I started mixing oyster shell in with their food at 18 weeks to boos calcium for their first eggs. I've noticed that they often times pick the pellets around it though, and don't really chow on the shell. I bet they know if they need it or not and if they start needing it they'll eat it more. Instincts are great, so the shell is there if they want it. It's so cheap I'm thinking better to have it for them than not.