I do recall hearing a news story some weeks ago saying some study had shown that robins do not actually migrate very far. Instead it said that they seclude themselves in sheltered wooded areas where they can find food, stay undercover, and try to remain aloof from predators etc. Maybe bogus, maybe not; but their sudden emergence would seem to support it.
All the precip we have had followed by a few days of warmish (40°) weather caused a couple of foolish bulbs to pop, and a bleeding-heart in the backyard to send up some tentative shoots. (I put a shovel-full of snow on it.) Maybe all that water forced the earthworms to the surface, and the reclusive robins couldn't resist a mid-winter feast.
(Sorry about the photo quality, but the camera on my Moto Droid sucks, imho.)