@ elaeserek:  Thank you so much for the info.
 
My concerns re oyster shell: Years back, in Germany, a hard-boiled egg was shelled within seconds. The last few years, here in the U.S., it takes ages to get the shells of a hard-boiled egg. (They even offer devices on TV to get the shells of hard-boiled eggs.) 
 
I suspect that oyster shell supplies chickens with more calcium than they actually need. I might want to try without it and see what happens. We never had a problem with soft and breaking egg shells, when I was a child. And my grandmother only fed our chickens wheat, not balanced chick-feed that contains calcium. Our chickens only got the shells of hard-boiled eggs, and I faintly remember that my grandmother heated the shells of raw eggs and soft-boiled eggs in the oven to sterilize them and, then, feed them to the chickens. 
 
So much of recommended nutrition (for animals AND humans) is nothing but a money-making business, and it is so very difficult for lay-people to make the right decisions.