The Mystery of Verditer Blue

The beautiful bird in the image above is the Verditer flycatcher (Eumyias thalassinus). It is a winter visitor to our parts and is typically known to hawk aerial insects from an exposed perch. Verditer blue is the dramatic blue-green color which adorns this bird thus naming it. While reading up more on this eye-catching beauty, I came across some interesting information about how this blue looks, well, blue. I have attempted to elaborate and put together some of the information I gleaned as simply as I could.

Most of the plumage colors in birds are, as we know, due to the different pigments that are present in the feathers. Interestingly, blue and white plumage colors do not occur because of the pigments but due to the small structural changes in the feathers that alter their property of light reflection. Thus when we see blue feathers, it is not the pigment but the modification in the feather structure that causes the blue light to be selectively reflected thus causing the feather to appear blue.  White feathers appear white because they reflect all the visible light. The verditer blue is thus a structural color, a schemochrome. Blues, violets and whites are structural colors. Of course, as always, there are exceptions to every rule – the Turaco birds produce their own green and blue pigments and are thus unique.

Let us recall a bit of high school physics. Does the name Tyndall ring a bell?

Visible light is composed of many colors, each having a distinct wavelength. Red light has a longer wavelength (~700 nm) and blue light has a much shorter one (~400 nm). So when visible light encounters particles with same or larger diameter than the component wavelengths, that light or those photons are reflected. So, particles that are 400 nm or slightly larger will selectively reflect blue light and allow other light to pass. Such reflected light photons are collected and seen by the observer’s eye.

Following is a very interesting link explaining why the sky appears blue –

The structural blue is often referred to as Tyndall Blue and the Tyndall scattering can be demonstrated by a simple experiment. When a few drops of milk are added to a glass of water and the glass is kept in a darkroom with a flashlight focused on it, the fluid appears bluish. The bluish color results from blue light bouncing off the milk particles while the other longer wavelengths pass through.

Structural and Pigment Color

Similarly, the surface layer of keratin of a blue feather when seen under a microscope appears cloudy or milky due to the presence of small air cavities. These small air cavities in the middle of the feather barb or keratin particles selectively scatter blue light while the underlying dark melanin layer absorbs the longer wavelengths. This results in blue coloring.

On the other hand, the surface of a red feather when seen under a microscope is transparent and colorless, while the underlying structures are filled with red pigment granules that reflect only red light.

Another simple experiment demonstrates the difference in structural and pigment colors. Because structural color is entirely dependent upon reflective structure, a blue feather becomes dark when it is ground into a powder. However, a red or yellow feather will retain its original color when subjected to the same treatment because the pigments are not damaged even though the feather structure was destroyed.

Iridescent feathers get their color due to the phenomenon of interference, which is due to waves of light interacting with each other to produce either reinforcement or cancellation of particular wavelengths or colors.

The same principle of blue light scattering also gives blue eyes their blue color, there is no blue pigment. To read more:

The unanswered question – Why do birds rarely produce the blue pigment?

-Until the next post…Ciao friends!!!