Schnee-Eule

Bestimmung

The snowy owl is mostly white. They are purer white than predatory mammals like polar bears (Ursus maritimus) and Arctic fox (Vulpes lagopus). Often when seen in the field, these owls can resemble a pale rock or a lump of snow on the ground. It usually appears to lack ear tufts but very short (and probably vestigial) tufts can be erected in some situations, perhaps most frequently by the female when she is sitting on the nest. The ear tufts measure about 20 to 25 mm (0.79 to 0.98 in) and consist of about 10 small feathers. The snowy owl has bright yellow eyes. The head is relatively small and, even for the relatively simply adapted hearing mechanism of a Bubo owl, the facial disc is shallow and the ear is uncomplicated. 1 male had ear slits of merely 21 mm × 14 mm (0.83 in × 0.55 in) on left and 21 mm × 14.5 mm (0.83 in × 0.57 in) on the right. Females are almost invariably more duskily patterned than like-age males. In mature males, the upper parts are plain white with usually a few dark spots on the miniature ear-tufts, about the head and the tips of some primaries and secondaries whilst the underside is often pure white. Despite their reputation for being purely white, only 3 out of 129 Russian museum specimens of adult males showed an almost complete absence of darker spots. The adult female is usually considerably more spotted and often slightly barred with dark brown on the crown and the underparts. Her flight and tail feathers are faintly barred brown while the underparts are white in base color with brown spotting and barring on the flanks and upper breast. In confusingly plumaged snowy owls, the sex can be determined by the shape of wing markings, which manifest as bars more so in females and spots in males. However, the very darkest males and the lightest females are nearly indistinguishable by plumage. On rare occasion, a female can appear almost pure white, as has been recorded in both the field and in captivity. There is some evidence that some of the species grow paler with age after maturity. One study's conclusions were that males were usually but not always lighter and that correctly aging is extremely difficult, sometimes individuals either get lighter, darker or do not change their appearance with age. On the other hand, with close study, it is possible to visually identify even individual snowy owls using the pattern of markings on the wing, which can be somewhat unique in each individual. After a fresh moult, some adult females that previously appeared relatively pale newly evidenced dark, heavy markings. On the contrary, some banded individuals over at least four years were observed to have been almost entirely unchanged in the extent of their markings. In another very pale owl, the barn owl (Tyto alba), the sexual dimorphism of spotting appears to be driven by genetics while, in snowy owls, environment may be the dictating factor instead. <p>The chicks are initially grayish white but quickly transition to dark gray-brown in the mesoptile plumage. This type of plumage camouflages effectively against the variously colored lichens that dot the tundra ground. This is gradually replaced by plumage showing dark barring on white. At the point of fledging, the plumage often becomes irregularly mottled or blotched with dark and is mostly solidly dark gray-brown above with white eyebrows and other areas of the face white. Recently fledged young can already be sexed to a semi-reliable degree by the dark marking patterns about their wings. The juvenile plumage resembles that of adult females but averages slightly darker on average. By their second moult fewer or more broken bars are usually evidenced on the wing. The extent of white and composition of wing patterns become more dimorphic by sex with each juvenile moult, culminating in the 4th or 5th pre-basic moult, wherein the owls are hard to distinguish from mature adults. Moults usually occur from July and September, non-breeding birds moulting later and more extensively, and are never extensive enough to render the owls flightless. Evidence indicates that snowy owls may attain adult plumage at 3 to 4 years of age, but fragmentary information suggests that some males are not fully mature and/or as fully white in plumage that they can attain until the 9th or 10th year. Generally speaking, moults of snowy owls occur more quickly than do those of Eurasian eagle-owls.<p>The toes of the snowy owl are extremely thickly feathered white, while the claws are black. The toe feathers are the longest known of any owl, averaging at 33.3 mm (1.31 in), against the great horned owl which has the 2nd longest toe feathers at a mean of 13 mm (0.51 in) Occasionally, snowy owls may show a faint blackish edge to the eyes and have a dark gray cere, though this is often not visible from the feather coverage, and a black bill. Unlike many other whitish birds, the snowy owl does not possess black wingtips, which is theorized to minimize wear-and-tear on the wing feathers in the other whitish bird types. The conspicuously notched primaries of the snowy owl appear to give an advantage over similar owls in long-distance flight and more extensive flapping flight. The snowy owl does have some of the noise-canceling serrations and comb-like wing feathers that render the flight of most owls functionally silent, but they have fewer than most related Bubo owls. Therefore, in combination with its less soft feathers, the flight of a snowy owl can be somewhat audible at close range. The flight of snowy owls tends to be steady and direct and is reminiscent to some of the flight of a large, slow-flying falcon. Though capable of occasional gliding