Satellite tracking has replaced the older picture of a straightforward north-south migration. Tagged snowy owls have bred in widely separated locations in successive years, following pulses in lemming and vole abundance rather than returning to a natal territory.
Winter is similarly loose. Some birds move south into southern Canada and the northern United States, occasionally in large irruptions. Others remain in the Arctic, and tracking has recorded owls spending winter on sea ice, apparently hunting seabirds at openings in the ice such as leads and polynyas.
A common misreading of irruption years is that starving owls are fleeing a food crash. Evidence from the large 2013 to 2014 irruption points the other way: an exceptional lemming year in eastern Canada produced many well-fed young, and it was that surplus that spread south. Global population estimates have been revised downward, which is why the species is now listed as Vulnerable.