THE Mergansers or Saw-bills differ from all other Ducks in having a narrow bill, quite unlike that of an ordinary Duck, and furnished on the margins of both mandibles with a series of distinct sharp-pointed, saw-like teeth, curving backwards. The Mergansers have longer wings and lighter bodies for their size than the diving Ducks, and are more powerful on the wing. They have the primaries coloured as in the True Ducks, but the contrast between the two webs of each quill is perhaps not quite so strong. Like the Ducks also, the males have a post-nuptial plumage, apparently very closely resembling that of the female. The Mergansers have strong feet, the toes being much longer than the tarsus, and these birds possess unrivalled powers of diving.
The Mergansers may be divided into two minor sections : the Mergansers and the Smews.
The Mergansers are of about the size of an ordinary Duck. The bill is about the length of the head, and extremely narrow. The crown is furnished with a conspicuous crest in both sexes. In the Goosander, the crest of the male is thick and bushy, and about one and a half inch in length; that of the female is longer, some of the feathers measuring two and a half inches in length, and more pointed. In the Red-breasted Merganser, the crest of the male is very narrow and straight, and in a measure double, the longest feathers measuring about three inches. The female has a much shorter crest, also narrow and straight, the feathers seldom exceeding a length of an inch and a half.
Mr. W. P. Pycraft has kindly drawn my attention to the very peculiar formation of the trachea in the Mergansers, by means of which the sexes may be determined without dissection or regard to plumage. The trachea of the male Goosander has two enlargements ; that of the female only one. In the male Red-breasted Merganser, the trachea has one enlargement; that of the female has none at all. These enlargements, or the absence of them, can be easily felt by passing the forefinger and thumb down the sides of the neck of the birds, from the head to the base of the neck.
The Smew is about the size of a Wigeon. The bill is very small, and much broader at the base than at the tip. The crown of the head of both sexes is furnished with a bushy crest, about one inch in length.
The differences between the Goosander of India and the Goosander of Europe and Northern Asia are very small, and I do not follow Count Salvadori in keeping the two races distinct. The male of the Indian bird has ordinarily broader black margins to the inner long secondaries; the female has the crown of the head tinged with grey.
More than thirty years ago, Gould described a species of Merganser from China. The bird is now in the British Museum, and is the only specimen of the species known. In the general colour of its plumage, it resembles the female of the Red-breasted Merganser. It is obviously an immature male, and has already acquired some black feathers on the back and the double-barred wing of the adult male, This species, named Mergus squamatus by Gould, is remarkable in that it has the sides of the breast and the sides of the body pale buff, each feather with a double crescentic black band, one within the other, and parallel to the margin of the feather. If this bird really came from China, and Gould expresses no doubt on the matter, it may be looked for in Upper Burma and the Shan States.
No other species of Merganser can be expected to occur within our limits.