flight, there is no evidence that snowy owls will soar. It is said that the species seldom exceeds a flying height of around 150 m (490 ft) even during passage. While the feet are sometimes described as "enormous", the tarsus is in osteological terms relatively short at 68% the length of those of a Eurasian eagle-owl but the claws are nearly as large, at 89% of the size of those of the eagle-owl. Despite its relatively short length, the tarsus is of similar circumference as in other Bubo owls. Also compared to an eagle-owl, the snowy owl has a relatively short decurved rostrum, a proportionately greater length to the interorbital roof and a much longer sclerotic ring surrounding the eyes while the anterior opening are the greatest known in any owl. Owls have extremely large eyes which are nearly the same size in large species such as the snowy owl as those of humans. The snowy owl's eye, at about 23.4 mm (0.92 in) in diameter, is slightly smaller than those of great horned and Eurasian eagle-owls but is slightly larger than those of some other large owls. Snowy owls must be able to see from great distances and in highly variable conditions but probably possess less acute night vision than many other owls. Based on the study of dioptres in different owl species, the snowy owl was determined to have eyesight better suited to long-range perception than to close discrimination, while some related species such as great horned owls could probably more successful perceive closer objects. Despite their visual limits, snowy owls may have up to 1.5 times more visual acuity than humans. Like other owls, snowy owls can probably perceive all colors but cannot perceive ultraviolet visual pigments. Owls have the largest brains of any bird (increasing in sync with the size of the owl species), with the size of the brain and eye related less to intelligence than perhaps to increased nocturnality and predatory behavior.<p>The snowy owl is a very large owl. They are the largest avian predator of the High Arctic and one of the largest owls in the world. Snowy owls are about the sixth or seventh heaviest living owl on average, around the fifth longest and perhaps the third longest winged. This species is the heaviest and longest winged owl (as well as the second longest) in North America, the second heaviest and longest winged owl in Europe (and third longest) but is outsized in bulk by about 3 to 4 other species in Asia. Despite being sometimes described as of similar size, the snowy owl is somewhat larger in all aspects of average size than the great horned owl while the similarly specialized taiga-dwelling great grey owl (Strix nebulosa), is longer in total length and of similar dimensions in standard measurements, but is shorter winged and much less heavy than the snowy owl. In Eurasia, the Eurasian eagle-owl is larger in all standards of measurements than the snowy owl not to mention two additional species each from Africa and Asia that are slightly to considerably heavier on average than the snowy owl. Like most birds of prey, the snowy owl shows reverse sexual dimorphism relative to most non-raptorial birds in that females are larger than males. Sexual dimorphism that favors the female may have some correlation with being able to more effectively withstand food shortages such as during brooding as well as the rigors associated with incubating and brooding. Females are sometimes described as “giant” whereas males appear relatively “neat and compact”. However, the sexual dimorphism is relatively less pronounced compared to some other Bubo species.<p>Male snowy owls have been known to measure from 52.5 to 64 cm (20.7 to 25.2 in) in total length, with an average from four large samples of 58.7 cm (23.1 in) and a maximum length, perhaps in need of verification, of reportedly 70.7 cm (27.8 in). In wingspan, males may range from 116 to 165.6 cm (3 ft 10 in to 5 ft 5 in), with a mean of 146.6 cm (4 ft 10 in). In females, total length has been known to range from 54 to 71 cm (21 to 28 in), with a mean of 63.7 cm (25.1 in) and an unverified maximum length of perhaps 76.7 cm (30.2 in) (if so they would have the second longest maximum length of any living owl, after only the great grey owl). Female wingspans have reportedly measured from 146 to 183 cm (4 ft 9 in to 6 ft 0 in), with a mean of 159 cm (5 ft 3 in). Despite one study claiming that snowy owl had the highest wing loading (i.e. grams per square cm of wing area) of any of 15 well-known owl species, more extensive sampling demonstratively illustrated that the wing loading of snowy owls is notably lower than Eurasian eagle- and great horned owls. The conspicuously long-winged profile of a flying snowy owl compared to these related species may cause some to compare their flight profile to a bulkier version of an enormous Buteo or a large falcon. Body mass in males can average from 1,465 to 1,808.3 g (3.230 to 3.987 lb), with a median of 1,658.2 g (3.656 lb) and a full weight range of 1,300 to 2,500 g (2.9 to 5.5 lb) from six sources. Body mass in females can average from 1,706.7 to 2,426 g (3.763 to 5.348 lb), with a median of 2,101.8 g (4.634 lb) and a full weight range of 1,330 to 2,951 g (2.932 to 6.506 lb). Larger than the aforementioned body mass studies, a massive pooled dataset at six wintering sites in North America showed that 995 males averaged at 1,636 g (3.607 lb) while 1,189 females were found to average 2,109 g (4.650 lb). Reported weights of down to 710 g (1.57 lb) for males and of 780 to 1,185 g (1.720 to 2.612 lb) for females are probably in reference to owls in a state of starvation. Such emaciated individuals are known to highly impaired and starvation