Common mergansers are the largest of the three merganser species in North America; adults weigh 1,230 – 1,700 grams (2.7 – 3.7 lbs) and are 54 – 71 cm (21.2 – 27.9 in.) long. Males are somewhat larger than females. Adult males have an iridescent, greenish-black head, with white neck, under parts, and secondaries against black upper wings, gray back and tail. Unlike red-breasted mergansers, male common mergansers in breeding plumage have no crest on head. In late summer during molt, males resemble breeding females in overall coloration, except the male’s wing has a larger white patch and they develop a crest shorter than the female’s.
Sea Duck Information Series fact sheet
Common Merganser Species Knowledge Summary and Information Needs [PDF]