We have seen Gang Gangs with feather picking that had underlying avian tuberculosis. So the first thing to do is to get the bird examined or, at the very least, catch it and examine its general body condition. A feather picker with poor body weight is likely to be diseased. Damage or disease to feathers and feather follicles can initiate plucking. This can be due to enclosure-related damage, exaggerated preening by self or mate or chewing of feathers by adjacent birds.
Damaged quills irritate the bird and serve as a continual focus for plucking and mutilation.
Allergic dermatitis is a cause that has yet to be proven but most likely exists, given all other animals can suffer from it. Psychological causes of feather picking are many and varied. In the wild these birds are flock animals with a highly developed social structure and a large territory through which they forage. It is not a surprise that they feather pluck when in captivity. It is more surprising that more of them don t do it.