240 THE ZOOLOGIST. 



birds reproduce less persistent sounds, which will be subsequently- 

 mentioned.* 



It seems that certain species never advanced beyond the cry 

 of distress; while others have acquired one or two unvarying, 

 or as we call them, " arbitrary phrases," which are respectively 

 employed under definite circumstances, such as the presence of 

 an enemy or of a bird of the opposite sex. Others, again, may 

 display a voice of great range in pitch, and variety of tone. 



The songs of birds are not immediately acquired, but are 

 developed in each individual by practice and cultivation ; and the 

 more frequent the practice the more elaborate the song. In 

 autumn the young male Sky Larks, Thrushes, and Blackbirds 

 begin to sing; and their first efforts result in objectless variations 

 of tone, which, however, soon betoken traces of imitation. Their 

 songs are generally lost during winter, but are regained, and 

 with increased power, at the approach of spring. 



Among birds, as among men, leisure is necessary to the 



development of song, and may to some extent induce it. The 



cause of the frequent imitativeness of captive birds f may perhaps 



be found in their security and idleness. It is not contended that 



leisure would make a bird of harsh voice musical ; but none will 



dispute that if a song-bird be constantly harassed it will not sing. 



The constant employment of the brain in detecting and avoiding 



danger, or in the occupation of getting food, hinders any tendency 



to develop song. It is possible that want of leisure, and a feeling 



of insecurity, may have prevented the elaboration of song amongst 



birds of torrid regions, where they are continually pursued by 



enemies, engaged in battle among themselves, or are labouring 



to obtain food, and where their increase is checked by violence 



rather than by the climatic influences that in temperate zones 



periodically lessen their numbers. If the suggestion that leisure 



is necessary to song be correct, we should find limited voices in 



birds that are much occupied either in obtaining food or avoiding 



enemies. Such is the fact. Of the former class are the Raptores 



and Picidce, of the latter the Rasores. The Anatidce may belong 



to both. 



* Tho Squirrel and the Snake reproduce in their respective alarm-cries 

 the sounds made by these animals during rapid retreat ; the former making 

 a snapping and swishing noise, and the latter a hissing, rustling sound. 



•| This paper docs not otherwise refer to the notes of biids in captivity. 