deaths are probably not infrequent in winters with poor food accesses. <p>Standard measurements have been even more widely reported than length and wingspan. The wing chord of males can vary from 351 to 439 mm (13.8 to 17.3 in), averaging from 380.1 to 412 mm (14.96 to 16.22 in) with a median of 402.8 mm (15.86 in). The wing chord of females can vary from 380 to 477.3 mm (14.96 to 18.79 in), averaging from 416.2 to 445 mm (16.39 to 17.52 in) with a median of 435.5 mm (17.15 in). The tail length of males can vary on average from 209.6 to 235.4 mm (8.25 to 9.27 in), with a full range of 188 to 261 mm (7.4 to 10.3 in) and a median of 227 mm (8.9 in). The tail length of females can average from 228.5 to 254.4 mm (9.00 to 10.02 in), with a full range of 205 to 288 mm (8.1 to 11.3 in) and a median of 244.4 mm (9.62 in). Data indicates that slightly longer wing chord and tail lengths were reported on average in Russian data than in American research, however the weights were not significantly different in the two regions. Less widely taken measurements include the culmen, which can measure from 24.6 to 29 mm (0.97 to 1.14 in) with a median average of 26.3 mm (1.04 in) in males and 27.9 mm (1.10 in) in females, and the total bill length which is from 25 to 42 mm (0.98 to 1.65 in), with an average in both sexes of 35.6 mm (1.40 in). Tarsal length in males averages about 63.6 mm (2.50 in), with a range of 53 to 72 mm (2.1 to 2.8 in), and averages about 66 mm (2.6 in), with a range of 54 to 75 mm (2.1 to 3.0 in), in females.<p>The snowy owl is certainly one of the most unmistakable owls (or perhaps even animals) in the world. No other species attains the signature white stippled sparsely with black-brown color of these birds, a coloring which renders their bright yellow eyes all the more detectable, nor possesses their obvious extremely long feathering. The only other owl to breed in the High Arctic is the short-eared owl (Asio flammeus). Both species inhabit open country, overlap in range and are often seen by day, but the short-eared is much smaller and more tan or straw-colored in coloration, with streaked brown on chest. Even the palest short-eared owls conspicuously differ and are darker than the snowy owl; additionally the short-eared most often hunts in extended flights. More similar owls such as the Eurasian eagle-owl and the great horned owl attain a fairly pale, sometimes white-washed look in their northernmost races. These species do not normally breed nearly as far north as snowy owls but overlaps certainly do occur when snowy owls when the latter owl sometimes comes south in winter. However, even the most pale great horned and Eurasian eagle-owls are still considerably more heavily marked with darker base colors than snowy owls (the whitest eagle-owls are paler than the whitest great horned owls), possess much larger and more conspicuous ear tufts and lack the bicolored appearance of the darkest snowy owls. While the great horned owl has yellow eyes like the snowy owl, the Eurasian eagle-owl tends to have bright orange eyes. The open terrain habitats normally used by wintering snowy owls are also distinct from the typical edge and rocky habitats usually favored by the great horned and Eurasian eagle-owls, respectively.<p>The snowy owl differ in their calls from other Bubo owls, with a much more barking quality to their version of a hooting song. Perhaps as many as 15 different calls by mature snowy owls have been documented. The main vocalization is a monotonous sequence that normally contains 2–6 (but occasionally more), rough notes similar to the rhythm of a barking dog: krooh krooh krooh krooh... The call may end with an emphatic aaoow, which is somewhat reminiscent of the deep alarm call of a great black-backed gull (Larus marinus). They will call mainly from a perch but also sometimes do so in flight. The krooh call of the male snowy owl may perform multiple functions such as competitive exclusion of other males and advertising to females. The calls of this species may carry exceptionally far in the thin air of Arctic, certainly over more than 3 km (1.9 mi), and maybe even to as much 10 to 11 km (6.2 to 6.8 mi) away. The female has a similar call to male but can be higher-pitched and/or more guttural as well as single notes which are often disyllabic, khuso. Female snowy owls have also been known to utter chirps and high screaming notes, similar to those of the nestlings. Both sexes may at times give a series of clucking, squeals, grunts, hisses and cackles, perhaps such as in circumstances when they are excited. The alarm call is a loud, grating, hoarse keeea. Another raspier bark is recorded, sometimes called a "watchman's rattle" call, and may be transcribed as rick, rick, rick, ha, how, quack, quock or kre, kre, kre, kre, kre. A female attacking to protect her nest was recorded to let out a crowed ca-ca-oh call, whilst other owls attacking to protect the nest did a loud version of the typical call while circling before dropping down. They may also clap their beak in response to threats or annoyances. While called clapping, it is believed this sound may actually be a clicking of the tongue, not the beak. Though largely only vocal in the breeding season, leading to some erroneous older accounts describing the snowy owl as completely silent, some vocalizations have been recorded in winter in the northern United States. Initially, the young of the snowy owl have a high pitched and soft begging call which develops into a strong, wheezy scream at around 2 weeks. At the point when the young owls leave the nest around 3 weeks, the shrill squeals they emit may allow the mothers to locate them.

