One thing about the oyster shell that I haven't seen mentioned is that it also helps do the job of grit in the gizzard. Everything goes to the gizzard to get ground up by the grit and other hard materials. I've even seen small bits of glass, metal, etc. in there. In the process of grinding, the oyster shell, and whatever else is there, gets ground small enough to be absorbed in the stomach/intestines or passed on through.
Rodney
Rodney