Hi Ewelina,
Some cases of feather plucking are behavioral. But Vets are finding that more often there is a medical cause. Now it can become a habit once they begin, which is why the diagnoses can be so challenging. Treating the issue does not always stop the plucking. If a medical cause is found and treated, the bird usually still needs a collar until the feathers have grown back in. Any bird that is feather plucking should have tests run. Typically there will be blood tests, including screening for some viruses, and often a skin biopsy to see if there is an infection. If you are not satisfied with what the Vet said, I would recommend a second opinion from another Vet. We are working to schedule a webinar on feather destructive behavior, hopefully in July.
Thank you for asking Lafeber,
Brenda