<\/a>This diagram shows the avian airways. (Courtesy Image \/ KM Hocker)<\/p><\/div>\n\t\t\t\t
Birds have a very different breathing arrangement from that of mammals. Movement of air is powered by expanding and contracting the ribcage (birds do not have diaphragms). The lungs aren’t suspended in the thorax; instead, they are anchored to the dorsal body wall of the thorax. They are relatively small and dense: Instead of having thin-walled spaces that inflate and deflate with each breath, they consist of many narrow air channels intertwined with blood capillaries. The body contains air sacs (which have several functions, and the number of them differs among species) that allow a continuous flow of air through the lungs. Inhaled air comes in through mouth or nostrils to the trachea; it bypasses the gas-exchange part of the lung and goes to a posterior pair of air sacs; exhalation then moves the air into the lungs for gas exchange. A second inhalation moves the ‘used’ air into anterior air sacs and a second exhalation moves it out through the trachea to the outside. In other words, it takes two breaths to move air through the full cycle. Thus, instead of the in\/out cycle of mammals, the very efficient avian arrangement allows one-way flow through the gas-exchange part of the lung, and no residual air is left behind.<\/p>\n\t\t\t\t
Breath can also be used to make sounds. In mammals, vocalization typically occurs with exhaling, as the passage of air vibrates membranes in the larynx, which is located at the back of the throat where the trachea and esophagus separate. Birds use a different system: avian vocalizations are typically produced by the syrinx, which is located where the trachea bifurcates, sending branches to right and left lungs. Sound production is controlled by syringeal muscles, pressure from an adjacent air sac, and the diameter of the air passageway. Because the syrinx has two separately-controlled sides, one on each tracheal branch, birds can produce two notes at once.<\/p>\n\t\t\t\t
Breath is also important to the sense of smell. Most non-human mammals have a much better sense of smell than we do, thanks largely to a larger number of olfactory receptors and more of the brain devoted to this function. In addition, although most inhaled air goes directly to the lungs, some is shunted to special areas in the nose that have concentrations of receptors (unlike in humans, in which incoming air passes over receptors on its way to the lungs). Some mammals also have modified nostrils that further increase the capacity to sense smells. Look at the nostrils of a dog, for example.<\/p>\n\t\t\t\t