Most feather problems are due to one of five things; psychological factors/boredom, nutritional deficiencies, infections, hormonal imbalances, or parasites. Here's a list of things you should be doing to prevent feather loss.
Feed a varied diet as per your veterinarians recommendations. Seed and water is not enough, a bird's diet should encompass commercial bird food pellets, seed, grit, water, greens such as silverbeet, carrot, broccoli, native tree branches and their flowers and fruits, mineral and salt blocks.
Allow daily time out of the cage if possible (but make sure there are no fans on, hotplates are switched off and windows are closed.
Leave a radio/TV on if your bird is alone in the house during the daytime.
Ensure your bird is receiving enough natural photoperiod, which is normal length of day and night. Cover the cage at dusk, and relocate it to a quiet room.
Create a spacious environment for your birds so that each bird has their own territory. Provide areas of camouflage in cages with several birds, bushy shrub cuttings (bottlebrush are good).
Clean the cage daily, feathers left in the cage may stimulate further self-mutilation.
Provide a birdbath where possible or spray with fine water mist.
Protect your bird from stressful situations such as from predators including cats, dogs and wild birds and extreme temperatures.
As a last resort you can try plastic collars, we do not recommend doing so until you have had your bird examined by a veterinarian for parasites and infections diseases which may not respond to the general changes as suggested above and may require medication.