Now, I know spring is here! Not only are all those early mustards (discussed in nature notes in years past) in bloom and fruit, but male goldfinches are turning bright yellow.
You can identify birds by their vocalizations, but you can also make use of the striking colors of some birds to distinguish one species from another. The American goldfinch is a fine example: the males turn bright yellow as spring develops. The yellow is accented by black wings, tail, and forehead patch. In the winter, however, both sexes are basically a grayish, dull, yellow-olive color with dull black wings.
Birds show many different variations in the color and pattern of their plumage. Very simply, plumage color is either produced by pigments or by special structural particles embedded within the feathers.
As you know, different pigments absorb different wavelengths of light energy and reflect others. We see reflected wavelengths of light as color. Thus, male cardinals appear red because red wavelengths of light are not absorbed by the pigments of their feathers, but are reflected back to be perceived by our retinas.
Some birds also display blue and iridescent colors and these are considered “structural” colors. The blues are produced by particles while the iridescent colors are caused by differential reflection of wavelengths from modified barbules of the feathers. The structure and angle of these barbules affects the reflecting surface the light hits and this, in turn, affects what we see.
The pigments that develop are determined to a certain extent by diet. Certain dietary amino acids, vitamins, etc. affect the production of various pigments. Thus the plumage colors can be changed depending upon what the bird is eating. In the spring, the colors of the male goldfinch changes as diet changes, in combination with changing hormone levels.
Plumage coloration is useful for several purposes, among them, species recognition, courtship, threat displays, and camouflage for concealment from prey and predators.
Something to think about in the field of evolutionary biology: If you were a female goldfinch, which male would you choose to mate with: one with dull yellow plumage or bright yellow plumage? You would probably choose the highly colored male since the color indicates a well nourished bird. A well nourished male would be a good “provider”. It would follow that offspring survival would also be affected favorably.