Family Typical Owls (Strigidae)

Vulnerable

Snowy Owl (Bubo scandiacus)

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Taxonomy

French: Harfang des neiges German: Schneeeule Spanish: Búho nival
Taxonomy:

Strix scandiaca

Linnaeus

, 1758,

Lapland, north Finland

.

Previously placed in monotypic genus Nyctea, primarily on basis of skull characters; recent genetic study, however, indicated that it is part of the Bubo lineage#R. Monotypic.

Distribution:

Breeds from W & N Scandinavia E across N Russia and N Siberia, including Novaya Zemlya, to Chukotskiy Peninsula, Anadyrland, N Koryakland and Commander Is; then North America in W Aleutians (Attu and Buldir), Hall I (in Bering Sea) and from W Alaska E through N Canada (including Banks I, Prince Patrick I and N Ellesmere I) to N Labrador; also N & E Greenland#R. Has bred occasionally in Iceland and N Britain (Shetland Is). In winter some wander S into N USA (very occasionally as far S as California), to S Baltic region of Europe and, in Asia, to N China and SE Russia, rarely farther S.

Descriptive notes

Male 53–64 cm, 710–2500 g#R; female 59–66 cm, 780–2950 g#R; wingspan 125–166 cm. Large white owl with small eyes, rudimentary ear tufts normally not erect. Male almost entirely white, sometimes with narrow, sparse, pale grey or brown barring on back, wings and/or tail; tarsus and toes with thick white feathering; irides brilliant golden-yellow, at times almost orange, eyelids blackish-edged; cere dark grey and covered by dense feathering; bill blackish, nearly concealed by feathering; claws horn-coloured to black. Female noticeably larger, crown spotted dark, moderate to extensive barring above and below, wings and tail more prominently barred blackish, but variable, sometimes almost male-like. Juvenile dark mouse-brown, feathers tipped and speckled greyish white, barred; immature male like adult female, immature female even more heavily barred.

Drawing by Tim Worfolk
Descriptive notes:

Male 53–64 cm, 710–2500 g#R; female 59–66 cm, 780–2950 g#R; wingspan 125–166 cm. Large white owl with small eyes, rudimentary ear tufts normally not erect. Male almost entirely white, sometimes with narrow, sparse, pale grey or brown barring on back, wings and/or tail; tarsus and toes with thick white feathering; irides brilliant golden-yellow, at times almost orange, eyelids blackish-edged; cere dark grey and covered by dense feathering; bill blackish, nearly concealed by feathering; claws horn-coloured to black. Female noticeably larger, crown spotted dark, moderate to extensive barring above and below, wings and tail more prominently barred blackish, but variable, sometimes almost male-like. Juvenile dark mouse-brown, feathers tipped and speckled greyish white, barred; immature male like adult female, immature female even more heavily barred.

Drawing by Tim Worfolk
Descriptive notes:

Male 53–64 cm, 710–2500 g#R; female 59–66 cm, 780–2950 g#R; wingspan 125–166 cm. Large white owl with small eyes, rudimentary ear tufts normally not erect. Male almost entirely white, sometimes with narrow, sparse, pale grey or brown barring on back, wings and/or tail; tarsus and toes with thick white feathering; irides brilliant golden-yellow, at times almost orange, eyelids blackish-edged; cere dark grey and covered by dense feathering; bill blackish, nearly concealed by feathering; claws horn-coloured to black. Female noticeably larger, crown spotted dark, moderate to extensive barring above and below, wings and tail more prominently barred blackish, but variable, sometimes almost male-like. Juvenile dark mouse-brown, feathers tipped and speckled greyish white, barred; immature male like adult female, immature female even more heavily barred.

Voice

When disturbed, a repeated “kre” call, uttered in flight; female may produce a loud, intense whistling or mewing note, during such situations as coition and when agitated at nest; when excited, male may utter low, rapid, and repeated cackling “ka”; female produces a higher pitched but similar “ke” note. Male song a loud, booming “hoo, hoo,” usually double, but sometimes numbering six or more, the last often the loudest, and 1–2 second intervals after each; female song higher-pitched, notes often disyllabic; alarm a rapid, repeated cackling “kre-kre-kre”.

