I'm new here, so please forgive me if this topic has already been covered. I looked first but didn't see anything. For many years, I have been reluctant to feed my chickens oyster shells because of where they come from. That is, oysters come from bays and estuaries, which is also where heavy metals end up being deposited. It's not that I've ever heard that oyster shells are laced with heavy metals, I just assume they are given the environment they live in. I give ground granite for grit, but haven't found another source of calcium. So I just feed a good laying mash and scratch grain. Am I all wrong about oysters and heavy metals? Does anyone know of another source for calcium? My birds always lay well and I've never had trouble with thin egg shells, so maybe it's a moot point. Thanks. Capt.