Average lifespan
Status: wild: 153 months.
In North America, Strix nebulosa inhabits dense coniferous forests in Canada, and montane coniferous forests of the western States. It usually prefers pine and fir forests, rarely straying far out onto tundra barrens and muskeg marshes. Nests in mature poplar woodlands, well secluded from human activities, and in spruce stands with islands of tamarack. In winter, it may inhabit forests, sparse woodland edges bordering open fields, weedy fields with posts or scattered low trees or bushes, or brackish tidal meadows (Baetsen 2000; The Owl Pages).
Terrestrial Biomes: tundra ; forest ; mountains
Strix nebulosa resides in Alaska, Canada, the higher elevations of the Rocky Mountain States, northern Minnesota and northern Wisconsin. Also, S. nebulosa breeds from northern Yukon to northern Manitoba and northern Ontario, south locally to central California, northern Idaho, northwestern Wyoming, central Saskatchewan, northern Minnesota, and south central Ontario. Winters generally through the breeding range, wandering south irregularly to the northern tier of States. It also occurs widely across Europe and Asia. (Osborne 2001, UNEP World Conservation Monitoring Center 2001).
Biogeographic Regions: nearctic (Native ); palearctic (Native )
In the wild, Strix nebulosa feeds primarily on small rodents such as voles and pocket gophers. Small rodents composed 80-90% percent of the diet while other mammals (mainly shrews) and birds composed the remainder (The Owl Pages).
The Great Gray Owl hunts by perching on a tree overlooking a meadow or open area. The owl's keen hearing enables it to accurately determine the location of its prey, even under two feet of snow or in tunnels. Once the owl locates some food, it silently glides from its perch and plunges into the snow to grab the rodent with its sharp talons. Fresh "plunge marks" will occasionally show an imprint of the owl's outstretched wing feathers where the owl dropped into the snow. In many areas these marks are often the only indication that Great Gray Owls are in the area (MacBride Raptor Project, Wolf 2000, Baetsen 2000).
Strix nebulosa is important in the ecological balance; they scavenge and dispose of carrion and provide a check on rodent populations. Also, this rare owl draws many bird-watchers from everywhere. In Alaska, Athabaskans may use it as a food source due to its stored winter fat.(Compton's Encyclopedia 1998; Osborne 2000).
The restrictions enforced on the logging industry because of the threatened status, will most likely be costly, and will potentially set back any business that relies on these resources for lumber and wood products. (Mann)
The U. S. Department of Interior Bureau of Land Management considers Strix nebulosa to be a sensitive species, and it is protected in the United States on both federal and state levels. It has been the subject of extensive management studies (Huff et al. 1997)
The Great Gray Owl is rarely seen by people and is very vulnerable to human disturbance such as clearing of forests for timber or farming. The main limit to its distribution is the availability of nest sites. If there are sufficient nest sites, then other factors such as food supply, determine how many owls live in the area. Chemicals used in exterminating mice and other pests have detrimentally affected the food chain of Strix nebulosa
Wildfires can increase the availability of nest sites by creating suitable stumps, and they can also increase mouse populations.
(Idaho Conservation Data Center 2001, Huff et. al 1997, Mann,
Osborne 2000).
US Migratory Bird Act: protected
US Federal List: no special status
CITES: appendix ii
State of Michigan List: no special status
IUCN Red List of Threatened Species: least concern
Perception Channels: visual ; tactile ; acoustic ; chemical
The Great Gray Owl is the tallest owl in Alaska standing at a length 24-33 inches high, with a wing span of 54-60 inches, depending on degree of maturity. Strix nebulosa is larger and grayer than other owls and its round head does not have any ear tufts. Its bill and eyes yellow.
The owl has a distinctive facial disk, with two obvious gray concentric circles. The feathers of the disk help direct sounds toward the ear openings that are hidden by feathers. The owl also has an asymmetrical skull with large bony cups surrounding the ear openings.
In addition to the predominately gray plumage and distinctive facial disk, the bird has a black chin spot just above two white-feathered mustaches and it has a prominent white collar on the front of the neck. Ventrally, the owl is exhibits varying shades of dark and light grays, browns, and white. The dorsal side has a little less white than the ventral side. The tail is long and extends beyond the folded wings.
Adaptations for hunting include the facial disk, soft feathers so flight is silent, and the ability to turn its head three quarters of a circle (270 degrees).
(The Owl Pages; Compton's Encyclopedia 1998; MacBride Raptor Project 1997; Baetsen 2000; Wolf 2000).
