I have one problem feather picker. Flo is a two-year old EE. She was precocious as a tiny chick, full of energy, always trying something new, pushing the envelope. By the time she was four-months old, she was racing around the pen at top speed, yanking feathers from backs, and butts and necks. She had so many victims, at one time I had saddles on half the flock to try to protect them from Flo shaving them bald.
Over all this time, I have learned a lot about feather-picking. I'm convinced that it's more stress-related than anything. However, I do believe much of Flo's problem was originally nutritional. She was under-sized until she was eighteen months old. I began feeding her a supplement and she went through an astonishing growth spurt, stopped plucking feathers, and calmed down.
However, she has had several relapses and remissions. The common denominator to all her relapses, has been stress. She was doing well, but relapsed a couple months ago when I began remodeling the pen. This threw the whole flock into stress. About the time the pen was completed, the flock all went into molt, including Flo, and she relapsed again.
Currently, Flo has gone back into remission again since she's completed molt, and everyone is enjoying the wonderful new pen.
So, feather-picking is all of that - it's a mental fixation, it's dietary, it's stress-related. And it's the most aggravating problems there is that we who love chickens have.