Fight Club

I encountered these birds outside my building.  At first I thought they were mating, it is spring after all. But on closer inspection I think they were having a vicious fight! They were rolling around and making squawking noises, I was standing really close and they didn't seem to care. As you can see from the pix they had their claws on each others faces and legs. They'd roll around and then get tired and just lay there. I watched them awhile and then got tired myself and moved on. I wonder what they were fighting about?