One of my cats was once growling at something out the window. I looked out to see what it was, and there was a hawk perched on a fence below. (second-floor apartment) I picked her up and moved her away from the window, and she just went right back and continued growling.
There was something sickly funny about the cat being aggressive toward what would normally be a predator, if the glass of the window wasn't in the way. I wish I had had time to get out the camcorder and film it, but I had to leave for work.
