[3] Adults have large rounded heads with grey faces and yellow eyes with darker circles around them.

[3] The call of the adult is a series of very deep, rhythmic 'who's, which is usually given in correlation to their territories or in interactions with their offspring.

Once believed to require a cold climate, it is now known that this bird survives in a few areas where summer temperatures exceed 100 °F (38 °C).

[citation needed] They breed in North America from as far east as Quebec[3] to the Pacific coast and Alaska, and from Finland and Estonia across northern Asia.

Even though the species occurs in Europe, the first great grey owl recognized by science was found in Canada in the late 18th century.

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.

[7][full citation needed] The California range for the species includes a small extension of the Oregon population, north of Alturas.

Biologists working on that study suggest that the Yosemite population should be considered a separate sub-species (Strix nebulosa yosemitensis).

In Washington state, great grey owls are found in the southeast where mountains extend up from Oregon and in the Colville National Forest.

Farther east in the western United States, great grey owls breed in Idaho, Montana, and Wyoming, as far south as the Grand Teton and Yellowstone National Parks.

In northeastern North America, the owls are found year-round in southern Quebec and Ontario, but individuals will sometimes move further south in winter into New York and New England, apparently in pursuit of more abundant prey.

[9] In winter 2017, the birds were recorded in northern New York in Robert Moses State Park and central Maine.

The erection of nest platforms for great grey owls was pioneered by Robert Nero in central Canada in the 1970s.

Unlike, for example, osprey or white storks, the great grey owl does not predictably re-use nest sites over consecutive years.

Immediately after fledging, the white, fuzzy young must use beak and feet to climb back into trees.

The abundance of food in the area usually affects the number of eggs a female lays, a feature quite common in northern owl species.

Their large facial discs, also known as "ruffs", focus sound, and the asymmetrical placement of their ears assists them in locating prey, because of the lack of light during the late and early hours in which they hunt.

They have excellent hearing, and may locate (and then capture) prey moving beneath 60 cm (2.0 ft) of snow in a series of tunnels solely with that sense.

Locally, alternative prey animals (usually comprising less than 20% of prey intake) include squirrels, hares, moles, shrews, weasels, thrushes, grouse, Canada jays, mountain quail, small hawks, ducks, frogs and large beetles.

In mated pairs, the male is the primary hunter who provides food for the entire family while the female guards and broods the eggs, nestlings, and flightless fledglings.

Livestock grazing in meadows also adversely affects great grey owls, by reducing habitat for preferred prey species.

[14]Other dangers to great grey owls include rodenticides, collisions with vehicles, and the West Nile Virus.

This lack of territorial aggressiveness makes the great grey owl difficult to find in the field.