° 1918 I Hawkins, Sexual Selection and Bird Song. 423 



and attributes the gay colors of the males to selection on the part 

 of the female. Wallace starts with the gorgeous colors and de- 

 clares that the gray colors of the females are due to natural selection. 

 Bright plumage would render the mother bird sitting on her nest 

 conspicuous and make her the easy prey to hawks and other 

 natural enemies. Hence all the highly colored females, through 

 generations have been destroyed, only the more sober colored birds 

 remaining. "The original brightness has been forfeited by the 

 sex as a ransom for life. Female birds in open nests are similarly 

 colored like their surroundings; while in those birds where the 

 nests are domed or covered, the plumage is gay in both sexes." 



The same principle of natural selection may be attributed to the 

 call of birds. "These are evidently a valuable addition to the 

 means of recognition of the two sexes, and are a further indication 

 that the pairing season has arrived ; and the production, intensifica- 

 tion, and differentiation of these sounds and odours are clearly 

 within the power of natural selection. The same remark will apply 

 to the peculiar calls of birds, and even to the singing of the males. 

 These may well have originated merely as a means of recognition 

 between the two sexes of a species and as an invitation from the 

 male to the female bird. When the individuals of a species are 

 widely scattered, such a call must be of great importance in enabling 

 pairing to take place as easily as possible and thus the clearness, 

 loudness, and individuality of the song becomes a useful character, 

 and therefore the subject of natural selection." 



The increase and development of beautiful plumage is caused by 

 the superabundant energy of the male bird. " During excitement 

 and when the organism develops superabundant energy, many 

 animals find it pleasurable to exercise their various muscles, often 

 in fantastic ways, as seen in the gambols of kittens, lambs, and 

 other young animals. But at the time of pairing male birds are 

 in a state of the most perfect development, and possess an enormous 

 store of vitality, and under the excitement of the sexual passion they 

 perform strange antics or rapid flights, as much probably from the 

 internal impulse to motion and exertion as with any desire to please 

 their mates." So, also, " the act of singing is evidently a pleasurable 

 one, and it probably serves as an outlet for superabundant nervous 

 energy and excitement, just as dancing, singing, and field sports 



