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
In terms of sheer quantitative abundance, rodents are by far the most dominant prey group in the diet of the Eurasian eagle-owl. Roughly 130 species of rodent are known to have been taken by these owls. Due to their small sizes, however, rodent prey often comprise a relatively small portion of the prey biomass, excepting relatively large rodents such as rats and hamsters, some of which weigh at least . In many parts of Europe, the most numerous prey in dietary studies are the brown rat and the common vole, followed by the nearly as widespread field vole ("Microtus agrestis"). Among the areas where the common vole was the leading prey species included southern Sweden, Germany, Poland, the Czech Republic, and Slovakia. In areas that have undergone more heavy development, the brown rat predominates, being the leading prey item in several parts of France including Côte-d'Or, Lozère and even the remote Pyrenees, Belgium, the Italian Alps, and Brașov, Romania. The brown rat is even the primary prey in Lebanon despite it being tied solely to human habitations as the environment is often too arid to allow it to survive further afield. Black rats ("Rattus rattus"), at , are also hunted but rarely in equal number to the generally more successful brown rats. In Norway and Finland, field voles were the most numerous prey species, per one study in the latter country it made up 30.1% of the food but only provided 3.3% of the prey biomass. The European water vole, averaging at , was the main prey species in Styria, Austria, although considerably secondary in biomass to hedgehogs, as well as in central Sweden. The water vole often the second or third most regularly found prey species in several other European studies. In Samara, Russia, the diet is dominated by the European water vole, although other species such as European hamsters ("Cricetus cricetus") and European hares ("Lepus europaeus") (both also significant prey there) probably outweigh them in biomass contribution. The common vole and European water vole together made up roughly 80% of the prey by number in the High Tatras of Slovenia and more than 60% of the diet in bordering Poland.
In the northern extremes of their range, Eurasian eagle-owls tend to have a less varied diet than those to the south and the need to catch significant numbers of rodents rises as they are often the only regularly accessible prey in sparse sub-Arctic environments. In northern Sweden, more than 60% of the food was made up of microtine rodents, i.e. the Norway lemming ("Lemmus lemmus"), the field vole, the European water vole and the grey red-backed vole ("Myodes rufocanus"). In one study from western Russia, rodents comprised more than 90% of the diet of the eagle-owls, led by the steppe lemming ("Lagurus lagurus") at 24% by number, the narrow-headed vole ("Microtus gregalis") at 21% and the European water vole at 17.9%. Similarly in high altitude tundra of the East Austrian Alps, nearly 90% of the food was found to be comprised by microtine rodents, the European snow vole ("Chionomys nivalis") being the most frequently identified species in the diet. In Spain, the Algerian mouse ("Mus spretus"), at up to 41.8% of the prey by number, and wood mouse ("Apodemus sylvaticus"), at up to 10.5% by number, are often regularly taken, especially in regions where rabbit numbers have declined. In Romania, the Romanian hamster ("Mesocricetus newtoni") and European hamster are the most prominent species found in the eagle-owl's diet, alongside the brown rat. The Romanian hamster is also dominated the prey list per a study in Bulgaria, only the European water vole follows distantly in number. Dormice are readily taken when available in Europe, especially the edible dormouse from central Europe to the eastern countries. In Slovenia, the edible dormouse was the most regularly-recorded prey species, making up 20.2% of the food but it comprised 9.4% of the biomass. The edible dormouse was also the most regular prey species in Trentino-Alto Adige/Südtirol, Italy, making up 22.5% of the diet. The smaller garden dormouse ("Eliomys quercinus") and considerably smaller hazel dormouse ("Muscardinus avellanarius") are also occasionally predated in Europe, plus at least a half dozen other dormouse species in eastern Europe, Asia Minor and several points to the east.