I have noticed that a few of my hens have been laying eggs with very soft shells, well they are hard but any slight pressure on the shell and they crack. They are free range with free choice layer feed. They all have access to oster shell and grit. I give them green veg. and fruit when I have it. They are all health...at least act like they are. I have noticed the shells getting softer in my red sex link in the summer, but now notice a few of my other girls are also laying fragile shelled eggs. Any ideas?