Habitat

Breeds on open Arctic tundra from near tree-line to edge of polar seas, with hummocks, rocks or other low prominences#R, and sparse low vegetation and dwarf shrubs and lichen; lowland salt meadows and poorly drained freshwater meadows; in general, breeds in areas with plentiful supply of Arctic or sub-Arctic rodents, usually at elevations below 300 m, except in Norway (where lemmings occur only on mountains at 1000 m or higher). In winter coastal fields and dunes, open moorlands and grasslands, marshes, agricultural fields; also winters on sea-ice at Arctic latitudes#R.

Food and feeding

Lemmings (Lemmus, Dicrostonyx) and other voles (Microtus, Clethrionomys) major food items, at times probably exploited exclusively; other prey taken according to availability, include other mammals to size of hares, birds to size of ptarmigans (Lagopus), ducks#R, medium-sized geese, seabirds#R#R, occasionally fish, amphibians, crustaceans and beetles. In Fenno-Scandia, of 2700 identified prey items, about a third were lemmings, and most (50·6%) were voles; in Shetland Is, where lemmings and voles absent, fed on rabbits or, when those rare, on chicks of various wading birds. In non-breeding season, mainly small mammals and birds, and some carrion: in British Columbia, grebes (mostly Podiceps) and ducks comprised 80% of prey by weight; in NE USA, where mammals abundant, rats and mice 35%, snowshoe hares 20%, and passerine birds 10% of diet. Two species of Microtus accounted for 99% of 5400 prey items identified from pellets collected in winter in Montana; other prey included pocket gophers (Thomomys), deer mice (Peromyscus), a weasel (Mustela), Grey Partridges (Perdix perdix), Common Pheasants (Phasianus colchicus), Northern Harriers (Circus hudsonius), and a Ring-billed Gull (Larus delawarensis)#R. Most active at dusk and dawn, but forages throughout day in summer; on Baffin I, hunts in all weathers and at all hours during continuous summer light, seemingly less around noon and midnight; details of activity during winter darkness uncertain. Hunts from perch, making low, often long flight to capture prey on ground; acute vision and hearing enables it to locate lemmings beneath snow; takes birds also from water’s surface, or pursues them in air; sometimes hovers; also pounces while standing or walking; said to catch fish while lying lengthwise belly down on rock by water hole. Small prey eaten whole, head first; larger ones first torn into pieces; birds sometimes partly plucked.

Breeding

Season May–Sept. Monogamous, often pairs for life; occasional polygyny and polyandry. Nest a shallow scrape on ground, usually in slightly elevated site (e.g. hummock, boulder) providing good view and drier conditions relative to lower sites#R; territory c. 3–4 km² but highly variable according to food abundance, can be much larger when prey scarce. Undulating courtship flight by male a series of flaps followed by glide with wings held in dihedral. Clutch size and nesting density#R vary with food supply, clutch 3–5 eggs when rodents limited, 7–11 when plentiful, average in Finnish Lapland 7·74; mean egg size 56·4 mm × 44·7 mm#R; eggs laid at 2-day intervals; incubation 31–33 days, by female, fed on nest by male; chick with greyish-white down; young brooded and fed by female, male delivers food to nest; young can walk outside nest at 14 days, most remain a few days longer, leave at 20–28 days still unable to fly; first attempts at flight at age of 36–43 days#R; fly well after 50 days, cared for by parents for at least further 10 weeks. Success very variable, even among neighbouring pairs: in Canada, 1 nest produced 11 young from 11 eggs, another 4 young from 10 eggs; in ideal conditions can be very high, e.g. 31 fledglings from 32 eggs in Canada; success dependent on food supply and timing of spring thaw, nesting aborted where food scarce; main causes of nestling mortality starvation, and chilling. In N Norway and Finland, two nests failed because of blackfly (Simuliidae) attacks on incubating females#R. First breeding probably at 2 years. Over a 3-year period, annual survival of breeding females equipped with satellite transmitters in the Canadian Arctic was 85·2–92·3%#R. Longevity record in wild a female ringed in Massachusetts and recaptured and released in Montana when at least 23 years and 10 months old; captive bird lived at least 28 years.

