Great horned owl

Name and taxonomy

Johann Friedrich Gmelin described the great horned owl in 1788 and placed it in the genus Strix. He based his account on a bird from Virginia that George Edwards had drawn in 1747. The species moved to the genus Bubo, which André Duméril set up in 1805. The species name virginianus appears to record that first bird from Virginia.

Early naturalists called the bird the winged tiger, and the name tiger owl came from that. Country people in North America often call it the hoot owl. In Spanish it is the buho cornudo, and in Portuguese the jacurutu. More than 20 subspecies have been named, and 15 are now accepted. The rest were only local shades of colour, not true subspecies.

Evolution and relatives

The genus Bubo reached the Americas across the Bering land bridge from Asia. Genetic work puts the lesser horned owl of southern South America closest to this bird. The snowy owl is the next closest living relative. The Eurasian eagle-owl looks similar and fills the same role in Eurasia, but it sits in a separate clade.

Fossil Bubo owls from the Pleistocene turn up across North America, mostly in and west of the Rockies. Nearly all of those birds were larger than the great horned owls alive today. The lesser horned owl was once treated as a subspecies of this bird. Genetic evidence now separates the two, and the great horned owl is the parent species. Dudley Zoo in England once raised a healthy hybrid of this bird and a Eurasian eagle-owl.

Appearance

The plumage is mottled brown and grey above, and pale below with dark horizontal bars. A white patch sits on the throat and can run down the breast as a pale streak. The round facial disc is brown, grey or reddish, with a dark rim and heavy black brackets at the sides. The eyes are yellow in almost every population, and amber in the South American birds. The bill and talons are a dark gunmetal grey.

This is a bulky owl with a barrel-shaped body, a big head and broad wings. A male averages about 1,224 grams and a female about 1,608 g. The heaviest birds recorded reach 2,503 g. Even tropical birds have feathered legs and feet, and only the snowy owl has longer foot feathers. A fully spread foot spans about 20 centimetres from talon to talon.

Habitat and range

The breeding range runs north into the subarctic of Canada and Alaska. It continues south through the United States and Mexico, thinly through Central America, and into South America. In the far south the lesser horned owl takes over and reaches Tierra del Fuego. The bird is missing from the West Indies and from almost every offshore island in the Americas. It is the second most widely spread owl of the Americas, after the barn owl.

Few birds accept as many habitats as this owl does. It lives in deciduous, coniferous and mixed woodland, in prairie, in desert margins, in mangrove and on rocky coasts. It prefers a patchwork of open ground to hunt over and woodland to roost and nest in. Thinly settled farmland therefore appears to suit it well. It is rare in unbroken old-growth forest and has been recorded only a handful of times in true rainforest.

In North American mountains it usually stops at the tree line. It reaches 3,300 metres in the Rockies, and the Andean birds live higher still. In Ecuador and Peru they are regular on treeless puna grassland at 4,100 to 4,500 m. Snowy owls replace them on the high Arctic tundra. Some birds live in city parks, though they avoid the busiest suburbs.

Diet and hunting

In North America mammals make up 87.6 per cent of the diet by number. Birds add 6.1 per cent, and reptiles, amphibians, fish and invertebrates make up the rest. Small rodents are the commonest prey, above all deermice and voles. Rabbits and hares supply most of the weight of food across the continent. Prey has ranged from a 2 gram shrew to animals of 6.8 kilograms.

The owl is one of the few predators that attacks skunks, because its sense of smell is poor. All six North American skunk species appear in its diet. The remains of 57 striped skunks were counted at a single nest. A nest may therefore smell of skunk long after the meal. It also kills more kinds of bird of prey than any other American predator.

Hunting peaks between about half past eight in the evening and midnight. The owl watches from a snag, pole or rock, then drops with its wings folded. The useful range of such a dive is about 90 metres. In level flight it can pass 65 kilometres per hour. Its feet crush prey with a force of at least 2,100 kilopascals. Small animals go down whole, and the bones return hours later in a large dark pellet.

Behaviour and social life

The great horned owl is active at night and roosts by day in a large tree. Dense conifers are favoured, because they give cover in every season. A roosting bird may sit bolt upright and pull itself thin to look like a broken branch. Males often keep a favourite roost near the nest and use it for years. Outside the breeding season a bird simply roosts wherever dawn finds it.

Pairs hold a territory all year and rarely leave it. Territory size ranges from an average of 16 square kilometres in the Yukon to 2.1 sq km in Wyoming. Borders are set by hooting, and most of that work falls to the male. A dispute that is not settled by voice can turn into a fight with feet and claws. Birds that cannot win ground live quietly as floaters along the edges of held territories.

Crows find roosting owls by day and mob them for hours. Dozens or even hundreds of crows may join in. Hawks, falcons, ravens and jays dive at the owl as well. A mobbed bird usually flies to the nearest hidden perch. If it is caught in the open it raises its wings and threatens the mob.

