78 EVOLUTION OF BIRD- SONG 



pnrases in the birds of the tropical regions, in which 

 locaHties a redundant population of birds is kept in 

 check by the attacks of enemies, or by mutual 

 contests, rather than by changes of climate, by which 

 in temperate lands it is periodically reduced. We 

 must remember that in the latter places the arrival 

 of warm weather calls forth sustenance not only 

 sufficient for the birds which have wintered there, 

 but also enough for swarms of immigrants who 

 crowd to the feast, and even rear their young with 

 this superabundant food. When the winter in a 

 temperate land is severe, so greatly are the food 

 supplies lessened that vast numbers of hardy birds 

 perish by starvation, though they seek never so 

 persistently for food ; but directly the weather 

 becomes genial, the creatures upon which they 

 subsist spring into life on every side, and then some 

 of the birds can spend long hours in doing nothing, 

 or in singing. This increased leisure is a first 

 necessity to the singing of our native species ; and I 

 submit that the want of it may fairly be suggested 

 as a cause of the lack of varied song not only 

 amongst tropical birds, but also in those which in 

 any part of the earth obtain their food in a par- 

 ticularly laborious manner — such as the woodpeckers 

 and creepers, which are practically always at work. 



