Eurasian eagle-owl

• 1. Introduction

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  The Eurasian eagle-owl, known by science as Bubo bubo, is an animal large enough to interrupt a football match. In June 2007, one landed on the pitch of the Helsinki Olympic Stadium during a European Football Championship qualification game between Finland and Belgium. The match was interrupted for six minutes. When the bird finally left, it was after Finland had scored their first goal, and the Finnish national football team has carried the nickname Huuhkajat, the Finnish word for Eurasian eagle-owls, ever since. The bird itself was later named Helsinki Citizen of the Year.

  What kind of creature commands that kind of attention? An owl with a wingspan that can reach 188 cm, orange eyes that glow in low light, and a voice so deep and resonant it can be heard across vast distances. A predator that hunts in the dark across a territory spanning roughly 51.4 million square kilometres of Eurasia. An animal first formally described by the Swedish naturalist Carl Linnaeus in 1758, and one whose evolutionary line eventually gave rise to some of the most recognizable birds on the planet, including the great horned owl of North America.

  How does an owl this large hunt, and what does it eat? Why does its range extend from Helsinki rooftops to Himalayan passes at 4,500 metres? And what does the genetic story of Bubo bubo reveal about the deep history of owls across the entire planet?

• 2. Size and Form

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  Females can weigh up to 4.6 kg and measure 75 cm from bill to tail. Males are noticeably smaller, typically weighing between 1.2 and 3.2 kg, and the size difference is among the most pronounced of any large bird. Researcher Heimo Mikkola recorded that the largest eagle-owl specimens reach the same upper body mass as the largest Blakiston's fish owl, and attain a length around 3 cm longer than that species.

  The wingspan of 131 to 188 cm places this bird among the largest flying predators in Eurasia. Its wings are, paradoxically, the smallest in proportion to body weight of any European owl, with a wing loading measured at 0.72 g per square centimetre. This means the eagle-owl must work harder in flight than many smaller owls. Barn owls and snowy owls, with lower wing loading, can fly faster, with more agility, and for longer stretches. The eagle-owl compensates with brute force and stealth rather than aerial dexterity.

  The plumage shifts across the species, but some features are consistent: ear tufts that stand erect on the head, upper parts mottled in tawny and dark blackish colouring, and those immediately recognizable orange eyes. In some European birds, the iris deepens to a bright reddish, blood-orange, while in subspecies from arid, desert-like habitats, it can tip into an orange-yellow. The facial disc, which in some owls forms a deep concave dish to funnel sound, is relatively shallow and poorly defined in the eagle-owl. This is not a bird that hunts primarily by hearing.

  The talons tell a different story. In the relatively small Spanish subspecies B. b. hispanus, the middle claw was found to measure from 21.6 to 40.1 mm. A 3.82 kg female examined in Britain had a middle claw measuring 57.9 mm, comparable in length to the hallux-claw of a large female golden eagle. Unlike the accipitrids, which use their talons to inflict organ damage and blood loss, the eagle-owl constricts its prey to death; the talons are primarily for holding, not cutting.

• 3. The World It Inhabits

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  Rocky terrain is the essential ingredient. Studies in most known populations have confirmed a preference for irregular topography, steep slopes, cliffs, gullies, and ravines. The benefit is practical: a nest wedged into a cliff ledge is nearly inaccessible to ground predators, including humans. The eagle-owl can be found up to 2,100 metres in the Alps, 4,500 metres in the Himalayas, and 4,700 metres on the adjacent Tibetan Plateau.

  Wet areas also attract them, not because of water itself but because the soft soil near rivers and wetlands encourages burrowing by voles and rabbits, the small terrestrial mammals that form the core of their diet. A study in Spain found that only 2.7% of the territorial range of eagle-owls consisted of cultivated or agricultural land, and farmland was only regularly used where it was less intensively farmed and held fallow or abandoned fields with more prey.

  In the Italian Alps, where almost no pristine habitat remained, eagle-owls nested near towns, villages, and ski resorts. Since 2005, at least five pairs have nested in Helsinki, drawn in part by feral European rabbits that had established themselves in the city after pet rabbits were released into the wild. In 2020, a brood of three chicks was raised by their mother on a large, well-foliaged planter on an apartment window in the city centre of Geel, Belgium.

  At the far extreme, the western Siberian subspecies B. b. sibiricus holds the most northerly distribution known in the species, reaching into the Ural Mountains and areas north to the limits of the taiga. In the Russian Far East, the Yakutian subspecies B. b. jakutensis ranges through northeastern Siberia, with reports placing some birds as far as the upper Kolyma River. The total range in Europe and Asia covers about 51.4 million square kilometres.

