How can I drive birds out of my parking lot where there are trees?

Hello, I have a plague, with me in the parking lot are trees that overshadow the place and I always 1 to max 2 days after the car wash the car is covered with bird droppings and the laundry was free. Is there a cheap, effective method to prevent the birds from drifting around there? Bird distributors at Ama are all rated as ineffective, since someone knows a good product?