Feather destructive behavior is very often a combined physical and behavioral issue. It is best to seek out a Vet who is Board Certified in Avian Medicine. Generally quite a few tests are necessary and if a cause is determined, the bird still may need a collar until all of the feathers have regrown, because the plucking has become a habit. If this just began in December, it is far too soon to expect him to be back to normal already. This is generally a long term treatment process which also involves the time it can take to regrow the feathers and determine if he has been successfully treated or broken the habit.
It may be helpful to pass along these notes to your Vet from Dr. Susan Orosz, in regards to a 20 year old Eclectus Parrot who began feather plucking. Dr. Orosz has her practice in Ohio and your Vet may want to contact her to discuss recent findings regarding these issues: One area that we are looking at(with feather destructive behavior) is the effect of avian bornavirus and other agents to produce avian ganglioneuritis. These agents can attack the nerves that travel to the skin and it may be one reason for the cause. Some of these birds will be negative for the virus but be positive for the antibodies produced from the myelin wrapping of the nerves and often can make the bird appear to be itchy. The type of testing for this is important to determine if this might be one of the reasons for plucking. The problem is that many vets do not understand the AGAA test and what it means and pursue other avenues. In the case of AGAA, of the parrots that were feather plucking about 70% of those parrots tested positive for ABV or AGAA. So we know that that is one strong relationship. While there are others, I would suggest making sure the skin and feather follicle is free of yeasts and bacteria; and then try giving robenicoxib for a period of a month or 2 to see if your Eclectus improves.
Thank you for asking Lafeber,