82 BRAIN MECHANISMS AND LEARNING 



and learned components, the former constituting the basis for the latter. 



A good example of a normal song of Friin^illa coclchs qciiojcri is given in 

 Fig. I. The inborn component of the chaffinch song can be revealed by 

 hand-rearing the young birds from early nestling life either in acoustic 

 isolation or at least out of contact with all chaffinch song. Six birds thus 

 individually isolated produced songs of an extremely simple type (Figs. 

 2-4) consisting of a song-burst of approximately the correct length (2-2.5 

 seconds) made up of about the right number of notes. The pitch, or funda- 

 mental frequency, of these notes was somewhat lower than normal and 

 the songs produced by these isolates lacked the division into the three 

 phrases so characteristic of the normal chaffinch song. Moreover, the final 

 flourish and all the other tme details by which the chaffinch song is 

 normally recognized as such were also absent. Of all the hundreds of wild 

 and aviary-kept birds which have passed through our hands during the 

 course of these experiments, none has produced songs of such extreme 

 simplicity as these six birds, although such undeveloped songs are known 

 to occur in the wild at times. 



In contrast to the simple, restricted song produced by the isolated birds, 

 we find that if, after babyhood, two or more such birds are put together 

 in a room but still without the opportunity of hearing experienced 

 chafhnches, they will develop more complex songs. It seems that the 

 attempt to sing in company produces mutual stimulation which en- 

 courages the development of complexity. The members of each group of 

 hand-reared birds thus kept together will, by mutual stimulation, build 

 up a distinctive community pattern. The birds conform so closely to this 

 pattern that it is sometimes barely possible to distinguish the songs one 

 from another even by electronic analysis. The song of such birds may be 

 quite as complex as that of a normal wild chaffinch, but its complexity 

 tends to be of a different kind and a song thus produced may bear little 

 resemblance to the characteristic utterances of the species. From further 

 experiment it is clear that, in the wild, young chaffinches learn some 

 features of the song from their male parents or from other adults during 

 the first few weeks of life. But most of the finer details of the song are 

 learned by the young bird when, in its first breeding season, it first comes 

 to sing in competition with neighbouring territory-holders. There is little 

 doubt that this is the way in which local song dialects are built up and 

 perpetuated. In addition to this true song, the chaffinch, like many other 

 species, has what has been called a sub-song, which is a much quiet and 

 less aggressive affair than is the full song. It is, however, not merely a song 

 of low intensity: it is of an entirely different pattern from the true song and 



