Mergus Linn., Syst. Nat., 10th ed, i, p. 129 (1758).
Type by orig. desig., Mergus merganser Linn.
The genus Mergus differs from Mergellus in its much longer bill, which is narrow, strongly hooked at the tip, with a nail occupying the whole width of the bill; the serrations are strong, wide apart and set with their tips pointing backwards; the nostril is situated between one-quarter and one-third the length of the bill from the base; the wings as in Mergellus; the tarsus short and strong and placed very far back as in that genus; the feet large; the tail of eighteen feathers cuneate ; there are two caeca.
The genus is found throughout the Northern Hemisphere, two species entering India, though one is of the greatest rarity.
Key to Species.
A. Head and neck black glossed with green.
a. Lower parts white throughout, or merely flushed with rosy………… M. merganser, p. 469.
b. Upper breast rufous with black marks………… M. serrator, p. 473.
B. Head and upper neck rufous. (Adult females and young males.)
e. Chin white; back grey………… M. merganser, p. 469.
d. Chin streaked with rufous ; back brown………… M. serrator, p. 473.