Peering into his tripod-mounted Swarovski scope, Hal Wierenga tries to explain why he's out in the cold along the Magothy River, searching for a big white bird that might never appear.

"For those of us who do what we do, it's an event," he says, scanning the river for a glimpse of the snowy owl.

Normally found in the Arctic, snowy owls have expanded their range this winter and have been sighted in decidedly un-Arctic places, including Frederick, Havre de Grace, Gibson Island and Middle River. The birding website eBird has logged sightings along the Northeastern United States, Canada, in the Great Lakes region and even one in Bermuda.

While the phenomenon has excited birders, it has downsides.

Scientists say factors including changes in food supply and habitat may have prompted more frequent southerly migrations. And at least five of the owls have struck planes at New York City airports, according to the Port Authority of New York and New Jersey. Officials at the Baltimore-Washington International Thurgood Marshall Airport are preparing for the birds' possible arrival there.

The snowy owl — which has gained some fame as Harry Potter's pet — is as large as a great horned owl but strikingly white, with markings on the chest. When there's no snow, the owl is easy to spot, often sitting in fields or along sandy beaches.

"It's kind of one of those once-in-a-lifetime opportunities you get," Dave Brinker, a regional ecologist for the Maryland Department of Natural Resources, said of spying a snowy owl.

Anyone who spots a dead snowy owl should contact the state Department of Natural Resources, he said, so scientists can collect and study the bird. And anyone who spots a live snowy owl should respectfully enjoy it from a distance, Brinker said.

Snowy owls are not endangered, but are protected from hunting under the federal Migratory Bird Treaty Act, which makes it illegal to possess, transport or sell the bird, its nests or eggs without a permit.

Snowy owls are occasional visitors to places like Maryland in the winter because they are looking for food, specifically small mammals and waterfowl. This year, an unusually high number of snowy owls have migrated here. The scientific term for the population boom is "irruption."

Brinker grew up in Wisconsin and frequently saw snowy owls in the winter. But in three decades in Maryland, he's never seen this many. "It may be the largest irruption that I'll ever see," he said.

Department of Natural Resources officials have heard reports of five owls at Hart-Miller Island near Essex, three on Assateague Island and others in Frederick, Harford County, Calvert County and around Annapolis.

For several weeks now, bird-watchers have been enthusiastically trading reports and pictures of snowy owl sightings.

Dan Haas, who lives in the St. Margaret's community near Annapolis, runs two Facebook pages for birding. He said 200 people have joined in the past few weeks, and he credits the snowy owl.

"There's so much talk about these birds," he said.

Snowy owls mostly live in Arctic places such as Russia, Siberia and Canada. One of their main food sources is lemmings, small Arctic rodents. Lemming populations are cyclical and can have an effect, in turn, on the snowy owl population, Brinker said.

When lemming populations boom, snowy owls feast on the little critters. Then, instead of laying clutches of three or four eggs, the owls may lay eight or more eggs. When those eggs hatch and the baby owls fledge, there's a whole lot of owls and not enough lemmings to go around.

So the young snowy owls must search farther and farther afield to find food, Brinker said. "The young don't compete very well, so they just keep looking," he said.

The owls who find Maryland a fine place full of small mammals and waterfowl will likely camp out for the winter, not returning to the Arctic until March, Brinker said.

Officials are already preparing for the possibility that snowy owls will be attracted to the wide-open spaces at BWI and other airports.