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.
Dietary biology of the Eurasian eagle-owl
Larger and more dangerous mammalian prey is not infrequently tackled by Eurasian eagle-owl. Nearly thirty species of mammalian carnivores have been reported in their diet. Many of these are mustelids, which are the smallest type of carnivore but can defend themselves viciously, even the tiny least weasel ("Mustela nivalis"). Nonetheless, unlike some smaller birds of prey, the Eurasian eagle-owl seems to have no problem subduing various weasels and even martens, some of the latter can nearly equal to their own weight, and will kill the young of much larger species such as European otter ("Lutra lutra") and Eurasian badger ("Meles meles"). However, the single most widely reported carnivore is the red fox ("Vulpes vulpes"), as they've been recorded in the diet from Denmark and Spain to the Russian Far East. Many fox specimens taken by eagle-owls are young individuals or subadults. Some studies posited that the average fox taken as weighing about , about equal to the eagle-owl's own weight, but another noted fox specimens killed that were estimated to weigh , much larger than the eagle-owls. At least four other species of foxes have been verified as prey in the Middle East and Asia. Raccoon dogs ("Nyctereutes procyonoides"), averaging , have been preyed on in Asia as well. Other relatively large carnivores have been hunted as well, including invasive Egyptian mongoose ("Herpestes ichneumon") in Spain and masked palm civets ("Paguma larvata") in China, both being roughly cat-sized at averages of , respectively. Domestic carnivores including cats ("Felis silvestris catus") of all sizes and small dogs ("Canis lupus familiaris") or puppies may on occasion become prey for Eurasian eagle-owls. Cats killed have been estimated to weigh from . The dogs killed by Bavarian eagle-owls were estimated to average . Non-native carnivores such as American mink ("Neovison vison") and raccoon ("Procyon lotor") have also been preyed on by eagle-owls, which is presumed to incidentally benefit the prey taken in considerably numbers by these rarely checked invasive fur-bearers.
The Eurasian eagle-owl is perhaps the only living owl widely reported (if not commonly) to kill the young of ungulates. At least nine species have been identified as their food. It is possible that some ungulates are eaten as carrion but this species, like most owls, normally kills its own food, unlike many eagles many of which consume carrion regularly. Among the ungulate prey recorded are three species of deer and five species of goat-antelopes, in addition to piglets of wild boar ("Sus scrofa"). Only the weight of the roe deer ("Capreolus capreolus") has been widely approximated when taken and specimens killed have included: a deer, a specimen and even, in the small adult size range for this deer, , all considerably larger than the eagle-owls themselves and far larger than any prey known for other owls. In studies from Bavaria and Austria, the average weight of roe deer caught was only , indicating very young deer being typically taken. Even if taken as newborn kids, fawns or calves, the young of other ungulates taken would weigh at least , from the birth weight of ibex ("Capra ibex") to that of red deer ("Cervus elaphus"). Perhaps even larger, the rare domestic sheep ("Ovis aries") taken by eagle-owls can reportedly include “half-grown” individuals.