My two year old Pekin ducks refuse to eat their oyster shells. The shells are fairly new to their diet because they had strong rock hard eggs in the past but after one became egg bound we started feeding them small dishes of their egg shells. They ate some of them but we never worried because their eggs were still healthy. A week or two ago we started noticing their egg shells were getting thinner then usual, so we bought some oyster shells. I know-we should've bought them a long time ago. Anyways I've been putting out separate dishes of oyster shell and the girls won't even touch it. They'll put their beaks in it and dig around but then dismiss them as rocks and leave. At some point they get tipped over anyways. Their eggs are getting thin to the point that when they're laid they dent from the floor. They really need to eat them but I don't know how to give it to them in a way that won't make the boys want to eat it too.