Top Photo: Male mallard swims away. Do you notice anything odd about this duck?
Everyone has seen a mallard. To most people mallards are the duck. Let’s face it, they’re everywhere. They’re the most common duck across the northern hemisphere and part of the southern. They can be found in North and South America, Europe, Asia and parts of the African continent. They’re even in Greenland, Australia and New Zealand.
If asked, what color is a mallard’s head? most people would quickly say green, and they’d be right. Field guides to birds repeat each other over and over with “Glossy green head,” “green head,” “Bright green head,” green, green, green. No other color is mentioned.
But first, let’s be sure, there is no color pigment in the head feathers of mallards, green or otherwise. The color you see is structural and the result of the interaction of light with the surface of the feathers and the angle the light strikes the feather in relation to the viewer’s position. Seen in bright light the head shines green, iridescent green.
(All birds shown are different individuals from different years except where noted)
But some mallards’ heads appear blue, along with the green, with some showing little or no green at all. Is this strictly the quality or direction of the light source causing the color differences?
In searching the internet I found numerous examples of blue-headed mallards, across the globe. I could not, however, find a good reason for its occurrence other than it being simple iridescence. Some authors suggested diet while others guess at genetic abnormalities. I’m not convinced.
I can see where diet would affect the color of feathers which contain pigment, but the color on a mallard’s head is structural in nature, not from any amount or kind of pigment in the feathers. A scarlet ibis’s brilliant red plumage comes from the carotene found in the crustaceans on which it feeds. The variation in a cedar waxwing’s tail-tip color and wing appendages has been suggested to be caused by carotenes in its diet. Male house finches’ heads, rump, and breasts are sometimes yellow or orange instead of red for the same reason, the presence or lack of carotenes in their diet. This makes sense. What doesn’t make sense is diet playing a role in the color of a bird’s iridescent feathers.
I’ve taken photos of a number of mallards over the years here at the museum. It doesn’t seem uncommon at all for some blue iridescence to appear on their head feathers. After all, the feathers are iridescent, and iridescent means “like a rainbow” (iris, irid = rainbow & escent = tending toward or becoming). This rainbow leans towards the blue end of the spectrum. Blue and green are neighbors on the light spectrum.
My copy of “The Birds of Britain and Europe with North Africa and the Middle East” when describing the male mallard states, “…Drake’s combination of green head (shot with purple towards the moult), narrow white collar…” So, there is at least one reference which acknowledges the blue sheen to the iridescence of the mallard’s head feathers where every other identification guide I’ve seen mentions only green when referring to the male mallard’s head color. I’m not sure though, what “shot with purple towards the moult” actually means.
The drake mallard which is floating around our wetlands at the moment is not involved in a molt, nor will it be for some time. These birds have a post breeding molt in summer after departing the female as she lays her eggs. This molt is a complete molt where he will become drab brown and flightless. Another, body feather molt quickly follows with the male molting back into the green headed, chestnut, gray, white and black plumaged duck we’re most familiar with.
There are many examples of blue/green iridescence in nature including the six-spotted tiger beetles in the photos below. The first image is an example of the species itself in diffuse, indirect light. The second photo is a combination showing a different individual in bright, direct sunlight.
I never really thought it unusual for the head color to vary, or paid much attention to it in the past. After all, many duck species have iridescent head coloration in reds, blues, greens and more.
I will probably take more photos of mallards in the future making note of the various lighting conditions and angle of view.