鸟类具有许多适应飞翔生活的身体构造特征：全身披羽毛，保持体温，前肢变成翅，胸肌发达，绝大多数善飞。躯体呈流线型，能减少飞行阻力。骨骼坚硬而轻，空腔内贮有空气，可增加浮力，减轻体重。肺较发达，与许多气囊相通，气囊遍布全身内脏、肌肉之间和骨髓腔里，能贮藏空气，辅助呼吸并增加浮力。心脏有两个心房和两个心室，动脉血和静脉血被分隔开，为飞翔提供更充足的氧气和能量。肠道短小，不积存粪便，可以减轻体重。鸟类的种类很多，现存种数约为8600种，中国所产鸟类约有1100种。鸟类与人类生活密切相关，它们中有的是著名的观赏鸟，如孔雀，有的是除害虫的益鸟，如啄木鸟，还有大量的家禽可供人类食用。
Pretend you are on a TV quiz show.The question is，“What is a bird？”Think carefully before you answer.Is a bird an animal that can fly？Bees and butterflies are animals that fly，but they’re not birds.Penguins，emus，and ostriches are birds，but they can’t fly.You answer，“A bird is an animal with feathers and wings.”“Correct！”says the quiz show host.
Baby birds，known as chicks，hatch from eggs.To escape from inside an egg，chicks use their beaks to break the eggshell
Bald eagles are majestic birds that can soar high in the air.The bald eagle is the national bird of the United States.Encarta EncyclopediaAnimals Animals/Ken Cole/Library of Natural Sounds，Cornell Laboratory of Ornithology，Clayton Reeves.All rights reserved.
Before mating，western grebes perform unique courtship dances.A pair will dance and run across the water，as shown here.After mating，the pair builds a floating nest out of plant material.
Hummingbirds are tiny birds with slender beaks.They are attracted to bright red or pink flowers.They use their long beaks to feed on the sweet nectar found deep inside these flowers.
Feathers，Wings，and Beaks