A bit of record keeping: A flock of about 15 robins visited my backyard on Feb. 27, accompanied by one hairy woodpecker. It was appealing to speculate that the woodpecker had some agreed-upon function for the flock, such as lookout, but this may be asking too much of birds' organizational skills. One online source describes mixed flocks as being composed of a "nuclear species", in this case the robins, and "attendants," (the woodpecker). Attendants tend to join a flock only while the flock is passing through the attendant's territory.
One website, called American Robin, Journey North, says that the whereabouts of robins in the winter has less to do with temperature or migratory habits than where food can be found.