As breeder of Exhibition fantails, I can assure you that the bird is not suffering from any health problems. The answer is as simple as Bruno describes it. If you just break away a small amount of the sheath at a time, you can feel the piping drying up. The feather will eventually look normal. Pigeons normally moult 2 tail feathers at a time but because fantails have a lot more tail feathers, they tend to moult them in batch of about 6.Just rub the quill with your finger and thumb once a day, but only about ½ inch each time, allow them to have plenty of baths and they will moult out perfect.