I have 20 grown chickens in a good sized coop. About every month, one will get dull, sit with ruffled feathers and then die within a few weeks. Their droppings look normal to me. What could this be? We have lost three hens in the last six months. I noticed one this morning in a nest box all fluffed up. These girls are anywhere from 2 years old to five years old. Thanks for any insight.