when i started reading this thread i was thinking "I wonder what they feed".A few extras in the diet can really help.
i would enhance their feed with omega 3's which are known to have benefits to inflammatory arthritis. Something like cod liver oil, sunflower seeds, various nuts. The contents of supplement capsules, or liquid from the chemist could be mixed with feed.
Omega 3's are long chain fatty acids, whereas the other oils in the feed you use may be the short chain type which may or may not be metabolized to energy, or converted to the long chain forms that benefit the body's joints/brain.. Depending on how much you add you may wish to reduce oil rich treats such that the birds don't get too much oil in their diet. Bear in mind the suggested human dosage of whatever you buy, and dose by combined body size of the birds.
Additional vitamin e would also help. Good vitamin rich food could consist of ground up breakfast cerial, (be sure to choose something low sugar)
Oils will take a few weeks to get into the animals systems so there will not be immediate difference. Vitamins will also help the animals feel more alert so you may notice this change before the joints appear different. It has worked with my dogs, and hamster, (And grandparents!!) so it should work for the birdies.
Good luck,
Dr S.