° '1919" J Correspondence. 149 



Evolution of Bird Song. 



Editor of 'The Auk': 



I was much interested in the article in the October number of 'The Auk, ' 

 on " Sexual Selection and Bird Song," by Chauncey J. Hawkins. Mr. Haw- 

 kins' conclusions are interesting, and perhaps entirely correct, but there are 

 one or two weak points in his arguments, that I should like to point out. 

 I have also some observations of my own, which it seems to me, point to 

 the opposite conclusion, that sexual selection is the primary cause of the 

 evolution of bird song. 



Bird voice and bird song are two different things, the evolutions of which 

 have not necessarily been brought about in the same manner. Yet Mr. 

 Hawkins, in portions of his paper relating to his own observations, does 

 not carefully distinguish between voice and song. His remarks concerning 

 the calls of Crows and Jays will not apply to a discussion of song. His 

 observations of Robins and Goldfinches in winter are not so stated as to 

 make it clear whether the birds were really singing or merely indulging in 

 rather musical call-notes. No one supposes that bird-voice, call notes, 

 alarm notes or notes of female to young have been evolved by sexual 

 selection. But when it comes to the true song of the male bird, there seems 

 to be, in my opinion, good reason to suppose that sexual selection had 

 at least some part in its evolution. 



In order to avoid mistaken ideas it would be well to have a definition of 

 bird song. I am not sufficiently well acquainted with the literature of 

 this subject to know whether anyone has attempted such a definition, so 

 I will give what I should consider a proper definition in my own words. 

 Bird song is a vocal performance produced by the male bird during a definite 

 season of the year, that season including the period of courtship, mating and 

 nesting. Such a definition would imply that a vocal performance in which 

 the female indulges regularly is not a song. Similarly a vocal performance 

 not confined to a definite season of the year is not a song. I am aware that 

 there are cases where an individual female has sung the song of the male. 

 I have met with a single instance of this sort in my own experience in the 

 case of the Slate-colored Fox Sparrow (Passerella iliaca schistacea) (Condor 

 XII, 80). I believe that such instances are merely those of individuals 

 showing a tendency toward masculine traits, and that such things may 

 occur in any singing species. Such isolated facts do not hurt the definition 

 or make it less plain. In the same manner a single individual might sing 

 outside the regular song period of its species, or might prolong its nesting 

 a little beyond the limits of its period of song. So long as such occurrences 

 are not general the definition of song remains clear. I doubt if anyone 

 could find an authentic instance however of two individuals of a singing 

 species which mated and began nesting when the male was not in full song. 



It is to be noted that musical quality is not part of the definition of song. 

 Many bird calls are exceedingly musical without being songs. Such is the 



