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.Andrews, P. (1990). Owls, caves and fossils: predation, preservation and accumulation of small mammal bones in caves, with an analysis of the Pleistocene cave faunas from Westbury-sub-Mendip, Somerset, UK. University of Chicago Press.
It is one of the largest species of owl. Females can grow to a total length of , with a wingspan of . Males are slightly smaller.Owls of the World: A Photographic Guide by Mikkola, H. Firefly Books (2012), 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.Penteriani, V., & del Mar Delgado, M. (2019). The eagle owl. Bloomsbury Publishing. 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.Obuch, J., & Karaska, D. (2010). The Eurasian eagle-owl (Bubo bubo) diet in the Orava Region (N Slovakia). Raptor Journal, 4(1), 83–98.
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.Voous, K.H. (1988). Owls of the Northern Hemisphere. The MIT Press, .
In addition to being one of the largest living species of owl, the Eurasian eagle-owl is also one of the most widely distributed.Hume, R. (1991). Owls of the world. Running Press, Philadelphia. With a total range in Europe and Asia of about 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,Mikkola, H. (2017). The Eurasian Eagle Owl is the largest living owl species! Tyto, 12(1): 15–18. 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, , as the largest Blakiston's fish owl and attained a length around 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 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.Bull, Evelyn L. and James R. Duncan. (1993). "Great Gray Owl (Strix nebulosa)", The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology. The Eurasian eagle-owl typically has a wingspan of , with the largest specimens possibly attaining .Schuchmann (1999). "Eurasian Eagle Owl (Bubo bubo)", pp. 186 in: del Hoyo, Elliott & Sargatal, eds (1999). Handbook of the Birds of the World. Vol. 5. Lynx Edicions, Barcelona. The total length of the species can vary from .Grimmett, R., Roberts, T. J., Inskipp, T., & Byers, C. (2008). Birds of Pakistan. A&C Black. Females can weigh from , and males can weigh from . In comparison, the barn owl (Tyto alba), the world's most widely distributed owl species, weighs about and the great horned owl (B. virginianus), which fills the eagle-owl's ecological niche in North America, weighs around .Dunning, John B. Jr. (Ed.) (1992). CRC Handbook of Avian Body Masses. CRC Press, .
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 , the tail measures long, the tarsus measures , and the total length of the bill is . 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 in length. A female examined in Britain (origins unspecified) had a middle claw measuring , on par in length with a large female golden eagle hallux-claw.MacGillivray, W. (1836). Descriptions of the Rapacious Birds of Great Britain.[With plates.]. Maclachlan & Stewart. 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.Ward, A. B., Weigl, P. D., & Conroy, R. M. (2002). Functional morphology of raptor hindlimbs: implications for resource partitioning. The Auk, 119(4), 1052–1063. 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.