I have been reading quite a bit about calcium additives for ducks, oyster shell and the like, but I have never felt the need to do this with my ducks. My Harlequins have been laying now since mid September, and they all have been producing eggs with shells so thick and membranes so tough that I need to crack them with a knife! Do others here also have this problem?? Would the calcium supplement be better suited for mature or aging ducks??