Great Gray Owl
Strix nebulosa
Summary 2
Adults have a large rounded head with a grey face and yellow eyes with darker circles around them. The underparts are light with dark streaks; the upper parts are grey with pale bars. This owl does not have ear tufts and has the largest facial disc of any raptor. There is a white collar or "bow tie" just below the beak. The long tail tapers to a rounded end.
In terms of length, the great grey owl is believed to exceed the Eurasian eagle-owl and the Blakiston's fish owl as the world's largest owl.[3] The great grey is outweighed by those two species as well as several others, including most of the genus Bubo.[4] Much of its size is deceptive, since this species' fluffy feathers, large head and the longest tail of any extant owl obscure a body lighter than that of most other large owls. The length ranges from 61 to 84 cm (24 to 33 in), averaging 72 cm (28 in) for females and 67 cm (26 in) for males. The wingspan can exceed 152 cm (5 ft 0 in), but averages 142 cm (4 ft 8 in) for females and 140 cm (4 ft 7 in) for males. The adult weight ranges from 580 to 1,900 g (1.28 to 4.19 lb), averaging 1,290 g (2.84 lb) for females and 1,000 g (2.2 lb) for males.[5] The males are usually smaller than females, as with most owl species.[2]
The call of the adult is a series of very deep, rhythmic whoos, which is usually given in correlation to their territories or in interactions with their offspring. At other times, adults are normally silent. The young may chatter, shriek or hiss. Tame owls may produce higher-pitched hoots when given food by humans.
In northern areas their breeding habitat is often the dense coniferous forests of the taiga, near open areas, such as meadows or bogs. In Oregon and California this owl has been found nesting in mixed oak woodlands. Once believed to require a cold climate, it is now known that this bird survives in a few areas where summer temperatures regularly exceed 100 °F (38 °C).
Great grey owls do not build nests, so they typically use nests previously used by a large bird, such as a raptor. They will also nest in broken-topped trees and cavities in large trees. In southwestern and northeastern Oregon, the great grey owl has been using man-made platforms for nest sites since the 1980s. The erection of nest platforms for great grey owls was pioneered by Robert Nero in central Canada in the 1970s. Nesting may occur from March to May. Unlike, for example, osprey or white storks, the great grey owl does not predictably re-use nest sites over consecutive years. Four eggs are the usual clutch size. Eggs average 42.7 mm (1.68 in) wide and 53.5 mm (2.11 in) long. The incubation period is about 30 days, ranging from 28 to 36 days. Brooding lasts 2 to 3 weeks, after which the female starts roosting on a tree near nests. The young jump or fall from the nest at 3 to 4 weeks, and start to fly 1 to 2 weeks after this. Immediately after fledging, the white, fuzzy young must use beak and feet to climb back into trees. The female is on guard at this time and may be aggressive toward potential predators. Most offspring remain near their natal sites for many months after fledging. Normally the male hunts for his mate and the young throughout the nesting period. Once the young begin the fly, the female typically withdraws and the male continues to feed the young until they can hunt on their own in the autumn. The young owls go off on their own by winter.
Windblown juvenile great grey owl
The abundance of food in the area usually affects the number of eggs a female lays, a feature quite common in northern owl species. In years when small mammal populations are very low the great grey owl may not attempt nesting; thus their reproduction is connected to the sometimes extreme fluctuations of small mammal populations. Also, great grey owls may not nest in years of drought. If food is scarce, they may travel a fair distance to find more prey, with considerable movements by large numbers in some years of particularly scarce prey. Though they do not migrate, many are at least somewhat nomadic.
One alert landowner in northeast Oregon, Andy Huber, helped a female great gray owl raise her four nestlings after a great horned owl killed the male. Huber live-trapped small mammals and then released them at the nest site, and the female took these to the nestlings. As the owlets matured and began flying they came for the prey on their own. Huber and the mother owl raised all four owlets successfully.[11]
These birds wait, listen, and watch for prey, then swoop down; they also may fly low through open areas in search of prey. They frequently hunt from a low listening post which can be a stump, low tree limb, fence post, or road sign. Their large facial disks, 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. On the nesting grounds, they mainly hunt at night and near dawn and dusk; at other times, they are active mostly during the night.
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. They then can crash to a snow depth roughly equal to their own body size to grab their prey. Only this species and, more infrequently, other fairly large owls from the genus Strix are known to "snow-plunge" for prey, a habit that is thought to require superb hearing not possessed by all types of owls.[12]
Unlike the more versatile eagle and horned owls, great grey owls rely almost fully upon small rodents. What species they eat depends on which small mammals are most abundant and available. In northern Canada they eat lemmings primarily. In dry parts of California's Sierra Nevada they eat mostly pocket gophers. In some areas voles are the predominant prey. Locally, alternative prey animals (usually comprising less than 20% of prey intake) include hares, moles, shrews, weasels, thrushes, grouse, grey jays, mountain quail, small hawks and ducks. This species is not known to scavenge or steal from other predators. 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.
Sources and Credits
- (c) Greg Lasley, some rights reserved (CC BY-NC), uploaded by Greg Lasley
- Adapted by calgaryzoo from a work by (c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Strix_nebulosa
