Flying requires a great expenditure of energy from an active metabolism. Bird are endothermic. they use their own metabolic heat to maintain a warm constant body temperature. feather and in some species layers of fat provide insulation that enables bird to retain their metabolically generate heat an efficient respiratory system and a circulatory system with a four chamber heart keep tissue well supplied with oxygen and nutrient supporting a high rate of metabolism the lung have tiny tube leading to and from elastic air sac that help dissipate heat and reduce the density of the body.