Don't they all migrate?
The northerns do or migrators. The locals stick it out,
A lot of our local birds migrate south as well, some birds even migrate north in the fall. I once got a hen mallard on the Snohomish in November that was banded in Northern California three months earlier.
Banding studies have shown that many local bird populations, waterfowl and songbird, shift their their ranges south even if their local habitat is still supportive of wintering populations of birds.