Eurasian eagle-owl (Bubo bubo)
The Eurasian eagle-owl (Bubo bubo) is a species of eagle-owl resident in much of Eurasia. It is sometimes called the European eagle-owl and is, in Europe, where it is the only member of its genus besides the snowy owl (B. scandiacus), occasionally abbreviated to just eagle-owl. In India, it is often called the Indian great horned owl, though this may cause confusion with the similarly named American bird. It is one of the largest species of owl, and females can grow to a total length of 75 centimetres (30 in), with a wingspan of 188 centimetres (74 in), males being slightly smaller. This bird has distinctive ear tufts, the upper parts are mottled black and tawny and the wings and tail are barred. The underparts are buff, streaked with darker colour. The facial disc is poorly developed and the orange eyes are distinctive.
The Eurasian eagle-owl is found in a number of habitats but is mostly a bird of mountain regions, coniferous forests, steppes and remote places. It is a mostly nocturnal predator, hunting for a range of different prey species, predominately small mammals but also birds of varying sizes, reptiles, amphibians, fish, large insects and earthworms. It typically breeds on cliff ledges, in gullies, among rocks or in some other concealed location. The nest is a scrape in which up to six eggs are laid at intervals and which hatch at different times. The female incubates the eggs and broods the young, and the male provides food for her and when they hatch, for the nestlings as well. Continuing parental care for the young is provided by both adults for about five months.
There are about a dozen subspecies of Eurasian eagle-owl. With a total range in Europe and Asia of about 32 million square kilometres (12 million square miles) and a total population estimated to be between 250 thousand and 2.5 million individuals, the IUCN lists the bird’s conservation status as being of “least concern“.
The Eurasian eagle-owl is a very large bird, smaller than the golden eagle (Aquila chrysaetos) but larger than the snowy owl. It is sometimes referred to as the world’s largest owl, although Blakiston’s fish owl (B. blakistoni) is slightly heavier on average and the great grey owl (Strix nebulosa) is slightly longer on average. The Eurasian eagle-owl has a wingspan of 160–188 cm (63–74 in), with the largest specimens attaining 200 cm (79 in). The total length of the species can range from 65 to 75 cm (26 to 30 in). Females weigh 2.27–4.5 kg (5.0–9.9 lb) and males weigh 1.8–3.5 kg (4.0–7.7 lb). In comparison, the barn owl (Tyto alba), the world’s most widely distributed owl species, weighs about 500 grams (1.1 lbs) and the great horned owl (B. virginianus), which fills the Eurasian eagle-owl’s ecological niche in North America, weighs around 1.6 kg (3.5 lbs). Among standard measurements, the tail measures 23–31 cm (9.1–12.2 in) long, the tarsus measures 7.4–8.8 cm (2.9–3.5 in) and the bill is 4.2–5.8 cm (1.7–2.3 in).
Based on the wing chord length (the only measurement taken for many of the less studied subspecies), there is considerable variation across the races, with owls at higher altitudes and more Northern latitudes being the larger varieties. The smallest race is B. b. nikolskii, found in warm, rocky desert-like habitats from eastern Iraq and Iran to Pakistan and Afghanistan and measuring 37.8–46 cm (14.9–18.1 in) in wing chord length. The largest race is B. b. yenisseenis of the icy forests of central Siberia to northern Mongolia at 44.3–51.8 cm (17.4–20.4 in). Many subspecies still require detailed description and study.
The great size, barrel-shaped build and conspicuous ear tufts make this a distinctive owl. In the nominate subspecies, the upper parts are mottled brownish-black and tawny-buff while the wings and tail are barred in similar shades. The facial disc is poorly developed, cream above and tawny-buff below, and incomplete above the eyes, which have orange irises. The beak and the feet are black. The crown of the head is brownish-black, each feather having a buff or cream-coloured edge. The long feathers making up the ear tufts extend about 60 mm (2.4 in) above the rest of the plumage on either side of the crown. The feathers on the back of the neck, the scapulars and the mantle are brownish-black with tawny edges. The back, rump and upper tail coverts are tawny-buff with wavy bars and streaks, and the tail feathers are dark brown with cream, buff and tawny-brown irregular lines.
