Male common mergansers have a striking appearance with a green, iridescent head, white body, and gray back, while females are primarily gray with a reddish-brown head and a white throat. Both sexes share a long, slender, serrated, red bill and are large diving ducks known for their streamlined shape.

Male Common Merganser:
• Head: Dark green and iridescent, often appearing black.
• Body: White with a gray rump and black back.
• Bill: Red, long, and slender, with a serrated edge.
• Wings: Dark gray with a prominent white patch on the inner half.

Female Common Merganser:
• Head: Reddish-brown, with a shaggy crest and a sharp contrast between the reddish-brown and the white throat.
• Body: Gray with a white breast and belly.
• Bill: Red, long, and slender, with a serrated edge.
• Wings: Gray, with a white patch on the inner half.

Both sexes can be identified by their long, slender, red, serrated bill. In flight, both sexes also show a distinctive white patch on their wings.