Birds and others use structural trickery to produce vibrant blue colours

2014-hm-Charles Krebs-737

Peacocks, kingfishers and bluebirds are among some of the most eye-catching of birds, and in many cultures are particularly admired for their rich blue colours. We have long known that red and orange birds take their colour from their food, extracting pigments in the plant and animal matter they consume. Flamingoes are possibly the best-known example, owing their pink colouring to the carotenoids in the shrimp and red algae that make up a large part of their diet. On the other hand, blue pigments are not only relatively rare in the natural diet of birds but they are digested soon after ingestion. And birds can’t produce blue pigment themselves – in fact, no terrestrial vertebrates can! Instead, blue feathers are coloured by a neat structural trick, no fancy pigments required. After studying 230 species of birds, ornithologist Dr Richard Prum and colleagues found that the barbs that make up the feathery filaments contain tiny bubbles of keratin, the protein that makes up our own hair and finger nails. The precise size and structure of these bubbles causes the keratin to scatter light in such a way that it appears blue to us – in the same sort of way the sky appears blue. It turns out these structural colour strategies are found in countless animals, both vertebrates and invertebrates, terrestrial and aquatic (blue-eyed humans included!). Some even combine their structural blue with a yellow pigment, to produce a vibrant green characteristic of several snakes and frogs. Now that’s getting creative!

Image courtesy of Charles Krebs and Olympus Bioscapes.

Glossary

Barbs: Bird feathers are made up of paired tiny filaments (barbs) which are connected together by barbules.

Carotenoids: Pigment molecules produced by plants which give a yellow, red or orange colour.

Ornithologist: A scientist studying birds.

Structural colour: Colour generated by secondary or higher-level physical structure, rather than by light-reflecting pigments alone.

Terrestrial: Living on land.

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