Taxonomie

The snowy owl was one of the many bird species originally described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae, where it was given the binomial name Strix scandiaca. The genus name Bubo is Latin for "horned owl" and scandiacus is Neo-Latin for "of Scandinavia". The former generic name Nyctea is derived from Greek meaning "night". Linnaeus originally described the different plumages of this owl as separate species, with the male specimens of snowy owls being considered Strix scandiaca and the likely females considered as Strix nyctea. Until recently, the snowy owl was regarded as the sole member of a distinct genus, as Nyctea scandiaca, but mtDNA cytochrome b sequence data shows that it is very closely related to the horned owls in the genus Bubo and the species is now thusly often considered inclusive with that genus. However, some authorities debate this classification, still preferring Nyctea. Often authorities are motivated to retain the separate genus on the grounds of osteological distinctions.<p>Genetic testing revealed a reasonably distinct genetic makeup for snowy owls, being about 8% genetically distinct from other Bubo owls, perhaps giving credence to those who count the species as separate under Nyctea. However, a fairly recent shared origin in evolutionary history has been illustrated through a combination of genetic study and fossil review and there is little, other than osteology of the tarsometatarsus, to outright distinguish the snowy owl from other modern species like the Eurasian eagle-owl (Bubo bubo). Genetic testing has indicated that the snowy owl may have diverged from related species at around 4 million years ago. Furthermore, it has determined that the living species genetically most closely related to the snowy owl is the great horned owl (Bubo virginianus). On a broader scale, owls in general have, through genetic materials, been determined to be a highly distinct group, with outwardly similar groups such as Caprimulgiformes revealed to not be at all closely related. Within the owl order, typical owls are highly divergent from barn-owls. Furthermore, the Bubo genus likely clustered at some point during the evolutionary process with other largish owls, such as Strix, Pulsatrix and Ciccaba, based on broad similarities in their voice, reproductive behaviors (i.e. hooting postures) and a similar number and structure of chromosomes and autosomes. A number, but not all, of extant typical owls seem to have evolved from an ancient shared common ancestor with the Bubo owls. In addition to the question of relationship of the traditional Bubo owls to the snowy owls, ongoing ambiguity of the relationship of other similarly large-sized owls has been persistent. These have sometimes either been included in the genus or within separate genera, i.e. the Ketupu or fish owls and the Scotopelia or fishing owls. Despite the adaptive distinctions, the grouping of these large owls (i.e. Bubo, snowy, fish and perhaps fishing owls) appears to be borne out via research of karyotypes.<p>The fossil history of snowy owls is fairly well documented despite some early confusion on how to distinguish the skeletal structure of the snowy owls from eagle-owls. It was determined that the snowy owl once was distributed much more widely and far farther to the south during the Quaternary glaciation when much of the Northern Hemisphere was in the midst of an ice age. Fossil records shows that snowy owls once could be found in Austria, Azerbaijan, Czechoslovakia, England, France, Germany, Hungary, Italy, Poland, Sardinia and Spain as well as in the Americas in Cape Prince of Wales, Little Kiska Island, St. Lawrence Island, and in Illinois. In the Late Pleistocene the range expanded southward even more so to Bulgaria (80,000–16,000 years, Kozarnika Cave, W Bulgaria). and much of the Italian Peninsula. Pleistocene era fossils from France, i.e. B. s. gallica, showed that the snowy owls of the time were somewhat bulkier (though still notably smaller than contemporary eagle-owls of the times, which were larger than the eagle-owls of today) and osteologically more sexually dimorphic in size than the modern form (9.9% dimorphism in favor of females in the fossils against 4.8% in the same features today). There are no subspecific or other geographical variations reported in the modern snowy owls, with individuals of vastly different origins breeding together readily due to their nomadic habits. Despite apparent variations in body size, environmental conditions are the likely variant rather than genetics. No evidence could be found of phylogeographic variation in snowy owls upon testing. Furthermore, the snowy owl appears to have a similar level of genetic diversity as other European owls.<p>Snowy owls are not known to interbreed with other owl species in the wild, and accordingly, no hybrids of snowy owls and other owl species have yet been sighted in the wild. However, a hobby falconer in Kollnburg, Germany, bred hybrids from a male snowy owl and a female Eurasian eagle-owl (Bubo bubo) in 2013. The two resulting male hybrid owls possessed the prominent ear-tufts (generally absent in snowy owls), general size, orange eyes, and the same pattern of black markings on their plumage from their Eurasian eagle-owl mother, while retaining the generally black-and-white plumage colours from their snowy owl father. The hybrids were dubbed "Schnuhus" from the German words for snowy owl and Eurasian eagle-owl (Schnee-Eule and Uhu, respectively). As of 2014, the hybrids had grown to maturity and were healthy.

