I have three backyard hens all just around 4 months old. None of them are laying yet but I want to make sure that they have enough calcium for when they do. They are currently on regular layer feed pellets for food. A couple of times I have put out a small tray of oyster shells for them that is separate from their automatic feeders. And I swear they eat it all by the next day! I don't want to be having to buy a new bag of it before I see any eggs. Is there such a thing as over eating for oyster shells? And should I not be feeding it to them until they are laying?