Hi Gary, the bird will be seeing it's own reflection. A common thing for birds. It will think it is another bird. This can be quite distressing for them. You can try a few things. Attach somehow, paper, foil, clingfilm to the inside of the window or if you google, window stickers you will see a huge choice of assorted ones to stick to the glass. After a short while hopefully it will get fed up and stop doing it.
Beauty is in the eye of the beholder.