Birdwatchers may get a special treat this winter. They might just spot a snowy owl.

Most of the year, these huge, white owls live on the treeless Arctic tundra. In the winter, some migrate to southern Canada and the northern U.S. But every three to five years, hundreds or even thousands of snowy owls fly south. This is called an irruption. 

What causes an irruption?

This year may be an irruption year.