The first of the snowy owls were spotted in Colorado in December, so birder Susan Perry was a bit late to the game, but she didn’t miss the party.

Local birders were atwitter at the sudden appearance of a snowy owl along the shores of Standley Lake in Westminster late last month, posting photos of this white visitor from the far north on birding sites and Facebook.

The fuss over the bird still hadn’t cooled by Saturday, when the owl decided to hang out in plain sight on suburban rooftops in the neighborhood next to the lake, just east of Wayne Carle Middle School. Perry had no trouble finding the bird when she pulled into the neighborhood — a crowd of 30 or so people were out on the street talking, taking photos and just watching the yellow-eyed interloper.

Another birder, who had set up a spotting scope on a tripod, was offering up-close peeks at the owl. Perry shot a photo through the eyepiece.

“A bird like this could make you a birder,” Perry said Tuesday evening, when she was back in the neighborhood, trying to catch one more glimpse. “It’s a pretty big deal for me.”

Since December, Colorado birders have reported snowy owl sightings in several places around the state at eBird.org, a worldwide citizen-science reporting project of the Cornell Lab of Ornithology. Only two of the birds seem to be hanging out, though: the one at Standley Lake and another at Lake Pueblo State Park.

This winter is turning out to be an “irruption” period, a winter when many snowy owls migrate far south from their usual wintering grounds, which are in Canada, Alaska and the northernmost regions of the lower 48 — North Dakota, Minnesota, Wisconsin, Michigan, New York and New England.

These southern incursions occur about every four years, said Scott Weidensaul of Project Snowstorm, an all-volunteer nonprofit in Pennsylvania that tracks snowy owls. Although scientists recently believed irruptions happened in winters when owls didn’t have enough to eat in their typical wintering grounds, they now believe irruptions are tied to cyclical booms in the lemming population in the arctic and subarctic during the owl’s breeding season, he said.

When there are more lemmings, more owlets survive. And when winter comes, those young owls wander south.

“There was a cyclical peak in the lemming population in the Ungava this summer, and there were lots of snowy owls breeding up there, and they had a lot of babies,” he said, referring to the Ungava Peninsula on Hudson Bay’s eastern shores.

“What drives these big invasions is not too little food in the arctic,” he said. “It’s that there’s too much. Some years, you do get irruptions of snowy owls that come south in fairly thin condition, and that may be exacerbated by lack of food in the north. But these movements that have been driven by young birds — those owls are young and healthy.”

This winter’s irruption has brought the owls into the Midwest, as far south as Oklahoma City, and down the Atlantic seaboard to Virginia. During previous irruptions, snowy owls have traveled as far south as Florida.

Project Snowstorm started during the irruption of 2013-14, when Weidensaul’s friend David Brinker called him and said, ” ‘We have to drop everything and try to find out what we can from a scientific perspective,’ ” Weidensaul said. Placing radio transmitters on the owls and tracking them for the past four years has shown scientists that snowy owls are more nomadic than previously thought, he said.

“Snowy owls, in general terms, are not like other birds,” Weidensaul said. “Most birds show a lot of what’s called site fidelity.”

But the snowy owls may be breeding in central Canada one year and Greenland the next, he said. “They just shift around a lot.”

Lake Pueblo State Park manager Monique Mullis has been posting updates about the snowy owl there on the park’s Facebook page, Parks and Wildlife spokesman Bill Vogrin said. “She has also directed the public to the best viewing area and cautioned visitors to stay back and not approach the owl in boats, as some have tried.’

“Luckily, the owl has stayed mostly on a breakwater, far from easy access by the public,” he said in an email. “And birdwatchers have been sharing their scopes with visitors so they can enjoy better views of the owl.”

Gail Garber, executive director of Hawks Aloft, a raptor research center in Albuquerque, noted that she didn’t think a snowy owl had ever wandered all the way down to New Mexico during an irruption.

“It’s a great tourism opportunity for Colorado, because all of us Westerners drive in to see them,” Garber said. “I got my last snowy owl up there at Barr Lake about five years ago. It was another irruption year. My friend and I drove up one day, and we saw the owl, and it was in a blizzard.”

For birders, an irruption year is a special year. “It’s just so unusual, it’s like the grail bird, it’s the grail of the owl world in North America,” Garber said.

Parks and Wildlife biologist April Estep said people should give the owls plenty of space. “About a half a mile — which seems like a long way away, but these birds have good vision.”

Getting too close can cause the owl to flush, which can be dangerous for the bird if there are cars nearby, or other raptors that prey on it. “Plus, we don’t want them using extra fat reserves,” Estep said. They’ll need that energy for their return home.

“If they start to look agitated — bobbing their head a lot, fidgeting — I would recommend (people) move back,” she said.

In the arctic, owls hunt by listening to the movement of prey beneath snow, Garber said. “If you have birders and photographers and they’re tromping around, you’re preventing that owl from getting anything to eat that day.”

Weidensaul also cautioned against disturbing the owls all day — which can be easy since unlike their regional cousins, they aren’t nocturnal, and they sometimes perch conspicuously on open ground in the middle of the day.

“They’re incredible birds,” Weidensaul said. “They‘re charismatic, they’re beautiful, they’re mysterious. I understand why people go a little bit gaga over snowy owls. It’s a little bit frustrating, but it’s also completely understandable. Because you’re not going to see a polar bear walking through your backyard, but these snowy owls bring a piece of the arctic with them.”

How you can help owls and other raptors

Don’t throw food out the window when you’re driving. “Throwing your food out the window because someone will eat it? Yeah, someone will eat it” — rodents, said Gail Garber of Hawks Aloft, a raptor research and rescue facility in Albuquerque. Rodents attract raptors. “The major cause of birds admitted to rehab is birds getting hit by cars,” she said.

Don’t use glue traps for mice. “Those glue traps are absolutely barbaric,” Garber said. “We had a little screech-owl that came in that was absolutely stuck.”

Don’t use rodent poison. “If you use poison to control rodents, it just climbs up the food chain,” Garber said. “We see a lot of birds, owls especially, that are poisoned.” Instead of glue traps and poison, Garber recommends “those old-fashioned snap traps, and it may sound barbaric, but it’s quick.”