As anyone who has ever parked their car near a seaside area with lots of gulls around can tell you, this can cause damage to the paintwork very quickly. It used to be believed that the problem was caused by acid in the bird dropping eating away at the paint, but in fact this is not the reason.
What actually happens is that the warmth (all droppings are warm when they first come out, though I don't recommend that you test this for yourself!) of the bird droppings causes the lacquer on the car paint to expand and makes it grow softer. Then as it cools down it will shrink, harden and mould itself to the shape left by the droppings. This makes the surface less smooth, which in turn means that the sunlight doesn't reflect off it so well, making the paint look dull.
You can read more about this here.