I was wondering about this. Our three hens started laying in late January, and two months leading up to that, I supplemented the grower feed with egg shells. When we got the first egg, it seemed just fine - not extra thick, but not thin. As more eggs came, same thing, acceptable shell. There became a point, though, that they were gobbling down those egg shells, and I didn't think I had enough, so last Saturday, I bought a 50 lb. bag of oyster shells, and just in a few days, their shells became noticeably thicker. I just couldn't believe it could happen that quickly, but I found this thread (I was going to start a new one) to answer my question. I was wondering, too, why most people put their oyster shell in a container, as I just throw it on the ground like scratch.