The development of song in most species follows three distinct phases. In the first, the sensory phase, the bird hears songs produced by tutor birds around him. In temperate species, this usually occurs soon after hatching in the spring or summer, almost a year before the bird begins to sing himself. The requirement for tutor songs was first shown experimentally by Thorpe (1958), who found that chaffinches (Fringilla coelebs) that had been reared in the laboratory without hearing the songs of adults produced very abnormal songs when they matured. The syllables of their songs were simple and disorganised. However, chaffinches that had heard tape recordings of adult chaffinch song as fledglings produced normal songs when they matured. Young chaffinches did not learn from other sounds, including the songs of most other bird species, so a mechanism for recognising the song of members of its own species must be built into the brain of the young bird. The timing of exposure to tutor song is critical. A young chaffinch must hear adult songs before the middle of its first summer if it is to sing normally the next year.
The second period of song development, the sensorimotor phase, begins early in the spring of a chaffinch's second year. The bird starts to sing spontaneously, at first producing quiet and variable songs. Gradually, song becomes louder and, because it is variable, it is called plastic. Plastic song is often a mixture of two different types of song, and includes repeated syllables. A bird must be able to hear its own plastic song for normal development (Konishi, 1965a, 1965b). In the final period, in late spring, song crystallises to its mature form, probably triggered by a rise in the level of the steroid hormone testosterone in the blood. In most species, deafening at this stage does not cause abnormalities in song.
This sequence of three phases is found in most species of song bird, but there are many variations in detail. Some species, including starlings and canaries, are called open-ended learners because they go through the processes of song practising and crystallisation every season. Zebra finches follow the general pattern, except that young males can still hear the songs of tutors while they start to produce plastic song, which is associated with their rapid development and social lifestyle. Their sensitive period for song learning starts at 20 days old, when they fledge, and lasts until they are 40 days old. Young zebra finches begin to sing 25 days after hatching and song crystallises 90-110 days after hatching, at sexual maturity.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.