Dietary biology of the Eurasian eagle-owl
In the Iberian peninsula, the Eurasian eagle-owl is part of a complex guild of predators that have evolved to survive mostly on European rabbits. Among this guild, golden eagles ("Aquila chrysaetos") were recorded per one study to rely on rabbits for 40% of the diet, the eagle-owl for 49% of the diet, the Spanish imperial eagle ("Aquila adalberti") for 50% of the diet, the Bonelli's eagle ("Aquila fasciata") for 61% of the diet and the Iberian lynx ("Lynx pardinus") for 79% of the diet. Elsewhere, the Spanish imperial eagle and eagle-owl are considered to outflank the golden and even the Bonelli's eagle as the most specialized avian predator of rabbits in the Iberian region. Other predators, such as common buzzard ("Buteo buteo"), northern goshawk ("Accipiter gentilis") black kite ("Milvus migrans") Iberian wildcat ("Felis silvestris tartessia"), red fox, stone marten ("Martes foina") and introduced Egyptian mongoose also prey heavily on rabbits in Spain, but are more generalized and less reliant than the above predators. All these powerful predators do not generally compete directly for food but conflicts may ensue over the rights to territories and nesting sites amongst the birds. Being nocturnal in activity, however, the Eurasian eagle-owl does not tend to compete as directly as do the three eagle species and even these are discreetly segregated by habitat preferences, the goldens preferring open, steep cliffs, the Bonelli's favoring densely vegetated areas mixed with rocky spots and the imperial favoring relatively flat open woodlands. Among these eagles, the golden eagle tends to have the most similar habitat preferences to the Eurasian eagle-owl and across a broad band of overlapping distribution, the two are considered nearly ecological equivalents by day and night. However, good-sized female golden eagles are up to twice as heavy as an average eagle-owl, so seem to have a considerable advantage in size and power. Conversely, to support their large size and large territory needs, the golden eagle cannot survive on small and diverse prey nearly as successfully as the eagle-owl. Among the Iberian peninsula rabbit-eating guild, the species with wider ranges, the eagle-owl and the golden and Bonelli's eagles, have had some degree of success living off of alternate prey following the devastation of the rabbit population due to rabbit haemorrhagic disease. The two rabbit predators endemic to Iberia, the Iberian lynx and Spanish imperial eagle, have been devastated by the reduction and other causes and both are threatened species today.
Eurasian eagle-owl
The fourth and most famous derivation of the evolutionary line that includes the Eurasian eagle-owl is the great horned owl, which appears to have been the result of primitive eagle-owls spreading into North America. It has been stated that the great horned owls and Eurasian eagle-owls are barely distinct as species, with a similar level of divergence in their plumages as the Eurasian and North American representations of the great grey owl or the long-eared owl. There are more outward physical differences between the great horned owl and the Eurasian eagle-owl than in those two examples, including a great size difference favoring the Eurasian species, the great horned owl’s horizontal rather than vertical underside barring, yellow rather than orange eyes and a much strong black bracket to the facial disc, not to mention a number of differences in their reproductive behaviour and distinctive voices. Furthermore, genetic research has revealed that the snowy owl is more closely related to the great horned owl than are Eurasian eagle-owls. The most closely related species beyond the Pharaoh, Indian and Cape eagle-owls to the Eurasian eagle-owl is the smaller, less powerful and African spotted eagle-owl ("B. africanus"), which was likely to have divided from the line before they radiated away from Africa. Somehow, genetic materials indicate the spotted eagle-owl appears to share a more recent ancestor with the Indian eagle-owl than with the Eurasian eagle-owl or even the sympatric Cape eagle-owl. Eurasian eagle-owls in captivity have produced apparently healthy hybrids with both the Indian eagle-owl and the great horned owl. The Pharaoh, Indian and Cape eagle-owls and the great horned owl are all broadly similar in size to each other, but all are considerably smaller than the Eurasian eagle-owl, which averages at least 15–30% larger in linear dimensions and 30–50% larger in body mass than these other related species, possibly as the eagle-owls adapted to warmer climates and smaller prey. Fossils from south France have indicated that during the Middle Pleistocene, Eurasian eagle-owls (this paleosubspecies given the name "B. b. davidi") were larger than they are today, even larger were those found in Azerbaijan and in Caucasus (either "B. b. bignadensis" or "B. bignadensis"), which were deemed to date to the Late Pleistocene. About a dozen subspecies are recognized today.
Eagle-owls are distributed somewhat sparsely but can potentially inhabit a wide range of habitats, with a partiality for irregular topography. They have been found in habitats as diverse as northern coniferous forests to the edge of vast deserts. Essentially, Eurasian eagle-owls have been found living in almost every climatic and environmental condition on the Eurasian continent, excluding the greatest extremities, i.e. they are absent from humid rainforest in Southeast Asia, as well as the high Arctic tundra, both of which they are more or less replaced by other variety of "Bubo" owls. They are often found in the largest numbers in areas where cliffs and ravines are surrounded by a scattering of trees and bushes. Grassland areas such as alpine meadows or desert-like steppe can also host them so long as they have the cover and protection of rocky areas. The preference of eagle-owls for places with irregular topography has been reported in most known studies. The obvious benefit of such nesting locations is that both nests and daytime roosts located in rocky areas and/or steep slopes would be less accessible to predators, including man. Also, they may be attracted to the vicinity of riparian or wetlands areas, due to the fact that the soft soil of wet areas is conducive to burrowing by the small, terrestrial mammals normally preferred in the diet, such as voles and rabbits. Due to their preference for rocky areas, the species is often found in mountainous areas and can be found up to elevations of in the Alps and in the Himalayas and in the adjacent Tibetan Plateau. They can also be found living at sea-level and may nest amongst rocky sea cliffs. Despite their success in areas such as sub-arctic zones and mountains that are frigid for much of the year, warmer conditions seem to result in more successful breeding attempts per studies in the Eifel region of Germany. In a study from Spain, areas primarily consisting of woodlands (52% of study area being forested) were preferred with pine trees predominating the oaks in habitats used, as opposed to truly mixed pine-oak woodland. Pine and other coniferous stands are often preferred in great horned owls as well due to the constant density, which make it more likely to overlook the large birds. In mountainous forest, they are not generally found in enclosed wooded areas, as is the tawny owl ("Strix alucco"). Only 2.7% of the habitat included in the territorial ranges for eagle-owls per the habitat study in Spain consisted of cultivated or agricultural land. On the other hand, compared to golden eagles, they can visit cultivated land more regularly in hunting forays due to their nocturnal habits, which allows them to largely evade human activity. In the Italian Alps, it was found that almost no pristine habitat remained and locally eagle-owls nested in the vicinity of towns, villages and ski resorts.
Although found in the largest numbers in areas sparsely populated by humans, farmland is sometimes inhabited and they even have been observed living in park-like settings within European cities. Since 2005, at least five pairs 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. The number is expected to increase due to the growth of the European rabbit population in Helsinki. European hares ("Lepus europaeus"), the often preferred prey species by biomass of the eagle-owls in their natural habitat, live only in rural areas of Finland, not in the city centre. In June 2007, an eagle-owl nicknamed 'Bubi' landed in the crowded Helsinki Olympic Stadium during the European Football Championship qualification match between Finland and Belgium. The match was interrupted for six minutes. After tiring of the match, following Jonathan Johansson's opening goal for Finland, the bird left the scene. 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.