Old World Aviaries
Reprinted from the Alaska Bird News, March 1995.
The avian respiratory system is unique. It differs from mammals in that birds have no diaphragm, have a syrinx at the end of the trachea instead of vocal cords in the larynx, and have no epiglottis. Birds also have air sacs, limited lung expansion, and air capillaries instead of alveoli. It is important to understand how this system works in order to prevent, diagnose and treat disease.
Air enters the nares and is moistened and warmed as it travels through the nasal cavity (which connects with the infraorbital sinus and the cervicocephalic air sacs) and exits the nasal cavity through the choanal slit. When a bird breathes, the mouth is closed and the glottis creates a seal with the choanal slit, allowing air to travel from the nasal cavity to the trachea. From the trachea the air travels through the syrinx into the bronchi, which connect the trachea to the lungs. The lungs, which are attached to the backbone, have air capillaries that are interwoven with blood capillaries for gas exchange.
Most birds have four paired and one unpaired pulmonary air sacs that connect to the lungs to create a large respiratory capacity. The cranial air sacs are the two cervical, single clavicular and two cranial thoracic air sacs. The caudal air sacs are the two caudal thoracic, and two abdominal air sacs. Depending on the species of bird, the respiratory tract communicates with the humerus, clavicles, coracoids, vertebrae, ribs, sternum and femurs.
Six inspiratory muscles move the ribs outward expanding the chest, increasing the volume of the thoracoabdominal cavity which creates negative pressure inside, compared to outside, the bird, and causes air to flow into the respiratory tract. There are nine expiratory muscles which cause the ribs and sternum to move inward causing expiration by creating an increase in internal pressure within the air sacs. If a bird is unable to move its ribs, it will suffocate.
When a bird breathes in fresh air, half of the fresh air goes to the lungs and half goes to the caudal air sacs. The air already in the lungs moves to the cranial air sacs. On expiration a bird moves air out of the air sacs. Air from the caudal air sacs moves to the lungs. The air that was in the lungs is moved out of the body via the trachea, along with the air from the cranial air sacs. Therefore, with both inspiration and expiration, the lungs are filled with air, but the air sacs fill and empty with inspiration and expiration respectively. This system is much more efficient than the mammalian system. Birds have fresh air (high in oxygen) in the lungs on both inspiration (from the trachea) and on expiration (from the caudal air sacs). Also the air capillaries are thinner and smaller than alveoli so the blood gas barrier is more efficient.
With disease in any part of this system, problems develop. Careful observation of the nares, choanal slit and trachea, as well as posture (i.e., tail bobbing) and body swellings can tell you a lot about respiratory health. Further information can be gathered by ascultation, radiography and collecting samples for cytology and microbiology.
Diseases of the respiratory tract can be nutritional, infectious, toxic, neoplastic or even blockage by foreign bodies. By understanding how a bird breathes we can better resolve respiratory problems.