My chickens are starting to pluck each others fearthers out and eat them. They mostly pick at the base of the tail on each other and the hens pick at the roosters comb. They eat Layena and oyster shell as well as scraps from the kitchen and when the weather is better they free range when I am home. Are they missing something in their diet? When I was growing up I remember this happening and when we gave them oyster shell the problem stopped. Any ideas on what to try?