I have six Rhode Island Reds that I raised from chicks I bought down at the feed store about three years ago. They have been good egg layers, but about mid summer this year they began to lay really thin misshapen eggs. At first I thought it was the extreme summer heat, but they continue to lay them even though it is almost winter. I also moved them from being penned to completely free range this year, but they were laying the thin misshapen eggs before I did this. I feed them layer, but when I moved them to free ranging I put out a pan of oyster shell (they don't seem to care for it that much). They seem healthy otherwise, but continue to lay thin misshapen eggs. What could be the problem?