421 



As a rule, the drake in this subfamily assumes by moult 

 .a special nuptial plumage for a longer or shorter period, but 

 the exceptions are numerous, especially amongst resident Indian 

 species. There is, in the forms assuming a nuptial plumage, 

 a double moult of the body-feathers, the two moults some- 

 times, as in the Common Mallard, occurring at an interval of 

 only three or four months. 



Another character found in almost all 

 drakes of this subfamily is the development 

 of a bulba ossea or labyrinth at the base of 

 the trachea, close to the bifurcation of the 

 bronchi. This bulba ossea is a bony, or 

 partly bony and partly membraneous, dila- 

 tation of the lower tracheal rings, and is 

 often lateral, as in the common Wild Duck 

 (tig. 104) ; but sometimes the expansion is 

 medial or in two directions, the different 

 forms being characteristic of particular 

 genera. This peculiar conformation is 

 found in the Merging but not in the 

 Anserine. 



, The bill throughout the subfamily is 

 bulba ossea. \. depressed, and, especially near the tip, much 



broader than high. The legs are rather 



short and more adapted for swimming than for walking. In the 

 majority the exposed portions of the secondary quills, with their 

 larger coverts, are distinguished by conspicuous and often metallic 

 coloration. The brightly coloured patch thus formed is known as 

 the " speculum." 



Key to the Genera.* 



.a. Hind toe not lobed or narrowly lobed. Breadth 

 of lobe or web less than one-third length of toe. 

 a'. Nostril one-third length of bill from fore- 

 head. 

 a". Size large : wing over 10 inches. 



a 3 . Breast and abdomen white ; male with [p. 422. 



a large fleshy comb at base of culmsn SARCIDIORNIS, 

 b 3 . Breast and abdomen brown tinged with 



rusty red ASARCORNIS, p. 424. 



c 3 . Breast and abdomen dark brown, head 



pink REODONESSA, p. 425. 



d 3 . Abdomen black in the middle, white 



at the sides TADORNA, p. 427. 



e 3 . Breast and abdomen ruddy ochreous . . CASARCA, p. 428. 

 b". Size smaller : wing under 10 in. ; tarsus [p. 430. 



reticulated throughout DENDIIOCYCNA, 



b'. Nostril less than one-third length of bill 



from forehead, 

 c". Size very small wing under 7 in. ; bill 



high at base and short NETTOPUS, p. 433. 



* This Key does not always serve for species not found in India. 