Communication and senses

The song is a low run of four or five hoots, deep and carrying. The female answers in a higher voice that lifts at the end. Her voice is higher because the larger sex has the smaller syrinx. Hooting peaks after midnight and drops away once the eggs are laid. Angry birds add growls, screeches, bill-clapping and a cat-like wail.

The ear openings lie hidden under feathers and sit at slightly different heights. That difference lets the owl time a sound at each ear and fix its source. It can find a vole moving under snow. The dish of the face gathers sound and steers it towards the ears. Its hearing is thought to be about ten times sharper than a human's.

The eyes are nearly as big as human eyes and rank among the largest of any land vertebrate. They are cylindrical rather than round, which lengthens the distance from lens to retina. Colour vision is weak, but night vision is excellent. The eyes cannot swivel, so the bird turns its neck instead, through 270 degrees. Comb-like edges on the flight feathers break up the air and let the owl fly in near silence.

Reproduction and life cycle

No other North American bird starts its nest this early in the year. Courtship runs from October to December, and pairs are usually settled by January. The male hoots with his white throat puffed out like a ball in the low light. He brings the female fresh prey and bows to her again and again. Pairs often stay together for many years.

No owl builds its own nest, and this one is no exception. It takes over the old stick nest of a hawk, eagle, crow or heron, often 4.5 to 22 metres up. Tree hollows, cliff ledges, cactus cavities and bare ground are all used as well. Urban pairs favour tall ornamental trees and nest higher than country pairs do. In a poor year for prey a pair may skip breeding altogether.

A clutch is usually 2 eggs, and the full range is 1 to 6. An egg is about 55 millimetres long, 46.5 mm wide and 51 grams in weight. Incubation takes 28 to 37 days and averages 33, with the female sitting and the male hunting. Owlets hatch at about 34.7 g and gain roughly 33 g a day for four weeks. They climb onto nearby branches at six weeks and fly a week later. Full flying skill comes at 10 to 12 weeks, and breeding starts at two years old.

Predators and threats

Adults have almost no natural predators. The exceptions are eagles and other great horned owls, which kill rivals in territory fights. Dead owls are sometimes eaten by the owl that killed them. Eggs and small owlets are taken by crows, ravens, raccoons and black bears when the parents are driven off. Owlets that fall early are caught by foxes, bobcats, coyotes and cats.

Prey can hurt the hunter. Five owls were found blind after skunks sprayed them in the eyes. Porcupine quills have disabled and killed others. Blackflies spread a blood parasite that causes anaemia and kills many young birds in the Yukon. More than half of young owls die during their first autumn away from the nest.

Population and conservation

The IUCN lists the great horned owl as Least Concern. Counting the lesser horned owl, about 5.3 million horned owls live in the Americas. Audubon puts the great horned owl alone at about 5.7 million birds. The picture is different in places, and a French Guiana list rates the species Endangered there. NatureServe ranks it as vulnerable in Vermont and secure across Canada.

Most deaths now come from people rather than from nature. Cars, wires, fences and buildings kill the largest share. Rat poison passes up the food chain and reaches owls that live on city rats. The owl also presses rare species, and it slowed the return of peregrine falcons and ospreys after DDT. Great horned owls hunt 50 species on the American Bird Conservancy's list of birds in decline.

Relationship with humans

Farmers once treated the owl as vermin because it killed poultry and game birds. Small bounties were paid for dead birds, and even early conservation campaigns left this species out. Michigan counted it as endangered around 1900 after heavy illegal collecting. Killing and trapping the owl is now illegal in most countries. Chickens shut in a covered coop at night are safe from it.

The owl also works for farmers by taking rats, mice and pest insects. It is easy to find and popular with birdwatchers, with 92,318 observations logged on iNaturalist. Alberta made it the provincial bird. Captive birds resent handling and often attack their keepers once grown. They are seldom trained for falconry with the success seen in hawks and falcons.

In culture and myth

Many warrior nations of North America admired the owl for its strength and courage. The Pima held that owls carried the spirits of warriors killed in battle. Arikara owl societies gave new members a face mask made from the wing and tail feathers of this bird. The Passamaquoddy of Maine heard its call as a love flute that could stir human passion. A Passamaquoddy story also makes the owl a deceiver who chases a woman by trickery.

The Hopi linked the owl to summer heat and to a good peach crop. At the winter solstice they used its feathers in a ceremony to call the summer back. Nations in New Mexico fletched arrows with owl feathers so the shot would make less noise. Zuni fighters held owl feathers in the mouth to borrow the bird's silence. In an Iroquois story the creator Raweno covered a pestering owl in mud and left it hooting by night.

Owl traditions outside the Americas appear just as old. The owl of Athena stands for wisdom in Greek tradition. Welsh myth changes Blodeuedd into an owl to punish her betrayal. Hindu tradition seats Lakshmi on an owl, yet still reads the bird as bad luck. In several Native American traditions an owl heard at night warns of a coming death.