During the breeding season the Great Gray Owl is found from central Alaska, northern Yukon, northern Mackenzie, northern Manitoba, northern Ontario south in the interior along the Cascade Mountains and Sierra Nevada Mountains to central California, and in the Rocky Mountains from northern Idaho and Montana to western Wyoming. Some individuals are found in far northern Minnesota and rarely in northern Wisconsin and Michigan. This species usually winters in its breeding range but wanders south to Montana, North Dakota, southern Minnesota, southern Wisconsin and central Michigan. Some individuals at high elevations may move to lower elevations to avoid heavy snow during the winter. Great Gray Owls are irregularly irruptive, with periodic invasions into various northern states. A combination of high reproductive rates the year before followed by a winter prey decline has been thought to explain these movements. Perhaps the accumulation of icy crusts on the snow cover will also drive some individuals further south. It is mostly young birds that make up the irruptions to the south. These irruptions happen in different regions in different years, suggesting that local prey availability drive these movements.
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