prey larger than the typical voles and mice far less so than in larger owls such as eagle-owls.Korpimäki, E., Huhtala, K., & Sulkava, S. (1990). Does the year-to-year variation in the diet of eagle and Ural owls support the alternative prey hypothesis? Oikos, 47-54. Among these are a few species of squirrel, most of which weigh more than as adults, in Europe exclusively the far-ranging red squirrel (Sciurus vulgaris), as well as rats, especially the brown rat (Rattus norvegicus) which made up nearly 20% of the diet in a Hokkaido study. Adult hedgehogs of a couple species are opportunistically taken at times in Europe and European hedgehogs (Erinaceus europaeus) taken in Finland were estimated to average about . Introduced muskrats (Ondatra zibethicus), which weigh on average an estimated when caught, are also taken in Finnish studies. They have been known to prey upon the Japanese giant flying squirrel (Petaurista leucogenys), which weighs around a median of . One large mammalian prey widely associated with Ural owls are hares, though they seldom occur in substantial numbers in the diet, they appear to be opportunistically taken in most parts of the range. As much as nearly 25% of their biomass may be hares and hares are the largest known prey to be taken by Ural owls. While hunting hares, Ural owls usually focus their hunts on relatively young specimens. In Europe, both European hares (Lepus europaeus) and, especially, mountain hares (Lepus timidus) may be hunted. The mean size of mountain hares taken in Finland have variously been estimated at , with a common median being about . Exceptionally, Ural owls may take even adult mountain hares, which average about , but this may be unconfirmed.Angerbjörn, A. (1995). Lepus timidus. Mammalian species, (495), 1-11. Japanese hares (Lepus brachyurus) may also be taken in Japan as well. Other prey The second most important prey group behind mammals (albeit considerably behind) is birds. Birds are also the most diverse prey group in the Ural owl’s diet, with well over 100 avian prey species recorded. Birds usually compromise less than 10% of the dietary intake by number in studies from Europe. Exceptionally high numbers, relatively speaking, were reported in Fennoscandia, especially when vole numbers go down. In bad vole years in Finland, of 860 prey items, 32.2% by number and 40.25% by biomass were birds. In particular, thrushes of the Turdus genus were the most reported