Hi. Wecome from OHIO. You got some really good replies that I agree with 100%. Here in yucky weather Ohio we have to get creative with the forage almost year round and have found, between asking for the old produce at our local grocery and also giving the birds the same high protein hay that our livestock receives , quality poultry feed and feeding back dried egg shells to them they pretty much remain the same year round. Yes they will lighten during egg cycle and that will vary according to the breed. Thin egg shells are most likely a calcium deficiency but exposure to pesticides and herbicides will also do that. Good luck and again WELCOME.