I have found that warm RUNNING water is the best. Just hold their little butt under and with your fingers (yes, I know, gross) loosen any dried poop, trying not to lose any feathers (that way no bare skin shows to get pecked) then blow dry their butt so feathers are puffy again. If the area doesn't look different, they won't get pecked. You may have to do this for several days and maybe several times a day but after a couple of days you should be past this problem.