How old is he? If he is young he may be shedding some "fluff". If he isn't young then to me it does look like plucking. You really need to take him to a vet to make sure it is not health related. If it is behavioral you may be able to stop it from becoming a habit and long lasting if you start now. After the vet check and if he sees nothing, what size cage is he in? Does he get enough attention? Is there something causing him anxiety? (loud noise, children, strangers always passing through, not enough to keep him busy (toys, foraging etc)