So they drove past the sign and parked. And as she got out of the car, Ms. Pourchot told her husband, “Babe, I think we have a snowy owl here!”

“You’re kidding,” he replied, but acknowledged that while they might have been looking for the avian equivalent of a zebra, they had happened on a unicorn.

On Dec. 27, she posted her report to ebird, an online tracking system created by Cornell University’s department of ornithology and the National Audubon Society. The next day, members of the local Audubon chapter were doing their annual Christmas bird count when word spread, said Kevin E. Dailey, the leader of one team. The news was so exciting that volunteers left their assigned zones in search of the truly rare bird. “That’s kind of heresy to leave your count circle area,” he said.

Some people are less happy to encounter snowy owls — particularly, the managers of airports that the birds are drawn to. With wide open spaces and short grass, “the airports, to them, look more like the Arctic tundra than anything else,” said Norman Smith, who runs the Snowy Owl Project for Mass Audubon. Birds at airports, however, pose a threat to planes, not to mention to themselves. The Port Authority of New York and New Jersey took heated criticism in December when it shot three snowy owls. Since then, the authority has tried trapping the birds, with limited success, and harassing them away from the airport by shooting off fireworks, said Ron Marsico, a spokesman for the Port Authority.

Mr. Smith had trapped the female owl at Logan one recent morning and had driven her here to the beach at Duxbury, about 40 miles southeast, for release into the wild, with hopes that she would continue heading south, away from the airport. In most years, Mr. Smith makes a trip from Logan to Duxbury or other release sites a half-dozen times. Since the start of the fall migration, however, the number has climbed above 75. “And the season is only half over,” he said.

When he released his grip on the owl’s legs, the bird flapped her broad wings and headed to the southwest, toward a small cluster of homes. Suddenly, another snowy owl sailed down from the houses and met the newcomer in midair, their talons locking. An aerial territorial skirmish followed as the two wheeled overhead, with the newcomer finally heading off to the west.