Birds are frequently referred to as “flying dinosaurs”. Flight enabled birds to overcome geographical obstacles and radiate widely into various ecological niches. Among the air-breathing vertebrates, the lung-air sac system of birds is structurally the most complex and functionally the most efficient respiratory organ. The avian lungair sac system is considered to have developed from the complex multicameral reptilian lungs like those of the monitor lizard, turtles, and snakes. The neopulmonic set is located ventrocaudally and comprises about onethird of the lung volume. In the avian lung, the geometric arrangement between the flow of air in the parabronchial lumen and the centripetal flow of venous blood from the periphery of a parabronchus is described by a crosscurrent model. Lungs of the small energetic species of birds show distinct pulmonary morphometric specializations. The small passerine species that have a high metabolic rate and operate at a higher body temperature have superior lungs.