Birds system. Birds need a tremendous amount of oxygen Birds have unique respiratory respiratory system to gain a have developed a different in and energy for flight, so they or deflate as they breathe Unlike mammals, the lungs of birds do not inflate, 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