VARIA TION IN BIRD- VOICES 1 43 



like circumstances, displayed by the male greenfinch. 

 In all these instances no hawk appeared, nor did the 

 other birds near betray any sign of fear. Pigeons 

 often swoop at each other in flight, as though in 

 play : I am not sure that in these birds the action 

 is induced by love ; but the same habit in the 

 kestrel appears to be due to this emotion, when 

 sometimes in early spring the birds call to each 

 other, and swoop playfully at each other in the air. 

 The cushat and the cuckoo also rise and fall in 

 graceful swoops before others of the opposite sex, 

 and some birds indulge in a jerky flight while sing- 

 ing : these have been already mentioned. The 

 cock ruffles his quill feathers with his foot when 

 playing up to the hen ; some woodpeckers rattle 

 their bills in cracks in the bark of trees as a means 

 of calling their mates. In these modes of expression, 

 as in song, extravagance would attract the notice 

 of a bird for which they were adopted ; and the 

 fact of their wide-spread occurrence proves them to 

 be thus attractive. Darwin mentions as " instru- 

 mental music " the rattling of quills by birds-of- 

 paradise and peacocks, the scraping of wings against 

 the ground by turkey cocks, the buzzing produced 

 in a similar way by some kinds of grouse, and 

 drumming by the North American grouse {Tetrao 



