WATER FOWL OF INDIA AND ASIA. 39 
cases only, and then not much. The armature of its 
edges is a sifting apparatus in the form of rows of 
plates or ridges, the ends of those belonging to the 
upper chap being sometimes, but rarely, visible when 
the bill is closed. 
All true Ducks have a bill of this type, but of the 
other sub-families the Mute-Swan and the Bar-headed 
Goose havea very similar one ; these, however, need no 
more to be mistaken for Ducks than the latter, when 
possessed of an eccentric beak, for Swans, Geese or 
Mergansers. 
* The males, or drakes, in this sub-family generally 
possess the bulb or drum at the base of the windpipe 
alluded to in my account of the Mergansers, and this 
varies in form according to the genus and species ; the 
voice of the drake differs accordingly from that of the 
duck. They are larger than the females, sometimes 
only a little, but occasionally very muchso. They may 
either resemble them closely in plumage, or differ from 
them so much as hardly to be recognizable, and there 
are gradations between these two extremes. As a rule, 
where the difference exists, the male after breeding 
assumes a plumage resembling more or less completely 
that of the female. The young usually resemble her, 
but may have a special plumage of their own, differing 
from that of either parent. 
In consequence of all these variations, although the 
drakes in full plumage are remarkably easy to distin- 
guish, it is a matter of some difficulty to find characters 
which will discriminate every species irrespectively of 
age, sex, orseason. This, however, can be done if atten- 
tion be paid to details. The fact should be specially 
noted that in the true Ducks, as in the Mergansers, the 
secondary quills and their coverts are very commonly 
so coloured as to produce a wing-bar (speculum) or 
ribbon mark, especially conspicuous when the wing is 
closed. Thisiscommon as arule to all individuals of 
