The most common cause for feather plucking is due to a lack of calcium in a birds diet.
It can happen with all birds from small finches but is often seen in Parrots and mistaken for boredom.
Adding Dr Rob Marshall Hi Cal or Morning Birds Calcium Plus to the birds water or Calcium Plus Powder added to your birds food is the first and best approach.
Most feather cases have been down to a nutritional problems, a weekly multi vitamin like Dr Rob Marshall Ioford & Dufoplus will help with any lack of vitamins problem.
Feather Plucking also can be down to medical conditions or parasites, so veterinary testing may be required to see if there is any underlying medical conditions.
Treating the feathers with S76 and spraying the cage/aviary with Avian Insect Liquidator will also help.