Hi XXXXXXXX,
Your little cockatiel friend sounds very ill. Birds try very hard to hide their symptoms from us, but when they become less active and fluffed up, it means that they have become very sick. I recommenmd that you take him in to your regular vet first thing tomorrow. Tonight, I recommend keeping him warm and encourage him to keep eating and drinking. You can give him supplemental heat by placing a heating pad under the cage on the side that he is staying in and wrap a towel around that side of the cage also to help hold the heat in. I would also move his food and water dish down to the bottom of the cage near the corner where he wants to stay. This will make it easier for him to access his food and water if he wants it. I would also offer any of his favorite foods right now to tempt him to eat (seeds, millet, etc). I will keep my fingers crossed that he will be ok, but you definitely need to take him in to the vet tomorrow morning.
Dr. Kris
Veterinarian
Veterinarian for 16 years