Wissenschaftlicher Name

Bubo scandiacus

Autorität

(Linnaeus, 1758)

Ordnung

Strigiformes

Familie

Strigidae

Gattung

Bubo

Verbreitung und Lebensraum

The snowy owl is typically found in the northern circumpolar region, where it makes its summer home north of latitude 60° north though sometimes down to 55 degrees north. However, it is a particularly nomadic bird, and because population fluctuations in its prey species can force it to relocate, it has been known to breed at more southerly latitudes. Although the total breeding range includes a little over 12,000,000 km2 (4,600,000 sq mi), only about 1,300,000 km2 (500,000 sq mi) have a high probability of breeding, i.e. breeding at no more than 3–9-year intervals. Snowy owls nest in the Arctic tundra of the northernmost stretches of Alaska, Northern Canada, and the Euro-Siberian region.<p>Between 1967 and 1975, snowy owls bred on the remote island of Fetlar in the Shetland Isles north of mainland Scotland, discovered by the Shetland RSPB warden, Bobby Tulloch. Females summered as recently as 1993, but their status in the British Isles is now that of a rare winter visitor to Shetland, the Outer Hebrides and the Cairngorms. Vagrant snowy owls have occasionally been found as far south as Lincolnshire. Older records show that the snowy owls may have once semi-regularly bred elsewhere in Shetland. They range in northern Greenland (mostly Peary Land) and, rarely in "isolated parts of the highlands", Iceland. Thence, they are found breeding at times across northern Eurasia such as in Spitsbergen and western and northern Scandinavia. In Norway, they normally breed in Finnmark and seldom down as far south as Hardangervidda and in Sweden perhaps down to the Scandinavian Mountains while breeding is very inconsistent in Finland.<p>They also range in much of northern Russia, including northern Siberia, Anadyr, Koryakland, Taymyr Peninsula, Yugorsky Peninsula, Sakha (especially the Chukochya River) and Sakhalin. Breeding has also been reported sporadically to the south in the Komi Republic and even the Kama River in southern Perm Krai. Although considered part of their regular range, the last breeding by snowy owls in the Kola Peninsula was in the early 1980s; similarly, breeding maps show the species in Arkhangelsk Oblast and the Pay-Khoy Ridge but no breeding records are known in at least 30 years in either. They range throughout most of the Arctic isles of Russia such as Novaya Zemlya, Severnaya Zemlya, New Siberian Islands, Wrangel Island, Commander and Hall Islands.<p>In North America, the breeding range has been known in modern times to include the Aleutians (i.e. Buldir and Attu) and much of northern Alaska, most frequently from the Arctic National Wildlife Refuge to Utqiaġvik, and more sporadically down along the coastal-western parts such as through Nome, Hooper Bay, the Yukon Delta National Wildlife Refuge, and rarely even south to the Shumagin Islands. The snowy owl may breed extensively in northern Canada, largely making its home in the Arctic Archipelago. Their Canadian breeding range can include broadly Ellesmere Island up to Cape Sheridan, north coastal Labrador, the northern Hudson Bay, perhaps all of Nunavut (especially the Kivalliq Region), northeastern Manitoba, most of northern mainland and insular Northwest Territories (including the delta of the Mackenzie River) and northern Yukon Territory (where breeding is mostly confined to Herschel Island). Since breeding and distribution is very small, local and inconsistent in northern Europe, northern Canada and northern Alaska represent the core part of the breeding range for snowy owls along with several parts of northern and northeastern/coastal Russia.<p>During the winter months, many snowy owls leave the dark Arctic to migrate to regions further south. Southern limits of the regular winter range are difficult to delineate given the inconsistency of appearances south of the Arctic. Furthermore, not infrequently, many snowy owls will overwinter somewhere in the Arctic through the winter, though seldom appear to do so in the same sites where they have bred. Due in no small part to the difficulty and hazardousness of observation for biologists during these harsh times, there is very limited data on overwintering snowy owls in the tundra, including how many occur, where they winter and what their ecology is at this season. The regular wintering range has at times been thought to include Iceland, Ireland and Scotland and across northern Eurasia such as southern Scandinavia, the Baltics, central Russia, southwestern Siberia, Sakhalin southern Kamchatka and, rarely, north China and sometimes the Altai Republic. In North America, they occasionally regularly winter in the Aleutian island chain and do so broadly and with a fair amount of consistency in much of southern Canada, from British Columbia to Labrador. Recent research has indicated that snowy owls regularly winter in several of the northern seas during wintertime, following the leads of sea ice as perching sites and presumably hunting mostly seabirds in polynyas. In February 1886, a snowy owl landed on the rigging of the Nova Scotia steamship Ulunda on the edge of the Grand Banks of Newfoundland, over 800 km (31,000,000 in) from the nearest land. It was captured and later preserved at the Nova Scotia Museum. Surprisingly, some studies have determined that after a high lemming year in North America, a higher percentage of snowy owls were using marine environments rather than inland ones.