The Elements of Evolution (23-7-1) Coloration


Avian coloration represents a compromise between the brightness that fires courtship display and the dullness needed for camouflage. Acuteness of color vision and peak coloration commonly go together in birds.

Feather colors are produced by various pigments deposited in feather barbs and barbules, and by structural tweaks at the feather surface. Most greens and blues result from structural alignments. Pigments produce the other colors.

There are 2 kinds of structural color: iridescent and non-iridescent. Both result from the microscopic structures of feathers which reflect only certain light wavelengths. Iridescent colors change with viewing angle. Iridescence usually only occurs on body feathers, rather than those used to fly, as the affects which give rise to iridescence weaken the feather. Some iridescent colors result from reflections at the interface of a feather’s melanin (pigment) granules and keratin layers.

Non-iridescent feathers manage their color display via tiny air pockets (vacuoles) within barbs which scatter incoming light. Such scattering produces the blues on some birds, including bluebirds.

The most peculiar aspect of color is the relative wavefront coherence by which it is generated. The blue sky is produced by incoherent interference of reflected light waves. By contrast, the blue of bird feathers occurs when light is reflected in phase, via coherent interference.

In feathers producing color structurally, color purity reflects the orderliness of the material employed. Iridescence arises from tightly packed arrays.

The colors and patterns of birds can vary as a function of sex, age, nutrition, and season. While some birds are similarly colored regardless of sex, plumage dimorphism is common. In some birds, such as gulls, color patterns differ each year until adulthood (which for gulls is 4–5 years).

In birds with elaborate courtship displays, the displayer is brightly adorned, while the watcher is typically dull-colored. Males are typically the exhibitionist, but in phalaropes (a shorebird), dull-colored males incubate the eggs whereas females perform courtship displays.

Many brightly colored birds shed their florid display at the end of breeding season, replacing it with a dull winter coat. Prominent adornment feathers used for courtship may also be lost.