you may want to offer oyster shell free choice but stick with normal feed. If the one chicken that's laying doesn't get enough calcium it can cause shell-less eggs, and I think it's a good sign that her body can handle oyster shell if she's laying. I could be wrong, maybe more ppl will post, but chickens are pretty good at knowing what to eat and what not to eat. I have 4 ages in my big coop, and only the older girls are eating the oyster shell. I have normal all purpose feed out there (the younger ones need to have the choice of calcium or no calcium) and the bigger ones are the only ones eating the oyster shell. Good luck!!!