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.