Northern Flicker is a familiar bird throughout the lower 48 states and southern Canada. Within that wide range it occurs in two strikingly different forms, long ago considered separate species: “Red-shafted Flicker” in the west, “Yellow-shafted Flicker” in the east.
The two forms differ most obviously in the color of the large feathers of the wings and tail, either red or yellow, respectively. They also differ in head pattern. In the middle of the continent, however, the forms mix, and it is common to see flickers with intermediate wing and tail color. Anywhere between the eastern edge of the Rocky Mountains and the eastern edge of the Great Plains you should expect to see some flickers with intermediate colors. On closer inspection, these birds also show mixed head patterns, confirming their identity as intergrades.
The red and yellow colors in birds are produced by carotenoid pigments. All carotenoids come from the birds’ diet, and (usually) any difference in color results from a difference in the way each population processes those carotenoids to produce pigments. There are variations in the saturation and shade of color depending on the age, sex, and health of the bird, but in general these red and yellow colors are quite consistent. Given the same diet, an eastern flicker will produce yellow feathers, a western flicker will produce red feathers, and an intergrade flicker orange feathers.
The exception to that rule involves a chemical compound found in the fruit of some Asian honeysuckles, now common across much of North America. When birds consume this new compound, it produces a red pigment, even in species that are normally yellow. This has led to the appearance of Cedar Waxwings with orange tail bands, Baltimore Orioles with red breasts, etc. It also leads to the occasional appearance of eastern “Yellow-shafted Flickers” with a few red feathers in their wings.