Fuck that bird.
Make it your new life goal to hunt down and shit on that bird. I know it will prove difficult to find the exact bird that did the pooping so just start pooping on all the birds in the area. let them know you are out for justice. Maybe one of them will panic and rat the culprit out. If you feel it's necessary you should capture a bird and beat some answers out of him. Water board the bastards. Go gorilla style.
But you need to establish your goals, and soon while it's fresh in the air. Provoke some fear, take away his celebration, get him sweating. That bird is still talking about it with his bird friends and they need to know that you are coming for him. Take the item of clothing or whatever he pooped on and hang it high on a post along with a fellow bird covered in your own excrement. His crime will not go unpunished.
Good luck bing.