Although many people believe that seeing an American Robin is one of the signs of spring, that's not technically true. Chances are good that your robins never left. You just didn't notice them.
We're accustomed to seeing robins "bob, bob, bobbing along" on the ground, pulling worms from the lawn all summer long. Plus, during the warmer months they usually appear as individuals rather than as a dense flock.
In winter, however, their behavior is quite different. If you want to see robins, rather than looking for individual birds on the ground, you'll have to look for a flock of birds in the trees, especially fruiting trees.
So in terms of predicting the arrival of spring, spotting an American Robin is about as accurate as declaring a ROUS to be an expert meteorologist. No, I take that back. Punxsutawney Phil is wise and all-knowing. Trust the pampered rodent.