Great horned owl

• 1. Introduction

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  In one great horned owl nest, scientists found the remains of 57 striped skunks. That single nest says a great deal about Bubo virginianus, a large owl of the Americas with a taste for prey that almost everything else avoids. Its nests often smell so strongly of skunk that the odor clings to kill sites and to the leftovers of its meals. What gives this bird its edge over such a well-defended animal? Where does it live, and how does it find food in the dark? And is anything at all off its menu?

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• 2. Tiger of the Air

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  Early naturalists looked at this owl and saw a "winged tiger," or a "tiger of the air." That image gave the bird one of its other names, the tiger owl. Many people simply call it the hoot owl.

  The formal scientific name arrived in 1788. The German naturalist Johann Friedrich Gmelin was preparing a revised and expanded edition of Carl Linnaeus's Systema Naturae. He placed the bird with the other owls in the genus Strix and named it Strix virginia.

  Gmelin built his description on the earlier work of an English naturalist named George Edwards. In 1747, Edwards had described and illustrated the owl in the second volume of A Natural History of Uncommon Birds. He had seen a live specimen from Virginia at the house of the Earl of Burlington in Chiswick. Edwards also owned a preserved specimen, and another formed part of the Leverian collection.

  The owl no longer sits in Gmelin's genus. In 1805, André Duméril introduced the genus Bubo, and the great horned owl now belongs there. That genus carries a long story of travel, and it begins with a crossing from Asia.

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• 3. Family Tree

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  The Bering land bridge carried at least one lineage of Bubo owls from Eurasia into the Americas. The great horned owl represents one of the one or two radiations of the genus that made the crossing. Once the owls had spread through the Americas, the lesser horned owl clearly split away. The snowy owl took a different path. The consensus seems to be that it divided from the great horned owl back in Eurasia. It then spread across the Arctic and the northernmost reaches of North America on its own.

  Pleistocene fossils of Bubo owls have turned up in North America, from as far east as Georgia. Most come from the Rocky Mountains and the lands to their west. They may be separate species or paleosubspecies. Almost all point to owls larger than the ones living today.

  The Eurasian eagle-owl, Bubo bubo, is a close cousin that fills the same ecological niche in Eurasia, despite being notably larger. One older study even suggested that the two might be a single species, based on shared life history, range and appearance. Genetic evidence does not support that idea. It places them in separate clades. The closest living relatives are the lesser horned owl, and then the snowy owl.

  By day, the red-tailed hawk, Buteo jamaicensis, plays a similar role. It often shares the owl's habitat, prey and nesting habits, so it is something of a daytime equivalent.

  More than 20 subspecies of great horned owl have been named. Many are only examples of individual or clinal variation. The most conservative treatments recognise as few as 10, and fifteen are recognised today. The differences are mainly in color and size, and they follow Gloger's and Bergmann's rules. One of the fifteen, pinorum of the Rocky Mountains, was named only in 2008, by Dickerman and Johnson. It filled the missing piece in a once-muddled picture of the western owls.

  The lesser horned owl, also called the Magellanic horned owl, was once counted as a great horned subspecies. Genetic evidence now supports treating it as a distinct species. In the field, the Magellanic owl shows finer and more numerous brownish bars on the underside, rather than the blotchy, irregular barring of the great horned.

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• 4. Tundra to Andes

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  Fort Chimo in Ungava and the Mackenzie Mountains mark places where great horned owls breed in the subarctic. Other northern records come from Keewatin, northern Manitoba, Anticosti Island and Prince Edward Island. From there the species runs through most of North America, then very spottily through Central America. In South America it reaches the uplands of Argentina, Bolivia and Peru. Beyond that point it gives way to the Magellanic horned owl, which ranges all the way to Tierra del Fuego. Only the barn owl is more widely distributed among the owls of the Americas.

  The gaps in the map are telling. The great horned owl is absent or rare from southern Guatemala to Panama, where there are only two records. It is also missing from the West Indies, from Haida Gwaii, and from almost all offshore islands. Its ability to colonize islands appears far weaker than that of barn owls and short-eared owls.

  Within its range, few birds are as flexible about where they live. The great horned owl takes up residence beside deciduous, coniferous and mixed forests, tropical rainforests, pampas, prairie, deserts, subarctic tundra, rocky coasts and mangrove swamps. It manages in some urban areas too.

  Still, it has limits. In the Mojave and Sonora Deserts, it keeps to vegetated or rocky fringes and avoids the heart of the desert. It is rare in landscapes that are more than 70% old-growth forest, such as the aspen forest of the Rockies. In south-central Pennsylvania, owls use cropland and pasture more than forest, which suggests a preference for broken-up landscapes. They like open ground for hunting beside woods for roosting and nesting, so lightly populated rural regions can be ideal. In cities they favor park-like settings, unlike screech owls, which readily live in busy suburbs.

  Altitude shows the same flexibility. In North America the owls are usually absent above the tree line, though they occur up to 2100 m in California and 3300 m in the Rockies. In the Andes they are a true mountain species. They are regularly recorded in treeless Puna grassland at 4100-4500 m in Ecuador and Peru, a height at which few owls live.

