How to Deal with an Underweight Bird
If your pet bird is underweight there could be various reasons behind this problem. The most likely is that your bird is not ingesting enough calories to keep it at a healthy and adequate weight. In many species, if you can feel the breast bone protruding, this indicates that the bird is underweight and may be ill.
Check the label on your bird food to make sure you are feeding your bird a sufficient amount. If the bird shows no interest in this brand of bird food for sale, try buying a new type of bird feed.
If your bird’s environment has been changed recently – a new smaller cage, removal of natural light, a noisier room – then this could be causing problems. It can make your bird depressed which in turn stops your bird from eating. If you recently moved the cage, try moving it back to where it was before. This may stimulate the bird's appetite.
Check for signs of illness – mites, lesions, loss of feathers, a lack of focus and alertness, wet nose or eyes and strange breathing patterns. Any of these could be a symptom of something graver that needs to be diagnosed by a veterinarian as soon as possible.
Inspect the bird's droppings for changes in frequency or appearance. Runny feces or a decrease in the amount of droppings may indicate an illness that needs a vet's attention.