Well, your between a rock and a hard place with getting bird poo.
You need to get it off the paint as soon as you can really. If you see it on the paint, you want to take it off the same day however you can.
Best bet if your out and about is to use a pressure washer at the shell garage or something, but just use the sprayer with clean water, no shampoo. That should get rid of it.
Failing that, you could spray the poop with quick detailer or something, let it loosen off a bit then wipe. Yes, this might marr the paint but doing this would be a lot less harmfull than leaving it on the paint.
The preferable choice though would be a full wash routine with snow foam and wash mits etc. but who has time to do that everytime a bird poos on your car?
Its about damage limitation really.