Before I bought my own chickens, I purchased eggs from a nearby farm. The shells were paper thin and broke easily - but they were delicious and I knew where they came from (as opposed to store bought eggs). Now that I have my own flock, I keep a dish of oyster shell in the coop at all times. I also give my chickens kitchen scraps. When all the chickens in the flock are at least 17 weeks old, their feed is layer feed. When I introduce younger chickens to the flock, I give them all "flock feed" until the younger ones are 17 weeks old. The eggs have a good strong shell. Sometimes I observe one of the hens eating a lot of the oyster shells. I figure they instinctively know what's best for them - as long as they have a choice. So I let them eat as much or as little as they want.