We had a robin that wouldn't quit - constantly bonking into one of our basement windows (I still need to clean all the marks off it - it's covered!). For a while we put a piece of cardboard against the outside, but it blocked the light and our view - took it away after a few days and the bird-brain came back! I finally took one of ds's stuffed animals with big beady eyes (actually a parrot puppet) and set it on the sill looking out. The bird hasn't been back since.