The Eurasian eagle-owl (Bubo bubo) may well be the most powerful extant species of owl, able to attack and kill large prey far beyond the capacities of most other living owls. However, the species is even more marked for its ability to live on more diverse prey than possibly any other comparably sized raptorial bird, which, given its considerable size, is almost fully restricted to eagles. This species can adapt to surprisingly small prey where it is the only kind available and to large prey where it is abundant. Eurasian eagle-owls feed most commonly on small mammals weighing or more, although nearly 45% of the prey species recorded have an average adult body mass of less than . Usually 55-80% of the food of eagle-owls is mammalian.Schuchmann (1999). Eurasian Eagle Owl (Bubo bubo). pp. 186 in: del Hoyo, Elliott & Sargatal, eds (1999). Handbook of the Birds of the World. Vol. 5. Lynx Edicions, Barcelona. Voous, K.H. 1988. Owls of the Northern Hemisphere. The MIT Press, 0262220350.
Hunting and digestion
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 genus Bubo, except that the snowy owls (Bubo scandiacus) regularly watch for prey from a ground position.Jaksić, F. M., & Carothers, J. H. (1985). Ecological, morphological, and bioenergetic correlates of hunting mode in hawks and owls. Ornis Scandinavica, 165-172. 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 of take off after spotting 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.Martínez, J. A., Serrano, D., & Zuberogoitia, I. (2003). Predictive models of habitat preferences for the Eurasian eagle owl Bubo bubo: a multiscale approach. Ecography, 26(1), 21-28.Martinez, J. A., Martinez, J. E., Manosa, S., Zuberogoitia, I., & Calvo, J. F. (2006). How to manage human-induced mortality in the Eagle Owl Bubo bubo. Bird Conservation International, 16(03), 265-278.Owls of the World: A Photographic Guide by Mikkola, H. Firefly Books (2012), 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.Sergio, F. (2002). Biases associated with diet study methods in the Eurasian Eagle-Owl. J. Raptor Res, 36(1), 11-16. 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).Cramp, S. 1985. Handbook of the birds of Europe, the Middle East and North Africa. Vol. 4. Oxford Univ. Press. Oxford. 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.Laroulandie, V. (2002). Damage to pigeon long bones in pellets of the eagle owl Bubo bubo and food remains of peregrine falcon Falco peregrinus: zooarchaeological implications. In ZM Bochenski, Z. Bochenski, JR Stewart (Eds.), Proceedings of the 4th Meeting of the ICAZ Bird Working Group, Kraków, Poland, 11–15 September 2001. Acta Zoologica cracoviensia (Vol. 45, pp. 331-339). 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.De Cupere, B., Thys, S., Van Neer, W., Ervynck, A., Corremans, M., & Waelkens, M. (2009). Eagle owl (Bubo bubo) pellets from Roman Sagalassos (SW Turkey): distinguishing the prey remains from nest and roost sites. International Journal of Osteoarchaeology, 19(1), 1-22.Lloveras, L., Moreno‐García, M., & Nadal, J. (2009). The eagle owl (Bubo bubo) as a leporid remains accumulator: taphonomic analysis of modern rabbit remains recovered from nests of this predator. International Journal of Osteoarchaeology, 19(5), 573-592.Willgohs, J. F. (1974). The eagle owl Bubo bubo (L.) in Norway; Part I- Food Ecology. Sterna, 13, 129-177.
Summary of general diet 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.Hume, R. (1991). Owls of the world. Running Press, Philadelphia, PA. 1991. 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;Delattre, P., Giraudoux, P., Baudry, J., Quéré, J. P., & Fichet, E. (1996). Effect of landscape structure on Common Vole (Microtus arvalis) distribution and abundance at several space scales. Landscape Ecology, 11(5), 279-288. 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.Donázar, J. A., Hiraldo, F., Delibes, M., & Estrella, R. R. (1989). Comparative food habits of the Eagle Owl Bubo bubo and the Great Horned Owl Bubo virginianus in six Palearctic and Nearctic biomes. Ornis Scandinavica, 298-306.Jánossy, D., & Schmidt, E. (1970). Die Nahrung des Uhus (Bubo bubo). Regionale und erdzeitliche Änderungen. Bonner zool. Beitr, 21, 25-31. 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.
A very large balance of the prey range can also be comprised by birds and other prey including reptiles, amphibians, fish, insects and other invertebrates are seemingly taken whenever they are available. All told, well more than 600 different species have been identified as prey of Eurasian eagle-owls. Their diet includes basically all the same prey as all sympatric diurnal raptors in the genera Buteo, Aquila and Haliaeetus. The total number of prey species may even surpass that of the great horned owl and may well be the greatest of any raptorial bird on the Eurasian continent. Among five segments of Europe the average prey diversity per region was 63 species, a higher number than any of the other 40 raptorial bird species studied there.Marti, C. D., Korpimäki, E., & Jaksić, F. M. (1993). Trophic structure of raptor communities: a three-continent comparison and synthesis. In Current ornithology (pp. 47-137). Springer US. The mean mass of prey per study has varied from as little as , near the now former Aral Sea in Turkmenistan with the Turkmen jerboa (Jaculus turkmenicus) being the primary prey species, to , in Spain with the European rabbit being the primary prey species.Jaksić, F. M., & Marti, C. D. (1984). Comparative food habits of Bubo owls in Mediterranean-type ecosystems. Condor, 288-296. Remarkably, in northern Spain, where the rabbit is now extirpated (as was the eagle-owl for a time in the 20th century), the average weight of prey can dip as low as .Serrano, D. Interhabitat differences in the diet of the Eagle Owl (Bubo bubo) in the mid Ebro river valley (NE Spain): effect of European rabbit (Oryctolagus cuniculus) availability. Ardeola. Volume 45(1), 1998. Pages 35-46. In comparison, the lightest mean prey mass in dietary studies of the great horned owl was while the highest was .Donázar, J. A., Travaini, A., Ceballos, O., Delibes, M., & Hiraldo, F. (1997). Food habits of the great horned owl in northwestern Argentine Patagonia: the role of introduced lagomorphs. Journal of Raptor Research, 31(4), 364-369.