Range mass: 790 to 1454 g.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Breeding takes place in late winter with the pair generally utilizing an abandoned hawk or crow's nest. The female Strix nebulosa lays eggs in March- June, depending on temperature range (egg laying may be delayed in deep snow years). Two to five dull white oval eggs are laid and are incubated by the female Strix nebulosa for a period of 28-29 days. The owlets hatch covered by soft white down with their eyes open. Both parents feed the young by bringing food to the nest, tearing into very small pieces that are eagerly consumed by the little ones. Soon the down begins to disappear and is replaced by feathers. Once the owlets are 'feathered out' they begin the pre-flight exercises. They can be observed walking around the top of their nest flapping their wings and gripping the nest edge with their talons. Young leave nest after three to four weeks with the ability to climb well. (The Owl Pages).
Key Reproductive Features: iteroparous ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; oviparous
Average time to hatching: 30 days.
Average eggs per season: 2.
The great gray owl is a native, permanent resident of the boreal forests of North America and Eurasia [27,53,59,75]. Strix nebulosa nebulosa inhabits North America and S. n. lapponica inhabits Eurasia [27,53]. Bird Web provides a distributional map of the great gray owl in North America, as well as photos.
The great gray owl is unevenly distributed and variable throughout its North American range [27]. Information on population status is currently lacking in many geographic areas [11,22,38,53].
The known breeding range in North America is from the boreal forests of Alaska east to Ontario and south to northern Minnesota and Wisconsin [3,4,28,53]. Great gray owls also breed in northern Idaho, western Montana, northwestern Wyoming [3,48], northeastern Utah [3,28], west-central Nevada [53], and the Sierra Nevada [3,48].
The wintering range is generally the same as the breeding range, except for a tendency for great gray owls to wander south in search of prey [27,68,100]. Great gray owls may wander to southern Canada, [4,69], New England, New York, and northern Michigan [69].
The following lists are speculative and are based on the habitat characteristics and species composition of communities great gray owls are known to occupy. There is not conclusive evidence that great gray owls occur in all the habitat types listed, and some community types, especially those used rarely, may have been omitted. See Preferred Habitat for more detail.
Food supply may be a critical factor regulating the population size of the great gray owl [27].
Diet: Primary foods eaten are small mammals, especially rodents such as voles (Microtus spp. and Clethrionomys spp.), mice (Peromyscus spp.), and pocket gophers (Thomomys spp.) [22,23,37,40,43,46,77,100,106]. Other prey items include shrews (Soricidae) and birds [37]. When desperate for food, great gray owls may eat frogs (Ranidae, Hylidae) [74].
In California, the diet of the great gray owl shifts between microtine rodents and pocket gophers as prey abundance changes [106]. Even when pocket gophers are abundant, great gray owls will not breed in the absence of microtine rodents. This may be due to the difficulty of catching pocket gophers [106].
As of this writing, no information is available on the effects of fire on the great gray owl. Despite the lack of information, some generalizations based on their habitat requirements may be possible. Following fire, modifications in the food supply and habitat of great gray owls may occur, as well as changes in the abundance of competitors and predators [85]. According to Finch and others [41], the effects of fire on birds and their habitat vary with: 1) the severity and extent of the fire; 2) temporal scales; 3) life history characteristics of the bird species; and 4) whether or not salvage logging occurs following fire. Severe fires alter the forest structure more than low-severity fires, and a stand-replacing fire may result in the replacement of a bird species with a different bird species. Large, severe fires may greatly alter bird habitat in the short term but be necessary for long-term maintenance of some forest types. Salvage logging may reduce the benefits of fire to birds that utilize snags for foraging and nesting [58]. Prior to European settlement, Native Americans managed forests that provided great gray owl habitat by frequent understory burning. With fire exclusion, habitat for the great gray owl has become less suitable due to decreased forest structure diversity and the encroachment of conifers into meadows [19].
Great gray owls require mature and old-growth forests for breeding and natural forest openings for foraging [21,22,22,51,52,57,63,72,93,106]. On the forest floor, downed woody material and tree stumps are used by adults for perching [28,29]. Leaning trees are needed by owlets for roosting and perching before they learn to fly [25,28,43] (see Preferred Habitat). The availability of nesting sites, perching sites, and foraging habitat are greatly affected by natural forest disturbances such as fire, succession, and disease outbreaks [38]. In the boreal mixed woodland on the plains of Alberta, habitat would probably not be suitable for 25 years after stand-replacing fire [87].
