I feed my pullets layer feed, but they also do eat some oyster shell. I think it's a good idea to have it available, just in case they might need a little more, or get other food that cuts down the percentage of calcium. Then the eggs will probably be good and hard, and cut down on the chances of egg-eating. I believe that free-rangers are able to supplement with insect "casings" (can't remember the term, early Alzheimers...), but chickens in a run are dependent on us for all their requirements.