Medications are just one cause of fret lines... stress when molting can do it and also it could be genetic...
What would the genetic aspect be? I just assumed this was completely normal because all four of my roos were the same...They have extremely good food, get extra meal worms, soldier worms, crickets(on occasion), scrambled eggs, and whatever other critters they eat outside...It has been hot/warm and only just starting to get colder at night, but their tail feathers have been streaked like this since they first grew longer tail feathers.
It was difficult for me to even find a post about this, but everyone else's comments primarily mentioned medication(of which mine have never had any, including in food) and weather...These four(and the pullet I got with them) were all severely dry-skinned and took months to grow their feathers, despite the fact that they were still very young and had never molded and even adding in crushed mealworms to their already high protein feed didn't seem to speed the feather growth. They are beautiful birds, fully feathered now and you wouldn't even know they'd had that issue seeing them now, but the possibility of it being hereditary seems very likely with mine. I just don't know what that really means, I guess? I ended up buying four more of the same breed a few months ago from the same person I bought these from and two had the same skin/feather issue, one roo, one pullet, but the roo that didn't has such an incredible tail at 3 months old, the same age as my Sumatra, yet an even longer tail! None of the older ones that have this streaking had a tail like his when they were that age...I'm wondering if it is possible that one of her roosters or maybe even some of her hens, but not all, have whatever this marker is? Sorry, that was very lomg winded, but I figured I'd put some back story into why I was curious about the hereditary aspect if you might know or if anyone else might?
These photos are all from the same roo, but all four(told they were pure Ayam Cemani) have exactly the same issue. Wing feathers do not have this, and they all have plenty of room to roost away from walls and fencing.