The chin and throat are white with a brownish central streak. The feathers of the upper breast have brownish-black centres and reddish-brown edges except for the central ones which have white edges. The lower breast and belly feathers are cream to tawny buff with dark barring. The underwing coverts and undertail coverts are similar but more strongly barred in brownish-black. The primaries and secondaries are brown with broad dark brown bars and dark brown tips, and grey or buff irregular lines. A complete moult takes place each year between July and December.
Eurasian eagle-owls are distributed sparsely through rocky areas but can potentially inhabit a wide range of habitats. They have been found in habitats as diverse as Northern coniferous forests and the edge of vast deserts. They are often found in the largest numbers in areas where cliffs and ravines are surrounded by a scattering of trees and bushes. Taiga, rocky coast lines, steppe and grasslands, may also be visited, largely while hunting. Their territories cover on average about 42.5 square kilometres (16.4 sq mi). Due to their preference for rocky habitats, the species is often found in mountainous areas and can be found at elevations of up to 2,000 m (6,600 ft) in Europe and 4,500 m (14,800 ft) in Asia. However, they can also be found at sea level, on islands and even over extensive reed beds. In all, they can hunt between sea level and the snow line. The birds roost by day in rock clefts, ruins, large hollow trees and dense foliage.
Although found in the largest numbers in areas sparsely populated by humans, farmland is include in their habitat types and they have even been observed living in park-like settings within European cities. Since 2005, at least five couples have nested in Helsinki. This is due in part to feral European rabbits (Oryctolagus cuniculus) having recently populated the Helsinki area, originally from pet rabbits released to the wild. In June 2007, a Eurasian eagle-owl nicknamed ‘Bubi’ landed in the crowded Helsinki Olympic Stadium during the European Football Championship qualification match between Finland and Belgium and interrupted the match for six minutes. Finland’s national football team have had the nickname Huuhkajat (Finnish for Eurasian eagle-owls) ever since. The owl was named “Helsinki Citizen of the Year” in December 2007.
The Eurasian eagle-owl is largely nocturnal in activity, as are most owl species. It has a number of vocalizations that are used at different times. The song, which can be heard at great distance, is a deep resonant ooh-hu with emphasis on the first syllable for the male, and a more high-pitched uh-hu for the female. These calls are repeated at intervals of up to a minute. Other calls include a rather faint OO-OO-oo and a harsh kveck-kveck. Annoyance at close quarters is expressed by bill-clicking and cat-like spitting, and a defensive posture involves lowering the head, ruffling the back feathers, fanning the tail and spreading the wings. When perching it adopt an upright stance with plumage closely compressed and may stand tightly beside a tree trunk in a similar fashion to a long-eared owl.
This broad-winged species has a strong direct flight, usually consisting of shallow wing beats and long, fast glides. It has, unusually for an owl, also been known to soar on updrafts on a few occasions. The latter method of flight has led them to be mistaken for Buteos, which are smaller and quite differently proportioned.
The Eurasian eagle-owl can live for up to twenty years in the wild. However, like many other bird species in captivity they can live much longer without having to endure difficult natural conditions, and have possibly survived up to 60 years in zoo collections. Healthy adults normally have no natural predators and are thus considered apex predators, although they can be mobbed by a variety of smaller birds, including smaller hawks and owls. The leading causes of death for this species are man-made: electrocution, traffic accidents and shooting sometimes kill or injure it.
