THE SONG OF BIRDS 155 



intensifying the male's excitement, especially in cases where 

 what looks like coquetry is exhibited. Thus, as Groos 

 says, " the female cuckoo answers the call of her mate 

 with an alluring laugh that excites him to the utmost, 

 but it is long before she gives herself up to him. A mad 

 chase through tree-tops ensues, during which she con- 

 stantly incites him with that mocking call, till the poor 

 fellow is fairly driven crazy. The female kingfisher often 

 torments her devoted lover for half a day, coming and calling 

 him, and then taking to flight. But she never lets him out 

 of her sight the while, looking back as she flies and measuring 

 her speed, and wheeling back when he suddenly gives up 

 the pursuit." 



The general conclusions of Professor Hacker's admirable 

 study of the song of birds may be briefly stated. 



(1) Love-calls and song probably had their roots in 

 the simple recognition-call or characteristic signal of the 

 species. Gulls and guillemots, hawks and crows do not get 

 beyond this grade. 



(2) The calls of the two sexes diverged and a certain 

 amount of specialisation appeared. Thus there came to 

 be special pairing-calls, serving not only for recognition, 

 but also for expressing and suggesting amatory excitement. 

 Cuckoos and woodpeckers illustrate this grade. 



(3) As sex dimorphism became more accentuated the 

 females becoming more passive and coy, the males more 

 active and insistent the song acquired more and more 

 of its secondary significance as a suggestive excitant, 

 and (unconscious) Sexual Selection came to the aid of 

 Natural Selection. Thrush and nightingale are familiar 

 instances. 



(4) Sometimes, however, the singing activity outlasts 

 the breeding or nuptial period. When it overflows into 

 other seasons it partakes of the nature of play, it acquires 



