As one of the most common ducks in North America, most everyone is familiar with the iridescent green head of the male mallard (Anas platyrhynchos). Feather color, as I mentioned in an earlier post, is determined by pigments, the structure of the feather or a combination of both.
While watching the ducks on a pond at Crystal Lake Fish Hatchery (Shasta County CA) one mallard caught my attention. The head of this mallard was blue. Because the iridescence and color of birds is affected by the angle of observation, I watched this particular mallard for nearly fifteen minutes, yet the blue color never varied. This was not a blue-green color nor was the mallard’s head simply “dark” due to shadows – the head was a constant, beautiful, rich blue. On occasion I hear about a blue-headed mallard and have even seen mallards whose heads seem blue in certain light. But the blue head color proves in reality to be brilliant green when the duck moves. No matter how this duck moved or how the light changed, its head was blue. I wondered why?
I certainly have not done a comprehensive review of the literature, but I could find no explanation for this blue-headed mallard although as I thought about the little information I had, I came up with several theories, one of which I will mention. Please remember, what follows are simply my musings, not, to my knowledge, scientific fact.
According to Nina G. Joblonski in her book “Living Color”, the intensity of the mallard’s iridescent green head feathers is related to the level of testosterone, higher levels of this hormone resulting in brighter green color. But since a non-breeding or eclipse male has a nondescript brown head similar to the female, where does a blue head enter the picture?
Eberhard Haase et. al. (reported in “Pigment Cell Research”) studied the pheomelanin and eumelanin levels in mallard duck feathers. Melanin is a pigment in feathers. Pheomelanin is a more yellow form and eumelanin is more brown. Without going into the entire content of their research, let me note, in a very superficial manner, that Haase and his colleagues found that males, as their testosterone levels rose in the spring, developed significantly more of the yellowish pheomelanin in their feathers while the brown eumelanin declined. Females displayed more of the brown eumelanin in their feathers. Following so far?
Remember from high school physics that green is a combination of the primary colors blue and yellow. Is the blue color (structural or pigmented?) of mallard head feathers disguised by the brown eumelanin in females and in males whose testosterone is low outside of the breeding season? Then as the male’s testosterone level begins to increase in the spring, the blue shows through as the eumelanin decreases. After the amount of yellowish pheomelanin gets high enough, the head feathers appear green since blue and yellow make green.
A mallard with a blue head could, repeat could, simply not have a high enough testosterone level to produce enough yellow pheomelanin in the head feathers to make the feathers appear green. This lack of testosterone could result from a genetic inadequacy or perhaps the mallard was a young male that still was not making enough of the hormone. Or. . . ?
I certainly do not know the correct answer. Yet speculating is an interesting mental exercise.
I would love to know why this mallard’s head appeared to be truly blue and not simply an optical trick.