fertility, albeit of a different kind: they believed the calling of the owls into summer predicted hot weather, which produced good peach crops. During the , the Hopi performed a ceremony with great horned owl feathers in hopes of summoning the heat of summer. Tribes in were known to use owl wing feathers to produce arrows which could strike their enemies with a minimum of sound. The held owl feathers in their mouths hoping to gain some of the silence that owls use in ambushes while striking their own enemies from other tribes. The felt the origin of the great horned owl was due to an unformed owl annoying , the almighty creator, while Raweno created the , causing Raweno to make the owl "covered with mud" (dark camouflage) and doomed to ceaselessly call "whoo whoo", which he used while harassing Raweno, by night because Raweno was active during the day.

Provincial bird[]

The great horned owl is the of .

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Further reading[]

  • Accordi, Iury Almeida; Barcellos, Andre (June 2006). (PDF). Revista Brasileira de Ornitologia (in Portuguese and English). 14 (2): 101–115. Archived from (PDF) on 2008-12-19.
  • Banks, R.C.; Cicero, C.; Dunn, J.L.; Kratter, A.W.; Ouellet, H.; Rasmussen, P.C.; Remsen, J.V. Jr.; Rising, J.A.; Stotz, D.F. (2000). (PDF). The Auk. American Ornithologists' Union. 117 (3): 847–858. :.
  • Dickerman, Robert W. (1 July 2002). "The Taxonomy of the Subarctic Great Horned Owl (Bubo virginianus subarcticus) Nesting in the United States". The American Midland Naturalist. 148 (1): 198–199. :.
  • Dickerman, Robert W. (29 January 2004). (PDF). . 124 (1): 5–6.
  • Ganey, J. L., W. M. Block, J. S. Jenness, R. A. Wilson (1997). "Comparative habitat use of sympatric Mexican spotted and great horned owls." J. Wildl. Res. 2(2): 115-123.
  • Holt, Denver W.; Berkley, Regan; Deppe, Caroline; Enríquez Rocha, Paula L.; Olsen, Penny D.; Petersen, Julie L.; Rangel Salazar, José Luis; Segars, Kelley P. & Wood, Kristin L. (1999). "69. Great Horned Owl". In: del Hoyo, J.; Elliott, A.; Sargatal, J. (eds): (Volume 5: Barn-owls to Hummingbirds): 185, plate 10. Lynx Edicions, Barcelona.  
  • Houston, C. S.; Smith, D. G.; Rohner, C. (1998). Great Horned Owl (Bubo virginianus). In: Poole, A.; Gill, F. (eds.): 372. , Philadelphia, PA & , Washington, D.C. Online version, retrieved 2006-12-05.
  • (1947). (PDF). Condor. 49 (1): 10–13. :10.2307/1364422.
  • Johnson, D. H. (1993). Thesis. Oregon State University, Corvallis, USA.
  • Rohner, C. (1997). . In: Duncan, JR; Johnson, DH; Nicholls, TH (eds): Biology and conservation of owls of the Northern Hemisphere, 2nd International Symposium. St. Paul, MN: US Department of Agriculture, Forest Service. pp. 347–362.
  • Rohner, C., and F. I. Doyle (1992). "Food-stressed great horned owl kills adult goshawk: exceptional observation or community process?" J. Raptor Res. 26(4): 261-63.
  • Rohner, Christoph, and Charles J. Krebs (1996). Oecologia 108(2): 303-310.

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