Migration south isn't a mandatory vacation. Migratory birds may linger in the north as long as there are still a lot of natural food sources available. If the snows every begin to blow, some birds may find they have to move further south.
NOAA's official Winter Outlook for 2011-2012 is that La Niña will produce a winter that is drier and warmer than normal in the Northeast and some birds have chosen to stay north longer than usual. However, a seesawing of atmospheric pressure between the Arctic and the mid-latitudes creates a less predictable Arctic Oscillation that could produce dramatic short-term dips in temperatures this winter which may create a feeding frenzy at the feeders.