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• 5. Built for the Dark

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  Adult great horned owls measure 43-64 cm in length, averaging 55 cm, with wingspans of 91-153 cm. Females run somewhat larger. The mean weight is 1608 g for females and 1224 g for males. This is the heaviest owl in Central and South America, and the second-heaviest in North America after the snowy owl. The body is barrel-shaped, the head large and the wings broad.

  The plumage is built for camouflage. The upper parts are a mottled brown with dark, complex markings, and the underparts are lighter with brown horizontal barring. A white patch on the throat varies in size. The owl's famous "horns" are tufts of feathers called plumicorns. Their purpose is not fully understood. The accepted hypothesis is that they act as visual cues in territorial and social encounters with other owls. Researchers agree that the tufts play no role in hearing.

  A fully spread foot measures around 20 cm from talon to talon. A long-eared owl manages about 8 cm, a barn owl 13-15 cm, and a great grey owl 18 cm. The talons can apply at least 300 psi of crushing power, far more than a human hand. In some big females the grip may rival that of the much larger golden eagle. Even the feet are feathered, with the second-longest foot feathers of any owl after the snowy owl.

  The flight of this heavy bird is nearly silent. Three features of the wing make that possible. Serrations on the leading edge of the feathers disrupt the turbulence from flapping. Softer feathers deaden sound. A fringe along the trailing edge cuts the last of it. The wing loading is described as proportionately the highest among raptors, yet the owl can glide on breezes at as little as 2 miles per hour.

  The ears matter even more in darkness. They sit hidden by feathers on the sides of the head, and the left opening is slightly larger than the right. Each opening is about 2.3 cm tall. Because the two openings differ in height, the owl can compare the timing and direction of a sound at each one and triangulate its source. That lets it locate prey even under snow. The disc-shaped face funnels sound toward the ears. Its hearing is estimated at up to ten times that of a human.

  The eyes are just slightly smaller than a human's, which makes them proportionately among the largest of any land vertebrate. They are cylindrical rather than round. The extra distance from lens to retina makes them behave like a telephoto lens. The retina is mostly rods, and color vision is relatively weak, with the cones peaking at 555 nm. The night vision, however, is excellent. The eyes cannot turn in their sockets, so the owl turns its whole head, up to 270°.

  Only one subspecies breaks the pattern of the iris. In South America, B. v. nacurutu has amber eyes, while every other great horned owl has a yellow iris.

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• 6. Voices and Borders

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  The song of the great horned owl is low-pitched and loud, and it lasts four or five syllables. People have called the call "solemn" and also "terrifying." The female's voice is higher and rises in pitch at the end. Her syrinx, the bird's voice organ, is smaller than the male's, and she is the larger of the two sexes. Calling seems to peak after midnight. Territorial hooting usually drops off in February or March, as egg laying begins.

  The repertoire goes well beyond hooting. One description speaks of "an indescribable assemblage of hoots, chuckles, screeches, and squawks, given so rapidly and disconnectedly that the effect is both startling and amusing." Owls make these noises when disturbed and angry at the nest, often just before attacking an intruding human or animal. They also use them in courtship and in disputes with other owls. Young owls still in their parents' care hiss or screech loudly, and people often mistake the sound for barn owls.

  Yukon and Wyoming show how much territory size can vary. A pair holds about 16 km2 on average in the Yukon, and only 2.1 km2 in Wyoming. Mated pairs stay on their territories year-round and for the long term. Unmated and younger birds move freely in search of a mate and a territory, and they leave regions with little food in winter. Hooting marks the boundaries, with the most activity before egg laying and a second peak in autumn, when juveniles disperse. Males do most of the defending, though females often join in hooting contests, even during incubation.

  When calling is not enough, the threat escalates. The most serious display involves spread wings, bill-clapping, hissing and longer, higher screams, with the body poised to strike with the feet. If the intruder keeps pressing, the defender hops forward and rakes at it with its claws.

  Territories limit how many pairs can breed in an area. Owls shut out of a territory live a silent life as "floaters." Radio-telemetry showed that floaters gather along the boundaries of established territories. At Kluane Lake in the Yukon, researchers saw incursions into neighboring territories only twice. Both were by females, when a neighboring female had died or left. At least four dead owls at Kluane appear to have been killed by other great horned owls in territorial conflicts. Owls killed this way are sometimes eaten.

  The pairs are also unusually early nesters. They are among the first birds to nest in North America, often laying eggs weeks or even months before other raptors.

  By day, the danger comes from American crows. Crows will spot a roosting owl, gather from considerable distances and caw angrily at it for hours. Owls are, next to red-tailed hawks, perhaps the main predator of crows and their young. When the owl tries to escape, the crows often follow it.

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• 7. Hunting by Night

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  Between 8:30 pm and midnight, hunting by great horned owls tends to peak. It can resume from 4:30 am until sunrise. Winter hunts last longest, because prey is scarcer. Some owls learn to hunt in the afternoon, when certain prey is vulnerable. Examples include eastern fox squirrels as they build their leaf nests, and chuckawallas sunning themselves on desert rocks.

