Owls, Owls Everywhere
Jan. 25, 2012
Snowy owls have been descending on the northern United States this winter by the thousands, to the delight of bird watchers and scientists. These graceful birds of prey usually live on the Arctic tundra in Canada, where they feed mainly on lemmings. Normally, only a relatively small number of the owls migrate south in the winter. The owls have also spread out farther than usual, appearing from the Atlantic coast to the Pacific and as far south as Kansas. One owl even reached Hawaii, where authorities shot it out of concern it would interfere with aircraft. Such a rapid increase in the numbers of a bird in an area where it is not usually found in winter is called an irruption. Scientists have been tracking the locations of the birds with great precision thanks to a citizen science project called eBird. Run by the Cornell University Lab of Ornithology and the National Audubon Society, the online project (at ebird.org) uses sightings collected by bird watchers across North America to create maps showing where species can be found.
Unlike most owls, snowy owls are active during the day, making them easier to observe. The owls reach about 23 inches (58 centimeters) long and have mostly white plumage with brown markings. Adult females have more brown markings than the adult males. The owl’s call features several rough notes and sounds somewhat like a dog’s bark. In addition to lemmings, the owls may feed on mice, voles, ducks, and hares.
Scientists are not certain why the owls have migrated south in such numbers. However, many animals in northern regions follow a boom-and-bust population cycle. For example, lemmings quickly grow in numbers when there is abundant vegetation. A boom in the population of lemmings provides abundant food for snowy owls. However, the lemmings soon eat much of the available food, and their numbers plunge. At that point, there may not be enough lemmings to support the large number of snowy owls. These conditions may cause snowy owls to fly farther south than usual, as they seek other sources of food. The irruption is expected to end by March or April.
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