The white, two-foot-tall birds, which live in the Arctic the rest of the year, are known to fly south in large numbers every few winters in what is known as an irruption. But this year, the numbers are unusually high, said Denver Holt, director of the Owl Research Institute in Charlo, Mont.  “There are so many across the country, everywhere, by the thousands,” Mr. Holt said. “It’s unbelievable. They are being seen from Boston, to the Great Lakes, the Ohio River Valley, Kansas, Vancouver and Seattle.”  “One showed up at the airport in Hawaii, and they shot it,” he added in astonishment. “It’s the first ever in Hawaii and they shot it!”  The owl was killed on Thanksgiving by federal officials who feared that the bird would interfere with landings and takeoffs.  Why so many more of the birds are showing up is largely a mystery, Mr. Holt said. “We do know they had a really good breeding year, and there was plenty of food last year,” he said. Instead of no chicks, or one or two, a single nest will produce five, six, seven or more fledglings in a good breeding year, he said.  The owls’ Arctic diet is 90 percent lemmings, although the birds, which are powerful hunters, also eat mice, voles, ducks, hares and even fish when they migrate south. Some ornithologists speculate that lemming populations crashed recently after a boom, which could have led to the push south, but researchers have not confirmed such a decline.  The irruption started in late fall and is expected to end by March or April. In few places are people as excited as in Kansas and Missouri, where snowy owls are exceedingly rare. Ninety have shown up in Kansas this winter and 40 in Missouri. Until this year, the highest number counted in Missouri had been eight.  “It’s a massive movement,” said Mark Robbins, the ornithology collection manager at the University of Kansas.  When five of the birds took up residency at Smithville Lake, near Kansas City, Mo., it created an “owl jam,” Mr. Robbins said. Thousands of people have driven there to see them, he said, and hundreds of owl seekers have shown up at Clinton Lake near Lawrence, Kan.  Unlike many owls, the snowy variety are diurnal, or active during the day, which accounts for some of the hubbub. Their blinding white coloring, sometimes with brown barring, and piercing yellow eyes are a magnet for birders and nonbirders alike.  Adding to the allure for children, the owls are of the same species as Hedwig, the faithful companion of the fictional wizard Harry Potter, which perished defending him in the final book of the series.  Geoff LeBaron, director of the Audubon Society’s Christmas bird count, said that it was hard to estimate how many snowy owls flew south in this irruption because the latest data has not been tallied, but that the overall number was probably a few thousand. Despite the surge, the society says, snowy owls are thought to have been in decline since 1945.  There is far more data on the scope of this migration than in years past, thanks to a citizen science project based at Cornell called eBird, which is run by the Cornell Lab of Ornithology and the Audubon Society. Bird-watchers around the country call in sightings, which are plotted on a map that shows precisely where the birds are wintering.  “A lot of people who have never seen one before have rushed out and seen multiples,” said Marshall Iliff, an ornithologist at Cornell and the project’s leader. “And photographers are having a field day.”  Additional hot spots include the mouth of the Columbia River in Washington State, with 10 to 13 birds; 20 at Lake Andes National Wildlife Refuge in South Dakota, and 30 in Boundary Bay, near Vancouver in British Columbia.  The owls are even showing up in urban and suburban areas, along highways, on signs and fence posts, and in other places where people can more easily spot them. It has been a good snowy owl year at Logan Airport in Boston, too. Because the airfield looks like tundra, snowy owls tend to flock there, and they must be trapped and removed.  “We’ve removed 21 so far this year, and the average is six,” said Norman Smith, who works for the Massachusetts Audubon Society and traps the birds. The most ever trapped was 43 in 1986, Mr. Smith said, “but the year’s not over.”  Mr. Holt, who has journeyed to the Arctic tundra to study snowy owls and their food and nesting habits for the last 20 years and is one of world’s leading experts on the bird, said he had seen no evidence that the owls, most of them young, are stressed. “They are not all here starving to death,” he said. “The birds appear to be in good physical condition.”  But Mr. Robbins said he had seen some evidence to the contrary. Of five dead birds he examined — three hit by cars, one hit by a train and one that was electrocuted — there was “no question” that “some of these birds are starving to death,” he said, probably because they have been unable to find enough food.  Whatever the causes of the irruption, owl watchers are making the most of what they suspect may be a unique opportunity.  Mr. Holt suggests that the draw of the snowy owls may be partly a fascination with the birds’ coloring. “White wolves, polar bears, white whales, white buffalo — there is something about white plumage that signifies innocence or purity,” he said. “People don’t flock to see any other animal the way they do white ones.”
High numbers of the white birds have been spotted from coast to coast, stirring speculation about the cause of the spike.