Since you have no roosters and havnt seen any external parasites, have you seen feathers on the ground? If not, you have a feather picker(s) and they are eating the feathers. This behavior can (and probably has) spread throughout your flock. If this is the case, it will be difficult to stop your birds from feather picking. You can purchase gamebird feed and give it to them for about one month. Then wean them off the gamebird feed back to regular layer feed. Eating feathers is an indication of a lack of enough protein in the diet. Gamebird feed has a higher protein content and will hopefully help stop the feather picking.