Eurasian Eagle-owl

The Eurasian eagle-owl (Bubo bubo) is a species of eagle-owl, a type of bird that resides much over Eurasia. It is often just called the eagle-owl in Europe and Asia.

It is one of the largest species of owl. Females can grow to a total length of 75 cm (30 in), with a wingspan of 188 centimetres (6 feet 2 inches). Males are 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 colouring. The facial disc is not very defined. The orange eyes are distinctive. At least 12 subspecies of the Eurasian eagle-owl are described.

Eurasian eagle-owls are found in many habitats; mostly mountainous and rocky areas, often near varied woodland edge and near shrubby areas with openings or wetlands. They also inhabit coniferous forests, steppes, and remote areas. Occasionally, they are found in farmland and in park-like settings in European and Asian cities and, very rarely, in busier urban areas.

The eagle-owl is mostly a nocturnal predator. Predominantly, they hunt small mammals, such as rodents and rabbits, but also birds and larger mammals. Secondary prey include reptiles, amphibians, fish, large insects, and invertebrates.

The species typically breeds on cliff ledges, in gullies, among rocks, and in other concealed locations. The nest is a scrape containing a clutch of 2–4 eggs typically, which are laid at intervals and hatch at different times. The female incubates the eggs and broods the young. The male brings food for her and for the nestlings. Continuing parental care for the young is provided by both adults for about five months.

In addition to being one of the largest living species of owl, the Eurasian eagle-owl is also one of the most widely distributed. With a total range in Europe and Asia of about 51.4 million km2 (19.8 million sq mi) and a total population estimated to be between 100,000 and 500,000 individuals, the IUCN lists the bird's conservation status as being of least concern, although the trend is listed as decreasing. The vast majority of eagle-owls live in Continental Europe, Scandinavia, Russia (which is almost certainly where the peak numbers and diversity of race occurs), and Central Asia. Additional minor populations exist in Anatolia, the northern Middle East, the montane upper part of South Asia, China, Korea and in Japan; in addition, an estimated 12 to 40 pairs are thought to reside in the United Kingdom as of 2016 (where they are arguably non-native), a number which may be on the rise, and have successfully bred in the UK since at least 1996. Tame eagle-owls have occasionally been used in pest control because of their size to deter large birds such as gulls from nesting.

Description

The Eurasian eagle-owl is among the larger birds of prey, smaller than the golden eagle (Aquila chrysaetos), but larger than the snowy owl (Bubo scandiacus), despite some overlap in size with both of those species. It is sometimes referred to as the world's largest owl, although Blakiston's fish owl (K. 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 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 2 m (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 1.2 to 3.2 kg (2.6 to 7.1 lb). In comparison, the barn owl (Tyto alba), the world's most widely distributed owl species, weighs about 0.5 kg (1.1 lb) and the great horned owl (B. 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, little external sexual dimorphism is seen 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, distinguishing the individual's sex is generally not possible. Sex determination by size is possible by in-hand measurements. In some populations, the female typically may be slightly darker 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).

Standard measurements and physiology

Among standard measurements for the Eurasian eagle-owl, the wing chord measures 378 to 518 mm (14.9 to 20.4 in), the tail measures 229–310 mm (9.0–12.2 in) long, the tarsus measures 64.5–112 mm (2.54–4.41 in), and the total length of the bill is 38.9–59 mm (1.53–2.32 in). The wings are reportedly the smallest in proportion to the body weight of any European owl, when measured by the weight per area of wing size, was found to be 0.72 g/cm2. Thus, they have quite high wing loading. The great horned owl has even smaller wings (0.8 g/cm2) relative to its body size. The golden eagle has slightly lower wing loading proportionately (0.65 g/cm2), so the aerial abilities of the two species (beyond the eagle's spectacular ability to stoop) may not be as disparate as expected. Some other owls, such as barn owls, short-eared owls (Asio flammeus), and even the related snowy owls have lower wing loading relative to their size, so are presumably able to fly faster, with more agility, and for more extended periods than the Eurasian eagle-owl. In the relatively small race B. b. hispanus, the middle claw, the largest talon, (as opposed to rear hallux-claw, which is the largest in accipitrids) was found to measure from 21.6 to 40.1 mm (0.85 to 1.58 in) in length. A 3.82 kg (8.4 lb) female examined in Britain (origins unspecified) had a middle claw measuring 57.9 mm (2.28 in), on par in length with a large female golden eagle hallux-claw. Generally, owls do not have talons as proportionately large as those of accipitrids, but have stronger, more robust feet relative to their size. Accipitrids use their talons to inflict organ damage and blood loss, whereas typical owls use their feet to constrict their prey to death, the talons serving only to hold the prey in place or provide incidental damage. The talons of the Eurasian eagle-owl are very large and not often exceeded in size by diurnal raptors. Unlike the great horned owls, the overall foot size and strength of the Eurasian eagle-owl is not known to have been tested, but the considerably smaller horned owl has one of the strongest grips ever measured in a bird.

The feathers of the ear tufts in Spanish birds (when not damaged) were found to measure from 63.3 to 86.6 mm (2.49 to 3.41 in). The ear openings (covered in feathers as in all birds) are relatively uncomplicated for an owl, but are also large, being larger on the right than on the left as in most owls, and proportionately larger than those of the great horned owl. In the female, the ear opening averages 31.7 mm (1.25 in) on the right and 27.4 mm (1.08 in) on the left, and in males, averages 26.8 mm (1.06 in) on the right and 24.4 mm (0.96 in) on the left. The depth of the facial disc and the size and complexity of the ear opening are directly correlated to the importance of sound in an owl's hunting behaviour. Examples of owls with more complicated ear structures and deeper facial disc are barn owls, long-eared owls (Asio otus), and boreal owls (Aegolius funereus). Given the uncomplicated structure of their ear openings and relatively shallow, undefined facial discs, hunting by ear is secondary to hunting by sight in eagle-owls; this seems to be true for Bubo in general. More sound-based hunters such as the aforementioned species likely focus their hunting activity in more complete darkness. Also, owls with white throat patches such as the Eurasian eagle-owl are more likely to be active in low-light conditions in the hours before and after sunrise and sunset rather than the darkest times in the middle of the night. The boreal and barn owls, to extend these examples, lack obvious visual cues such as white throat patches (puffed up in displaying eagle-owls), again indicative of primary activity being in darker periods.

Distinguishing from other species

The great size, bulky, barrel-shaped build, erect ear tufts, and orange eyes render this as a distinctive species. Other than general morphology, the above features differ markedly from those of two of the next largest subarctic owl species in Europe and western Asia, which are the great grey owl and the greyish to chocolate-brown Ural owl (Strix uralensis), both of which have no ear tufts and have a distinctly rounded head, rather than the blocky shape of the eagle-owl's head. The snowy owl is obviously distinctive from most eagle-owls, but during winter the palest Eurasian eagle-owl race (B. b. sibiricus) can appear off-white. Nevertheless, the latter is still distinctively an ear-tufted Eurasian eagle-owl and lacks the pure white background colour and variable blackish spotting of the slightly smaller species (which has relatively tiny, vestigial ear tufts that have only been observed to have flared on rare occasions).