I am getting fairly good egg production, but I have one hen (I think it is the same one, color the same, size the same) who lays an extremely thin shelled egg. It often has streaks of blood, and sometimes little crusty spots that makes you think you can scratch them off. But if I do, or clean off the blood, the shell crumbles it is so thin. All the rest seem to be laying beautiful strong shelled eggs. I feed oyster shell free choice, grind up eggshells, put it in their feed. I have never had this problem before. This group is about 9 months old, and I think it is just one hen. Could this be something more than slow development?