The best I have found if your car is attacked by birds take it to a self service car wash, ASAP, wet the hood top or any flat surfaces first let them soak.
Then when you start rinsing off the soap, the droppings will wash off, (Please don't aim the spray wand directly at the droppings, it will damage the clear coat.)
Be proactive wax the flat areas of your car, hood top and trunk lid, you don't have to wax the whole car the flat areas are what take the beating.
With a decent wax job on the flat areas every few months, those bird droppings will wash off easily.