It can be of nervous origin, parasite infestation, or poor nutrition. It is definitely possible to treat it, but it is necessary to visit a specialized veterinarian. The most common cause is nervous origin. Parrots and budgies in particular, because they are very intelligent birds, respond to various stressful stimuli by self-plucking, which can later develop into an unpleasant and difficult-to-learn habit. For these birds, losing a partner or assigning another may be a very frustrating affair. But other influences, perhaps invisible to humans, can also cause this behavior, such as moving a cage, changing feeds, a new piece of furniture, a new family member, etc. Another possibility is that they are bored. Try hanging a piece of rag in his cage if he doesn't have fun and cup a cloth rather than himself. Give him more toys, such as a mirror, a bell, willow twigs, etc.