<p>Large winter irruptions at temperate latitudes are thought to be due to good breeding conditions resulting in more juvenile migrants. These result in irruptions occurring further south than the typical snowy owl range in some years. They have been reported, as well as in all northerly states in the contiguous states, as far south as the Georgia, Kentucky, South Carolina, nearly all the Gulf Coast of the United States, Colorado, Nevada, Texas, Utah, California and even Hawaii. In January 2009, a snowy owl appeared in Spring Hill, Tennessee, the first reported sighting in the state since 1987. Also notable is the mass southern migration in the winter of 2011/2012, when thousands of snowy owls were spotted in various locations across the United States. This was then followed by an even larger mass southern migration in 2013/2014 with the first snowy owls seen in Florida for decades. The nature of irruptions is less well-documented in Eurasia, in part due to the paucity of this owl in the European side, but accidental occurrence, presumably during irruptions, has been described in the Mediterranean area, France, Crimea, the Caspian part of Iran, Kazakhstan, northern Pakistan, northwestern India, Korea and Japan. Stragglers may too turn up as far south as the Azores and Bermuda.<p>Snowy owls are one of the best known inhabitants of the open Arctic tundra. Frequently, the earth in snowy owl breeding grounds is covered with mosses, lichens and some rocks. Often the species preferentially occurs in areas with some rising elevation such as hummocks, knolls, ridges, bluffs and rocky outcrops. Some of these rises in the tundra are created by glacial deposits. The ground is usually rather dry in tundra but in some areas of the southern tundra can also be quite marshy. Not infrequently, they will also use areas of varied coastal habitat, often tidal flats, as a breeding site. Breeding sites are usually at low elevations, usually less than 300 m (980 ft) above sea level, but when breeding to the south in inland mountains, such as in Norway, they may nest at as high as 1,000 m (3,300 ft). Outside the breeding season, snowy owls may occupy nearly any open landscape. Typically wintering sites are rather windswept with meager cover. These open areas can include coastal dunes, other coastal spots, lakeshores, islands, moorlands, steppes, meadows, prairies, other extensive grasslands and rather shrubby areas of the Subarctic. These may be favored due to their vague similarity to the flat openness of the tundra. Manmade open sites are now perhaps even more used than natural ones, often agricultural fields and rangeland, as well as large areas of cleared forests. During irruption years when they are found in the northeastern United States, juveniles frequent developed areas including urban areas and golf courses, as well as the expected grasslands and agricultural areas that older birds primarily use. On the plains of Alberta, observed snowy owls spent 30% of their time in stubble-fields, 30% in summer fallow, 14% in hayfield and the remainder of the time in pasture, natural grasslands and sloughs. The agricultural areas, large untouched by the farmers in winter, may have had more concentrated prey than the others in Alberta. Perhaps the most consistently attractive habitat in North America to wintering snowy owls in modern times may be airports, which not only tend to have the flat, grassy characteristics of their preferred habitats but also by winter host a particular diversity of prey, both pests which rely on humans as well as wildlife attracted to the extensively grassy and marshy strips that dot the large airport vicinities. For example, Logan International Airport in Massachusetts has relatively one of the most reliable annual populations known in the United States in winter. All ages spend a fair amount of their time over water in the Bering Sea, the Atlantic Ocean and even the Great Lakes, mostly on ice floes. These marine and ocean-like freshwater areas were observed to account for 22–31% of habitat used in 34 radio-tagged American snowy owls over two irruptive years, with the tagged owls occurring a mean of 3 km (1.9 mi) from the nearest land (while 35–58% used the expected preferred habitats of grassland, pasture and other agricultural land).

