Anatomy and physiology
The respiratory system of birds is significantly different, both physiologically and anatomically, from that of mammals. These
differences are a result of adaptations to the demands associated with flight resulting in an extremely efficient gas exchange
system allowing them to breathe at high altitudes (low O2 levels) and maintain their high metabolic rate. The nares are the beginning of the avian respiratory tract and may be located
anywhere along the beak from the tip (kiwis) to the base. Some species have an operculum to filter large particles from inspired
air. Birds have three conchae (rostral, middle, and caudal). The infraorbital sinus is the only paranasal sinus of birds and
has a rostral diverticulum within the beak, the preorbital diverticulum rostral to the eye, the infraorbital diverticulum
ventral to the eye, the postorbital diverticulum caudal to the eye and the mandibular diverticulum that extends into the caudal
mandible. The nasal passages and sinus communicate with the choana, a normal anatomic cleft in the roof of the mouth. The
choana has fine papillae along its margins. During respiration, the glottis abuts the choana allowing birds to breathe through
the nares. The glottis of birds is simple with no epiglottis making them more susceptible to foreign body aspiration. The
glottis is located at the base of the tongue and is usually accessible even in awake birds but is not involved in sound production.
The trachea is long compared with mammals with complete cartilagenous rings it bifurcates just cranial to the heart as the
syrinx. The syrinx produces sound and is composed of modified tracheal cartilages forming flexible membranes to which muscles
attach. The muscles are used to cause vibration producing sound. The trachea becomes narrower progressing caudally with the
syrinx being the narrowest portion of the trachea which predisposes foreign objects to lodge and granulomas to form there.
Each primary bronchus enters one lung and divides into secondary bronchi which further subdivide into parabronchi. Inspired
air moves from the parabronchi into atria which open along the walls of the parabronchi. Air then flows into air capillaries,
the avian analog of alveoli being the gas exchange site. However, unlike mammalian alveoli, air capillaries communicate with
each other and with other atria so air cannot become entrapped and emphysema does not occur in birds. The lungs of birds are
fixed adhered dorsally to the ribs and vertebrae. They do not expand and collapse during respiration. The lungs are directly
connected to the air sacs of which there are 4-5 sets (cervical, interclavicular, cranial thoracic, caudal thoracic, and abdominal).
The cervicocephalic air sac communicates with the infraorbital sinus and not directly with the lungs.
The respiratory cycle in birds is complicated but the majority of air requires two complete breathes to make it through. Most
of the inspired air bypasses the lungs and goes into the caudal thoracic and abdominal air sacs during the first inspiration.
During the first exhalation, air moves from these caudal air sacs into the lungs where gas exchange occurs. With the second
inspiration, air moves from the lungs into the cranial air sacs and during the second exhalation it leaves through the trachea.
This creates a unidirectional flow of air through the lungs. Blood moves in a direction opposite to that of air flow creating
a counter-current exchange which is very efficient for gas exchange. Air is moved through the respiratory system by movement
of the sternum so if a bird cannot move its sternum it will not be able to move air.