Hello, I have 13 adult hens, and now 11 chicks, 2-4 wks old. And 6 ducklings. I was wondering if I should give them grit or oyster shells? I have some Oyster shells for the hens, but the pieces are really big and they don't seem to really eat them. I looked at TSC today, and they have granite grit, and also oyster shells that are ground up finer than what I have. Which is best? or both? How old do they have to be before they get it? Sorry for so many questions. . . . P.S. the hens are free range, the chicks will be when they get older. Thank you!