I am going to be switching all my chickens over to flock raiser when my new chicks become integrated, and I have been told that I should also then offer oyster shells on the side for my layers. First of all, do I need to make these inaccessible to the younger chickens? And also, is there something I could give them instead of oyster shells, like their own eggs shells, or otherwise? And P.S. Is this still a necessary addition for free ranging birds? Thank you!