Movements

Mostly migratory and nomadic; some remain in breeding area all year if conditions allow. Movements are unpredictable, related in ways not fully understood to abundance of prey; thought to vary considerably from region to region across Arctic tundra, and intensity of movements fluctuates annually. Lemmings, the principal prey in the breeding season in many regions, fluctuate markedly in local abundance between seasons. A radio-tracking study of nine breeding females in the Canadian Arctic#R has recently shown for the first time that when settling to breed the birds search for long periods (up to 108 days) and may travel great distances (up to 4093 km) when searching for suitable locations. The time taken to settle, distance between searching areas, distance travelled and the duration of prospecting movements were longer in the year in which the density of lemmings recorded in the eastern high Arctic (Bylot I) was lowest. The owls eventually settled in areas where local lemming abundance was relatively high. Individual breeding dispersal distance between consecutive years averaged 725 km (range 18–2224 km). Nearctic populations periodically irruptive, with most individuals leaving breeding areas when lemming numbers crash. Winters irregularly S to C & SC USA, N & C Europe, NC Russia, NW & NE China and Japan, occasionally farther S; on average, immature males winter farthest S, adult females farthest N, with adult males and immature females in between. Vagrants recorded as far S as Bermuda, the Azores and Hawaii.

Status and conservation

VULNERABLE. CITES II. Generally uncommon to scarce. Until recently, due to marked population fluctuations and unpredictability of species’ prey-dependent breeding densities and wintering distribution, global population estimates have been very crude, ranging from 200,000#R individuals to 100,000–500,000 individuals#R. However, birds are now thought to be sparsely distributed, with subpopulations clumped around food resources, rather evenly distributed across the tundra. On this basis, global population is calculated to be c. 14,000 pairs or 28,000 mature individuals#R, which corresponds closely with the previous estimate of a maximum of 14,000 females produced by analysis of the genetic structure of Scandinavian, North American and Siberian populations#R. In North America, overall status has long been presumed little changed; however, given the challenges of estimating numbers, such statements of doubtful value and impossible to test. North American populations are now estimated at <30,000 individuals (probably an overestimate) and to have declined by 64% between 1970 and 2014#R. Many apparently die from starvation during movement S from Arctic regions; in examination of specimens in Alberta, however, 45% had moderate to heavy fat deposits, and only 14·1% believed to have starved; in that study, traumatic injuries were major cause of mortality, including collisions with unknown objects (46·5%), automobiles (14·1%), utility lines (4·2%) and airplanes (1·4%), also gunshot wounds (12·7%), electrocution (5·6%), and entanglement in fishing tackle (1·4%). In N Europe believed to have declined, but no real information on any long-term trends; rarity may be due to humans as much as to contraction of Arctic habitats; in N Scandinavia, breeding density 40–55 pairs/100 km² in one peak lemming year, but only 15 pairs/100 km² in next peak year; species no longer breeds in Shetland and rarely does in Iceland. No information on N Asian populations. Taking into account new estimates, global population is now thought to be much smaller than previously assumed and to be declining rapidly#R. Not previously considered of conservation concern, the species was therefore assessed as Vulnerable for the first time in 2017#R. Protective measures prohibit the shooting and trapping of owls for any reason; harvesting of species for food, feathers and claws by native peoples may have local impact on population, but unlikely to have any wider effect on total numbers; policies enacted to protect large birds from electrocution and airplane strikes should benefit this species.

Recommended citation

Holt, D.W., Berkley, R., Deppe, C., Enríquez Rocha, P., Petersen, J.L., Rangel Salazar, J.L., Segars, K.P., Wood, K.L., Garcia, E.F.J., Marks, J.S. & Sharpe, C.J. (2018). Snowy Owl (Bubo scandiacus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/55027 on 22 June 2018).