Snowy owls have been swooping into South Dakota in unusually high numbers this winter, delighting bird watchers and stirring up a hoot over their mysterious arrival.

Snowy owls typically nest in the Arctic tundra of Canada and migrate south for the winter. This season, however, these Arctic natives are participating in a dramatic migration called an irruption, during which large numbers of birds migrate to areas where they are not commonly found.

“Every year there’s a few that migrate south because there’s simply not enough food to get all of them through the winter, but normally they don’t come this far south,” said Kent Jensen, associate professor of ornithology at South Dakota State University.

Irruptions can cause snowy owls to migrate remarkable distances from their normal ranges and this season the owls have been spotted from coast to coast in the northern United States.

One of the nation’s hotspots has been Lake Andes National Wildlife Refuge, located on the Yankton Indian Reservation, where 20 different snowy owls have been spotted.

“At their peak, there were approximately 35 sightings a day,” recalled refuge project leader Mike Bryant, who said there has not been an irruption of this magnitude since 1955.

Bryant noted that the refuge saw a spike in visitors—an estimated 500 more than usual—from Dec. 21 through the first two weeks of January, when snowy owl sightings were at their highest due to lack of snow in the area, which makes it easier for snowy owls to hunt.

“We’ve had pretty good local interest,” said Bryant, adding that the curiosity surrounding snowy owls at the refuge transcends the surrounding states. “Even a guy from England came to see it.,” he said.

While most visitors have been birders and wildlife photographers, the unusually large presence of snowy owls in Lake Andes has drawn plenty of non-birders as well.

“A large white bird attracts interest,” Bryant said. “It’s something that people haven’t seen before.”

As their name suggests, snowy owls are easily recognizable by their snow-white coloration. Males have a shock of pure white plumage, while female and juvenile snowy owls demonstrate dark scalloping and barring. Their keen yellow eyes and sharply taloned feet festooned with thick feathers make them superior hunters in their native Arctic habitat. Standing nearly two feet tall, snowy owls are powerful birds of prey.

Snowy owl irruptions are associated with cyclical lemming population crashes that occur every four to five years, but the cause of this season’s irruption of unusually large proportions has generated much speculation among ornithologists.

Jensen suggested that lemming the population was very high last spring and summer and that the large food supply resulted in a very successful breeding season, during which one snowy owl can lay up to eight eggs. The surplus of lemmings then led to the survival of an unusually high number of snowy owl chicks, which have now migrated south for the winter due to a sudden food shortage.

“The situation was just right for a real high reproductive year and then as they moved down we ended up with this doughnut hole of no snow here in the Dakotas that really concentrated the birds,” said Jensen. “That’s my best guess.”

As snowfall has increased in South Dakota, snowy owls have dispersed and sightings have become less clustered at Lake Andes, but this doesn’t mean the owls have disappeared.

“They’re somewhere,” Jensen said, adding that people will continue to enjoy snowy owl sightings until they migrate back to the Arctic in March