Nature’s Palette in Flight

One morning at the Thane Creek Flamingo Sanctuary in Mumbai, we were out birding with a lively group of school children when we spotted a male purple sunbird (Cinnyris asiaticus) on a palash (Butea monosperma) tree. The scene (almost) had all the makings of a great photograph: bird on a bright orange flower, perfectly still, but against the light!

“That’s purple?” a child asked, puzzled. Fair enough, we thought. In that lighting, the bird looked like anything but its namesake.

Moments later, the bird darted across the unpaved path and landed on another tree. This time, sunlight hit it directly, and suddenly, what had seemed black turned into something dazzling. Its feathers shimmered with brilliant shades of blue, violet, and purple, like the shine on a polished pair of sunglasses.

The children stared, open-mouthed. One whispered, “It changed colour!”

In truth, it hadn’t. What changed was the light and how we saw it. That small moment became a quiet lesson in the science and wonder of feathers.

Birds showcase a range of colours for several reasons. These could aid in camouflage to blend into its surroundings. They may be present for thermoregulation or intra- and inter-species recognition. And they could also be used as a form of communication for mating or signalling.

Bird feathers typically appear coloured either due to the presence of pigments or due to structural colouration. Pigments can be melanin (blacks, browns, dull reds and yellows), carotenoids (reds, pinks, oranges, yellows), and sometimes porphyrins, pterins, or psittacofulvins (pinks, browns, reds, and greens). Structural colouration refers to microscopic structures within the feather that reflect and scatter white light to produce blues, greens, and other iridescent colours. Bird feathers may also use a combination of pigments and feather structure to create iridescent and vibrant hues that change with the angle of light falling on them. 

The greater flamingo (Phoenicopterus roseus) and lesser flamingo (Phoeniconaias minor) are two iconic wetland species we see in India, instantly recognisable by their elegant form and striking pink plumage.

But if you ever come across a juvenile flamingo, you might be in for a surprise. Far from the vibrant shades of pink, young flamingos are a dull mix of grey, brown, and white. They don’t look anything like the flamboyant adults they’ll one day become.

So, where does that famous pink come from? Interestingly, the answer lies in their diet.

Humans consume vegetables like carrots and pumpkins to obtain carotenoids, which our bodies utilise to produce vitamin A. When flamingos eat crustaceans and algae, carotenoid pigments present in these organisms are broken down and deposited in their skin and feathers over time. In fact, without a carotenoid-rich diet, even adult flamingos would fade to a paler version of themselves.

Structural colouration arises due to an orderly arrangement of tiny structures on certain parts of the feather. These features are generally a combination of thin keratinous films and sponge-like nanostructures. Incoming white light interacts and scatters off these features in specific ways, giving rise to brilliant and often iridescent hues. Stavenga and colleagues, in their 2011 study, investigated the feather colouration of the common kingfishers (Alcedo atthis). Using powerful microscopes, they found that the bird’s orange breast feathers contain minuscule hollow cells with tiny granules lodged on the walls of these cells. These granules are collections of chemical pigments that selectively absorb the blue-green colours of white light and reflect orange-red.

On the other hand, the bluish back and tail feather tips have keratinous structures dotted with hollow spaces that are only fractions of a micrometre wide, similar to the form of sponges. What is interesting is that the arrangement of these hollow spaces in the kingfisher’s feathers is not random; it has an order to it, causing the shorter wavelengths of light (bluish hues) to preferentially scatter at certain angles and give an iridescent blue appearance. 

The peacock (Pavo cristatus) feather is perhaps the most exalted: a symbol of beauty, grace, and opulence across cultures. Its vibrant shades of deep indigo, shimmering blue, emerald green, and hints of pink and violet blend so fluidly that it almost seems to be hand-painted. Unsurprisingly, its appeal makes it vulnerable to illegal feather collection for decoration, religious use, and fashion.

But what makes a peacock feather so dazzling?

Interestingly, its colour doesn’t originate from pigments at all. It is a result of structural colouration. The dance of shining light with the minute structures on these feathers brings out patterns of vivid, shifting, and iridescent colours, giving the feather a magical allure.

Unlike pigment-based colours that may fade as the pigment chemical degrades due to ageing, these structural hues don’t fade easily over time, adding to their enduring charm and value in trade. 

Most sunbird species, including the purple sunbird, show pronounced sexual dimorphism, i.e. differences between males and females. The male’s plumage generally consists of attractive and iridescent structurally coloured feathers, while the female is drab with dull olive, brown, and yellow hues arising from melanin-based pigments. Male sunbirds of certain species are known to occupy exposed perches to amplify and flaunt their structurally coloured iridescence under direct sunlight. 

The breeding male purple sunbird also possesses spectacular yellow and scarlet coloured pectoral (chest) feathers, which it emphatically displays to a potential mate. Yellow feather colouration in most birds (except parrots) comes from carotenoid pigments, which birds get from their diet. So, a male with bright yellow feathers likely eats well and is good at finding food, making him potentially a healthier mate. Research also suggests that carotenoids are physiologically beneficial for the bird as they act as antioxidants and enhance immunity. Therefore, only fitter (healthy) bodies will have available carotenoids that may be directed towards feather colouration. 

Parrots, of the family Psittacidae, are known for their vivid colours, including not just green but also bright reds, oranges, yellows, and blues. Most birds get the reds, yellows and oranges from dietary carotenoid pigments, but parrots are different. In the late 19th century, it was discovered that unique pigments called psittacofulvins are responsible for colouration in parrots. These pigments are not found in their blood or their liver, which suggests that parrots produce them directly in their feathers, rather than deriving them from food. Further, a combination of absorption of certain colours of light by psittacofulvin pigments and the optical effects due to feather microstructure give rise to green colouration in the plumage.

Studying avian plumage colouration has opened avenues and inspired lab-based innovations for societal benefit. In a fascinating study, Dakota E McCoy and colleagues examined the super black feathers found in some male birds of paradise, a family (Paradisaeidae) of small to medium-sized forest birds known for their vivid plumage and elaborate courtship displays. According to them, their feathers appear extra dark due to the tiny, curved spines on them. These spines create deep, curved pockets that trap up to 99.95% of the light hitting them. This property is sought after for use in various applications such as renewable energy. Harnessing such scientific knowledge to tailor new materials might provide elegant and efficient solutions to existing technological challenges.

For more information on bird feathers in India and the world, visit the Feather Library Initiative at featherlibrary.com.