ected 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 drafts folder ever since. The thought itself, on how and why the plumages of birds get their colours from, came about from a question asked by one of my friends after a good session of photography sometime mid-2014. That discussion triggered my inquisitiveness and I started reading up about schemochromes (or structural colours). Don’t recollect why I didn’t complete this blog at that time, I must have just been lazy I guess. But, better late than never.
- Properties of Light and examination of Isotrophic Substances, Prof. Steven A Nelson, Tulane University
- Schemochromes; The physics of Structural Plumage colours, scienceblogs.com
- How birds make colourful feathers, allaboutbirds.org