How old is your 'tiel?
Has he been in his cage all day?
Any other birds or animals around (do you think he was injured by something)?
What kind of food is he usually eating?
Any other symptoms, like not eating normally or changes in his droppings? Any signs of fluffing or sitting at the bottom of the cage?
It's a very good sign that she's eating. Are her droppings normal looking?
"Fluffing" is puffing up her feathers. It makes her look bigger, 'fluffy'. This is a bird's way of holding heat closer to their skin. Remarkable things, feathers. They can be air conditioners or down comforters.
What sort of food has she been on most of her life (seeds or pellets)?
Thanks again for the additional info.
The first suspicion when a female 'tiel seems off and includes limping is that she might be egg bound. Fortunately she's not showing other symptoms that would indicate this (very serious) condition.
Watch for any difficulty with her droppings, droppings pasting or sticking around her vent or any blood or protrusions there. If this occurs - you need to get her emergency care no matter what time of day or night and no matter how far you have to drive.
Now, that said, the next possibility is still something you need to have her seen for and I'd find someone to see her first thing in the morning. Even if she's acting better - it's very likely to come back and be worse.
A 'tiel on an all seed diet has a tendency to develop tumors and a condition called hepatic lipidosis (fatty liver disease). Both of these conditions are quite often seen initially (by very observant owners) as limping and acting a bit 'off'.
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.
http://www.cockatiels.org/articles/nutrition/diet.html cites feeding both seed and pellets, but only after weaning the ‘tiel from a mostly seed diet.
Also check:
The American Cockatiel Society http://www.acstiels.com/Articles/BasicCare/cockatie.htm
The Bird Care Library at Harmony Animal Hospital http://www.petvets.com/petcare/birds.html#nutrition
What you can do while securing a vet to see your companion is supportive care.
Putting a heat source into the cage may be necessary since a sick or distressed bird may lose body heat. I prefer a non electric source and use rice socks.
Use a thick, clean sock and fill it ¾ with plain, raw white rice. Knot the end and microwave it for about 1 ½ minutes. Shake it afterwards to distribute the heat and be sure it's not too hot.
You can layer a thick towel on one side of the cage, secure with clothespins out of the reach of the bird's beak - then clip a heating pad over the towel and set on low. Check often to be sure it's not overheating and that the bird isn't gnawing through. A side attachment like this will allow the bird to move closer or away as needed.
If she becomes bottom-bound, install a heating pad under just a portion of the base, again, set on low.
The rice sock option is far safer and more efficient though.
Cover the cage with a thick blanket, leaving all or part of the front door uncovered for better air circulation, to help keep the bird calm by being able to see out and of course so you can check on it regularly just by peeking in. The covering also helps hold some of the heat inside.
I'm very uncomfortable giving the heating pad suggestion because of the potential for the bird to bite through the bars and possibly catch a part of the heating pad. This is just one reason I prefer to avoid electric sources of heat. Another reason is that heating pads can suddenly become faulty and overheat. If ever using an electric source for heating anything in anyway, please be vigilant and constantly double checking carefully.
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 ever 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.
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.
These are just temporary supportive measures to keep your bird alive while you get professional, hands on care. These measures will not cure your bird.
Don't mistake what looks like recovery to be a real recovery. It is very common for a bird to regain enough strength to start hiding their illness again, but what's happening is that it's progressing and by the time you see symptoms again, it will be much worse or too late.
I'd rather take my bird in for an evaluation and find out everything is fine and there's no known explanation for the period of acting ‘off' (which does happen by the way), than to lose my companion.
Good luck with her and please let me know,
Certified Avian Specialist
Cert. Avian Specialist; Int. Assoc.Animal Behavior Consult; Pet Ind. Joint Advisory Council; author