Fire may reduce conifer invasion in natural forest openings [51]. Golden eagles in the Appalachian Mountains stopped breeding after open habitats in the mountains, which were required for foraging, reverted to brush following fire exclusion [92]. Fire exclusion in great gray owl foraging habitat may have a similar effect.
The spotted owl and great gray owl share similar habitat requirements such as large trees, snags, closed canopies, and multiple forest layers. Fire management strategies for the spotted owl may apply to the great gray owl [98]. A fire management suggestion made by the US Department of the Interior, Fish and Wildlife Service [96] and Rapp [83] for spotted owls was to isolate nesting sites from adjoining forest at high risk for fire by creating a buffer zone around sites to reduce flammability. This should be done without compromising the nest site. Verner and others [98] suggest low-severity underburning in spotted owl habitat in the Sierra Nevada, minimizing the removal of duff and large woody debris.
Effects on prey: Rodents use dead woody material on the forest floor and in forest openings for cover [26,28] and may either be positively or negatively affected by fire, depending on fire severity [28,60]. Immediately following prescribed fire in meadows, the availability of prey species increases for the great gray owl due to the removal of grass layers used for cover [10,67]. After the availability of prey increases following fire, prey populations may decrease due to lack of cover [28].
Shrews, deer mice (Peromyscus maniculatus), creeping voles (Microtus oregoni), and Townsend's chipmunks (Neotamias townsendii) were absent to rare on severely burned sites but comprised most of the small-mammal community in unburned clearcuts. The study was conducted following the Oxbow Fire, a large (170 km²), mixed-severity wildfire in a Douglas-fir and mixed-conifer forest in western Oregon [104]. Rodent populations slowly after the fire, and population numbers were similar between burned and unburned areas by postfire year 7 [18]. Populations of pocket gophers (Thomomys talpoides and T. mazama) generally increase following stand-replacing fire [13], particularly in lodgepole pine/western needlegrass (Achnatherum occidentale) communities in central Oregon. This may be due to the abundance of long-stolon sedge (Carex inops), a rhizomatous, fleshy-rooted graminoid that increases 200% to 400% following burning [99].
The following table provides fire return intervals for plant communities and ecosystems where the great gray owl is important. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) grand fir Abies grandis 35-200 [6] maple-beech-birch Acer-Fagus-Betula spp. >1,000 sugar maple Acer saccharum >1,000 sugar maple-basswood Acer saccharum-Tilia americana >1,000 black ash Fraxinus nigra 101] tamarack Larix laricina 35-200 [80] western larch Larix occidentalis 25-350 [7,15,35] Great Lakes spruce-fir Picea-Abies spp. 35 to >200 [36] Engelmann spruce-subalpine fir Picea engelmannii-Abies lasiocarpa 35 to >200 [6] black spruce Picea mariana 35-200 conifer bog* Picea mariana-Larix laricina 35-200 [36] jack pine Pinus banksiana <35 to 200 [34,36] Rocky Mountain lodgepole pine* Pinus contorta var. latifolia 25-340 [14,15,95] Sierra lodgepole pine* Pinus contorta var. murrayana 35-200 Jeffrey pine Pinus jeffreyi 5-30 western white pine* Pinus monticola 50-200 Pacific ponderosa pine* Pinus ponderosa var. ponderosa 1-47 [6] interior ponderosa pine* Pinus ponderosa var. scopulorum 2-30 [6,12,64] red pine (Great Lakes region) Pinus resinosa 3-18 (x=3-10) [33,44] red-white pine* (Great Lakes region) Pinus resinosa-P. strobus 3-200 [34,54,66] eastern white pine Pinus strobus 35-200 [101] eastern cottonwood Populus deltoides <35 to 200 [80] aspen-birch Populus tremuloides-Betula papyrifera 35-200 [36,101] quaking aspen (west of the Great Plains) Populus tremuloides 7-120 [6,49,70] mountain grasslands Pseudoroegneria spicata 3-40 (x=10) [5,6] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [6,8,9] coastal Douglas-fir* Pseudotsuga menziesii var. menziesii 40-240 [6,71,84] California oakwoods Quercus spp. <35 Oregon white oak Quercus garryana <35 [6] California black oak Quercus kelloggii 5-30 [80] western redcedar-western hemlock Thuja plicata-Tsuga heterophylla >200 mountain hemlock* Tsuga mertensiana 35 to >200 [6] *fire return interval varies widely; trends in variation are noted in the species reviewForest management practices impact great gray owl populations [38]. On the Salmon National Forest in Idaho, the great gray owl is an indicator species for healthy old-growth forests [57]. Because great gray owls occur in local concentrations, management is recommended in areas where they have been seen in the past or in prime great gray owl hunting habitats characterized as forest edges or meadows with deep soils [28,47].
