Why are Bluebirds Blue?
Physics of Structural Colors in Bird Feathers
avian colors are the result of different types of pigments that are deposited
into the developing feather. However, pigments alone do not produce all
avian feather colors. Blues and whites typically result from small changes
in feather structure that alters their light reflective properties. These
fundamental modifications cause blue light to be selectively reflected
from the feather surface in the case of blue feathers, while white feathers
reflect all visible light. In short, blues and whites are structural colors,
Visible light is composed of many colors of light, each with distinct wavelengths. Red light, for example, has a long wavelength (~700nm) while blue light has a much shorter wavelength (~400nm). When visible light encounters particles with the same diameter or larger than its component wavelengths, those specific light photons are reflected. Such reflected light photons are collected and seen by the observer's eye, thereby imparting color to the perceived image. Because blue light has a very short wavelength, it is selectively reflected more easily than other colors of light with longer wavelengths. This was first understood in 1869, when scientist John Tyndall noted that miniscule particles in earth's atmosphere preferentially scattered blue light resulting in the familiar "sky blue" of a clear summer day. Shortly afterward, Rayleigh demonstrated that Tyndall's "fine particles" are gases in our atmosphere, specifically, nitrogen and oxygen. Tyndall's contribution is widely recognized by describing this phenomenon as "Tyndall scattering" and referring to structural blue colors as "Tyndall Blues."
Tyndall scattering can be demonstrated at home using a simple experiment to produce a pale Tyndall blue color. First, mix one or two drops of milk into a glass of water then place this glass in a dark room and focus a flashlight upon it, and the fluid will appear bluish. This bluish color results from blue light bouncing off milk particles suspended in the water while other, longer, light wavelengths pass through the fluid, unobstructed. Of course, milk has some larger diameter particles in it that reflect other wavelengths of light that slightly longer than blue, thereby contaminating the pure "Tyndall" blue color.
Blue coloring in most bird species results from preferential scattering of blue light by the feather structure. When a blue feather is observed under a powerful microscope, the surface layer of keratin appears cloudy or milky due to the presence of small air cavities. A cross-section of the feather reveals an underlying layer of melanin granules and tiny air pockets in the middle of the feather barb. These small air cavities act like tiny particles because they selectively scatter blue light while the melanin granules absorb longer wavelengths of light, intensifying the blue. Structural differences are immediately obvious when a red feather, which derives its color from pigments, is viewed under the same microscope. The surface of the red feather is transparent and colorless while the underlying structures are filled with red pigment granules that reflect only red light.
The differences between structural and pigment colors can be demonstrated using several simple experiments. Because blue color is entirely dependent upon the reflective structure of the feather, it turns dark when ground up into a powder. However, a red or yellow feather retain their original color when subjected to the same treatment because pigments are not damaged when the feather structure is ruined. Pigments can also be removed from the feather without damaging its structure. When a red or yellow feather is placed into an appropriate solvent, the pigment granules will dissolve into the solvent, leaving behind a colorless feather. Blue feathers can also lose their blue coloring when placed into a liquid with a particular optical density, such as balsam, that fills the air cavities in the feather structure, thereby preventing reflection of blue light. Thus, such a feather appears dark when it is wet, but its lovely blue color returns after it has dried.
The physical phenomena that generate structural blue colors are similar, but not identical to, those that produce iridescent colors, such as those seen in purple martins and magpies. For example, iridescent feathers often appear to be very bright when compared to a structural blue feather viewed under the same light. Unlike iridescent feathers, blue feathers remain blue to the observer when the feather is rotated in relation to the light source whereas the coloring of iridescent feathers will vary and then become black as the angle of the light shifts. Similar to an iridescent feather, a structural blue feather will appear dark when it is placed directly between the light source and the observer because light cannot be reflected from the feather surface into the observer's eye.
White also is also a structural feather color that relies upon the same principles described for blue feathers, except that white is produced when all wavelengths of light are reflected. A white feather also shows comparable structural characteristics. When a white feather is observed under a powerful microscope, the surface structure appears crystalline, resembling cut glass or snow, clearly capable of reflecting all visible light. White feathers also contain many air cavities in the feather barbs that increase the total reflection of all spectrums of visible light. As previously described, both melanin granules and air pockets are found in the middle of blue feathers; however, white feathers lack melanin but contain many more air cavities. This lack of underlying melanin granules can be easily demonstrated because a lustrous white feather becomes transparent when it is immersed in balsam.
Turacos (Family: Musophagidae) are unique among birds because they alone produce their own green (and blue) pigments. However, all other birds make green feathers using a combination of both structural and pigment colors. Basically, the feather retains its blue-reflecting structures but embedded within its keratin structure are either yellow carotenoids (producing pure bright greens) or melanins (producing darker olive greens). Thus, it is possible to produce either blue or yellow birds from green parents, through the loss of either a yellow pigment or blue-reflecting feather structures -- a fact that has provided thousands of bird breeders with many decades of pleasure.
Surprisingly, despite humanity's deep appreciation for colors, there remain many questions yet to be answered about colors in animals. For example, why do birds rarely produce blue pigments? Why do they instead rely upon structural changes within their plumage to provide the lovely blue color that so many humans associate with "the bluebird of happiness?"
[1 March 2000]
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