Differences in appearance, different habits, different distribution ranges, etc.

Differences in appearance: the male of the mallard duck has a green head and neck with a metallic luster, black eyes, a waxy yellow beak, and the color of the feathers transitions from brown on the upper body to black on the tail; the female's plumage is duller in color, mainly brown, with a white neck ring.

Differences in habits: the mallard has webbing between its toes but rarely dives, while the greenhead is a deep-water animal that specializes in diving with its wings retracted.

Differences in range: The mallard is widely distributed in China and is a major economic waterfowl; the green-headed duck is a global critically endangered species.