This eagle-owl mainly feeds on small mammals in the 200–2,000 g (0.44–4.41 lb) weight range, such as voles, rats, mice, rabbits and hares. However, prey can be killed up to the size of both fully-grown foxes and marmots and young deer (up to a mass of 17 kg (37 lb)), if taken by surprise. In central Europe, hedgehogs are often a favorite prey item, being eaten after the owl skins off their prickly backs. Eurasian eagle-owls may habitually visit refuse dumps to feed on rats. The other significant group of prey for Eurasian eagle-owls is other birds and almost any type of bird may fall victim. Common avian prey includes corvids, grouse, woodpeckers, herons and, especially near coastal areas, ducks, seabirds and geese. Other raptors, including large species such as northern goshawks (Accipiter gentilis), peregrine falcons (Falco peregrinus) and the largest buzzards, are regularly attacked as also are any other species of owl encountered. When there is an opportunity, they will also prey on reptiles, including large and venomous snakes, frogs, fish and even large insects and earthworms.
The hunting method usually used by this owl is to watch from a perch for animal movement and then to swoop down swiftly once prey has been spotted. The prey is often killed quickly by the Eurasian eagle-owl’s powerful talons though alternatively it may be bitten on the head. Then the victim is carried off to be swallowed whole or torn into pieces with the bill. Occasionally, this owl may capture other birds on the wing, including nocturnal migrants which are intercepted in mid-flight. Larger prey (over 3.5 kg (7.7 lb)) is consumed on the ground which leaves the owl vulnerable to loss of their prey or even attack by predators such as foxes. The dietary preferences of the species frequently overlap with the larger golden eagle but direct competition is uncommon due to differing times of activity between the species.
Courtship in the Eurasian eagle-owl may involve bouts of “duetting”, with the male sitting upright and the female bowing as she calls. There may be mutual bowing, billing and fondling before the female flies to a perch where coitus occurs, usually taking place several times over the course of a few minutes. This owl usually nests on a cliff ledge, in a crevice, a gully or cave, but sometimes nests on the ground, between rocks or in some other concealed location. It often uses the same nest site year after year. Occasionally, it may take over a nest made by a large bird such as a common raven (Corvus corax) or golden eagle. Laying generally begins in late winter but may be later in the year in colder habitats. A single clutch of up to six white eggs is laid, each egg measuring 56–73mm x 44.2–53mm (2.2–2.9″ x 1.7–2.1″) and weighing 75–80 g (2.6–2.8 oz). The eggs are normally laid at intervals of three days and are incubated only by the female. The first egg hatches after 31 to 36 days. During the incubation period, the female is brought food at the nest by her mate.
The female continues to brood the young which are of different ages as a result of the eggs hatching successively. The male continues to bring prey and the female feeds the nestlings, tearing up the food into suitably-sized pieces. The chicks grow rapidly, being able to consume small prey whole after a few weeks. The female resumes hunting after about three weeks which increases the food supply to the chicks. These can walk around at five weeks and by seven weeks are taking short flights. Both parents continue to care for them for about five months, and in Europe the young disperse or are driven from their parents’ territory in the autumn. They reach sexual maturity by the following year, but do not normally breed until they can establish a territory at around two or three years old.
Feathers are lightweight and robust but nevertheless need to be replaced periodically as they become worn. In the Eurasian eagle-owl this happens in stages and the first moult starts the year after hatching with some body feathers and wing coverts being replaced. The next year the three central secondaries on each wing and three middle tail feathers are shed and regrow, and the following year two or three primaries and their coverts are lost. In the final year of this post-juvenile moult, the remaining primaries are moulted and all the juvenile feathers will have been replaced. Another moult takes place during years six to twelve of the bird’s life. This happens between June and October after the conclusion of the breeding season and again it is a staged process with six to nine main flight feathers being replaced each year. Such a moulting pattern lasting several years is repeated throughout the bird’s life.
The Eurasian eagle-owl was first described by the Swedish naturalist Carl Linnaeus in his 10th edition of Systema Naturae in 1758. It is a member of the horned owl genus Bubo which includes the eagle-owls, the horned owls, the fish owls and the snowy owl. About a dozen subspecies are recognised, but the exact number is unclear and they tend to intergrade where their ranges overlap. Two owls formerly considered subspecies of the Eurasian eagle-owl are now recognized as distinct species: the pharaoh eagle-owl (B. ascalaphus) and the rock eagle owl (B. bengalensis).