Scientific research has revealed that throughout the animal world, communication is just as important as it is to human beings. Countless animals lack the【C1】______for human speech, yet they employ entirely different methods in order to communicate【C2】______each other. Some of the most dramatic examples of this are provided by birds. There are roughly 10 000【C3】______of birds in the world, each of which has its own miraculous features.【C4】______you may live, you can see a great number of these feathered creatures and can【C5】______different and extraordinary properties. In addition to their flawless flight mechanisms, expertise【C6】______the routes and timing of migrations, and ability to build nests, their methods to communicate is【C7】______wonder. At critical times in birds' lives, their【C8】______of hearing becomes particularly important. Experiments have shown that in order for birds to learn the【C9】______song of their own species, they need an auditory feedback system.【C10】______this system, young birds learn to【C11】______the sounds they produce with the song they have memorized. If they were【C12】______, it wouldn't normally be【C13】______for them to recognize songs. Birds' extremely sensitive hearing functions perfectly. Clearly, if this sense failed to【C14】______properly, the bird would not be able to hear any sounds【C15】______Moreover, birds also produce【C16】______communications by their facial expressions, beak movements, feather ruffling, and【C17】______their wings. Although each species has its own body language, many different species【C18】______movements in the same way. Via facial expression, birds can【C19】______a variety of messages to those around them--negative feelings such as dislike and resentment, as well as【C20】______ones like pleasure, enthusiasm and curiosity. 【C1】