Mallard in Pennsylvania’s Blair County displays both male and female traits

Altoona, Pa. — A mallard observed this winter in central Pennsylvania sparked discussion among birders and waterfowl hunters after appearing to display traits of both sexes.

Gliding across the water alongside a flock of mallards and an American black duck, the bird was initially dismissed by some observers as a possible hybrid. Mallard–black duck hybrids are known to occur and often show blended characteristics, including muted coloration and partial green on the head.

Closer inspection, however, told a different story.

A faint green sheen washed over the back of the bird’s head, but the bill – bright orange and mottled with dark gray – matched that of a female mallard. Most striking was a single curled tail feather, known as a “drake curl,” a trait almost exclusively associated with male mallards.

The bird’s flanks added to the confusion. Gray feathers with fine, fingerprint-like vermiculation typical of drakes were visible, though they were mixed with brown female-type plumage. Subtle hints of chestnut coloration, another male-associated trait, appeared on the breast.

Despite its appearance, the duck was neither a hybrid nor a misidentified male. Instead, it was a female mallard expressing male plumage traits due to a hormonal imbalance – a documented but often misunderstood phenomenon in birds.

When plumage blurs traditional gender lines, biologists often rely on bill color as a reliable indicator of sex. In mallards, the bill rarely lies.

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How plumage is controlled

Mallards are among North America’s most familiar ducks and one of its most visually dimorphic species – those in which males and females look distinctly different.

Adult drakes are recognized by their glossy green heads, yellow bills, gray flanks and curled tail feathers. Hens wear intricately patterned browns and tans that provide camouflage while nesting, along with orange bills heavily mottled with dark markings.

These differences are influenced not only by genetics but also by hormones – particularly estrogen.

As long as a hen produces normal levels of estrogen, she retains typical female plumage. When estrogen production declines or stops, male characteristics that are normally suppressed can begin to appear.

Hormonal control of plumage occurs in many bird species, but it is most noticeable in species with strong visual differences between the sexes, such as ducks and pheasants.

What causes the change?

In birds, only one ovary –usually the left – is functional. If that ovary becomes damaged or ceases to function, estrogen levels can drop sharply. This may occur as a result of age, disease, injury, infection or other physiological stressors.

“The theory is that this can happen to a hen when it gets old, but I think that’s been disproven with some banded birds showing this a little over a year after they are hatched,” said Cole Tiemann, waterfowl and migratory gamebird biologist with the Delaware Division of Fish and Wildlife.

“Either way, this happens when their ovaries get damaged, and they start to show male plumage.”

When estrogen declines, testosterone-driven traits may emerge.

In female mallards, this can include a drake curl, partial green coloration on the head, grayer flank feathers and a brighter bill.

Despite these changes, the bird does not change sex. She remains genetically female and typically continues to behave like a hen, though she may no longer be reproductively viable.

“I’m not sure if they attempt to breed,” Tiemann said. “But if I had to guess, no. They still quack like a hen mallard, though.”

Vocalizations provided another important clue in the Blair County bird. The duck repeatedly gave the loud, familiar quacks associated with female mallards. Drakes, by contrast, are generally quieter, producing softer, raspier calls.

How rare is it?

Hens expressing male plumage traits are considered uncommon but not extraordinarily rare.

“I’ve only seen two of these, and I’ve looked at a lot of mallards,” Tiemann said. “If I had to put a number out there, something like one out of 10,000 hens. It’s rare, but it does happen.”

Similar hormone-related changes have been documented in domestic chickens, peafowl, pheasants and other waterfowl species. Life expectancy plays a role, as birds in captivity often live long enough for ovarian failure to occur. That makes the Blair County observation notable: the bird was fully wild.

In natural settings, these individuals often go unnoticed.

Female mallards typically draw less attention than brightly colored drakes, and birds with unusual plumage are frequently dismissed as hybrids or simple oddities.

Only careful observation reveals the underlying biology.