Thanks for the information.

Well there are dozens of causes of this problem and dozens of treatments for it unfortunately. Most of the time this is caused by an airsac infection (bacteria, or fungus) and it weakens the wall of the airsac causing the air to leak under the skin and cause this bubble. The second most common cause in my experience is... no identifiable cause. If there is no sign of illness either his outward activity and his white blood cell count is normal then often this has happened for an unidentifiable reasons. Some birds have been taken as far as surgery with a camera inside the airsac to see if there is a cause. Commonly none is found. In this case just time and no treatment will allow the airsac to go down on it's own. If he is bothering the area or can't move or eat because of it then it can have a surgical draining tract put in but this is for extreme circumstances. I would have him seen by a vet, a simple cbc blood test will tell us if we suspect infection or not. If not often it will heal on it's own over weeks to months. If there is a high white cell count then the infection needs to be identified as the infection itself can cause fatal pneumonia and the only sign would be this air pocket.

I hope this helps, best wishes.
Dr. Dan