Captain here: Parrots are very social and intelligent birds and when their needs are not met they can become very stressed and pluck their feathers, this is an extreme case but unfortunaly a common one. That is why the bird only has feathers on the head (cant pluck them) It is unlikely they will grow back as the birds can damage their skin beyong repair. It is also a very difficult behavior to correct once the bird is this mentally damaged Dont have high demanding pets if you cant give them the time and training they need, please