This is a dark, slender duck with a long crest. It has a dark hood with petroleum-green sheen, a pale grey breast and dark grey upperparts. It has a long, bushy hindcrest usually worn and shorter in females.
Their breeding habitat is shallow, fast-flowing rivers in south-central Brazil. They nest in tree-cavities and possibly rock-cavities. They mainly eat fish, small eels, insect larvae, dobson flies (Corydalis sp.) and snails.
This species is critically endangered. Numbers have reduced due to polluted rivers caused by forest clearance and agriculture. The current population is estimated at less than 250.
The Auckland Islands Merganser was not, as claimed by Luther (1967), flightless; individuals were observed in flight by Hutton and Ranfurly (fide Kear and Scarlett 1970).
Measurements taken on skins were as follows: total body length; length of exposed culmen on midline; nail width; length of the chord of the unstraightened wing; length of the tail, measured medially' length of the tarsus on anterior surface; and length of the middle toe, excluding nail.
Mergansers are capable divers and are largely piscivorous, but unlike several other genera of the Mergini (e.g., Somateria, Melanitta, and Clangula), are strictly foot-propelled divers, i.e., strokes of the feet provide the sole propulsion at submergence and for underwater loco- motion (Townsend 1909; Kelso 1922; Brooks 1945; Humphrey 1957, 1958).
The BrazilianMerganser is undoubtedly a year-round resident in Misiones.
In Misiones the BrazilianMerganser is not rare but has been found only along the small rivers and streams that flow from the highlands in the interior of the province into the Alto Paraná, which is a tributary of the La Plata River.
BrazilianMergansers are sedentary birds and probably spend their entire lives along a small part of one river.
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