One of my 18 month old Silkie roosters is chewing his tail feathers, not plucking them. The shaft (Rachis) of the feathers are still there, and cut short, but the vane of the feather is gone. I haven't actually seen him in the act, I just assume that's what he's doing, because my other rooster and my hen's feathers look just fine, so I didn't think it could be a parasite. Oyster shells are mixed in with the feed every day, so it can't be a calcium deficiency either. Any thoughts???