Plenty of water to loosen it. Soak the poo well and it should, mostly, wash off with a sponge / cloth / hose.
Bird poo is highly acidic, so time spent on the paintwork will attack the laquer / wax / finish, so be sure to keep the paintwork protected with your favourite brand of polish.
Also, the cleaner the paintwork (by this, I mean lowering the surface friction by liberal application of the shiny stuff), the easier it is to remove the poo / dirt / detritus.
Never pick at the poo with your finger nail as this will create tiny scratches in the paintwork, unless you're VERY careful.