I have six apparently healthy red sex link hens approx. 7 months old. All have been exemplary producers and I have been getting 6 eggs daily. They free range during the day and are fed about 30-40 ounces of "All grain Poultry Layer/Breeder" food in the feeder each day. I have not fed them any oyster shells. About 5 days ago, one of the eggs was noticeably lighter than the other five. I have a nice picture showing this problem but am restricted from uploading it because I am a new poster. So, the shell has gotten lighter and rougher and thinner each successive day with today's egg shell being almost nonexistent.....only the membrane holding the egg together but not enough shell to hold it in a round shape. All six hens appear perfectly healthy and energetic. Any ideas?