Remember, a duck's bill is like our hands. Ducks do everything with their bills, grooming, carrying things around, digging,.…#3 is out of the shell... hopefully we can keep up the good results!
Is it customary for the already hatched ducklings to "groom" the one that's trying to pull itself out of the shell? (Are they just going "oh look food"?)
I assume that this grooming of their new friends is their way to make contact and welcome them into the group.