Wow! You are an excellent parent! I'm so happy to hear you allow your birds to be birds during the day. You even have the daylight/nighttime hours divided appropriately.
All this praise said, let's look at the various possibilities behind what you're seeing:
Sometimes a tumor on the kidney will not appear on the outside of the body, but other symptoms such as those you’re describing. These things could be the result of the tumor pressing on certain nerves.
Tumors can also be in a male’s testes or female’s ovaries and there are not always obvious changes until later on when the growth is more dominant inside.
Other indications that there may be tumor activity would be a change in cere color, weight loss, changes in droppings (often becoming pasty, soiling around the vent) and just subtle, overall changes that owners may sense more than actually see or be able to describe.
Fatty liver disease is something that is often seen in a bird on a seed only or predominantly seed diet. No matter how much the manufacturer insists they are fortified and healthy, they are misleading all of us. This is what I expect is happening with Coco
Skeletal problems, deficiencies and even toxicities can cause various symptoms too, even though they seem very unrelated. Anything from loss of balance in some birds, as well as the more common symptoms such as breathing difficulties/breathing with beak open, restlessness, nervousness, acting afraid of usual things, startling easily and so on.
Zinc and other toxic metals or substances can be ingested slowly over time when toys, clasps, chains, links or even cages are chewed on or played with. Other poisonings occur when the bird actually swallows a toy, link or piece of one. Watch out for bell clappers for instance.
Take a look here under critical conditions to reassure yourself that urgent veterinary intervention may be necessary (these symptoms apply to all birds, not just ‘tiels)
Remarkably, birds don’t need much, if any vitamin C. It is a water soluble vitamin which means it passes out of the body after the body takes what it needs and C is available in a wide variety of both fresh and processed foods given to birds.
Vitamin A/Beta Carotene is frequently found to be deficient in birds. This is a fat soluble vitamin which means it gets stored in the fat cells of the body, so it’s possible to overdose on it. With our companion birds though, too little is the situation most often encountered.
The symptoms a bird will show when deficient are increased allergic reactions, respiratory/sinus infections, reproductive problems, skin and feather disorders, even cysts and tumors, as well as various intestinal complications.
Vitamin A is most ideally received from natural foods like sweet potatoes, yams, carrots, squash and other dark colored vegetables. If your bird doesn’t care for fresh vegetables, a ½ teaspoon of natural baby food (human baby food) of any of these vegetables. Again, it must be all natural and nothing but the vegetable with water sufficient for processing.
What you can do while securing a vet to see your companion is supportive care.
If your bird will drink and eat on their own, excellent. If not, have an eyedropper ready to administer a few drops of plain water, or better yet, children’s Pedialyte every 20-30 minutes. Put the dropper gently inside the beak and let the drops fall into the bottom beak under the tongue rather than trying to get into the back of the throat. We don’t want to chance the bird inhaling the fluid and developing pneumonia.
You can also make some sugar water with 1-2 tablespoons of natural white sugar (none of those sugar substitutes no matter how natural they claim to be) in ½ cup of water. Stir until the sugar is dissolved and offer the bird a couple drops of this along with the Pedialyte.
In a pinch, Gatorade or other sports drink without added zinc or at least with zinc listed low on the list of ingredients, can be used while you are getting Pedialyte.
Another feeding option is to offer ½ spoon of all natural, organic baby food (squash, yams, sweet potatoes, mixed vegetables) which many birds take readily; also try some pabulum or baby rice cereal and a few licks of natural (no artificial anything) yogurt.
Feel free to mix these or offer them one at a time.
This may actually 'fix' things rather quickly; however, liver or kidney disease (if that's the real internal issue) absolutely still needs a vet visit.
These days, with birds growing fast in popularity as in home companions, many DVM’s are quite experienced and able to see and treat many birds. If you have a pet store that sells birds or know of any bird breeders – ask them who they use for their bird care.
Pet Smart? in town you may have a vet for your bird. Most Pet Smart’s now have a veterinary clinic inside and many of them will see birds (open 7 days a week too).
Also, calling ANY vet and asking who they refer bird patients to should also glean you a few more options.
Will you let me know how it goes?