I think them damn birds do it intentionally and laugh their asses off afterwards. I think they have competetions to see who can drop the biggest deuce on a single vehicle.
Read Jonathon Livingston Seagull. Birds have a sense of humor. I once had a Cockatoo, Yellow Nape. He had learned how to cuss from his previous but dead owner. My wife and I bought him. One fine day, a neighbor friend of my wife came over and let herself in as she was wont to do. Sydney said "Hello Bitch" and started squawking vigorously .She busted a gut laughing. We all did.