I have never figured out why robins are considered to be harbingers of spring. In the places where they live (except for the far northern edge of their range), they don't migrate (far, at any rate). They're there all year 'round.
Nonetheless, here in Colorado, birders have been commenting that there are considerably MORE robins than we usually see in the winter ... and far fewer Townsend's Solitaires. Everyone's wondering what's going on.