Management of great gray owl habitat includes protecting nest sites and surrounding habitat, restricting activity around nest sites during the breeding season, retaining stands near foraging areas, and installing artificial nest platforms [42,106]. Artificial nesting platforms placed 30 to 49 feet (9-15 m) from the ground have been used successfully by the great gray owl in Oregon and California [16,28,29]. For detailed building instructions, see Bull and others [29]. Depressions cut into tree stumps make nest sites [30].
Duncan's [38] management recommendations include collecting occurrence data, limiting clearcutting area size to <10 ha within a mosaic of different aged forest stands, clearcutting in irregular shapes such as scalloped-shaped edges to reduce great gray owl predation, and leaving large-diameter dead trees for nest sites and perches [38].
According to Gould [47], management of the great gray owl "should follow in the steps of the spotted owl (Strix occidentalis)" due to similar habitat requirements. This includes managing for large-diameter trees in closed-canopy forest [98].
For general management recommendations and survey techniques for the great gray owl, see the Bureau of Land Management's Survey and Management Recommendations and the Forest Service's Management and Standards Guidelines.
Population trends: Nest site availability, suitable foraging habitat [42,53], and fluctuating rodent prey populations [42] are the most important factors limiting great gray owl populations [53]. Currently, no data substantiate historical population trends in great gray owl distribution [42,48,53], and a standardized survey protocol needs to be developed to determine long-term (>10 years) population trends [38,48,53]. Great gray owl populations in North America are believed to have been stable [27,28,106] for the previous 10 to 100+ years [38]. The population of great gray owls in south-central Oregon is probably declining due to habitat loss from timber harvest, urban sprawl into prime great gray owl areas, and mountain pine beetle (Dendroctonus ponderosae) infestations [21]. In 1987, Duncan [38] estimated that North American great gray owl populations ranged between 20,000 and 70,000 breeding pairs. These numbers reflect the dependence on microtine rodents, which exhibit population fluctuations over 3 to 5 year periods [38].
To sustain a viable population, 30 pairs of great gray owls are needed on the Salmon National Forest in Idaho, requiring a minimum total amount of 50,000 acres (1,670 acres/pair) of old-growth forest [57].
Silviculture: Timber harvest has great impact on the great gray owl. This species requires specific Habitat management to persist [48], and monitoring the effects of forest fragmentation is critical [93].
Intensive logging limits the number of nesting, perching, and roosting sites, and can be detrimental to great gray owl populations [22,27,38]. Great gray owl management guidelines emphasize the connectivity of old-growth forest as dispersal corridors [83]. Great gray owl populations may persist with "some amount" of logging [38]. Partial cuts are generally suitable for foraging; however, dead and downed material should be left to increase cover for great gray owl prey such as voles. For natural nests, dead, broken topped trees [31,94] >20 inches (50 cm) DBH should be retained, as well as any tree with old raptor nests, especially northern goshawk nests [28].
Downed wood is important in great gray owl habitat. In northeastern Oregon, downed wood was within 3 feet (1 m) of where prey was caught or attempted to be caught 80% of the time [28]. For detailed recommendations on managing trees and logs in the Interior Columbia River Basin, see Bull and others [29].
Clearcutting may increase the amount of viable foraging habitat for the great gray owl; however, it decreases nesting habitat quality. Clearcutting creates "temporary meadows" capable of supporting rodent populations consumed by the great gray owl, but these meadows undergo "rapid" succession to forests [51]. In southeastern Idaho and northwestern Wyoming, northern pocket gopher (Thomomys talpoides) populations increased in great gray owl diet proportionally to the amount of clearcut forest [43].
Clearcutting decreases the number of mature, large-diameter trees needed for nesting by the great gray owl, which could lead to a decline in great gray owl populations [46,51]. On the Targhee National Forest in eastern Idaho and northwestern Wyoming, great gray owl presence was related to the amount of clearcutting. Areas clearcut 39% to 62% (SD=11.68) did not contain great gray owls. Areas clearcut 14% to 26% (SD= 5.124) contained successful nesting in 2 locations [103].
