The Eurasian eagle-owl ( Bubo bubo) stands as one of the most formidable owl species on Earth, renowned for its size and adaptability. As apex predators, they play a crucial role in maintaining the balance of their ecosystems, particularly in controlling rodent populations. Their presence is invaluable, and their removal can lead to significant increases in rodent numbers, potentially classifying them as keystone predators.
Generalist Diet and Hunting Strategies
What truly sets the Eurasian eagle-owl apart is its incredibly diverse diet. It can adapt to surprisingly small prey where it is the only kind available and to large prey where it is abundant. While many owls have specific dietary preferences, the Eurasian eagle-owl exhibits a generalist approach, consuming a wide array of prey items depending on availability. They primarily feed on small mammals weighing 100 g (0.22 lb) or more, although nearly 45% of the prey species recorded have an average adult body mass of less than 100 g (3.5 oz). Their diet includes basically all the same prey as all sympatric diurnal raptors in the genera Buteo, Aquila and Haliaeetus. All told, well more than 600 different species have been identified as prey of Eurasian eagle-owls.
Hunting primarily consists 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.
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. 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.
Prey Consumption and Digestion
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 3,000 g (6.6 lb)) 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.
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Pellets of the Eurasian eagle-owl average around 75 mm × 32 mm (3.0 in × 1.3 in), 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 150 mm (5.9 in) 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.
Dietary Diversity and Regional Variations
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. 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. Most dietary studies place the average weight of Eurasian eagle-owl prey in the zone of 100 to 500 g (0.22 to 1.10 lb), thus the species fits well within the rule of thumb that most prey weigh no more than 20% of the raptorial bird's own weight. The average prey weight in comparison to the eagle-owl's body mass is reportedly average and unremarkable for both owls and raptors in general, the average prey per one study was 7.2% of the eagle-owls’ body mass.
The European raptorial birds most likely to attack prey of a relatively large size compared to themselves were the black kite (Milvus migrans), whose average prey was 35.7% of its own weight, and the Eurasian pygmy owl (Glaucidium passerinum), whose average prey was 31.7% of its own weight. On the other hand, when rabbit numbers were at their peak, the average prey mass in Spain may range up to at least 55% of the eagle-owls’ own body mass in that population. A raptorial prey comparison estimated that the mean prey weight for eagle-owls in Europe was 294.7 g (10.40 oz) as a mean among 5 main regions, the highest of any owl but similar or slightly lighter than the European average for northern goshawks (Accipiter gentilis) and considerably lower only than two widespread European eagles. Per that study the average prey weight of the great horned owl in 4 North American regions was much lower (averaging only 64 g (2.3 oz)). In Mediterranean scrub, the average mammalian prey of the Eurasian eagle-owl was nearly five times heavier than those of great horned owls from similar habitats in Chile, California and Colorado, where in combination the horned owl's mammalian food averaged 230 g (8.1 oz). However, a comparison between eagle- and horned owls based upon six comparable biomes in the Americas and Eurasia found the great horned owl's prey averaged 372 g (13.1 oz), improbably slightly higher than the eagle-owl at 327 g (11.5 oz). If known Asian studies are mixed with European ones, the average prey weight for the Eurasian eagle-owl overall may also roughly be 372 g (13.1 oz).
Among sympatric owls in Europe, the mean prey weight of long-eared owls (Asio otus) was found to be 32 g (1.1 oz), the tawny owl (Strix aluco) took prey weighing a mean of 34.5 g (1.22 oz) (though locally can be as high as 130 g (4.6 oz)) and short-eared owls (Asio flammeus) mean prey weigh was 35 g (1.2 oz). A study in Finland seeking to distinguish the differences in prey taken by females and smaller male eagle-owls found that the two sexes prey averaged 828 g (1.825 lb) and 575 g (1.268 lb), respectively.
Rodents
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 300 g (11 oz). In many parts of Europe, the most numerous prey in dietary studies are the 360 g (13 oz) brown rat and the 28.5 g (1.01 oz) common vole, followed by the nearly as widespread 40 g (1.4 oz) 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 200 g (7.1 oz), 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 75 to 140 g (2.6 to 4.9 oz), was the main prey species in Styria, Austria, although considerably secondary in biomass to hedgehogs, as well as in central Sweden.
