In addition to that..

I quote from Michael Stoops from
http://meguiarsonline.com/forums/sho...d.php?p=390667

Another reason why you should lift the bird dropping off of the paint rather than wiping, is because of how birds digest food. If you've ever had a pet bird you know that you need to add gravel to their food seed. Birds require gravel and/or even small stones to help them grind up and digest food, primarily seeds that are swallowed whole and need to be literally ground down in the gizzard. Inevitably, some of this gravel, sand or other abrasive particulate matter is going to be passed through their system and end up in that dropping on the hood of your car. Simply wiping the dropping off the paint can potentially lead to a grinding of this particulate against the paint, which is obviously not a good thing.