Snowy Owl

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Snowy Owl
At the extreme northern margins of the arctic tundra lives the Snowy Owl, the northernmost, heaviest, and most distinctively-attired owl of North America. Largely diurnal, Snowy Owls spend much of their time perched still and silent on prominent lookouts, waiting to make forays for prey. The habits, distribution, and population numbers of this species are closely linked to the erratic availability of their primary prey, lemmings.

Snowy Owls breed in open country between the tree line and the sea, from coastal Alaska across Canada to Labrador. The breeding range of the species also includes northern Greenland, Scandinavia, and Russia.

The migratory behavior of the species is complex and incompletely understood. In some years, some North American Snowy Owls remain on their breeding grounds year-round, while others migrate in winter to southern Canada and the northern half of the contiguous United States. In the northern plains, New York, and New England, Snowy Owls occur regularly in winter. Elsewhere, such as in the Pacific Northwest, the midwest states, and eastern Canada, Snowy Owls are irruptive, appearing only in some winters but not in others.
Snowy Owl Range Map

One long-prevalent hypothesis for Snowy Owl irruptions is that they follow cyclic population crashes of their main prey, lemmings, across the North American tundra every four years. Recent evidence suggests that the picture is more complex, and that lemming population dynamics are quite variable across the continent. Snowy Owls may therefore irrupt from some portions of their breeding grounds, but not from others, in any given year. Temperature and snowfall levels may also play a role in Snowy Owl migration.

When lemmings are available, Snowy Owls eat them in great numbers — by one estimate, approximately 3 to 5 per day, or up to about 1,600 lemmings per year, for an individual adult owl. Snowy Owls forage throughout the long stretches of arctic daylight in summer, in all kinds of weather. They generally wait on a perch until they locate prey, then pursue and seize prey in their talons. Snowy Owls evidently have remarkably sensitive vision. Snowy Owls can also locate prey by sound, even in dense grass or under thick layers of snow.

When hungry, Snowy Owls eat lemmings whole, head first; after eating a few in this way, Snowy Owls then pick at their prey, eating it in pieces. Snowy Owls often use snow to clean their bills and faces after eating.

When lemmings are not available, as on their southern wintering grounds, Snowy Owls eat a wide variety of prey, including rabbits, rodents, waterfowl, and other birds. On Agattu Island, in the Aleutian archipelago, one research team discovered that Ancient Murrelets ( Synthliboramphus antiquus ) made up about 70 percent of the diet of nonbreeding Snowy Owls there. John James Audubon reported seeing a Snowy Owl catching numerous fish while lying belly-down next to a water hole.

Male Snowy Owls commonly establish and maintain territories during the breeding season through booming hoots and threat postures, including puffing out their throats, raising their tails, and bowing deeply. Breeding density varies with the availability of food. Snowy Owls also defend territories on their wintering grounds.

Snowy Owl nests are scrapes on the ground, formed into a shallow, rounded depression by the female. Clutch sizes vary widely, from 3 to 11 eggs, depending largely on the abundance of prey. Nestlings are covered with short white, fluffy down for about the first 10 days after hatching, whereupon they begin to acquire a longer, gray coat of down. Only females incubate eggs and brood hatchlings, whereas males are primarily responsible for bringing food to mothers and young. Both sexes, especially males, fiercely defend nests against humans and other potential predators, including even wolves.

Description: A large, white owl. Length about 20 inches for adult males, about 24 inches for adult females. Plumage is snow-white, with variable amounts of gray or brown barring on head, wings, back, breast, and tail. Females bear more barring than males, with some overlap between the most sparsely-marked females and heavily-marked males. Some males are almost entirely white. Irises are yellow. Toes bear feathers, all the way to the talons.

Voice: Territorial call is a low, powerful hoot, often issued in pairs. Snowy Owls of both sexes hiss when threatened. Outside the breeding season, Snowy Owls are usually silent, but at least one researcher has documented hearing screams and grating noises from territorial Snowy Owls in winter.

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