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
Hunting mainly consist of the owl watching from a perch for prey activity and then swooping down swiftly once prey is spotted. The prey is often killed quickly by the eagle owl's powerful grip and talons though is sometimes bitten on the head to be killed as well. Then the prey item is swallowed whole or torn into pieces with the bill. The same basic hunting and killing methods are used by all owls in the "Bubo" genus, except that the snowy owls ("Bubo scandiacus") and fish owls regularly watch for prey from a ground position (on a bank in the case of fish owls). Most hunting occurs in wood-cloaked openings, often those carved out by wetlands or watersheds. While they can and do hunt within woodlands, they are not well suited to hunting in areas with dense understories, thick foliage or tree thickets, as they seem to hunt firstly by vision and only secondarily by sounds, unlike some other owls. Eurasian eagle-owls are too heavy with relatively modest wing areas to hunt extensively on the wing although this species’ relatively short, broad wings allow it low-speed maneuverability in the moments are it takes off after they spot a prey item. Because of the limits of its flying abilities, the Eurasian eagle-owl requires ample, elevated perches from which to watch for prey activity and thus in most areas it will not hunt extensively in open, treeless areas, unlike smaller owls like the barn owl ("Tyto alba"). Tree branches are often the main hunting perches used but they will also use rock formations, rubble with large rocks, hills with tall grasses or even a building, tall pole or other elevated manmade perch. Occasionally, they may capture other birds on the wing, including nocturnal migrants, taking advantage of their distraction and diminished visual acuity to intercept them in mid-flight. They seem to hunt mostly by opportunity, taking whatever detectable prey item that is available, preferably those that are active. Most prey, even up to the size of small rabbits, are swallowed whole. While a small rodent can easily be swallowed in an instant, if the prey item is large, the swallowing process can appear grueling and grotesque and in some cases eagle-owl nestlings have choked to death while attempting to swallow overly large prey (i.e. moorhens). Small prey is often swallowed immediately after capture, although occasionally it is carried in the bill to the nest for the young or to a roost for quick consumption. Larger prey is normally carried in the owl's feet and is more likely than smallish prey to be brought to the nest due to its nutritional value. Exceptionally large prey (probably any over ) is consumed on the ground where it is killed, possibly being torn apart into manageable pieces. This leaves the owl vulnerable to loss of their prey to scavengers or to confrontations with heavier predators. The undigestable portions of prey are regurgitated in the form of pellets as in all owls. Pellets of the Eurasian eagle-owl average around , thus they are around the same size as those of most other large "Bubo" owls, even the notably smaller great horned owl ("B. virginianus"). In some cases, pellets of "Bubo" owls can range up to in length. Eagle-owl pellets average slightly larger (about 10%) than those of great grey owls ("Strix nebulosa"). As is the case with all owls, pellets are indisputably the best method to examine the main diversity of prey consumed by an owl. However, it is also important to examine the prey remains left around a roost or nest when available. Larger prey, which may be torn apart before the eating of portions or fleshy parts of the prey item, is frequently determined solely by remains rather pellets as are birds, in which the wings, feet and some feathers may be removed before consumption. As in the great horned owl, after capturing larger prey, the Eurasian eagle-owl often beheads it. In part, this may be due to making prey easier to carry off and, in both large owl species, it is considered a signature indicator for predator identification.
The diet is generally more diverse than any other co-existing owl but often focuses predominantly on a particular range of prey species, with two to four species often making up the majority of the diet. Studies conducted over the course of years has shown that the regional diet often changes dramatically from year to year. This is partially driven by the cyclical population trends of much of their prey as well as by habitat changes, which are often due to human activities. The primary prey species varies by region but is comprised mainly by small mammals such as voles, rats, mice, rabbits and hares. In Europe, the five main prey species are, in order of extent of reporting and numeric abundance in dietary studies: the brown rat ("Rattus norvegius"), which has now occupied edge habitats (although generally associated with man) in almost every part of Europe excluding the colder north; the common vole ("Microtus arvalis"), often the most abundant native, small mammal in wild portions of Europe; the European hedgehog ("Erinaceus concolor"), which occupies wooded edges across western Europe alongside the eagle-owls; the European water vole ("Arvicola amphibius"), a unique, large vole occupying the wetland edges often hunted by eagle-owls; and the European rabbit ("Oryctolagus cuniculus"), which has a very close relationship with the eagle-owls found throughout the Iberian Peninsula and the massifs of France. As the distribution of the Eurasian eagle-owls moves west (and south), the primarily prey species gradually change but often continues to be dominated in number by small rodents and in biomass by slightly larger mammals such as hedgehogs and hares and rabbits.