224 THE STORY OF THE BIRDS 



vocally mute, as we have already seen, the organs 

 of voice being absent or in a rudimentary con- 

 dition only. These species succeed in making 

 much sound by smartly snapping the two man- 

 dibles together. The Storks furnish us with a 

 capital example of these bird mutes. What is 

 technically known as the "klappering" of the 

 Storks is a very remarkable performance, gone 

 through not only as the birds stand upon the 

 ground, but as they fly, often at vast heights. 

 In producing this singular sound the bill is 

 laid against the stomach or the back, and then 

 the two mandibles are struck together so rapidly 

 as to form a kind of trill, curiously modulated 

 both in speed and tone. During the love season 

 certain Game Birds produce various humming 

 or rattling sounds with their wings at the time 

 of courtship ; whilst some of the Nightjars make 

 a booming noise with their wings. The common 

 Nightjar of our English woodlands produces what 

 is apparently a wing sound, as male and female 

 chase each other in the evening gloom. 



We will now proceed to glance at a few of the 

 most curious and characteristic modifications of 

 plumage which are utilised in the production of 

 various sounds. Many of these special modifi- 

 cations are dimorphic and peculiar to the male 



