216 THE STORY OF THE BIRDS 



the origin and purpose of avine song. A very 

 popular idea is that song, or the instinct of song, 

 is but the outward expression of exuberant in- 

 ternal joy. The evidence, however, appears to 

 be overwhelming that song originated in sexual 

 rivalry, and has been developed and perfected, 

 fostered and increased, by that stronger and more 

 assertive vitality which is, without exception, so 

 characteristic of birds with any pretensions to 

 musical skill. Singing birds appeal to the female 

 through their sweet and joyful cries ; they sing 

 their best and loudest, and warble one against 

 the other in the heat of their passion, striving 

 their hardest to outsing all rivals within hearing. 

 Song also seems to be the agent of joy expres- 

 sion, and may also be indulged in as a pastime 

 or diversion, many species singing more or less 

 constantly throughout the year, and even when 

 sexual rivalry is dormant. But it is a rule, with- 

 out any exception whatever, that all song-birds 

 sing their best when inspired by love, and that 

 in no case does this music show the faintest 

 decline in power at a season when pairing 

 instincts are predominant. Song and sexual 

 rivalry are resumed together each season, and 

 the music dies away with the decline of that 

 sexual stimulus. Song, we may remark, declines 



