Snowy owls have been spotted in larger numbers across Dakota County this winter than in recent years, though the cause of the influx remains a mystery.

At least 13 of the enigmatic owls, a prized sighting for birders, have been spotted in the 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.

A snowy owl perched on a piece of driftwood at Presque Isle State Park’s Gull Point near Erie, Pa., in December.

"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.

Kevin Smith, an ardent bird-lover and a member of the Minnesota Ornithologists' Union, a local bird-watching group, reporting spotting his first of the bird on Dec. 26 near Vermillion.

So far, none have been seen in Washington County, which, Smith says, has to do with the birds' fondness of "open stretches. They don't want trees in the way," Smith said. "Washington County just doesn't have the habitat for it."

He continued: "They 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."

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 agrees, adding, "This is most snowy owls I've seen reported in Dakota County in … 20 years."

Libor Jany • 651-925-5033 Twitter:@StribJany