If you were to ask most people, they would say the bird of December is the Cardinal. At least, that's what I would think. But in doing some research I found several sources that list the American Robin as the bird of December.
Some reasons why the Robin is associated with winter is that they sing their song throughout winter to defend their territory. Some religious writers even say the Robin was associated with the Christmas season because it was given its red breast by pulling thorns out of Christ's forehead as he died on the cross.
Not only is the American Robin one of the most recognizable birds visually, but it's song is very recognizable as well. The adults are gray-brown above, with brick-red breasts, white bellies and black-streaked white throats. Juvenile robins have brown specks on their breasts.
Robins like to eat fruits and berries as well as insects and worms. Robins tend to eat more fruit in the winter than in other months of the year. Any berry producing tree can act like a magnet to these birds.
A Robin's nest is anywhere from 5 to 20 feet above the ground. The female makes the nest while the male searches for and brings back food. The nest is made up of mostly grass and mud as well as some twigs.
More Information on the American Robin
Robin Song File (.MP3)
How do Robins Find Worms?
European Robin Information