It was thought that acids and alkalis in bird droppings caused this problem. But research by Autoglym found that the damage actually occurs from the paint lacquer contracting upon cooling and moulding to the uneven texture of the hardened deposit. As paint lacquer warms - in the direct summer sunshine for example - it softens and expands. At the same time, that heat dries and hardens any bird droppings on the surface. Then as the paint lacquer cools, overnight for example, it contracts, hardens and moulds around the texture of the bird dropping. The advice is to always remove the deposit at the earliest opportunity with a moist cloth. Autoglym's Bird Dropping Wipes are ideal - to gently lift the deposit from the surface. Consult a valeter by all means, but there is probably nothing you can do except commission a repaint of the affected area.