My cockatiels spend quite a bit of time on top of the budgie cage. I was worried they were going to poop on the budgies' heads. But, this doesn't seem to happen.

I have never seen a bird with a dirty head or back. Why?

Birds actually do poop on each other every now and then, but the "victim" will preen it off before too long. I would guess that the weight of the poop tugs on the feather quill so it's more comfortable to get rid of it.

This picture looks photoshopped but it isn't - somebirdy really did drop a poop in the middle of Snowy's forehead. The glare of her bright white feathers tends to disrupt the quality of photos:

Griffin had poop on his head once. It was hilarious.

Beaker aims for Jaid. He hits him occasionally.

My goodness! I apparently have two very well-behaved tiels. Good to know LOL.