The shoulder position can cause a bird to think they are the boss, i.e. the dominant one. I noticed that with my bird, Abbie, that regular baths helped to decrease her feather plucking. I also moved her to the main room of the house where the action was and that seemed to help too. My Vet told me that feather plucking is like nail-biting to us. I bite my nails when I'm bored or anxious.
I wish I could be of further assistance. Good luck.