Birds
Birds have a unique respiratory system. ฺBirds need a tremendous amount of oxygen to again energy for flight, so they have developed a different respiratory system. Unlike mammals, the lungs of birds do not inflate or deflate 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 body in birds is done through small blood vessels.