Dietary biology of the Eurasian eagle-owl
Equally or even more so than the great horned owl, the Eurasian eagle-owl is a threat to any smaller type of raptor it encounters, whether other owls or diurnal birds of prey. All told, more than 20 species of accipitrid, 15 species of owl and 9 species of falcon have been found amongst their prey. The Eurasian eagle-owl is the primary predator of other birds of prey throughout Eurasia, not even the prolific raptor-killing northern goshawk ("Accipiter gentilis") equals the sheer number of raptors taken. Up to 6% of the overall food by number and 36% by prey biomass for Eurasian eagle-owls can be comprised by other raptorial birds. The primary raptorial prey taken by Eurasian eagle-owls in Europe are two species: the long-eared owl and the common buzzard. Both are taken in such large numbers that they rank in the top five most regularly hunted bird species in Europe. In Luxembourg, the long-eared owl and common buzzard were the fourth and fifth most regularly hunted prey species. Other high rankings have included the long-eared owl being the fourth most regular prey species in Bavaria and the buzzard being the third most regular prey species in a study from the Czech Republic. While migrating, long-eared owls appear to select areas to move through partially based on whether or not eagle-owls are detected, thus eagle-owls have a very serious influence on the behaviour of this prey. Despite the large numbers taken of both of these, the more scarce eagle-owl does not seem to have a serious deleterious effect on their overall population, especially compared to anthropogenic factors.
Other than these two species, a large share of the raptorial prey for eagle-owls is made up of other owls. Given that all European owls are to some extent nocturnal, they may be encountered and killed upon detection by the Eurasian eagle-owl. In Europe, it has killed every other species of owl, from the tiny pygmy owl and scops owl to the large great grey owl and the snowy owl. The Eurasian eagle-owl is the only raptor that has been known to prey on snowy owl on multiple occasions. However, the threat Eurasian eagle-owls poses to other raptors can be exaggerated. Occasionally, with adequate mammal prey populations, they can nest as close as a few dozen meters of other raptors and never harass them. When nesting in the same groves as the long-legged buzzards ("Buteo rufinus") in southeastern Bulgaria, Eurasian eagle-owls did not predate the buzzards at any point. Long-legged buzzards have been killed however in Kazakhstan. Even in those occasional cases where they pick off one or several raptors, they do not effect the overall population, unless the raptorial prey is already heavily diminished due to some other (often human-based) cause. For example, despite both being a known predator of both, they did not significantly depress numbers of either northern goshawks or tawny owls in southern Finland. One study showed that many raptors engage in spatial avoidance during potential encounters with Eurasian eagle-owls and while the predation by eagle-owls did decrease raptor population densities, they seldom caused declines in breeding success or altered habitat occupancy.
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.