This happened to my friend's brother with the majority of a slice of pizza once. Carried it off and everything. What made it really funny was that he'd been yelling insults at the birds for like fifteen minutes until it happened.
I once had a bird do a perfect divebomb poop into my tuna salad. I opened it, put the fork in, the bird just flew up off the ground and on its way, pooped in my salad. Perfectly. It didn't splatter out of the box onto my hands, clothes, face, nothing. Perfect accuracy.