Can you guess what made this mark on our window?

When I saw this I thought it was so sad that I had to laugh. Our neighbor is from Bosnia and raises pigions which are native to Bosnia. He is so proud of them, he trains them to do tricks and enters them in competitions and such (quite interesting to watch actually). The little shed in the picture is his pigeon house, unfortunately they are probably all on our roof right now so you can't see them. Well, the other day we were all on the main floor when we heard a very loud crash in our bedroom. I ran upstairs to see what it was--nothing. a few days later I noticed this ghost of a pigeon on our window. Although we have found a dead pigeon or two in our yard in the past, we didn't find one this time. Tough bird!