112 THE COURTSHIP OF ANIMALS 



but, as in the case of the Prairie-hen, by the enlargement 

 of the air-sacs of the neck. 



These air-sacs, which are present in all birds, are only 

 enlarged to further the ends of sexual display in a few 

 species, and, curiously enough, these are in no way related 

 one to another. The Adjutant storks, it may be remarked 

 in this connection, have used the air-sacs which are fed 

 by the nasal system instead of those fed by the lungs, 

 as in all the species so far described. When deflated this 

 pouch forms a quite inconspicuous conical swelling in 

 front of the neck ; under the stimulus of excitement, 

 it awakens as it were into activity, and is suddenly 

 transformed into a great red or red-and-black bag, en- 

 circling the neck and projecting far downwards in front 

 of it, only to be deflated an instant later with a speed 

 which leaves one gasping. 



The specialization of the air-sacs, that is to say their 

 transformation to perform new functions subservient to 

 the ends of sexual activities, is not exclusively confined 

 to display. In at least one instance an air-sac has been 

 specially developed to act as a voice resonator. This 

 is furnished by the Emu, wherein the wind-pipe, near the 

 middle of its length and on its anterior aspect, has a 

 number of incomplete rings forming a long slit. The 

 lining of the windpipe escapes from this slit in a hernia- 

 like pouch, and takes up a position beneath the skin. 

 Even when inflated this pouch gives no very obvious 

 sign of its existence, but it serves to produce a curious 

 hollow, drumming sound, like the boom of a big drum 

 softly beaten. But why it should have been developed, 

 when the Ostrich and the Cassowary produce similar 

 but louder " music " without any special apparatus 

 whatever, is a mystery. At least one species of Cas- 



