I've been thinking alot lately about oyster shells for calcium. I've read here that it's more bioavailable but what about mercury poisoning? We're growing our own food so we know what's in it but do we really know? Where do these oyster shells come from? I've read that oyster shell calcium for humans is not bioavailable and a waste of money, so wouldn't that be the same for chickens or are their systems different than ours? You'd think that the chicken's own egg shells would be more bioavailable. Does some one have study they could point me to? Thanks for helping me out.