Do you only ever use your own eggs? Do you ever buy eggs? Like in the winter during the slow period. Chickens tend to prefer eggshells to oyster shells, so if yours don't want to eat the oyster shells, you can still feed them only eggshells and be totally fine for calcium, so long as they have an unlimited supply that never runs out. If you only ever feed them their own, like others have mentioned, over time it won't be enough. However, if you buy supplemental eggs during the slow season, or even during the egg-laying season if you only have a few hens that don't meet your egg needs, you can save the shells from the eggs you buy, and provide a constant supply. That's what I do. My hens stop laying completely in the winter so I buy eggs during that time, bake and crush their shells and save them up (they can last a really long time once fully dry). I save my own shells once my hens resume laying, too. So I always have a plentiful supply to feed to them. They've never eaten oyster shell or layer feed (the kind with added calcium) and I've never had soft or brittle shells. So, to sum it up, eggshells only is totally enough, provided that the hens have an unlimited supply that never runs out.
