Hi Libby,
The preen glad is surrounded by feathers, so it can be hard to see. As for the feather destructive behavior, bloodwork is just a first step in diagnosing the issue. Not all Vets who see birds, are Board Certified as an Avian Specialist. Feather Destructive Behavior is a very complicated disorder that can have a medical cause, be purely behavioral, or most commonly, is both medical and behavioral. You really need a Board certified Avian Vet to see the bird. With that type of chewing, a nutritional issue should definitely be considered. Is he getting a nutritionally balanced diet such as pellets or our foraging diets, or does he only get seeds? If he is getting balanced nutrition, special bloodwork to check on levels like protein, calcium and other possible deficiencies should be run. I would take him to a specialist and let them know your concerns about the preen gland so it can be checked. Preen glands can get blocked, although that typically results in an enlarged gland which would make it easy to see. But there can be other issues with it, so it’s always good to express your concerns to the Vet so everything can get ruled out.
Thank you for asking Lafeber,
Brenda