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  3. Bird Song P1: Bird song has never lacked admirers drawn by its aesthetic qualities. But to scientists, bird song is also of inte

Bird Song P1: Bird song has never lacked admirers drawn by its aesthetic qualities. But to scientists, bird song is also of inte

admin2018-10-18  29
问题 Bird Song
P1: Bird song has never lacked admirers drawn by its aesthetic qualities. But to scientists, bird song is also of interest because it represents an evolutionary flowering of vocal learning. Hereditary information and environment have a crucial role to play in the behavioral patterns of voca learning. Since the pioneering work of W. H. Thorpe on chaffinches (a common European bird), many species of bird have been studied yielding a bountiful harvest of insights into both the learning process and the constraints on what they are able to learn.
P2: To determine how much learning was affected by various factors, Thorpe devised a series of experiments. He hand-reared a group of young chaffinches together in the same cage, keeping them isolated from their parents and any other audio role models. Thorpe found that the song they produced was about the right length and in the correct frequency range, and even structured similarly to those raised in the wild. And yet, the quality of the songs they produced was very poor. They were crude versions of the wild chaffinch’s song, lacking the refinement and detail characteristic of the typical wild adult song, and the song was not split up into distinct phrases as it usually is. In later experiments, researchers played recordings of songs to the chicks and discovered that many of them would learn the exact pattern of the acoustic source. This was particularly remarkable, as juveniles were able to copy the songs that they were only exposed to over the first few weeks of life with extreme precision, though they would not sing themselves until about eight months old. After that brief period, however, the windows of opportunity for the song-learning process apparently shut down for the rest of the birds’ lives.
P3: The songs of different species of birds vary and are generally typical of the species.
Species vary greatly in the complexity of their songs and in the number of distinct kinds of song they sing. In general, however, the constraints on learning which birds have ensure that they only learn songs appropriate to the species to which they themselves belong. The constraints may be innate in their brain’s circuitry. The chick hatches with a rough idea of the sounds that it should copy, and the singing of the parents and neighbors activates parts of the song template— syllables or song types used in composing his song. The crude song of a bird reared in isolation gives some clues as to what this rough idea may be: the length, the frequency range and the breaking up into notes are all aspects of chaffinch song shared between normal birds and those reared in isolation. In other cases the constraints are more social. During development, young birds may also rearrange the song notes they learn to generate into a new order, which includes many notes that are near or exact matches of material copied from one or more adult models. Thus, young birds are only able to learn from individuals with whom they have social interactions. Whatever the nature of the rules that direct song-learning in a particular species, there is no doubt that they are effective; it is very unusual to hear a wild bird singing a song which is not typical of its own species despite the many different songs which often occur in its social colonial environment.
P4: However, not all birds show the same learning pattern as do chaffinches. There are some species which produce normal sounds even if deaf, so that they are restricted from the auditory feedback of their own efforts, much less copy those of others. In other cases, such as parrots and hill mynahs, birds can be trained to copy a huge variety of sounds. The amazing capability of mynahs has apparently arisen simply because birds in an area learn more readily from live tutors, though those calls are highly varied in structure. The ability to master them has led the birds, incidentally, to be capable of mimicking a wide variety of other sounds.
P3: The songs of different species of birds vary and are generally typical of the species. ■ Species vary greatly in the complexity of their songs and in the number of distinct kinds of song they sing. In general, however, the constraints on learning which birds have ensure that they only learn songs appropriate to the species to which they themselves belong. ■ The constraints may be innate in their brain’s circuitry. The chick hatches with a rough idea of the sounds that it should copy, and the singing of the parents and neighbors activates parts of the song template—syllables or song types used in composing his song. ■ The crude song of a bird reared in isolation gives some clues as to what this rough idea may be: the length, the frequency range and the breaking up into notes are all aspects of chaffinch song shared between normal birds and those reared in isolation. ■ In other cases the constraints are more social. During development, young birds may also rearrange the song notes they learn to generate into a new order, which includes many notes that are near or exact matches of material copied from one or more adult models. Thus, young birds are only able to learn from individuals with whom they have social interactions. Whatever the nature of the rules that direct song-learning in a particular species, there is no doubt that they are effective; it is very unusual to hear a wild bird singing a song which is not typical of its own species despite the many different songs which often occur in its social colonial environment.
Why does the author mention that it is very unusual to hear a wild bird singing a song which is not typical of its own species?
选项 A、To explain why a variety of different bird songs are often heard in a relatively small area
B、To argue that social constraints have a greater impact upon learning than do genetic constraints
C、To provide an example of how the process of learning rules varies from one species to another
D、To illustrate how effective the different constraints upon learning are in young birds
答案D
解析 【修辞目的题】题目问这句话的作用，因为此句在文中的分号后，用来进一步说明前半句“there is no doubt that they are effective”，因此答案是D。
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