You might not know what causes your bird to lose weight, but it can be a sign of disease, stress, or malnutrition. Weight loss in birds can be caused by almost any illness. Even a small amount of weight loss is important to small birds because they have little to lose. Plus, they have a very high metabolism and their bodies don't store nutrients well.
Weight loss of just 5 - 10% can be a sign that your bird is in real trouble. Keep track of your bird's weight weekly. A keel test can also be used to determine if your bird is underweight. If your bird is underweight, take it to the vet to diagnose the problem.
When your bird is showing some of these signs of being sick, they should be seen by a veterinarian as soon as possible, as their condition might be far more serious than it appears.
For the time being, your bird should be kept in a warm, draught-free environment with good ventilation until you can visit your veterinarian.
Keep the bird in a quiet, stress-free environment with plenty of water and good quality feed. To reduce the stress on your sick bird and to protect your healthy birds from infectious disease, you might want to isolate your sick bird from other birds.