• 4. Voice and Territory

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  At 250 to 350 Hz, the territorial song of the Eurasian eagle-owl is deeper and considered more impressive than the call of the great horned owl or even the slightly larger Blakiston's fish owl. The male produces a deep resonant ooh-hu, with emphasis on the first syllable; the female answers with a more high-pitched uh-hu. The widely used German name for the bird, uhu, is drawn directly from this call.

  Song posts are carefully chosen: rocky pinnacles, stark ridges, and mountain peaks that sit along the outer edges of the eagle-owl's territory. The height of these posts is not incidental. Broadcasting from a prominent elevation allows the call to carry farther, reducing the need for dangerous physical encounters at territorial borders. A territory averages 15 to 80 km2, similar to or occasionally slightly larger than that of the great horned owl.

  Vocal activity is almost entirely confined to the colder months, from late autumn through winter. A Spanish study found that calls begin no sooner than 29 minutes after sunset and end no later than 55 minutes before sunrise. From October to December, the purpose is territorial. From January to February, the voice shifts to courtship.

  The white throat patch is nearly as important as sound in territorial signalling. When taxidermied specimens with flared white throats were placed around the perimeter of eagle-owl territories, males reacted strongly and often physically attacked the mounted bird. A specimen with an un-flared throat provoked a milder response. Females did not vary significantly in their reaction between the two conditions. In addition to the deep hooting call, the species produces a faint laughter-like OO-OO-oo, a harsh kveck-kveck, and, when confronted with intruders, an extremely loud and reportedly terrifying hooo.

• 5. Nesting and Offspring

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  The male selects breeding sites and presents them to the female by flying to each location, kneading a small depression in any soil present, and producing staccato notes and clucking noises. Several sites may be offered; the female makes the final choice. In Baden-Württemberg, Germany, the time the male spent at potential nest sites increased over the pre-laying period from a mean of 29 minutes per visit to as long as 3 hours.

  Eurasian eagle-owls do not build nests or add material. They lay directly onto whatever surface the chosen location provides: a cliff ledge, a rock crevice, the ground between boulders, or among the roots of trees. In the largely forested Altai Krai region of Russia, all recorded nests were found on the ground, usually at the base of pines. The species occasionally adopts nests built by other large birds, including common buzzards, golden eagles, common ravens, and black storks, though this is less common than in the great horned owl, which regularly prefers pre-built nests.

  A typical clutch holds 2 to 4 white eggs, each measuring from 56 to 73 mm in length by 44.2 to 53 mm in width and weighing roughly 75 to 80 g. They are laid at intervals of three days and hatch at different times. Only the female incubates; the male brings food to her and the nestlings. In Engadin, Switzerland, the male hunts alone until the young are 4 to 5 weeks old, at which point the female gradually resumes hunting for both herself and the brood.

  Parental care by both adults continues for about five months. There is one verified case of polygamy in Germany, where a male mated with two females simultaneously, and a documented case of cooperative brooding in Spain, where a third adult of undetermined sex helped a breeding pair care for their chicks.

• 6. An Evolutionary Family

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  Carl Linnaeus formally described the Eurasian eagle-owl in 1758, placing it in the tenth edition of his Systema Naturae under the binomial Strix bubo, specifying the habitat as Europa. The type locality was later restricted to Sweden. André Duméril introduced the genus Bubo in 1805, and the species was moved there, joining a genus that now includes roughly 10 extant species among the larger owls of the world.

  Recent genetic research has clarified old taxonomic boundaries significantly. Eight former members of the genus Bubo were found to belong instead in the related genus Ketupa, which contains the fish owls. The snowy owl appears to have separated from other Bubo species at least 4 million years ago. Genetic analysis has also revealed that the snowy owl is more closely related to the great horned owl than to the Eurasian eagle-owl.

  The great horned owl of North America is considered one of the most famous derivations of the Eurasian eagle-owl's evolutionary line, apparently the result of primitive eagle-owls spreading into the Americas. Despite this shared lineage, the two species differ in notable ways: the Eurasian eagle-owl averages at least 15 to 30% larger in linear dimensions and 30 to 50% larger in body mass. The great horned owl has yellow rather than orange eyes, horizontal rather than vertical underside barring, and a much stronger black bracket around the facial disc.

  The Pharaoh eagle-owl, found in the Arabian Peninsula and rocky outcrops of the Sahara, was until recently considered a subspecies of the Eurasian eagle-owl. Genetic testing placed it at approximately 3.8% divergence in mitochondrial DNA from the Eurasian species, well past the 1.5% minimum typically used to differentiate species. Fossils from southern France have shown that during the Middle Pleistocene, eagle-owls of the paleosubspecies B. b. davidi were even larger than today's birds, and specimens from Azerbaijan and the Caucasus, dated to the Late Pleistocene, were larger still.