84 BRAIN MECHANISMS AND LEARNING 



may be described cis a long succession of chirps and rattles (Thorpe, 1955, 

 1958; Thorpe and Pilcher, 1958). It seems to have no communicatory 

 function and is most frequently heard in the early spring, when it is 

 produced, so far as we know, much more by tirst-year birds than by older 

 ones — the latter seeming to come into full song with much less of this 

 preliminary sub-song. There seems little doubt, however, that the sub- 

 song provides in some degree the raw material out of which, by practice 

 and by the elimination of unwanted extremes of frequency, the full song 

 is 'crystallized'. The chaffinch has, of course, like other species, a number 

 of call notes which arc in the main signals for co-ordinating the behaviour 

 of the flock, mate and family; some of these may be used as components 

 of both sub-song and full song. It is interesting that call notes are much 

 more in evidence as components of the songs of isolated birds than they 

 are in the songs of normal ones. This is presumably because the isolated 

 birds have had a greatly restricted auditory experience to draw upon as 

 compared with normal wild individuals. 



Chaffinches are not imitative birds, in that they do not normally copy 

 anything but sounds of chaffinch origin. Once a chaffinch has heard a 

 chaftinch song as a young bird in the wild, it appears to have learned 

 enough about it to refuse to copy any sound pattern which departs far 

 from the normal chaffinch song; that is, it will learn only the finer indivi- 

 dual variations of the song of other chaftmches. So, in the wild, chaffinches 

 practically never, in their full songs, imitate anything but other chaffinches. 

 Hand-reared isolated birds will learn songs of far greater abnormality, 

 however, provided always that the tonal quality is not too far different 

 from that of a chaffinch song. Voices as 'abnormal' as that of a domesti- 

 cated canary may be learned by hand-reared birds (Thorpe, 1955), and very 

 occasionally by wild birds; but when this happens the alien notes are kept 

 as components of the non-communicative sub-song only; the full song is 

 not contaminated with them. 



If one catches wild chaffinches in their first autumn and keeps them 

 until the following spring with other chaffinches, the song of which is 

 already fixed, one gets clear evidence that the young birds have modelled 

 their songs, in some respects at least, on those of their associates, which 

 have probably come into song first as the spring season comes on, and 

 which they thus hear before they themselves have got very far along the 

 path of song production. Similarly, if, instead of exposing such wild- 

 caught first-year birds to the songs of other chaffinches, one plays such 

 songs to them by means of a repeating tape machine which we may call a 

 'song tutor', a clear positive effect is manifest. An experiment carried out 



