Autoglym active insect remover works for me! I also make sure it is HOT! before I put it on.

However I recommend a really good air rifle to discourage them from shitting on the car in the first place! Magpies and pidgins make HUGE ones, and if they have been eating the purple berries can take the paint off down to the bare metal (big thing on the Mazda forums)... my poor neighbor had this on his 1 year old car!