I can't speak for the BMW paint, but I have a speedy solution to bird dropping and many other deposits on the car.
I always carry a 2 liter bottle of seltzer water and paper towels in the trunk of the car. Whenever I get bird droppings on the car, I immediately get out the seltzer water and flood the stain. It bubbles up the droppings right away and then you can gently wipe away the dropping.
On any car, if you don't get the droppings up as soon as possible, you can permanently damage your paint. I learned about this in 1990 when a goose let loose on the hood of my new black Lincoln and a gas station attendant came out with a bottle of seltzer. I have been sharing this gem ever since. Make sure you get seltzer, not club soda as this will often contain sodium.
Author: BMW Digest
by Bob & Betsy Sawyer
Reprinted in Gesundheit - June 1997
St. Louis BMW Club