72 EVOLUTION OF BIRD- SONG 



insects are not sufficiently abundant to support any 

 but small creatures ; and most of those animals 

 which subsist upon this food have to work hard for 

 it, at least during the greater part of the year. Also, 

 small birds would have an advantage over larger 

 ones in the pursuit of insects among dense foliage. 

 Thus the trees can support a numerous population 

 of small birds, though offering but meagre fare to 

 larger kinds ; and the birds which dwell among the 

 trees have not only less need of caution in the 

 utterance of cries than have birds on open land, 

 which are visible from a distance, but their sense of 

 hearing is continually exercised, as we have seen. 



The ravens, crows, and magpies mentioned by 

 Darwin as being not given to song, are very observant 

 of sounds ; and many of them are garrulous, though, 

 on account of their timidity, their loquacity is not 

 readily observed by us when they are in a wild 

 state. If these larger birds, or their remoter 

 ancestors, had been compelled by their size to 

 abandon arboreal life for the open land, or had in- 

 creased in size in consequence of such abandonment, 

 they might have retained the apparatus (which they 

 actually possess) for singing, even when their more 

 exposed position rendered song an accomplishment 

 too dangerous to be continued. 



