Keep a couple of towels and a spray bottle filled with distilled/deionized/bottled water with a bit of car soap and a capful of rubbing alcohol in the trunk.
Spray the bird muck as soon as you can. Wait a couple of minutes and rub gently. Most, if not all should come off. After wiping, clean/polish the area with another dry, soft cloth. Repeat as necessary.
If the muck won't come off after the above, try some very warm water to soften it before attempting to wipe it.
This technique works for spots on the rest of your car as well. I pre-spray all the spots on my car before washing it. As always, pretest any new process in a small/inconspicuous area to make sure it works for your application.
btw, I use RainX self dry car soap for virtually spotless self drying (no wiping).
2004 E60 530is, Delivered 12/12/03, TiSilver/LGrey, PP, SP,PS, PDC, ACC, FRS, Step, Xenons, clear bra, body colored rear inserts. (Retired, 1983 henna red 320is)
2004 Corvette Z06 Commemorative Edition