There are many causes for feather plucking.  Birds pluck themselves if they are itchy from mites and lice.  If the diet is lacking in some ingredient, or if the air in a room is too dry, you might see some plucking.  We see this in winter when bird owners heat with wood and the air dries up.

Unfortunately, often birds pluck their feathers because they are bored.  Increasing contact with your bird helps as well as playing with them with interactive toys.  It is also good to rotate their toys as you would with a young child.

Also consider the possibility of allergies.  One of our customers, who was a cigaret smoker, had a bird that plucked.  They consulted with the veterinarian and tried various changes in diet, room humidity and play time.  Eventually their vet suggested they change out of their clothes that smelled of smoke before approaching the bird's cage, and the bird stopped plucking!