Explanation:
Birds are busy animals. Consider insect-hawking flycatchers, far-flying shorebirds, and hovering hummingbirds, and you get a picture of how active birds can be. That’s why they favor high-energy foods and have high metabolic rates that demand huge oxygen levels.
Mammals’ respiratory systems are woefully inadequate to provide the amounts of oxygen that birds require.
In humans, for example, a tube called the trachea connects our nose and back of the mouth to our lungs. The trachea splits into two primary bronchi that enter the lungs, where they divide into secondary bronchi, which in turn branch into tertiary bronchi. These give rise to small tubes called bronchioles, which terminate in thin-walled, grape-like clusters called alveoli (shown at lower left in the diagram above). Surrounded by capillary networks, alveoli are the sites for gas exchange. Because of pressure differences, oxygen diffuses from the alveoli into blood capillaries, and carbon dioxide diffuses from blood capillaries into the alveoli.
The cul-de-sac nature of our lungs means that air normally flows in two directions — that is, from our nostrils through the trachea to the alveoli and back again through the same ducts. It follows that the first air breathed into the alveoli is left over and stale, having given up most of its oxygen before the last exhalation. Bidirectional airflow, therefore, is inefficient.
