In mammals, breathing is accomplished by excursions of the diaphragm, which increases the volume of the chest, thereby drawing air into the lungs. Birds do not have a diaphragm, and the lungs do not expand and contract with respiration. Instead, large air sacs fill the thoracic (chest) and abdominal cavity. Birds use their breast and abdominal muscles in order to breathe. Contraction of these muscles expand the volume of the thoracoabdominal cavity, thereby inflating the air sacs, and drawing air past the lungs for oxygen exchange. If the abdominal cavity contains an enlarged organ, a tumor or excess fluid, there may not be sufficient room for expansion of the air sacs. If the air sacs cannot expand fully, air cannot be drawn past the lungs, and the oxygen exchange is insufficient. Birds with space-occupying lesions in the abdominal cavity will often become dyspneic or tachypneic. Occasionally, the space-occupying lesion will be contained within the air sacs in the form of exudate or fungus. This is most often seen with Aspergillosis, a fungal respiratory disease.
This concept should also be considered whenever a bird is restrained. If held too tightly around the chest, the bird cannot expand the thoracoabdominal cavity. It may become very difficult for a bird to breathe when improperly restrained. Very small birds, such as canaries and parakeets, could potentially suffocate.
Birds have an extremely efficient respiratory system, which is an adaptation for prolonged, rigorous exercise such as long flights. However, most pet birds do not exercise very much, spending most of their time sitting in cages or perches (activities requiring very little oxygen expenditure). Because of this, dyspnea and tachypnea are usually not seen until disease processes are quite advanced. Therefore, if you notice labored respiration in your bird, veterinary attention is immediately warranted.
The advanced efficiency of the avian respiratory tract can sometimes have deadly consequences. Exposure to gases and aerosolized fumes that are often relatively harmless to other species can be acutely fatal to birds. For example, inhalation of fumes from over heated Teflon and other non-stick pans can cause a fatal acute pneumonitis. Birds will die quickly following exposure to carbon monoxide. (You may be familiar with the practice of coal miners bringing canaries into the mines. The sudden death of these birds served as a warning of toxic gas concentrations). In general, all aerosolized sprays should be used with caution, or avoided entirely, around pet birds.