The common green-headed duck is the Mallard, a species found across the Northern Hemisphere in nearly any wetland habitat. This strikingly colored bird is often the first waterfowl people encounter, raising the question of whether the shimmering green head confirms the duck is always male. While the answer is complex, the vibrant head is a strong indicator of sex for most of the year.
The Green Head and Sexual Dimorphism
The iridescent green head is a clear visual signal of the male Mallard, known as a drake, and is a prime example of sexual dimorphism. This difference in appearance between the sexes is driven by sexual selection, where traits that enhance breeding success become pronounced. The male’s glossy head is not created by a pigment but by the microscopic structure of the feathers, which refracts light to produce the shimmering green effect.
This structural coloration is most vibrant during the breeding season and serves as a display to attract a mate. The drake also exhibits a chestnut-brown breast, a white neck ring, and gray flanks, creating a distinct silhouette. In contrast, the female’s muted colors are an adaptation for survival, providing camouflage while she performs the duties of nesting and raising young.
Identifying Female Ducks
Female Mallards, or hens, lack the brilliant green head and colorful body plumage, instead displaying a mottled pattern of browns and tans. This cryptic coloration is an adaptation that allows the hen to blend seamlessly into vegetation while she incubates her eggs, protecting her and her clutch from predators.
While the hen’s body is less colorful, she shares one prominent feature with the drake: a violet-blue patch on the secondary wing feathers called the speculum, bordered by white bands. Her bill color is typically a mottled orange and brown, often with dark blotches, unlike the male’s bright yellow bill. Furthermore, the female produces the loud, recognizable series of quacks most people associate with ducks, a sound that is deeper and quieter in the male.
Seasonal Changes in Male Plumage
The complexity of identifying sex is due to a temporary change in the male’s appearance known as the “eclipse plumage.” After the breeding season concludes, typically between late June and August, the male Mallard undergoes a complete molt. He sheds all his bright feathers, including the flight feathers, which leaves him temporarily flightless. During this vulnerable period, the male adopts a dull, female-like brown plumage to hide from predators.
For several weeks, the male in eclipse plumage closely resembles the female, as the green head is replaced by brown feathers. Identification is still possible by looking for subtle clues that persist through the molt. The male usually retains a bright yellow or greenish-yellow bill, which contrasts with the female’s mottled orange bill. Additionally, the male’s breast feathers often have a warmer, more rufous-brown tone than the female’s, providing a slight difference in coloration.