Well my car got hit about a month ago by a seagull/crow, something large and I thought I had gotten rid of the last layer (of crap) bonded to my paint by the sun.
Then washing the car this last Sat. with the sun just right I saw the faint remains were still there!
The only thing I hadn't tried I dug out. A ten year old can of Turtle Wax, Bug & Tar remover. The stuff was thick after all this time, but I shook it up and after 3 application I finally got ride of the bird crap. Ahhh success.

 

Zaino users probably know that already, though.

 

DITTO for the Zaino claybar..it'll take out the birds, PLUS it did a really great job of taking out a thin coat of red paint left on the surface of my car after some thoughtless person rubbed their car door up agains mine! Nothing worked on that for me cept the claybar!

 

Waxing every week has proven that even set bird poop will come off with a small stream of water. The thick layer of wax makes the poop slide right off when you wash even if it's been there for about a week. You can just take a cup of water and pour it over the poop spot...it should come off.