Are Bluebirds Blue? – The fascinating world of structural colours

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Photo Credit: Elaine R. Wilson

Take a look at the picture above. What colour are the feathers of that bird? They’re blue, right? Wrong.

If you were to take the red plumage from say, a cardinal, and grind it up into a powder, it would still look red. If you were to take the plumage of a bluebird, and grind it into a powder, it would look colourless.

The kind of cardinal that does not elect Popes

Wait, what?

The big difference between the colour of the two feathers, is that one is caused by pigments (in the cardinal case, by red pigments) absorbed by the bird through its food, and the other is caused, not by pigments, but by the structure of the feather.

Structural colours, as they’re called, are generated by light interacting with a three-dimensional structure (in the bluebird’s case, the structure of the feather). When the bluebird feather cell grows, complex keratin structures are formed. When the cell dies (feathers are made of dead material), the liquid inside them dries, leaving the keratin structure intact. Red and Yellow light cancel each other out, when they pass through the structure, but blue light is unaffected. It is quite a feat of evolution for organisms to be able to build such precise nanostructures.

Butterflies and Berries

Bluebirds are not the sole users of structural colours. Many butterflies use them, and so do some plants.

There are a few mechanisms that are used to generate structural colours, includiding diffraction grating (as seen in the butterfly picture below) and photonic crystals, which are ” optical nanostructures that affect the motion of photons in much the same way that semiconductors affect electrons”.

Butterfly wing at different magnifications reveals microstructured chitin acting as a diffraction grating – Source: Wikipedia

The Emerald Swallowtail butterfly has “micron-sized bowl-shaped pits” in its wing structure. These act as ‘selective mirrors’. The sides of the pits reflect only blue light, and the centres reflect only yellow light. Combined, the butterfly looks green.

Pollia Condensata berries, the most intense blue of any living material – Photo Credit: Juliano Costa

The berries of Pollia condensata have the strongest blue coloration in nature. Spiral chitin structures cause what’s called Bragg reflection.

Some animals, like cuttlefish or octopuses, can change colour at will, by altering the nanostructures in their skin that filter light.

-Francesco

Further reading:

Why are some feathers blue – Smithsonian.com