MINNEAPOLIS -- It's been a tough winter for owls in parts of North America, and the evidence is turning up on roadsides, at bird feeders and at a wildlife rehabilitation center in Minnesota.
The dead, injured and sick owls are symptoms of what ornithologists call an "irruption," a natural, cyclical phenomenon that happens when hungry owls that normally winter in northern Canada head south in search of food -- either because their normal food of mice, voles and lemmings are in short supply or heavy snow cover makes it difficult to hunt for small rodents. Other irruptions have been reported recently in New England, as well as southern Ontario and Quebec, and parts of British Columbia.
This year it happens to be northern Minnesota that's seeing much of the action and it's mostly tiny boreal owls.
"They're excruciatingly cute," said Geoff LeBaron, director of the Christmas Bird Count program at the National Audubon Society.
As the owls flock to Minnesota, so do bird watchers. The prime owl habitat of the Sax-Zim Bog is about 45 miles northwest of Duluth, and it has an annual birding festival in February.
Frank Nicoletti, director of banding at the Hawk Ridge Bird Observatory in Duluth, said he's been guiding owl watchers nonstop for the last couple of months.
They've been traveling to northern Minnesota from all over for the chance to spot visiting boreal owls, which are normally tough to see because of their size, and because they don't usually come out during daylight. They also sit very still when they perch.
Irruptions tend to involve young owls because older owls are more experienced hunters and know their territories better and so are better at finding food. Younger northern owls also don't know about dangers from humans -- like cars -- so they're more likely to end up as roadkill. They're often weak by the time they make it south, and some species might not recognize the local small mammals as food.
Nicoletti picked up three dead boreal owls on Tuesday alone, and evidence is also turning up at the Raptor Center on the St. Paul campus of the University of Minnesota. It was treating about 30 owls out of about 58 avian patients this week. With the help of volunteers to hold the birds still, veterinary technicians on Wednesday checked the wings, ears, eyes and weights of sick and injured owls as they tried to nurse them back to health.
"They definitely are not having a, quote, normal year," said the Raptor Center's executive director, Dr. Julia Ponder. She said nine of the 10 owl species commonly seen in Minnesota are represented there now for the first time in anyone's memory.
As thrilling as it might be to spot an owl in the wild, and as disappointing as it might be to find a dead one, experts stress that what's happening now happens regularly across the northern latitudes, and to a varying mix of owl species.
"It's definitely a natural cycle," LeBaron said. "The food sources for owls and other raptors are highly cyclical. It's a boom and bust thing for predators."
LeBaron said northern owls also started turning up last fall in New England, while central and southern British Columbia east of the mountains are seeing higher-than-usual numbers of snowy owls.
Barred owls have been seen widely in New England along with a lot of northern saw-whet owls, said Marshall Iliff, a leader of the eBird tracking program at Cornell University. He added that lots of great gray owls and a few northern hawk owls reached southern Ontario and southern Quebec this winter.
Nicoletti rates the irruption that has brought the boreal to Minnesota as "mild." Nicoletti noted that in 2004-05, northern owls turned up south of Minneapolis, which is in the southern part of the state.
Experts say the weather may be one reason why people in Minnesota are seeing more dead or starving birds in recent weeks. The winter started out relatively mild but turned snowy in February and the snow has turned crusty, making it harder for owls to catch the mice underneath. Starving owls are drawn to homes, where they try to prey on mice that gather spilled seed from bird feeders or that hole up in garages and woodpiles.
Now that spring is approaching, the owls seem to have started heading back north, Nicoletti said.
But people's fascination with the birds will remain.
"They capture the imagination," LeBaron said. "There's so much lore around owls it's hard not to become captivated by them."