Is there a reason you need soap? Grease or something like that? In general Ducks will wash themselves in clean water and prean their feathers. Their feathers need oil from their oil glands to keep them strong and waterproof, dish soap might interfere with that process. So I would say use no soap unless there is something worse than dirt/mud contaminating their feathers. If you do need soap, GardenGal's suggestion of a gentle pet soap might be a better way to go than dish soap. I can't claim to be an expert however, so if you have more questions about ducks, I would suggest you check out the Duck part of the forum, lots of very knowledgeble people there! Hope that helps!