This species account is dedicated in honor of George Wood, and for his support of the Cornell Lab of Ornithology.
The Great Gray Owl, the largest Strix in North America, is the only member of its genus that breeds in both the Old and New Worlds. Primarily a bird of dense, northern boreal forests, it finds suitable coniferous habitat south into the northern Rocky and Sierra mountains and along some central Asiatic mountain chains. A rodent specialist, this owl favors areas near bogs, forest edge, montane meadows, and other openings. In some winters, when its prey are scarce, individuals from northern populations of this elusive raptor wander south to the northern U.S. and southern Canada, often in considerable numbers and always to the delight of birdwatchers. A single Great Gray Owl wintering on a farm in Massachusetts for two months in 1973 attracted at least 3,000 birdwatchers.
Although it appears to be more massive than other owls of the northern forest, its actual body mass is at least 15% smaller than the more common Great Horned Owl (Bubo virginianus), with which it shares habitat. Thus its plumage makes up much of the bulk of the bird, allowing it to withstand the bitter cold of northern winters. In addition, the Great Gray Owl can accurately locate by ear its rodent prey under snow, plunging through the surface to grab the unsuspecting vole beneath. It has been reported to break through snow crust thick enough to support the weight of an 80 kg man.
The breeding density of this striking bird seems limited by both prey and nest site availability. It favors the abandoned nests of other birds of prey, but will nest on the tops of broken trees or on artificial platforms, the latter a valuable management tool allowing this species to nest more densely and in habitat that might otherwise be marginal. Although well studied in Scandinavia, less is known about this species in North America. This account, therefore, relies heavily on just a handful of North American studies ( Nero 1980 , Winter 1986 , Bull et al. 1988a , Bull et al. 1988b , Bull et al. 1989a , Bull et al. 1989b , Duncan 1992 ), plus references to Scandinavian populations ( Pulliainen and Loisa 1977 , Mikkola Mikkola 1981 , Mikkola 1983 , Cramp 1985a ).