I watched a redtail hawk knock itself out cold when it hit a tree next to my tree stand.

He was after a squirrel at the base of the tree. The squirrel jumped up the tree right as the hawk was a maximum velocity. Apparently, those things don't have air brakes.

He layed on his back in the snow with his wings spread out flat for 5 minutes. He twitched, stood up, gathered his thoughts and then flew up into the same tree. He sat there for at least 30 minutes before continuing his hunt.