Is it a baby? Can you tell what is injured?
I had an injured bird in my backyard a year or two ago. It was not a baby, and it would periodically flap it's wings and just generally act crazy. It appeared to be poisoned or something. I tried to put it in a shoe box with some towels, but it would eventually flap its way out. I wanted to put it out of its misery, but I just couldn't bring myself to hit it.