Dietary biology of the Eurasian eagle-owl
Throughout the western Mediterranean region, the Eurasian Eagle-Owl is something of a specialist on 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 (2471 square acres). In central Spain, up to 73.1% of the prey that were recorded for Eurasian eagle-owls, by number, can be 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. On the other hand, given the now much reduced numbers of rabbits, the eagle-owls still regularly select rabbits, seemingly out of proportion to their commonality. Although large mature wild rabbits commonly weigh , the eagle-owls usually hunt smaller specimens, often with an average weight around , with young rabbits usually selected in spring and summer and subadults selected in the autumn and winter. This is largely due to the behaviour of rabbits, with younger rabbits being forced to distribute following weaning, being less cautious in behaviour and occupying less primary habitat, thus being forced out of the confines of their burrows more regularly than mature adults. However, the estimated average weight of rabbits taken can be variable, reported from as little as in a study from Spain to as much as in the Netherlands.
Dietary biology of the Eurasian eagle-owl
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. from Asia Minor to the Middle East and Central Asia. Relatively large species such as Tristram's jird ("Meriones tristrami") and the Euphrates jerboa ("Allactaga euphratica") are often amongst the more prominent species to be hunted. In western Kazakhstan, the primary foods are the great jerboa ("Allactaga major") and northern mole vole ("Ellobius talpinus") while on the Buzachi and Mangyshlak Peninsulas of the Kazakh Caspian Sea, the primary foods were the Libyan jird ("Meriones libycus"), great gerbil ("Rhombomys opimus") and small five-toed jerboa ("Allactaga elater"). All told, in the peninsular Kazakh regions of the Caspian Sea, jirds, jerboas and gerbils made up about 92% of the food. In the Tibetan Plateau of China, the tundra vole ("Microtus oeconomus") is the leading prey species, making up 53.1% of the prey. In Mongolia, two rather small rodents, the Mongolian five-toed jerboa ("Allactaga sibirica"), at 23.6% of the prey, and the Campbell's dwarf hamster ("Phodopus campbelli"), at 14.5% of the prey, make up a majority of the prey by number. In the Transbaikal, the Chinese striped hamster ("Cricetulus barabensis") was the main prey species, making up 43.8% of the diet.
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 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 woolly flying squirrels ("Eupetaurus cinereus"), the largest of the world's flying squirrels and one of the rarest, were the only identified foods for Eurasian eagle-owls in northern Pakistan alongside some European hares. Although not common in the diet, a few species of marmot have been taken and may be an important aspect of the prey biomass. In Europe, adult-sized alpine marmots, ("Marmota marmota") have been killed. However, the average weight of Tarbagan marmots ("Marmota sibirica") taken in Mongolia was only and that of alpine marmots in Engadin, Switzerland was only . These obviously comprising young marmots as adults have average weights from , from spring low weights to pre-hibernation weights during autumn hypophagia. Other than marmots, the largest rodent known to be hunted by Eurasian eagle-owls is the introduced aquatic rodents, the muskrat ("Ondatra zibethica"), which in one study estimated to average when taken, and the nutria ("Myocastor coypus"), equal in size to the large marmots.