Well it is bird poo, but the effect it has on paint is that the area that was covered by the bird will be etched or have finish damage. The longer the dropping remains the more damage the finish takes
What causes it?
Birds flying or perching overhead do their business and it lands on your car
The organic etching process is sped up and intensified by heat
How is it prevented?
Avoid parking under trees or other things that birds tend to perch on