COURTSHIP IN BIRDS 271 



audibly vibrating wings, while the ruffed grouse 

 stands on a log and, by the rapid whirring of his 

 wings, emits his characteristic "drumming." 

 That this drumming is evolved from a flight 

 song and that there was once a vocal part of the 

 performance, I have little doubt. These ex- 

 amples show the stages in the evolution. 



The loud clapping together of the wings be- 

 hind the back in domestic pigeons during flight 

 and their habit of soaring with wings obliquely 

 upwards, although common at all times, are most 

 marked in the courtship season and are probably 

 of courtship origin. The V-shaped pose of the 

 tail-feathers of the bronzed grackle is probably 

 of the same nature, for it is discarded in mid- 

 summer. 



Both the Savannah and vesper sparrow stand 

 or walk on the ground and elevate and sometimes 

 vibrate their wings rapidly above their backs. 

 They also fly slowly a short distance above the 

 ground with head and tail up and wings rapidly 

 fluttering and deliver their song. 



The rapid headlong plunges of the nighthawk 

 may be classed as a display of motion, a form of 



