鸟类身体的每一个部分几乎都在进化中形成了适宜飞行的身体结构。翅膀和羽毛是最明显的特征——它们提供了可以克服地心引力的上升力。绝大部分鸟类有着流线的外形,重量集中在身体中间,这样能够很好地保持平衡。鸟类的骨骼上有许多凹下的小洞,用来减轻体重,而且许多块骨头坚实地契合在一起,这样就减少了沉重的关节组织和不必要的肌肉组织。鸟类的飞行肌巨大而有力,但它们运动时需要充足的氧气供应,因此鸟类长有特殊的肺囊,以便在空气中尽可能多地吸取氧气。 Almost every part of the bird's body evolves to form a flying body structure. Wings and feathers are the most obvious traits - they provide an ascending force that overcomes the force of gravity. The vast majority of birds have streamlined appearance, weight concentrated in the middle of the body, so that it can maintain a good balance. Bird bones have many concave holes to reduce weight, and many bones firmly together, thus reducing the heavy joint tissue and unnecessary muscle tissue. Birds have huge and powerful flying muscles, but they require adequate supply of oxygen when exercising, so birds have special lung pockets to absorb as much oxygen as possible in the air.