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.
Great grey owl
They breed in North America from as far east as Quebec to the Pacific coast and Alaska, and from Finland and Estonia across northern Asia. They are permanent residents, although northerly populations may move south and southeast when food is scarce. In Europe they are found breeding in Norway and Sweden and more numerously through Finland and Russia. Even though the species occurs in Europe, the first great grey owl recognized by science was found in Canada in the late 18th century.
There are sedentary populations in the Pacific states of California, Oregon, and Washington. The great grey owl in this region is found in disjunct areas of appropriate habitat. In winter these birds do not move far but may go downslope to escape deep snow as they must capture their prey on the ground.