The range of blues that you can see in the Verditer Flycatcher is quite stunning. From a more pale blue in the juveniles and the females to a more intense blue in the males. And the interesting thing is this – this birds do not get their dramatic plumage colour from pigments; instead, this is a structural colour that results from the feather scattering light such that it appears blue. Now, you would ask, what is a structural colour. Let me try and explain what I have understood about this.
To start with, we have to recap some basic Physics about light and wavelengths. Light is a form of electromagnetic radiation and, as you would know, all electro magnetic radiation propagate as waves with differing wavelengths – cosmic rays and gamma rays having the smallest wavelength on one end of this spectrum and long radio waves at the other. Visible light occupies a range of spectral wavelength between 380nm and 730 nm – Violet at the lower wavelength and Red at the higher wavelength. The image below, from this blogpost, should make this easy to visualise.
We see different colours because our eyes interpret these various wavelength into their respective colours. If we see a single wavelength, say 380 nm, our eyes will interpret that as Violet – and since it is a single colour (because it has a single wavelength) it is what is called a monochromatic light. If we see a source that has all the wavelengths of visible light, from 380 to 730nm, our eyes will interpret that as white. And conversely, black, when the source has no wavelength in the visible spectrum.
Moving on, how is it that we see objects in the colours that they are in? Simplistically, objects are said to be of the colour of the light leaving their surface (reflected or emitted) which depends on the spectrum of the incident light, reflective properties of the surface as well as the angle of incidence and viewing. A pigment is a material that changes the colour of transmitted or reflected light because it selectively absorbs or reflects certain wavelengths of reflected light (wavelength-selective absorption). When white light encounters a pigment, parts of the spectrum are absorbed by the pigment and the rest are reflected or scatted. This reflected light is what creates the appearance of colour.
Getting back to the original question, plumage of birds get their colours due to two reasons. The first one is quite straightforward to grasp – pigmentation. Basically, the feathers have pigments (like our skin has varying levels of melanin) that absorb certain wavelength of light and reflect the colours that we see. There are 3 types of organic pigments that are the basis of colours of their plumes – Carotenoids (yellows, oranges and reds), Melanins (blacks and browns) and Porphyrins (pinks, greens, browns, reds and some fluorescence). Depending on the type of pigment (and combinations) specific wavelengths are absorbed resulting in our eye perceiving the appropriate colour. This image that I found in one of the articles in a Cornell Lab site illustrates this well.
The second method is through structural colouration – basically this is production of colours by microscopically structured surfaces fine enough to interfere with visible light, sometimes in combination with pigments. Structural colouration is underpinned by the principle of Wave Interference – interference between the reflections from two or more surfaces of thin films combined with refraction as light entered and leaves these thin films. These reflections could interfere constructively at certain angles and destructively in other angles and this gives the effect of different colours at different angles. Blues and greens that we see in the plumage of birds like the Verditer Flycatcher or the Bee-eater are caused by this method. As also are the iridescent colours that we see in the plumages of the Indian Peafowl or the Himalayan Monal. This illustration, from the aforementioned article, makes it easy to understand the concept.
I started writing this blogpost around 4 years back and it has been lying around in my draft