Instead of heavy jaws with teeth, modern birds have toothless, lightweight jaws, called beaks or bills. The eyes of birds are protected by three eyelids: an upper lid resembling that of humans, a lower lid that closes when a bird sleeps, and a third lid, called a nictitating membrane, that sweeps across the eye sideways, starting from the side near the beak. This lid is a thin, translucent fold of skin that moistens and cleans the eye and protects it from wind and bright light.
- jaw [dʒɔ:] n. 颚
- beak [bi:k] n.喙
- bill [bil] n. 啄
- eyelid [ˈailid] n.眼睑
- nictitating membrane [ˈnɪktɪˌteɪtɪŋ - ˈmembrein] n.瞬膜：鸟类、爬行动物和一些哺乳动物的内生透明眼睑，闭上可保护和滋润眼睛
- translucent [trænzˈlu:snt] adj.半透明的
- moisten [ˈmɔisn] v.使潮湿