Why aren't trees covered in bird poop?

I mean, they fly over them and stuff and it always seems to just fly out, but I haven't seen any trees traumatized by bird poop. Or maybe the trees are like that freaky one from Harry Potter and eats any birds that might poop on it and shake it off? I see bird poop UNDER trees but not on them because they aren't like people and sit 24/7 so they fly and poop. so I dunno..