Removal as soon as possible will help minimize the damage. Instead of carrying a hose and bucket in your car, carry a bottle of no salt seltzer water. No salt seltzer water is nothing more than water and carbon dioxide which will not harm your paint. When needed, take off the cap, place your thumb over the top, shake well and you have a fire hydrant that will wash the worst of the bird's thoughtful gift from your paint. Try to rub this area as little as possible.
Birds use gravel to digest their food and grit is one of the major components of their presents.
If you try and rub off the solids, you may scratch the paint. When you have the time, give the area a coat of wax. If the acids have left a slight mark in the paint, see the article on cleaning your paint. 3M Imperial Hand Glaze will usually remove all traces.