It's been a good winter for spotting snowy owls in the Twin Cities.

But not in Washington County.

While reports of snowy owls in the metro area are up from previous years, none have been spotted so far gliding through the county's airspace.

Kevin Smith, an ardent bird-lover and a member of the Minnesota Ornithologists' Union, a local bird-watching group, blames the lack of sightings on too many trees and the birds' fondness for "open stretches."

"They don't want trees in the way," Smith said. "Washington County just doesn't have the habitat for it."

Smith added that owls "don't perch up on trees, even if trees are in a neighborhood. They'll perch on power poles, they'll perch on a building, and they'll perch on those irrigators that you see."

Meanwhile, at least 13 of the enigmatic owls, a prized sighting for birders, have been spotted in Dakota County since the beginning of December, particularly in and around Vermillion, according to a map that tracks reported sightings.

They normally stick to the Canadian forests where they do most of their hunting, except during "irruptions," when they expand their range for reasons ornithologists are still sorting out.

This sometimes arises "when the prey base crashes and they come south looking for food. They're somewhat nomadic in their predatory behavior," said Dr. Julia Ponder, executive director at the Raptor Center at the University of Minnesota. "Usually when we have an irruption, we see a remarkable number of starving birds."

Bird experts point to fluctuations in lemming populations in the Arctic as the single most significant cause of these unusual migrations, which lead to more sightings in northern states like Minnesota. In recent years, the stealthy birds have also been spotted in far-flung places such as Bermuda and Hawaii.

What is unusual about this year's migration is that lemming numbers appeared stable, raising the tantalizing question for wildfowl experts and amateur bird-watchers: What caused the latest influx?

One theory is that snowy owls are territorial and were forced to migrate in search of new breeding grounds.

"If there's a lot of prey, they'll often have a really good year, and they'll produce a lot of chicks. And in the winter they'll disperse," said Jen Vieth, executive director of Carpenter Nature Center in Washington County's Denmark County.

In Empire Township, one was spotted along Hwy. 3 near the Empire Transportation Facility on Jan. 2, while a female owl, identifiable by its dark markings, was seen perching near the intersection of 180th Street and Emery Avenue outside Vermillion on Dec. 30, according to the map.

Many owls were recorded during the Christmas Bird Count, an annual avian census that begins on Dec. 14.

Vieth said the birds have arrived in the United States in higher numbers than usual.

"Some of the birders think that it's going to be a record year, especially on the East Coast," Vieth said. "In a normal year there's often sporadic reports — you know, one or two — down in the Rochester area. But definitely not as many as this year."

Smith, who reporting spotting his first of the bird on Dec. 26 near Vermillion, agrees, adding, "This is most snowy owls I've seen reported in Dakota County in … 20 years."