Question

why the flying birds are known to have double respiration

Answer 1

To match up to the high metabolic rate, and consequently high oxygen demand, of flying birds, they are known to have a process of double respiration. Although birds have lungs, they are very dependant on their air sacs for ventilation. Compared to mammals, bird lungs are smaller. The air sacs make up 15% of the body volume, compared to 7% lung volume in mammals. But, the air sacs do not have good blood supply, therefore they act as bellows and the gas exchange actually takes place through the respiratory system. This is essentially what is referred to as double respiration.

Answer 2

Double respiration means inhaled oxygen is used twicely for respiration. In birds oxygenation of blood takes place twicely during inspiration and expiration and this double respiration in birds is taken by intercoastal muscles, abdominal muscles and sternum. In birds air sacks are unidirectional and allow flow of oxygen only during inspiration or expiration while in other mammals both take place at one time.