I had a cohort of chicks that suffered an outbreak of cocci a few weeks ago. Some died. Some became very sick but eventually recovered. Those that became sickest yet survived have a record of the stress in their feathers. Below is one of the more strongly affected survivors, a red jungle fowl x American game cockerol (or stag if you prefer). I prefer male juvenile. He is in his second feather set and has the third (first adult) starting to come in. The following is a closeup showing check marks on flight feathers of his tail. Notice the discoloration and narrowness of feathers roughly half way down their length. Those areas were grown in while bird was sick. The feathers are brittle in those locations.