The bird droppings are acidic and have etched into your the paint. The only way to remove the etching is to polish the paint where the etching is. To fully remove the marks the paint has to be leveled down even to the bottom of the etching. This could be next to impossible depending on how deep the etching is. At the minimum the appearance of the etching will be reduced with a good polish. Please PM me if I can be of assistance.
