A feather we could say is like a fingernail..it shows an illness or chemical exposure or just about anything in ridges in the fingernail..even a very mild illness . drs look at the fingernail to say there was an illness at some point buy the ridges, the feather as growing sometimes it is purley genetic to develop barring no illness involved which is the case most times but on occasion even a very mild stress or illness can change the growth structure of the feather for that period causing a barr..
I have 2 young birds that were attacked by weasle ..the stress of the event combined with long exposure to antibiotics one had quite severe infection cause just a little stress barring on growing baby wing feathers it was very faint but there..none on body..as they grew and wing feathers were replaced with next set, no barring..back to what would be normal for this breed.