Bufflehead males (Bucephala albeola) have iridescent purple-green heads that often, depending on the light, appear black. In April, I photographed a bufflehead at Manzanita Lake in Lassen Volcanic National Park CA. In every picture but one, the bufflehead has a brown patch on its head. In the one photograph without a brown spot, the bufflehead was swimming away from the sun while in all the other pictures the duck was swimming toward the sun.
Feathers color is achieved through two strategies – pigmentation and structure. Although the colors achieved through each method may look similar, the two approaches differ in the way light waves are treated.
In birds whose color is determined by pigmentation, the color is produced by selective absorption. The pigment absorbs part of the light spectrum and reflects the remainder of the spectrum back to our eyes. The pigment in a red bird feather absorbs all the light except red wavelengths and so we see the feather as red. A black feather absorbs all light wavelengths and a white feather reflects all wavelengths. There are three main groups of pigments: carotenoids, melanins and porphyrins.
There are two types of structural colors: iridescent and non-iridescent. Iridescent feathers achieve their colors by refraction of the incident light by the microscopic structure of the feather barbules (the smallest division of the feather). The color depends on and changes with the viewing angle.
Non-iridescent structural colors are the result of light being scattered by air pockets in the feather barbs. Unlike iridescence, where the color can change depending on the viewing angle, the color of non-iridescent feathers is specific. Almost all blue feathers use the air pocket strategy.
Another “color” variation in feathers results from the reflection of ultra-violet wavelengths. Birds can see in the UV range but humans cannot. Thus UV reflection may affect what a feather looks like to a bird. However, this UV reflection does not alter the color a human sees when looking at a feather.
The brown patch on the bufflehead was not some injury or genetic variation. Instead its iridescent head was viewed, and photographed, from an angle that made some of the feathers appear brown/reddish.