Large diving duck. Males are 60 to 71 cm, 1700 g; females 54 to 68 cm, 1230 g. Male has greenish-black head, white neck and underparts, black upper wings, gray back and tail, and long narrow scarlet-orange bill. Female has rusty-brown head with long crest and white chin patch, gray breast, back, wings and tail with white flanks and belly and scarlet-orange bill (Mallory and Metz 1999).
For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see Marks et al. 2016, Birds of Montana.
Western Hemisphere Range
During summer in the Bozeman area, Common Mergansers occur on large lakes and large rivers. During migration, most birds are on lakes (in the largest concentrations) (Skaar 1969).
The Common Merganser eats primarily small fish, but will also eat insects, mollusks, crustaceans, worms, frogs, small mammals, birds, and plants (Mallory and Metz 1999).
Pair formation appears to occur during late winter-early spring. Females breed in their second year, generally laying one clutch of 10 eggs with an incubation period of 32 days. Near Fortine, young about 1/3 grown were seen on June 14. Bozeman area migration periods are March 15 to May 5 and September 20 to December 15, with peaks April 5 and November 15. Young have been seen as early as May 31 (Skaar 1969). Nesting records statewide are from late March to late May.