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The Great Gray Owl is distinctly larger than the Great Horned Owl and Snowy Owl and much larger than the Barred Owl (although a surprising proportion of its bulk consists of feathers rather than bones and muscle). Its flat face is punctuated by small yellow eyes surrounded by concentric rings. Although Great Gray Owls are most active at dawn, at dusk, and at night, they hunt during daylight in summer (and in winter if food-stressed), although still generally near dawn or dusk. Wingbeats are deep and slow. Both sexes produce a call consisting of 5 to 10 very quiet (typically inaudible beyond 400 m) evenly spaced deep hoots at a rate of around one per second.
The diet of the Great Gray Owl consists mainly of small mammals, especially Microtus voles. These owls can detect prey under snow by sound and can plunge and break through snow crust hard enough to support 80 kg and as deep as 45 cm. Small mammals are swallowed whole and larger prey are pulled apart. Abandoned nests of other large bird such as Goshawks, Ravens, or Ospreys 10 to 50 feet above the ground are the most common nest sites, although these owls may sometimes nest on top of broken tree trunks and, rarely, on the ground; nests may be reused for several years. Typical clutch size is 2 to 5 eggs (clutch size may be larger in years with abundant food). Incubation (for 28 to 36 days) is by the female only, but the male brings food to the incubating female on nest. The female broods young for their first 2 to 3 weeks. In some areas, the adult female departs after young fledge while the male remains and feeds them for up to 3 months. In some winters, large numbers of Great Gray Owls may move south or southeast into eastern Canada and the extreme northeastern United States in North America and northern Germany and Ukraine in Europe (apparently in response to a sudden drop of rodent population). Great Gray Owls may live more than 20 years.
(Kaufman 1996; Dunne 2006; Svensson 2009; AOU 1998; Holt et al. 1999 and references therein)