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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. 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. 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 315 g (11.1 oz) 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 2,000 g (4.4 lb) woolly flying squirrels (Eupetaurus cinereus), the largest of the world's flying squirrels and one of the rarest, was the only identified foods for Eurasian eagle-owls in northern Pakistan alongside some European hares. Although uncommon, marmots are known to be taken in some occasions.
Lagomorphs (Rabbits and Hares)
Throughout the western Mediterranean region, the Eurasian Eagle-Owl is something of a specialist in a single prey species: the European rabbit. The rabbit historically reached extremely high densities in Iberian scrub, i.e. more than 40 per every 1 square hectare (2.471 square acres). In central Spain, up to 73.1% of the recorded prey for Eurasian eagle-owls, by number, are rabbits. Where still available, they are also the main prey in Portugal and central France. However, following the devastation of the wild rabbit due to rabbit haemorrhagic disease, the local eagle-owls have frequently had to adapt to other, often smaller, prey.
Other lagomorphs extensively overlap in range with Eurasian eagle-owls in Europe are hares. Hares are often considered secondary in prey remains in most European biomes by number but often are a significant contributor to prey biomass for eagle-owls. For example, in Finland, the mountain hare (Lepus timidus) made up only 5.3% of prey by number but made up 56.2% of the prey biomass as the hare specimens were very large, including adults weighing up to around 4,000 g (8.8 lb). On the contrary, the mountain hare was the numerous prey species in one study from the Swiss Alps, making up 28.3% of the prey by number. The European hare was also the second most common prey species in a study from the Czech Republic. In lower Austria, the European hare (Lepus europaeus) was also the second most abundant species in the diet but the estimated average weight of hares taken was rather low, at 850 g (1.87 lb), indicating young hares of probably less than a month old are most often taken there. Similarly, another area near Europe where hares were numerically important was in southwestern Turkey, only young European hares were taken, being the second most common prey species by number.
Outside of southwestern Europe, the next closest association between lagomorphs and this species is in the arid steppe of southern Russia, hares are apparently the primary prey of the local eagle-owls. Little has been published in recent ornithological reviews, but old studies have indicated that the eagle-owls in this region are so dependent on hares, primarily again the mountain hare species, that cyclical reductions in numbers of the hares are followed immediately by a reduction of the local eagle-owls. A similar phenomenon has been noted in the great horned owl in its association with the snowshoe hare in the American boreal forest. The same prey, the mountain hare, was the most important along the Pechora River in Russia, making up 34.1% of prey numbers. However, the general habitat is distinctly different from the steppe, as it consists of mountainous sub-taiga dense forests. Other than the prior mentioned species, at least five other species of hare have been found as occasional contributors to the eagle-owl's diet. The only other family in the lagomorph order, the pikas, have been found to factor regularly into the diet of Eurasian eagle-owls as w…
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Birds and Other Prey
While mammals, particularly rodents and lagomorphs, form the cornerstone of the Eurasian eagle-owl's diet, they are opportunistic hunters and will readily take birds of all kinds, including crows, ducks, grouse, seabirds, and even other birds of prey (including other owls). Other prey items include snakes, lizards, frogs, fish (caught by plunging into water), and crabs. Their adaptability extends to insects and other invertebrates, which are taken whenever available.
Conservation and the Eagle-Owl's Role
The Eurasian Eagle-owl underwent a significant decline in Europe during the 20th century, due mainly to human persecution. Pesticide use and poisoning from mercury seed-dressings have also been a problem, as have collisions with vehicles, barbed wire and power lines. In addition, diseases such as myxomatosis and rabbit hemorrhagic disease have decimated rabbit populations in some areas, with severe effects for the Eagle-owl population.
