Some winters, many more snowy owls than usual move south out of the Arctic, and these big moves happen every few years. Every four years or so there is a modest irruption of snowy owls, timed to four-year lemming cycles. The snowy owls that travel farthest south tend to be the young ones, and snowy owls that go south usually begin the trip in November, with most gone from their winter range by April.

Project SNOWstorm found that a big move south is a baby boom: young owls come south after summers of plentiful lemmings. Irruptions are typically thought to occur when lemming populations crash, but one reporter wrote that this did not appear to be the case in one big irruption year. Winter snowy owl numbers in the south probably relate to how much prey there is in the north, and scientists say snowy owl migration is not fully understood.

Snowy owls are nomadic: they wander to wherever prey is plentiful rather than returning to one home. Tracking showed snowy owls to be among the great nomads of birds, ranging thousands of miles across the Arctic, and a snowy owl breeding in central Canada one year may be in Greenland the next. Snowy owls do not show fidelity to a particular breeding site, so they do not return to the same place each year.

Snowy owls are hard to count because they are irruptive and nomadic and breed in remote Arctic tundra. Birdwatchers help count snowy owls by reporting sightings through eBird, and Project SNOWstorm tags snowy owls with a backpack harness of low-friction ribbon that goes over the bird's wings. Older satellite tracking logged a single location a day at best, but the cellular tags used by Project SNOWstorm can store up to 100,000 locations.

In winter, many snowy owls travel south to Canada and the northern United States. South of the tundra, the winter habitat of snowy owls is open, flat land that resembles tundra, and they live in open places such as prairies, farmland, beaches and airports. Some snowy owls spend weeks or months in winter on the frozen Great Lakes.

Words to know

• nomadic: always wandering
• migration: seasonal travel
• harness: body strap
• habitat: living place