Dwarf mistletoe: Trees infected with dwarf mistletoe are important nesting sites for great gray owl [28,43,78,103] (see Preferred Habitat). Bull and others [30] suggest that to manage forests containing dwarf mistletoe, choose groups of lightly-infected trees, where the brooms are large and dense enough for animals to build or use an existing nesting platform. Preserve old, infected larch trees when managing for western larch, because they produce high-quality snags sought by wildlife [30].
Agriculture and livestock grazing: Great gray owl nesting density is strongly influenced by the intensity of land use, including agricultural practices and human activity [107]. According to Young [107], the great gray owl is capable of persisting in agricultural areas. Livestock grazing may be harmful, however [32]. The great gray owl may abandon meadows that are heavily grazed by cattle and domestic sheep due to the reduction of rodent prey populations in grazed areas [106].Predators for adult great gray owls include common ravens, great horned owls [23,76,103], northern goshawks [23], broad-winged hawks, and American martens (Martes americana) [103]. Owlets may be killed by red-tailed hawks [28], American black bears (Ursus americanus) [37,76], fisher (Martes pennanti) [37,100], lynx (Lynx canadensis) [37], great horned owls [37], and golden eagles (Aquila chrysaetos) [100].
Other causes of mortality: At least 20% of adult great gray owl mortality is due to starvation during winter [100]. Collisions with vehicles and shooting by humans also cause mortality [103].
Strix nebulosa (Forster) is the scientific name for the great gray owl, a member of
the Strigidae family [1,4,90]. Listed below are the 2 recognized subspecies [2,4,90]:
Strix nebulosa nebulosa (Forster)
Strix nebulosa lapponica (Thunberg)
Migration: Great gray owls may be resident or seminomadic, depending on the availability of rodent prey [59,68,69,100]. They may move to low elevations during winter to avoid hunting in deep snow [43].
Mating: Great gray owls are generally not monogamous; however, pair bonds may be maintained in boreal forests if prey populations remain high over subsequent years [37]. Great gray owls probably first breed when 3 years old [22,23]. Pair formation begins in January and extends as late as 2 weeks before egg-laying in April and May [43].
Nesting: Nesting chronology data for the great gray owl are meager. Timing of egg laying may depend on the availability of rodent prey and habitat quality [100]. Egg laying may be delayed during years of heavy snow [43]. In California, great gray owls nest in April [106]. Egg laying occurs in April and May in southeastern Idaho and northwestern Wyoming [43]. In northern Alberta, the earliest egg-laying date was 23 March, but most eggs were laid by 15 April [76].
Eggs are laid at 1- to 3-day intervals [27]. Clutch size ranges from 2 to 5 eggs, and 3-egg clutches are most typical [76]. Average clutch size was 2.67 (n=6) eggs in the Targhee National Forest, Idaho [103] and 2.3 eggs (n=64, SD=0.87) in northeastern Oregon [23].
Incubation is performed only by the female [27,76], and brooding lasts 2 to 3 weeks [27,76]. Brooding lasted an average of 29.7 days in southeastern Idaho and northwestern Wyoming [43].
Development: Great gray owls are semiprecocial and nidicolous [27]. Adults may sometimes favor their smaller or weaker offspring [23].
Fledging: Young leave the nest 26 days [23] to 28.5 days [43] after hatching. Owlets learn to fly 7 to 14 days after leaving the nest. Before learning to fly, they climb leaning trees using their talons, wings, and bills, and roost off of the ground [28,43]. Females continue protecting their young after fledging and males bring prey items. Males may extend fledgling care longer than females. In Oregon, females abandoned their young between 3 and 6 weeks after fledging, but males provided care for up to 3 months after fledging [28]. Young are independent by late summer and leave the natal site in fall and winter. Maximum dispersal distance of radio-tagged juvenile great gray owls from their natal site in Oregon ranged from 4.7 to 19.9 miles (7.5-32.0 km) [24].
Survival: The survival of young depends on the availability of leaning and deformed trees as well as the presence of forested habitat within a 1,640- foot (500 m) radius around the nest for roosting and avoiding prey [43]. Owlets have a 63% chance surviving from the egg to the flight stage [43]. In northeastern Oregon, the probabilities of juveniles surviving to the following ages were: 12 months old=0.53; 18 months old=0.39; and 24 months old=0.31 (n=32). Mortality was due to starvation, falling from the nest [28], or predation) [23]. Survival rates are high if juveniles survive to breeding age [28].
