There are many weeds/seeds/vegetables that provide calcium, but because chickens have probably not evolved in the local area (They are most likely to have evolved in Asia and India) the local calcium products may not be enough to produce the type of eggs we would like to have.
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I have seen both oyster and clam shells advertised for chickens so I don't see why not. I'm not sure if snails have the same calcium content, but if they are washed I don't think they would pose a problem