birds have a unique respiratory system. Birds need a tremendous amount of oxygen to gain energy for flight, so they have developed a different respiratory system. Unlike mammals, the lungs of birds do not inflate as they breathe in and out. Instead, they have air sacs which are like tiny air bags in cars. These air sacs ventilate air, leaving their lungs full of air at all times. The process of oxygen circulating through the boy in birds is done through small blood vessels.