The Great Grey Owl is large grey to greyish brown owl with dense, fluffy plumage, long wings and tail, and a large head. Females are larger and heavier than males.
Mostly active at nights, but also at dusk and just before dawn; sometimes active during the day in breeding season. They fly with soft, slow wingbeats and generally do not often move more than short distances between perches and seldom glides.
When ground is covered with snow, a Great Grey Owl can hunt by hearing alone and often plunges into the snow to capture small rodents moving underneath as far as 30 cm.
The Great Grey Owl has a distinctive primary call which is a very soft, low-pitched hoot "whooo-ooo-ooo-ooo". This call is used as a territorial declaration and can be heard up to 800m away under good conditions.
Territorial calling begins after dusk, peaks before midnight, then peaks again later. Males and females also give a single hoot when near the nest. Females give an excited "ooo-uh" when the male arrives with food. When excited near the nest adults growl, shriek, hoot, wail, and snap their bills. When threatened, a Great Grey Owl will snap its beak, spread its wings, and growl.