  Most hunting starts from a perch. A snag, a pole or another high spot gives the owl a view. It often flies 50-100 m from perch to perch, stopping each time to survey. When it senses prey below, it dives, often with wings folded. The effective maximum hunting distance from a perch is 90 m.

  The owl also hunts on the wing. It flies low over open ground, quartering slowly, at speeds that can exceed 65 km/h in level flight. It can hover briefly, for about 6-18 seconds, especially in windy places. It sometimes walks after small prey, though it is a poor walker with a pronounced side-to-side gait, like a starling. Now and then it wades into shallow water. It can also snatch birds and tree-dwelling mammals straight from a branch.

  A great horned owl kills mostly by crushing with its feet, or by incidental stabbing from the talons. Some prey is also bitten about the face. Prey is swallowed whole when possible. Some 6-10 hours later, the owl coughs up a pellet of bone and other indigestible parts, usually where it ate. These pellets are dark gray or brown and large, measuring 7.6-10.2 cm long and 3.8 cm thick. Some contain skulls up to 3 cm wide.

  Large prey gets taken apart. The owl may behead it, remove its legs, and crush its bones to make the load more compact. If the kill is still too heavy to carry, the owl may return to eat more later. Nesting owls build up caches of prey, usually in the crotch of a tall tree. In northern regions, where large prey is common, an owl may let uneaten food freeze. It later thaws the food using its own body heat.

  Success depends on the setting. Experiments with five species of rodent showed that open areas gave better hunting success. Cloudy nights and denser bush foliage both lowered it. Researchers who study diet mostly examine pellets under perches and around nests, because they give the fullest picture of what the owl eats.

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• 8. An Enormous Menu

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  More than 500 species have been identified as prey of the great horned owl, the most diverse prey profile of any raptor in the Americas. Mammals account for more than 200 of those species, and birds for nearly 300. As one author put it, "Almost any living creature that walks, crawls, flies, or swims, except the large mammals, is the great horned owl's legitimate prey." In North America, the diet is 87.6% mammals, 6.1% birds and 1.6% reptiles and amphibians. Insects, other invertebrates and fish make up the remaining 4.7%. Prey weights range from 0.4 g to 6.8 kg. A single owl needs about 50-100 g of food per day, and it can live off a large kill for several days.

  Small rodents dominate by number. Deermice weigh 14-31.5 g and voles 20-58 g, yet they are a staple because they swarm the wooded edges the owls frequent. An owl family with two offspring needs roughly a half dozen voles or a dozen mice a night. In snowy regions, mice tend to outnumber voles in the winter diet. The mice travel on top of the snow, while the voles tunnel underneath it. A healthy owl family can decimate a colony of field rats, and owls living near garbage dumps may subsist mostly on rats.

  By weight, hares and rabbits matter most. In Utah, where the owls depend on jackrabbits, average brood size rose from 2 at jackrabbit lows to 3.3 at their peak. At that peak, jackrabbits made up 90.2% of prey biomass. In the Morley Nelson Snake River Birds of Prey National Conservation Area of Idaho, counted rodents outnumbered lagomorphs more than 10 to 1, with 1159 against 114. Even so, jackrabbits and mountain cottontails supplied about half the biomass.

  In the northern boreal forest, owls depend on the snowshoe hare, whose numbers follow a 10-year cycle. At the peak, snowshoe hares made up 90-99% of the winter diet in Alberta. At the low point, they made up only 0-16% of summer diets there. When hares were scarce, owls had to leave or suffer high mortality. In Alberta, the local owl population can triple from the hare lows to the peaks.

  Skunks are the best-known surprise. Because the owls have a poor sense of smell, they are the only predators that routinely attack these bold mammals with impunity. All six North American skunk species are recorded as prey. Full-grown striped skunks can be three times as heavy as the owl that attacks them.

  Some attacks go wrong. Owls with porcupine quills embedded in them have been found, sometimes dead. Yet the size of quills left at bloodied kill sites shows that owls sometimes kill even adult porcupines. Owls have also taken raccoons, bobcats, young or small dogs, and cats.

  Birds come second to mammals, but they outnumber mammals in variety, with more than 250 species killed in North America alone. On Protection Island in Washington state, rhinoceros auklets were the most numerous prey, present in 93% of 120 pellets. In prairie wetlands of North Dakota, birds, mainly ducks and American coots, made up 65% of the diet by number and 83% by biomass. The wild turkey is probably the largest bird whose adults the owl kills. Owls have also killed the nestlings of species as large as trumpeter swans and sandhill cranes.

  Reptiles and amphibians are taken too. More than a dozen snake species are hunted in North America, including venomous cottonmouths and prairie rattlesnakes. The hatchlings of loggerhead turtles and American alligators have been caught as they make their way to water. Insects are mostly eaten by inexperienced young owls, since raising chicks on such small food would be inefficient.

  Even carrion is sometimes eaten, especially in harsh northern winters. A motion-triggered wildlife camera filmed one owl scavenging a white-tailed deer carcass and tearing off a leg. Studies comparing rural and urban owls point to a simple rule: the most abundant rodent in the local environment fills most of the diet.

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