However, the Euroasian Eagle-owl has recovered to some extent in Europe as a result of improved protection, extensive reintroduction / restocking programs, and an increased food supply due to clear-felling and the proliferation of refuse dumps, with the resulting increases in rats and other rodents. Conservation measures recommended include protecting its habitat from development and extensive logging, as well as action to prevent collisions with power lines.
As apex predators, Eurasian eagle-owls play a critical role in maintaining ecological balance. They help to control pest populations and reduce the spread of disease. Their importance to their ecosystems cannot be overstated. It is essential to dispel myths and misperceptions about owls and recognize them as remarkable animals that inspire wonder and curiosity.
Eagle-Owls in Captivity
Eurasian Eagle Owls are a species that’s adapted to life in zoological facilities. Many facilities present this animal both on the glove or in free-flight shows, and it is a show-stopper in any situation. However, due to their size, behavioral traits, and training needs, this is a species only recommended for highly skilled, advanced animal handlers. It is important to note that Eurasian Eagle Owls are large and powerful birds, and, improperly handled, they could pose threat of injury to staff or guests. Handlers should always be aware of their individual owl’s preferences and comfort, and respond accordingly to avoid injuries. While many Eurasian Eagle Owls are comfortable on glove between multiple handlers, some individuals do demonstrate discrimination between handlers and are more suited to a limited number of handlers with whom they have a strong relationship. Additionally, it is highly recommended that they are hand-reared following the recommendations in the Eagle Owl Ambassador Animal Guideline.
Diet in Captivity
Wild eagle-owls are predatory, obligate carnivores with a generalist diet that varies throughout their range and includes both mammalian and avian prey items. From the Eagle Owl AAG: Typical whole prey diet items can include mouse, rat, rabbit, day-old chick, quail, fish, and commercially-made bird of prey diet. Note that chicks and fish are less nutritionally valuable than whole adult prey, such as mouse, rat, rabbit, and quail. A healthy diet should include a variety of prey items. When feeding previously frozen food items, it is recommended to include a vitamin supplement, such as Vitahawk, to compensate for potential loss of vitamins during the freeze/thaw process. If/when feeding items that are not whole prey (i.e. just the muscle meat), it is also recommended to dust with calcium supplement. From the Eagle Owl AAG: Diet amounts and food presentation may vary depending on the bird, but should be mindful of the appropriate weights and body condition scores of this species as well as the individual ambassador. See the full Eagle Owl AAG for more detail on diets and appetite management.
Enrichment
From the Eagle Owl AAG: The most successful environmental enrichment devices appear to be items that can be footed and shredded, such as paper products, cardboard boxes, lettuce, melons, squash or other produce. These items can be provided alone or stuffed with prey or diet, although owls should be monitored for ingestion of foreign objects if food is delivered with a novel item. Other successful items include tennis balls, holl-ee rollers, and canvas dog toys, which present opportunities for seizing, grabbing, mock “killing,” and mantling.Training for shows or public interactions is a form of enrichment and provides an opportunity for the bird to engage in both cognitive and physical activity.
Presentation
From the Eagle Owl AAG: Ambassador eagle-owls can be presented both on and off-grounds in formal (captive audience) and informal (casual walk-by programs, chats or displays) programs. They are successfully presented both indoors and outdoors, on a glove, perch, or in a free-flight program with proper training and attention to the safety. Free-flight programs in particular afford larger audiences the opportunity to see this large bird in action, making them a popular addition to amphitheater-style shows.
From the Eagle Owl AAG:Consideration should be given as to appropriate times for handling ambassador animals during presentations, and rest breaks scheduled accordingly. Program handlers should maintain the animal’s basic husbandry needs and a medical protocol should be in place in case concerns arise. Most eagle-owls appear comfortable participating on glove up to 30 minutes, some up to one hour. Many institutions have been successful working with ambassador eagle-owls regularly for to 2-4 hours per day, with rest periods in between programs. Many eagle-owls travel well, and overnight outreaches are acceptable as long as the owl’s basic husbandry needs are addressed and a medical protocol is in place in case of concerns. It is not recommended to allow public contact with ambassador eagle-owls, in the form of direct contact, such as petting or feeding the owl.