SALMON, Idaho (Reuters) - Bird enthusiasts are reporting rising numbers of snowy owls from the Arctic winging into the lower 48 states this winter in a mass southern migration that a leading owl researcher called "unbelievable."

Thousands of the snow-white birds, which stand 2 feet tall with 5-foot wingspans, have been spotted from coast to coast, feeding in farmlands in Idaho, roosting on rooftops in Montana, gliding over golf courses in Missouri and soaring over shorelines in Massachusetts.

A certain number of the iconic owls fly south from their Arctic breeding grounds each winter but rarely do so many venture so far away even amid large-scale, periodic southern migrations known as irruptions.

"What we're seeing now -- it's unbelievable," said Denver Holt, head of the Owl Research Institute in Montana.

"This is the most significant wildlife event in decades," added Holt, who has studied snowy owls in their Arctic tundra ecosystem for two decades.

Holt and other owl experts say the phenomenon is likely linked to lemmings, a rodent that accounts for 90 percent of the diet of snowy owls during breeding months that stretch from May into September. The largely nocturnal birds also prey on a host of other animals, from voles to geese.

An especially plentiful supply of lemmings last season likely led to a population boom among owls that resulted in each breeding pair hatching as many as seven offspring. That compares to a typical clutch size of no more than two, Holt said.

Greater competition this year for food in the Far North by the booming bird population may have then driven mostly younger, male owls much farther south than normal.

Research on the animals is scarce because of the remoteness and extreme conditions of the terrain the owls occupy, including northern Russia and Scandinavia, he said.

The surge in snowy owl sightings has brought birders flocking from Texas, Arizona and Utah to the Northern Rockies and Pacific Northwest, pouring tourist dollars into local economies and crowding parks and wildlife areas. The irruption has triggered widespread public fascination that appears to span ages and interests.

"For the last couple months, every other visitor asks if we've seen a snowy owl today," said Frances Tanaka, a volunteer for the Nisqually National Wildlife Refuge northeast of Olympia, Washington.

But accounts of emaciated owls at some sites -- including a food-starved bird that dropped dead in a farmer's field in Wisconsin -- suggest the migration has a darker side. And Holt said an owl that landed at an airport in Hawaii in November was shot and killed to avoid collisions with planes.

He said snowy owl populations are believed to be in an overall decline, possibly because a changing climate has lessened the abundance of vegetation like grasses that lemmings rely on.

This winter's snowy owl outbreak, with multiple sightings as far south as Oklahoma, remains largely a mystery of nature.

"There's a lot of speculation. As far as hard evidence, we really don't know," Holt said.

(Editing by Steve Gorman and David Bailey)