158 SEXUAL DIMORPHISM 



do not seem to be different in the two sexes, except, indeed, 

 that the duties of incubation are performed only by the 

 female. Another interesting unisexual peculiarity in drakes 

 is the modification of the lower end of the trachea, some 

 six or eight of the lowest cartilaginous rings being fused 

 together and dilated to form a bulb, the walls of which are 

 partially ossified. In the ordinary drakes of the genus Anas 

 there is a simple enlargement or ampulla, protruding usually 

 to the left side. In the Fuligulime, which includes Somateria, 

 the Eider Duck, the enlargement is more complicated, having 

 " fenestras " or apertures in the bony walls which are covered 

 by membrane ; the bulb thus forms a " tympanum " or 

 drum. 



Darwin remarks that the meaning of these differences in 

 the trachea of the two sexes of Anatidse is not understood, 

 for the male is not always the more vociferous. But 

 Macgillivray, as quoted in the Royal Natural History? says 

 that although both sexes quack when alarmed, and the 

 female the louder, the male also utters a rather low and 

 soft cry between a croak and a murmur, and the female a 

 louder and clearer jabber. 



Yarrell 2 considers that the internal edge of the constric- 

 tion between the osseous bulb and the trachea acts like 

 the reed or tongue of a wind instrument, and writes: "A 

 compound tone of voice is thus produced (in the male), by 

 which wild-fowl shooters can distinguish males from females 

 in the darkest night whenever the birds utter their 

 note." 



There is thus no doubt of the sexual difference in voice 

 in the Common Mallard, and as it is known that the court- 

 ship of the drake is an elaborate process, it is probable that 

 his peculiar voice is a kind of love-gurgle produced by the 

 osseous bulb or drum. Thus there must be special strains 



1 Vol. iv. p. 344. British Birds, vol. iii., 1856, p. 276. 



