New Zealand endemic diving duck. Widely but patchily distributed throughout North and South Islands; not found on Stewart Island, and no longer present on Chatham Islands. Unlike any other resident duck species. Compact and dark; floats on water with cork-like buoyancy, showing bath-toy duck silhouette. Forms large flocks, often congregating in sheltered areas near willows or reed beds. Spends a lot of time underwater, where it can travel considerable distances. Sexes alike but distinguishable. Male has dark black plumage, iridescent blue-green head and wings, and yellow iris. Female is duller brown with brown iris. Flies with very fast wingbeats, often just above the water’s surface. Vagrant Hardhead differs in its white iris, white undertail, and white “saddle” on bill.