Prank Fake Bird Poop
The beauty of prank Fake Bird Poop is that it's going to cause just as big, and almost as gross of a mess for the shat-upon to clean up, but will be significantly less repulsive for the administering jokester to handle than real bird poop. Also easier to harvest and transport in liquid form.
According to this brand of Fake Bird Poop's seller, it's the "realest looking fake bird poop out there guaranteed." And they wrote that in all caps, which probably means they have extensive experience working with bird shits both real and manmade.
Your Fake Bird Poop will arrive in powdered form. To activate it, just mix with water, and get your splatter brush (or finger) ready. Ideal bird poop targets include cars, home and office windows, and the top of my ex-girlfriend Karen's head.