On the one hand, a cockatiel may suddenly die with no symptoms at all. As alarming as that it is, it can and does happen. Anytime a bird is showing signs of illness, however, that should be taken seriously as it is possible the bird could end up critically ill and dying.he signs of severe illness in a bird are weight loss (you'll feel the breastbone sticking out like a knife blade), sitting fluffed on the bottom of the cage, a decrease in activity and vocalizations, refusing to eat, and labored breathing (often shown by tail-bobbing).