Dietary biology of the Eurasian eagle-owl
Generally speaking, there appears to be rather strictly adhered size hierarchy among birds of prey and the very largest raptorial prey attacked by Eurasian eagle-owl tend to roughly equal to their own size. Larger raptors, such as eagles and vultures, seem to be largely invulnerable to eagle-owls. However, in Scandinavia, there are two cases of eagle-owls preying on “fairly large” nestlings of white-tailed eagles ("Haliaeetus albicilla"). Given the size, average adult weight of , and formidable character of this eagle, it is possible that the young of other raptors larger than themselves may too be attacked. Eastern imperial eagles ("Aquila heliaca"), which are nearly as large as the white-tailed eagle at an average weight of , have been killed by eagle-owls in Kazakhstan. Remarkable, considering the greater threat that the eagle normally poses to the eagle-owl than vica versa, it appeared a golden eagle was amongst the prey remains found around a nest in the French Pyrenees. Cases in Spain where large, formidable birds of prey, the Egyptian vulture ("Neophron percnopterus") and Bonelli's eagle, that are roughly equal in size to the eagle-owls, both weighing just over , have been predated, these seem to be taken from the nest or shortly after independence and rare occurrences. However, in Turkey, the eagle-owl is apparently a regular and serious predator of Bonelli's eagles. The Eurasian eagle-owl predates the largest members of the most species-rich diurnal raptor genera: the northern goshawks from the "Accipiter" genus, the upland buzzards from the "Buteo" genus and the gyrfalcons ("F. rusticolus") of the "Falco" genus. Additional large raptors, i.e. those that can average or more in body mass, taken as prey include osprey ("Pandion haliaetus"), the red kites ("Milvus milvus"), short-toed eagles ("Circaetus gallicus"), the greater spotted eagle ("Clanga clanga"), the lesser spotted eagle ("Clanga pomarina"), and the rough-legged buzzards ("Buteo lagopus") and saker falcons ("Falco cherrug"), in both of which females average more than . The taking of rough-legged buzzards and snowy owls must be confined to full-grown victims, since they nest further north than Eurasian eagle-owls. Eurasian eagle-owls also hunt the smallest raptors available, including those such as lesser kestrels ("Falco naumanni") and Japanese sparrowhawks ("Accipiter gularis") that weigh less than . Some of these are cases of eagle-owls killing nestlings or fledglings but they can easily overtake adult buzzards, goshawks and falcons of any size.
Dietary biology of the Eurasian eagle-owl
Jirds, jerboas, gerbils and hamsters, particular the genera "Allactaga", "Ellobius", "Gerbillus", "Meriones", and "Pygeretmus", start to become noticeable in the diet in the more arid portions of this species’ range, i.e. from Asia Minor to the Middle East and Central Asia. Relatively large species such as Tristram's jird ("Meriones tristrami") and the Euphrates jerboa ("Allactaga euphratica") are often amongst the more prominent species to be hunted. In western Kazakhstan, the primary foods are the great jerboa ("Allactaga major") and northern mole vole ("Ellobius talpinus") while on the Buzachi and Mangyshlak Peninsulas of the Kazakh Caspian Sea, the primary foods were the Libyan jird ("Meriones libycus"), great gerbil ("Rhombomys opimus") and small five-toed jerboa ("Allactaga elater"). All told, in the peninsular Kazakh regions of the Caspian Sea, jirds, jerboas and gerbils made up about 92% of the food. In the Tibetan Plateau of China, the tundra vole ("Microtus oeconomus") is the leading prey species, making up 53.1% of the prey. In Mongolia, two rather small rodents, the Mongolian five-toed jerboa ("Allactaga sibirica"), at 23.6% of the prey, and the Campbell's dwarf hamster ("Phodopus campbelli"), at 14.5% of the prey, make up a majority of the prey by number. In the Transbaikal, the Chinese striped hamster ("Cricetulus barabensis") was the main prey species, making up 43.8% of the diet.
What all primary rodent prey species share is that they are either nocturnal or crepuscular and are often common to abundant in openings of woods and rocky environments. The reason Eurasian eagle-owls take rats, which associate rather closely with humans, much more regularly than they take house mice ("Mus musculus") is not their larger size (many other rodents regularly taken by eagle-owls are as small or smaller than a house mouse) but the rat's ability to flourish along the edges of crop fields, refuse dumps, roadsides and abandoned buildings and fields, whereas the mice are more closely tied to active human habitations. A single refuse dump can host several thousand rats and eagle-owl territories that include dumps can locally expect higher productivity due to the abundance of this food source. Squirrels are readily predated but, being diurnal, are largely unavailable to this and other owls. When they are captured, it is presumably around dawn or dusk from their tree hole, in species such as the widespread red squirrel ("Sciurus vulgaris"), or burrow entrance, in the case of ground squirrels, most regularly recorded as prey in Asia Minor and the Central Asia. A few specimens of the woolly flying squirrels ("Eupetaurus cinereus"), the largest of the world's flying squirrels and one of the rarest, were the only identified foods for Eurasian eagle-owls in northern Pakistan alongside some European hares. Although not common in the diet, a few species of marmot have been taken and may be an important aspect of the prey biomass. In Europe, adult-sized alpine marmots, ("Marmota marmota") have been killed. However, the average weight of Tarbagan marmots ("Marmota sibirica") taken in Mongolia was only and that of alpine marmots in Engadin, Switzerland was only . These obviously comprising young marmots as adults have average weights from , from spring low weights to pre-hibernation weights during autumn hypophagia. Other than marmots, the largest rodent known to be hunted by Eurasian eagle-owls is the introduced aquatic rodents, the muskrat ("Ondatra zibethica"), which in one study estimated to average when taken, and the nutria ("Myocastor coypus"), equal in size to the large marmots.