Zugverhalten

Complete migrants

Verhalten

Snowy owls may be active to some extent at both day, from dawn to dusk, and night. Snowy owls have been seen to be active even during the very brief winter daytime in the northern winter. During the Arctic summer, snowy owls may tend to peak in activity during the twilight that is the darkest time available given the lack of full nightfall. Reportedly, the peak time of activity during summer is between 9:00 pm and 3:00 am in Norway. The peak time of activity for those owls that once nested on Fetlar was reported between 10:00 and 11:00 pm. According to one authority, the least active times are at noon and midnight. As days become longer near autumn in Utqiaġvik, the snowy owls in the tundra become more active around nightfall and can often be seen resting during the day, especially if it is raining. During winter in Alberta, snowy owls were tracked in the daytime, despite being also active at night (as they were deemed too difficult to track). In the study, they were most active during 8:00–10:00 am and 4:00–6:00 pm and often rested mostly from 10:00 am to 4:00 pm. The owls were perched for 98% of observed daylight and seemed to time their activity to peak times for rodents. The variation of activity is probably in correspondence with their primary prey, the lemmings, and like them, the snowy owl may be considered cathermal. This species can withstand extremely cold temperatures, having been recorded in temperatures as low as minus 62.5 degrees Celsius with no obvious discomfort and also withstood a 5-hour exposure to minus 93 degrees Celsius but may have struggled with oxygen consumption by the end of this period. The snowy owl has perhaps the second lowest thermal conduction to the plumage on average of any bird after only the Adelie penguin (Pygoscelis adeliae) and rivals the best insulated mammals, such as Dall sheep (Ovis dalli) and Arctic fox, as the best insulated polar creature. Presumably as many as 7 rodents would need to be eaten daily to survive an extremely cold winter's day. Adults and young both have been seen to shelter behind rocks to shield themselves from particularly harsh winds or storms. Snowy owls often spending a majority of time on the ground, perched mostly on a slight rise of elevation. It has been interpreted from the morphology of their skeletal structure (i.e. their short, broad legs) that snowy owls are not well-suited to perching extensively in trees or rocks and prefer a flat surface to sit upon. However, they may perch more so in winter though do so only mainly when hunting, at times on hummocks, fenceposts, telegraph poles by roads, radio and transmission towers, haystacks, chimneys and the roofs of houses and large buildings. Rocks may be used as perches at times in all seasons. Though often relatively sluggish owls, like most related species, they are capable of sudden dashing movements in various contexts. Snowy owls can walk and run quite quickly, using outstretched wings for balance if necessary. This owl flies with fairly rowing wingbeats, occasionally interrupted by gliding on stretched wings. The flight is fairly buoyant for a Bubo owl. When displaying, the male may engage in an undulating flight with interspersed wingbeats and gliding in a slight dihedral, finally dropping rather vertically to the ground. They are capable for swimming but do not usually do so. Some seen to be swimming were previously injured but young have been seen to swim into water to escape predators if they cannot fly yet. They will also drink when unfrozen water is available. Snowy owl mothers have been observed to preen their young in the wild, while pairs in captivity have been observed to allopreen. In the period leading up to breeding, snowy owls switched regularly between searching (for nesting grounds) and loafing, often searching less when snow cover was less extensive. <p>Snowy owls will fight with conspecifics in all seasons occasionally but this is relatively infrequent during breeding and rarer still during winter. Dogfights and talon interlocking may ensue if the fight between two snowy owls continues to escalate. A study determined that snowy owls are able to orient the whitest parts of their plumage towards the sun, spending about 44% of time oriented as such during sunny days and much less on cloudy days. Some authors interpret this as a presumed signal to conspecifics, but thermoregulation could also be a factor. It is known that during winter in Alberta that female snowy owls are territorial towards one another and may not leave an area for up to 80 days but males are nomadic, usually only staying 1–2 days in an area (seldom to 3–17 days). The females spent on average seven times as long in a given area than did males. During threat displays, individuals will lower the front of the body, stretch the head low and forward, with partially extended wings and feathers on the head and raise their back. If continuously threatened or cornered, the posture in the threat display may become still more contoured and, if pressed, the owl will like back and attempt to slash with its large talons. The threat displays of males are generally more emphatic than those of females. Although snowy owls have been considered as semi-colonial, they do not appear to fit this mold well. Nesting sites can be loosely clustered but this is a coincidental response to concentrated prey and each pair tends to be somewhat intolerant of each other. During winter, snowy owls are usually solitary but some aggregations have been recorded, especially nearer the Arctic when more narrow food selection can lead to up to 20–30 owls gathering in an area of about 20 to 30 ha (49 to 74 acres). Congregations were also recorded in the winter in Montana, where 31–35 owls wintered in a 2.6 km2 (1.0 sq mi) area, owls mostly grouped in loose aggregations of 5–10 owls each or occasionally side-by-side or about 20 m (66 ft) apart. In extreme cases in Utqiaġvik, the owls may have exceptionally close active nests that may be down to only 800 to 1,600 m (2,600 to 5,200 ft) apart. Juvenile