Title says it all. He was being shouted at by a nice little collection of birds: blue jays, yellow-rumped warblers, Carolina chickadees, what I think may have been a titmouse, and a very upset ruby-crowned kinglet. Looked to be about the size of a crow.
Sidenote, what would be a good way to upload a couple of videos here? I took a few short videos of the birds accosting him, and got a decent recording of the interesting variety of sounds. The kinglet sounds like a ticking engine.