This sounds like a very loved bird. Being an only bird makes her even more special since she likely gets a lot of attention and indulgences. Though her age is considered 'geriatric' for a 'tiel, these days average lifespans have increased to 18+ years because of leaps and bounds in foods and vet care.
These are just some of the possibilities and I know how much you want to help your companion right away. At this point, not knowing exactly what the causation is, a nice steamy bathroom might provide some relief. Not only to provide overall comfort, but to help with her skin/feathers.
Run hot water in your shower with the doors shut and sit there with your friend providing some quality time and reassurance. If you have a vaporizer (not a humidifier), running it in the bird’s area (never put meds in it, just plain water) may provide similar relief as sitting in a steamy bathroom, but more ongoing.
Lightly misting your feathered friend with bottled or distilled water (for purity purposes) might help a little, but if it seems to scare the bird or be uncomfortable, don’t.
Yeast, Vitamin A deficiency may go hand in hand. When a bird’s diet is mostly seed, vitamin A tends to be lacking, along with calcium. I know the packaging makes it sound like it’s a great feeding mix, but spraying the surface of seeds with vitamin A and calcium doesn’t get it into the bird when they crack those seeds and leave the casings behind. Unfortunately, manufacturers are ‘bottom line’ oriented, after a profit, not necessarily the health or welfare of our birds.
A deficiency can open the bird up to being susceptible to opportunistic bacteria and fungus. Yeast is an infection often seen in the mouth, crop and intestinal tract, but also found on the bird’s skin and feathers.
Calcium deficiency may lead to spontaneous fractures or breaks of the bone and all sorts of serious complications in a female whether she lays eggs or not.
Since she hasn't been exposed to other birds, many of the more serious illnesses are unlikely, though 'anything is possible'. It would be well worth having your vet run a good genetic screening with a drop of blood they'll get from a toenail. It isn't much more traumatic than having her nails done.
You must see your vet about this. Left untreated, no matter what it is, has a significantly higher chance of becoming a serious health threat. Birds are masters at hiding illnesses. By the time you notice anything wrong, it’s usually gone on for quite a while longer than you imagine – so there’s no time to waste.
It’s generally recommended that most of today’s companion birds have a predominantly pelleted diet. Pellets have been continually updated since being introduced to the market years ago and today’s formulas are better than ever.
Supplementing this diet with fresh foods every day is ideal and many owners find they can re-introduce seeds - in limited amounts (perhaps once or twice a week) without the bird refusing the pellets overall.
It took me two years of constant, daily trying before one of the permanent on site rescues would eat fresh anything (other than cashews), but she eventually decided she likes raw carrots. Once in a while. In fact, she’ll try nearly everything once in a while when she sees me eating it too. Needless to say, my diet has improved dramatically since I’ve got to do something with all those veggies!
Whole grains, dark leafy vegetables, fruits and legumes. Include the colors orange, yellow , green, plus reds too! Think sweet potatoes/yams, squash, melons, oranges, peas, chard, beets and others.
Brown rice, quinoa, whole wheat couscous and natural, whole grain pastas are great choices.
Limit fats, especially the kind from animals. Good fats are most plant fats like soy, olive and canola oils. No fried anything
Even though this is put out by a cockatiel site, it’s applicable to all hook bills from budgies/parakeets to conures, greys and macaws.
http://www.cockatiels.org/articles/nutrition/diet.html cites feeding both seed and pellets, but only after weaning the bird from a mostly seed diet.
http://www.letstalkbirds.com/cockatiels.htm
Good luck and keep up the very good care
Certified Avian Specialist
Cert. Avian Specialist; Int. Assoc.Animal Behavior Consult; Pet Ind. Joint Advisory Council; author