Home range: The home range of 5 nesting males in northeastern Oregon averaged 4.5 km² (range 1.3-6.5 km²) [24].
A 1989 analysis of western raptor habitat concludes: 1) properly managed prescribed fires and wildfires are necessary within raptor habitat; 2) fire exclusion has led to ecosystem stagnation and will ultimately lead to "devastating" wildfires; 3) prescribed fire can be used to reduce fuels, maintain historic conditions in an ecosystem, and create mosaics of open and closed habitat, thus increasing raptor carrying capacities as well as prey abundance; and 4) unless vegetation types are very similar, it will be difficult to standardize fire management practices [65].
As of 2007, there was no research on the effects of wildfire or prescribed burning for the great gray owl. Data are needed to make detailed management recommendations. Based on the habitat requirements for the great gray owl, fire management generalizations are possible.
Great gray owls could be greatly affected by fire, depending on its severity. Stand-replacing fire may negatively affect the great gray owl due to the destruction of nesting sites such as vacated raptor nests [11,28,43,73,76,100,103] and dwarf mistletoe brooms [11,28,43,78,103,106]. Snag availability would probably increase, providing perching, roosting, and nesting sites [11,43,103,106]; however, great gray owls may not use these sites due to decreased canopy cover (see Preferred Habitat). Salvage logging could have negative effects. Mammalian prey species populations may be very low for the first several years following severe fire, but would eventually increase [13,18,28,38,99].
Low-severity wildfire and prescribed burns open dense forest structure and may benefit the great gray owl; however, the removal of large-diameter snags and dwarf mistletoe brooms used for nesting would be "detrimental" [81]. If prescribed surface burns are performed, some dead woody material on the forest floor needs to be retained due to its importance to the great gray owl [25,26,28,43,46] and mammalian prey species. To maintain prey species, prescribed burning should be restricted during the great gray owl's breeding season (see Timing of Major Life History Events) [46].
Great gray owls and spotted owls share similar habitat requirements, and may respond similarly to wildfire and prescribed fire. For detailed fire effects and use for the spotted owl, see spotted owl.
The great grey owl (Strix nebulosa) (also great gray owl in American English) 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 genus Strix found in both Eastern and Western Hemispheres. In some areas it is also called Phantom of the North, cinereous owl (name also used for S. sartorii), spectral owl, Lapland owl, spruce owl, bearded owl, and sooty owl.[3]
Adults have large rounded heads with grey faces 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. 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.[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. 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.
There are two recognized subspecies of the great grey owl spread across North America and Eurasia.[6]
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 exceed 100 °F (38 °C).
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. 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.
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 more than 25,000 years. Biologists working on that study suggest that the Yosemite population should be considered a separate sub-species (Strix nebulosa yosemitensis).[8] 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 grey owl nests have ever been found in Washington state.
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] Great grey owls are rarely sighted as far south as Pennsylvania and Long Island, New York.[9] In winter 2017, the birds were recorded in northern New York in Robert Moses State Park and central Maine.[10]
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.
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 grey 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 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. 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 and other parts of Scandinavia, 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 squirrels, hares, moles, shrews, weasels, thrushes, grouse, Canada jays, mountain quail, small hawks, ducks, frogs and large beetles.[13] 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.
The harvest of timber from the great grey owl's habitat is, perhaps, the greatest threat to this species.[14] 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.[14] 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.[14]
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Other dangers to great grey owls include rodenticides, collisions with vehicles, and the West Nile Virus. The 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 the virus. Testing of owls in the Yosemite area since 2005 has found evidence of the 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.[15] 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.[16]
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.
The great grey owl is the provincial bird of Manitoba.
As of 2016, there are four great grey owls captive in Oregon and California. Two live at Blue Mountain Wildlife near Pendleton, Oregon, a third lives at Lindsay Museum in Walnut Creek, California,[17] and the fourth lives at CuriOdyssey in San Mateo, California.[18]
A captive wild injured great grey 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.[19]
The Eurasian subspecies of the grey owl is very commonly kept in European zoos.
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{{cite journal}}: CS1 maint: multiple names: authors list (link) The great grey owl (Strix nebulosa) (also great gray owl in American English) 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 genus Strix found in both Eastern and Western Hemispheres. In some areas it is also called Phantom of the North, cinereous owl (name also used for S. sartorii), spectral owl, Lapland owl, spruce owl, bearded owl, and sooty owl.
