A sign that winter is progressing are the flocks of robins, descending on berry-laden hollies. They're not unlike the cedar waxwing flocks, taking turns swooping down on the hollies and back up to branches on the bare oaks. They're both sociable winter birds: a nice piece from Journey North that I just found describes their behavior.
The birds I heard and saw today outside my office in the Garden had a melodic murmur; without my (regular) glasses or binoculars, I couldn't see them clearly, but based on the sound, they may have been cedar waxwings (also from the Journey North site courtesy of Lang Elliot) rather than the robins I saw at home recently.
Both are welcome signs that winter is moving towards spring.