Our chickens have been laying eggs with very hard shells. We bought a bag of oyster shells for them for the calcium 2 weeks ago but never opened it and don't plan on opening it too soon. I'm getting egg mash from a local feed mill and it has 30%calcium added to it. Would that be too much? I also heard that free range birds get a lot of calcium from plants and stuff. Do you think they will get " softer" if I change their diet? And should I change it?