Birds respiratory system are much different than that of other mammals. They have lungs and air sacs. Dependent on the species they may have none or seven air sacs including; two posterior thoracic, two abdominal, two anterior thoracic, two cervical and one interclavicular. Although these air spaces don't play an immediate role in gas exchange but are vital for respiration.
The role of the air sacs is to allow unidirectional flow through the lungs- this means that the air moving into the lungs is mostly fresh air and thus has a higher oxygen content, but in mammals the air flow is bidirectional where the air moves backwards and forwards into the lungs includes air that has been in the lungs for a long period of time and is mixed with old air and consequently contains less oxygen. It also means that birds can take in oxygen even when they are exhaling.
The good thing about the respiratory system of the bird is that gas exchange can take place throughout the body and the air sacs reach into the bones and throughout the abdominal cavity. But the lungs of the bird do not expand and contract like that of humans
In the respiratory cycle of birds it is split into two cycles of inspiration and expiration. In inspiration it moves through the nares and into the nasal cavity and then passes through the larynx and into the trachea. It then passes through the syrinx and then splits into two as it enters the posterior air sacs and other air sacs and finally the lungs.
The gases then exchange in the lung through diffusion of oxygen diffusing onto the blood of the bird and carbon dioxide from the blood into the lungs.
During expiration the air moves from the posterior air sacs through into the lungs and then gas exchange takes place. Then when inspiration occurs again the gas is moved into the cranial air sacs from the lungs. In the second cycle of expiration the air moves from the cranial air sacs through the trachea and out of the nostrils.
When the hummingbird moves the muscles cause pressure changes within the air sacs which means that more oxygen can enter the respiratory system when the humming bird is most active and in need of the most oxygen.
The Hummingbirds respiratory system is similar to that of other birds but hummingbirds have the highest oxygen requirements of all vertebrates. To transport this level of oxygen to their vital organs they have the highest density of red corpuscles. At rest it's breathing rate is about 300 breaths per minute but it may rise to around 500 breaths per minute during flight this is large compared to around 14-18 breaths per minute of humans and the volume of air inspired is much higher than that of animals of the same size.
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http://www.peteducation.com/article.cfm?c=15+1829&aid=2721