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
·Depending on how bad the rust is it will appear in different degrees. It can appear anywhere from bubbles to blisters or even rust spots. There may also be corrosion underneath the paint surface.
Why does it happen?
Damage to the paint surface exposing bare metal
Flash rust formation on bare metal prior to primer application
How can it be prevented?
Thoroughly remove all rust prior to refinishing
Follow recommended procedures for refinishing bare metal substrates