228 THE STORY OF THE BIRDS 



' peculiar sounds are produced by the aid of these 

 feathers, for the birds are otherwise songless, 

 the vocal muscles being absent in this group, t 

 One species makes a sharp sound like the crack 

 of a whip, followed by a rasping sound similar 

 to the call of the Corn-crake ; another succeeds 

 in making a sharp and whirring sound like that 

 made by a small wooden rattle, followed by 

 several sharp snaps. We ought also to mention 

 in connection with the present subject that the 

 males of some other birds are provided with 

 air sacs or pouches, which they inflate during 

 the period of courtship, just like bagpipes, and 

 by the aid of which various booming or drum- 

 ming sounds are produced. 



We have now to consider that portion of the 

 subject which deals with Display rather than 

 with vocal or other sound ; with those curious 

 flights, posturings, love antics, and dances which 

 may justly rank as some of the most remark- 

 able phenomena in nature. This special kind 

 of display, broadly speaking, may be divided 

 into two distinct types : that indulged in by 

 aerial species, and that peculiar to terrestrial 

 species. In studying the subject we shall find 

 it impossible to draw any arbitrary line between 

 the various aerial movements of certain species, 



