When birds are sick, they tend to be subtle about showing any evidence, but there are some symptoms to look for. A healthy bird should have nostrils that are clean and free of any discharge, and no deviation between the top and lower bill. The bird should be alert and responsive to human attention, with bright, discharge-free eyes, and smooth, glossy plumage without bald spots or patches of distorted growth.
If the bird is losing feathers and doesn't regrow them, it can indicate a virus or thyroid problem. When a bird plucks out its own feathers, it is usually caused by anxiety rather than viruses, and anxiety often results from lack of attention or a change in environment. Self-plucking can also point to a hormonal imbalance. If you notice this behavior, take the bird to an avian veterinarian who will give your pet a thorough examination. If there are no physiological causes, the alternative is to manage the plucking, usually by using a collar which resembles an upside-down lampshade and prevents the bird from being able to reach his or her feathers with the beak.
You can aid your bird by misting its feathers each day and covering the cage each night so he or she gets 10 hours of rest. A bird also needs mental stimulation, which you can provide by rotating its toys every day. Be sure to provide a balanced, nutritious diet and get regular check-ups at the vet.