We had exactly this problem, only half with bald bums. Tried all sorts of stuff on them to no avail. Then we spotted the feather eater. She used to sneak up on them, pull a feather from their bottom and run off and eat it. Only half of the hens didn't spot her first -those higher in the pecking order she didn't go near and those lower were always nervous of everything and ran off. Those in the middle were the victims. She was removed and isolated for several weeks but on return did the same thing again. She was isolated for a couple of months but carried on. She didn't eat much when isolated and when with the others only ate feathers. We checked her records from hatching and she started feather eating at 8 weeks. She was isolated then but obviously it didn't work. She had never laid so she had to go, which was a shame because the three Gold Laced Wyandotte sisters were a close knit group -the last batch with only them hatching. All the hens bum feathers grew back in about three months.
Except for little Minnie's (her sister) that is -and the cannibals.
