I have 14 black Australorp hen that are contained in a cage run. They were purchased as "about 1 year old" 14 months ago. 2 have developed a issue recently: they are balding. The feathers on their heads are receding. There is no other signs or symptoms of illness. They are feed a free access of laying mash crumbles and a dish of crushed oyster shell is maintained in the run. My experience spans 14 months so I am green-horn chicken keeper so looking for some advice/help.