108 THE ANATOMY OF BIRD-SONG. 



the oval orifice formed for the purpose by the 

 lips, and that the pitch and tone quality are 

 governed by the cavity of the whistler's 

 mouth and throat. In other words, the notes 

 in whistling are caused by the breath from 

 the lungs passing through the opening be- 

 tween the lips, and they are given character by 

 the volume and length of the vibratory col- 

 umn of air in the mouth and trachea of the 

 whistler. Now make a deep cup of your 

 hand and hold it over your mouth while you 

 whistle. You find by careful experiment 

 that any interference with the column of air 

 in front of the mouth-orifice also curiously 

 modifies the sounds. 



The bird's glottis is the whistling orifice; 

 there the sounds are generated, and their 

 character is controlled by the vibrations of 

 two columns of air, one in the trachea, the 

 other in the cavity of the bird's mouth and 

 throat. The glottis is set in the midst of a 

 tangle of muscles, fibrous tendons, and elastic 

 membranes belonging to itself, the tongue, 

 the mouth, and the throat of the bird. Any 

 movement of the tongue, the throat, or the 

 mandibles of the bird affects the glottis in 

 some degree, so that in the case of the best 

 songsters some of the vocal movements are 

 intricately compound, and consequently the 

 sounds produced are likewise compound, 

 but not in origin, simply in modification. 

 For instance, in the croaking of a yellow- 

 billed cuckoo, the sounds are begun at the 

 glottis and would be clear whistle-notes but 

 for the form assumed by the bird's mouth and 

 throat, and the vibratory reflection caused by 

 the action of the elastic membranes against 

 which the bird's breath impinges as it leaps 

 from the trachea. The fact that the sounds 

 uttered by the crows, the cuckoos, and many 

 other oscines are gutteral in effect has led to 