males appear to be especially prone to loose associations with one another, appearing to be non-territorial and able to hunt freely in front of one another. In a 213 km2 (82 sq mi) area in and around Utqiaġvik, productive years may have about 54 nests while none may be found in poor years. Utqiaġvik may have about 5 owls in early summer every 1.6 km (0.99 mi), have a nest spacing of 1.6 to 3.2 km (0.99 to 1.99 mi) and the owls territory size is about 5.2 to 10.2 km2 (2.0 to 3.9 sq mi). In Churchill, Manitoba, nest spacing averaged about 3.2 km (2.0 mi). In Southampton Island in a year when the owls nested there, nest spacing averaged 3.5 km (2.2 mi), with the closest two 1 km (0.62 mi) apart and density per nest was 22 km2 (8.5 sq mi). In Nunavut, densities could go from 1 owl per 2.6 km2 (1.0 sq mi) in a productive year to 1 owl per 26 km2 (10 sq mi) in a poor year and from 36 nests in a 100 km2 (39 sq mi) area to none at all. Owl density on Wrangel Island in Russia was observed be a single bird each 0.11 to 0.72 km2 (0.042 to 0.278 sq mi). The first known study of winter territories took place in Horicon Marsh where owls ranged from 0.5 to 2.6 km2 (0.19 to 1.00 sq mi) each. In Calgary, Alberta, mean territory size of juvenile females in winter was 407.5 ha (1,007 acres) and adult females was 195.2 ha (482 acres). Wintering owls in central Saskatchewan were radio-monitored, determining that 11 males had an average range of 54.4 km2 (21.0 sq mi), while that of 12 females was 31.9 km2 (12.3 sq mi) with the combined average being 53.8 km2 (20.8 sq mi).<p>It is fair to say that the snowy owl is a partial, if fairly irregular, migrant, having a very broad but patchy wintering range. 1st year birds tend to disperse farther south in winter than older owls with males wintering usually somewhat more to the south than females of equivalent ages, adult females often wintering the farthest north. The snowy owl likely covers more ground than almost any other owl in movements but many complex individual variations are known in movements, and they often do not take the traditional north–south direction that might be assumed. Migratory movements appear to be somewhat more common in America than in Asia. A study of wintering owls in the Kola Peninsula determined that the mean date of arrival of owls was 10 November with a departure date of 13 April, covering an average of 991 km (616 mi) during the course of the wintering period and clustering where prey was more concentrated. Some variety of movements recorded each autumn and snowy owls winter annually in plains of Siberia and Mongolia and prairies and marshlands of Canada. The Great Plains area of southern Canada host wintering snowy owls about 2 to 10 times more frequently than other areas of the continent. Some weak correlation has made with individuals having some level of fealty to certain wintering sites. Wintering snowy owls, a total of 419, recorded in Duluth, Minnesota from 1974 to 2012 would occur in larger numbers in years where rats were more plentiful. The amount of individual returns among 43 Duluth-wintering owls was fairly low in subsequent winters (8 for 1 year, a small handful in the next few years, and 9 in non-consecutive years). Sometimes surveys appeared to reveal hundreds of wintering snowy owls on coastal sea ice during an irruptive year. Three siblings that hatched in same nest in Cambridge Bay were recovered in drastically different spots at least a year later: one in eastern Ontario, one in Hudson Bay and one in Sakhalin Island. A nestling banded in Hordaland was recovered 1,380 km (860 mi) to the northeast in Finnmark. In the Logan Airport, 17 of 452 owls were recorded to return, eleven the following year, three 2 years later, and then singles variously 6, 10 and 16 years later. A banded female from Utqiaġvik was recorded to migrate over 1,928 km (1,198 mi) along seacoast down to Russia, returning over 1,528 km (949 mi) and covering at least 3,476 km (2,160 mi) in total. Another banded young female from Utqiaġvik went to the same Russian areas, returned to Utqiaġvik and then onto Victoria Island, but did appear to breed, while another also covered a similar route but ended up nesting on Banks Island. Another female migrated to the Canada–United States border, then moved back to the Gulf of Alaska, then to winter in the same border areas and then finally to both Banks and Victoria Island. Snowy owls from the Canadian Arctic were monitored to have covered an average of 1,100 km (680 mi) in one autumn then covered an average of 2,900 km (1,800 mi) a year later. In late winter, owls from the same area were found to have covered a mean of 4,093 km (2,543 mi) of ground in the tundra and spent a mean of 108 days, apparently searching for a suitable nesting situation the entire time.<p>In no fewer than 24 winters between 1882 and 1988, large numbers have occurred in Canada and the United States. These were irruption years. Record breeding irruptive years were recorded in the winters of 2011–2012 and 2014–2015. In the 1940s, it was calculated that the mean gape in time between large irruptions was 3.9 years. Southbound movements as such are much more conspicuous after peak vole years, once thought to be separated by periods of around 3–7 years. However, more extensive research has weakened the argument that irruptions are entirely food-based and the data indicates that irruptive movements are far from predictable. This is because a statewide survey in Alaska found no statewide synchrony in lemming numbers. Therefore, rather than decline of lemmings, it is the successful productivity of several pairs that plays the role, resulting in a large number of young owls that then irrupt. However, the snowy owls cannot breed in high numbers unless lemmings are widely available on the tundra. This connection of irruptions to high years of productivity was confirmed in a study by Robillard et al. (2016). About 90% of the snowy owls seen in irruptive years from 1991 to 2016 that were ageable were identified as juveniles.

Schutzstatus

Bestandstrend

declining