Let’s look at one parrot feather. Since blue is my favorite color we will look at a blue feather. They are so colorful but what is their source of color? There are two possibilities. The color may be intrinsic, based on pigmentation, or it may be the result of structure causing refraction of light. Intrinsic color would be found if the structure of the feather would be examined microscopically using transmitted or incident lighting. The simplest thing to do is to look at the feathers with the microscope.
I made two questions for the observations.
- Is the feather one piece or is it made of multiple repeating parts that attach to one another forming one.
- What is the source of the color in the feather where it appeared blue on the outer most surface (dorsal side) and yellow on the inner most side (ventral surface)?
- I expected that under the microscope the feather might be simply dark grey. If it were blue then the color would be derived from refraction. If it were colorful (polychromatic) then the color would be intrinsic mineral or organic pigment.
To assist in observation, I used a microscope with cell phone camera which was already described in an earlier post. The two lenses used were 4x and 10x. These low power lenses are sufficient to give useful results. Incident and transmitted white light was used for illumination. No polarization was used.
These are paired photos of the same feather showing opposite sides with incident lighting
Note that the barbules and the rachis are darker brown pigment suggesting the presence of melanin pigment and not blue. The apparent blue color is derived mostly from refraction caused by light interacting with the structure of the feather not by pigment. The yellow is most likely derived from a carotenoid pigment. The barbs refract the incident light and thus constitute the majority of the color.
Parrots produce psittacofulvins, a type of red to yellow pigment that’s not found in any other type of vertebrate. These include carotenoids which are yellows and reds, malanoids which are browns and blacks and porphyrins which produce pinks, browns and greens in some birds.* The porphyrins may also fluoresce.** Blue color is the result of iridescence (additive interference), non-iridesence (destructive interference) and defraction. It is not the result of pigmentation. Interference and refraction are physical behaviors of light as it passes through certain media.***
Bird feathers are made of repeating barbed structures that can stick to one another like VelcroTM. The interlocking is very efficient. It is sufficiently strong to withstand the forces of gravity and flight generated by the bird. If some become disengaged they can be physically reconnected through the grooming process of preening. They are resistant to unzipping even when wet. However when wet, the air filled spaces can obviously entrap water. The weight of trapped water may be sufficient to destabilize aerodynamics and probably jeopardize the flight parameters of weight and balance. It is the repeating structural make-up of the feather that bends the light and allows it to fluoresce and at the same time permits flight.
The barbs are colored on the multiple surface areas. Apparently the color in the material contributes to the strength of the vane. Melanin and carotenoids improve the strength. This would suggest that birds with variegated color have strength distribution consistent with the colors. Colors are also helpful in bird identification by people. They are also important in bird-to-bird display for mate selection and for territorial dominance display.
In summary the answer to the color source question is both intrinsic and refracted light. The blue coloration in the feathers here is the result of optical interpretation of pigments and refracted light. The structure and color of the feather are directly related.
We will see the effects of viewing feathers under ultraviolet light in another posting.
We will look at the cormorant like Anhinga as they dive for fish in another posting.
*Cooke et al. Genetic Mapping and Biochemical Basis of Yellow Feather Pigmentation in Budgerigars. Cell, 2017 DOI: 10.1016/j.cell.2017.08.016
**Cornel lab/bird academy/articles/ how birds make colorful feathers
#bird blue color #refraction # feather photomicrograph #rachis #barbs #vanes #melanin #carotenins #porphyrins