The Eurasian eagle-owl (Bubo bubo) is a species of eagle-owl that resides in much of Eurasia. It is also called the European eagle-owl and in Europe, it is occasionally abbreviated to just eagle-owl. It is one of the largest species of owl, and females can grow to a total length of 75 cm (30 in), with a wingspan of 188 cm (6 ft 2 in), males being slightly smaller. This bird has distinctive ear tufts, with upper parts that are mottled with darker blackish colouring and tawny. The wings and tail are barred. The underparts are a variably hued buff, streaked with darker colour. The facial disc is poorly developed and the orange eyes are distinctive.
Besides being one of the largest living species of owl, it is also one of the most widely distributed. The Eurasian eagle-owl is found in many habitats but is mostly a bird of mountain regions, coniferous forests, steppes and other relatively remote places. It is a mostly nocturnal predator, hunting for a range of different prey species, predominantly small mammals but also birds of varying sizes, reptiles, amphibians, fish, large insects and other assorted invertebrates. It typically breeds on cliff ledges, in gullies, among rocks or in other concealed locations. The nest is a scrape in which averages of two eggs are laid at intervals. These 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 at least 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, the IUCN lists the bird's conservation status as being of "least concern". The vast majority of eagle-owls live in mainland Europe, Russia and Central Asia, and an estimated number of between 12 and 40 pairs are thought to reside in the United Kingdom as of 2016, a number which may be on the rise. Tame eagle-owls have occasionally been used in pest control because of their size to deter large birds such as gulls from nesting.
The Eurasian eagle-owl is a very large bird, smaller than the golden eagle (Aquila chrysaetos) but larger than the snowy owl, despite some overlap in size with both species. 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 much lighter weight great grey owl (Strix nebulosa) is slightly longer on average. Heimo Mikkola reported the largest specimens of eagle-owl as having the same upper body mass, 4.6 kg (10 lb), as the largest Blakiston’s fish owl and attained a length of around 3 cm (1.2 in) longer. In terms of average weight and wing size, the Blakiston’s is the slightly larger species seemingly, even averaging a bit larger in these aspects than the biggest eagle-owl races from Russia. Also, although 9 cm (3.5 in) shorter than the largest of the latter species, the Eurasian eagle-owl can weigh well more than twice as much as the largest great grey owl. The Eurasian eagle-owl typically has a wingspan of 131–188 cm (4 ft 4 in–6 ft 2 in), with the largest specimens possibly attaining 200 cm (6 ft 7 in). The total length of the species can vary from 56 to 75 cm (22 to 30 in). Females can weigh from 1.75 to 4.6 kg (3.9 to 10.1 lb) and males can weigh from 1.22 to 3.2 kg (2.7 to 7.1 lb). In comparison, the barn owl (Tyto alba), the world's most widely distributed owl species, weighs about 500 g (1.1 lb) and the great horned owl (Bubo virginianus), which fills the eagle-owl's ecological niche in North America, weighs around 1.4 kg (3.1 lb).
Besides the female being larger, there is little external sexual dimorphism in the Eurasian eagle-owl although the ear tufts of males reportedly tend to be more upright than those of females. When an eagle-owl is seen on its own in the field, it is generally not possible to distinguish the individual’s sex. Gender determination by size is possible via in hand measurements. Reportedly, in some populations the female may be slightly darker on average than the male. The plumage coloration across at least 13 accepted subspecies can be highly variable. The upper parts may be brown-black to tawny-buff to pale creamy gray, typically showing dense freckling on the forehead and crown, stripes on the nape, sides and back of the neck, and dark splotches on the pale ground colour of the back, mantle and scapulars. A narrow buff band, freckled with brown or buff, often runs up from the base of the bill, above the inner part of the eye and along the inner edge of the black-brown ear tufts. The rump and upper tail-coverts are delicately patterned with dark vermiculations and fine wavy barring, the extent of which varies with subspecies. The underwing coverts and undertail coverts are similar but tend to be 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. The facial disc is tawny-buff, speckled with black-brown, so densely on the outer edge of the disc as to form a "frame" around the face. The chin and throat are white with a brownish central streak. The feathers of the upper breast generally have brownish-black centres and reddish-brown edges except for the central ones which have white edges. The chin and throat may appear white continuing down the center of the upper breast. The lower breast and belly feathers are creamy-brown to tawny buff to off-white with a variable amount of fine dark wavy barring, on a tawny-buff ground colour. The legs and feet (which are feathered almost to the talons) are likewise marked on a buff ground colour but more faintly. The tail is tawny-buff, mottled dark grey-brown with about six black-brown bars. The bill and feet are black. The iris is most often orange but is fairly variable. In some European birds, the iris is a bright reddish, blood-orange colour but then in subspecies found in arid, desert-like habitats, the iris can range into an orange-yellow colour (most closely related species generally have yellowish irises, excluding the Indian eagle-owl).