Distinctive, large, rather gooselike duck of coastal wetlands, tidal mudflats, and nearby grassy fields. Less numerous and more local inland at lakes and along rivers. Often seen walking and grazing, at times in flocks. Striking plumage often looks simply black-and-white; at close range, head is deep glossy green, broad breast band dark rusty. Male has raised knob at base of red bill; juvenile duller overall, with whitish face and eyering.