Great grey owl

Great grey owl
Whitby, Ontario, Canada
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Strigiformes
Family: Strigidae
Genus: Strix
Species: S. nebulosa
Binomial name
Strix nebulosa
Forster, 1772
World distribution of S. nebulosa

The great grey owl or great gray owl (Strix nebulosa) is a very large owl, documented as the world's largest species of owl by length. It is distributed across the Northern Hemisphere, and it is the only species in the Strix (genus) found in both Eastern and Western Hemispheres. In some areas it is also called Phantom of the North, cinereous owl, spectral owl, Lapland owl, spruce owl, bearded owl, and sooty owl.[2]

Description

Cross sectioned great grey owl specimen showing the extent of the body plumage, Zoological Museum, Copenhagen

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 Bubo genus.[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 (60 in), but averages 142 cm (56 in) for females and 140 cm (55 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]

Taxonomy

There are two recognized races of the great grey owl spread across North America and Eurasia.[6]

Habitat

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).

Range

They breed in North America from as far east as Quebec[2] to the Pacific coast and Alaska, and from Finland and Estonia across northern Asia. They are permanent residents, although northerly populations may move south and southeast when food is scarce. In Europe they are found breeding in Norway and Sweden and more numerously through Finland and Russia. Even though the species occurs in Europe, the first great grey owl recognized by science was found in Canada in the late 18th century.

Adult female

There are sedentary populations in the Pacific states of California, Oregon, and Washington. The great grey owl in this region is found in disjunct areas of appropriate habitat. 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.

In Oregon, the great grey owl breeding range is scattered from the Siskiyou Mountains in the southwest to the Blue Mountains in the northeast. Two bird festivals each May feature field trips to try to see the species: the Ladd Marsh Festival in La Grande, OR and the Mountain Bird Festival in Ashland, OR.

A 2015 study in California estimated there were fewer than 300 birds in the state at the time. The species is listed as Endangered under the California Endangered Species Act.[7] The California range for the species includes a small extension of the Oregon population, north of Alturas. In addition breeding has been confirmed in the Tahoe National Forest east of Nevada City; in the Sierra Nevada foothills south of I-80 at around 2,000 feet (610 m) elevation; and for the population clustered around Yosemite. In Yosemite is where the first nest south of Canada was found in 1914. A study of the Yosemite owls shows that this population has been genetically isolated from populations in Oregon and farther north for over 25,000 years. Biologists working on that study suggest that the Yosemite population should be considered a separate sub-species. Nearby populations at lower elevations were not tested.

In Washington state, great grey owls are found in the southeast where mountains extend up from Oregon and in the Colville National Forest. Only a handful of great gray owl nests have ever been found in Washington state.

Farther east in the United States, great grey owls breed in Idaho, Montana, and Wyoming, as far south as the Grand Tetons.

Breeding

Egg, Collection Museum Wiesbaden

Great grey owls do not build nests, so 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.

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.

Feeding

Owl in flight

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 Strix genus are known to "snow-plunge" for prey, a habit that is thought to require superb hearing not possessed by all types of owls.[8]

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.

Although seldom preyed upon, great grey owl nestlings and juveniles may themselves fall prey to bears, fishers, and large hawks, especially Northern goshawks; while adults may fall prey to Bubo owls, golden eagles and lynxes.

The song of the male is a series of very deep, rhythmic hoots whoo, whoo, whoo, whoo, whoo.... At other times, adults are normally silent. The young may chatter, shriek or hiss. Tame male and female owls may produce higher-pitched hoots when given food by humans.

Threats

The harvest of timber from the great grey owl's habitat is, perhaps, the greatest threat to this species.[9] Intensified timber management typically reduces live and dead large-diameter trees used for nesting, leaning trees used by juveniles for roosting before they can fly, and dense canopy closures in stands used by juveniles for cover and protection.[9] If perches are not left in clearcuts, great grey owls cannot readily hunt in them. Although human-made structures (made specifically for use by this species) have been utilized by these owls, the species is far more common in areas protected from logging. Livestock grazing in meadows also adversely affects great grey owls, by reducing habitat for preferred prey species.[9]

Plumage of the face (Weltvogelpark Walsrode)

Other dangers to great grey owls include rodenticides, collisions with vehicles, and West Nile Virus. West Nile Virus is likely to become more prevalent with climate change. In Ontario and northeastern Oregon there are confirmed great grey owl deaths from West Nile Virus. Testing of owls in the Yosemite area since 2005 has found evidence of West Nile Virus in that population.

Due to their large size, great grey owls have few natural predators. Great horned owls, various small carnivores, and black bears have been documented preying on young, but such predators rarely threaten adults, and owls have been known to fend off animals as large as black bears when defending their nests.[10] The only known predator of adult great grey owls is the Eurasian eagle-owl (Bubo bubo), which occasionally preys on the former in parts of Europe.[11]

Territorial behavior

The great grey owl is not as aggressive as most other alpha predators. They are less likely to attack each other or potential threats than are other large predatory birds. They do not protect a large nesting territory, nor do they defend hunting territories through aggression. As an exception, the female is aggressive in protecting eggs and owlets. She is especially alert and aggressive when fledglings first leave the nest but cannot yet fly, and thus are extremely vulnerable.

This lack of territorial aggressiveness makes the great grey owl difficult to find in the field. Most owls respond to their own species calls if played back in a nesting territory. Great grey owls will often ignore such calls. They also do not flush every time a human approaches or drives past. The great grey owl often remains still even if a human is nearby and therefore they are often overlooked or unnoticed.

Provincial bird

The great grey owl is the provincial bird of Manitoba, where it is known as the great gray owl.

Captivity

As of 2016, there are three captive great gray owls in Oregon and California. Two live at Blue Mountain Wildlife near Pendleton, Oregon, and a third lives at Lindsay Museum in Walnut Creek, California.

A captive wild injured great owl, Boris, resides at the Alaska Raptor Center in Sitka, Alaska. Boris was found north of Anchorage with a trauma to his right wrist, leaving him flightless.[12]

Footnotes

  1. ↑ BirdLife International (2012). "Strix nebulosa". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
  2. 1 2 3 Great Gray Owl, Owl Pages
  3. ↑ http://animals.jrank.org/pages/786/Owls-Strigiformes-PHYSICAL-CHARACTERISTICS.html
  4. ↑ Great Grey Owl, The Owl Foundation
  5. ↑ CRC Handbook of Avian Body Masses by John B. Dunning Jr. (Editor). CRC Press (1992), ISBN 978-0-8493-4258-5.
  6. ↑ König, C.; Weick, F.; Becking, J.-H. (2008). Owls of the World (2nd ed.). A&C Black Publishers Ltd., London, UK. ISBN 978-0-7136-6548-2.
  7. ↑ BirdLife International (2012). "Strix nebulosa". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
  8. ↑ Lynch, Wayne, Owls of the United States and Canada: A Complete Guide to Their Biology and Behavior. The Johns Hopkins University Press (2007), ISBN 978-0-8018-8687-4
  9. 1 2 3 Great Gray Owl, Sierra Forest Registry
  10. ↑ "Great Gray Owl - Strix nebulosa". The Owl Pages.
  11. ↑ International Masters Publishers. Wildlife Explorer. Connecticut, Group 7, Card 89. ISBN 1-886614-77-6.
  12. ↑ "Boris - Alaska Raptor Center". Retrieved 3 November 2016.

References

Further reading

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