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Question of the Week: Why does the Kingfisher look blue?

Cerys RJones25 September 2018

The Common Kingfisher is one of Britain’s most colourful native birds and a personal favourite of mine. Despite the name, the Common Kingfisher isn’t actually all that common. I’ve only been lucky enough to see one in the wild and it was a brief encounter; I still vividly remember the bright blue flash of its feathers. Although these creatures are known for their striking colours, the blue feathers down the back of the Kingfisher are actually brown.

The bright blue colour you perceive is due to a phenomenon called structural colouration. Structural  colouration is seen throughout the animal kingdom and makes creatures appear much more colourful than they actually are. So while the coloured pigments in the kingfisher’s feathers are brown, you actually view them as a brilliant blue.


The brightly coloured Common Kingfisher (Image: Avijan saha)

Structural colouration, first described by Robert Hooke and Isaac Newton, is when the observed colour of an object is not due to the pigment but rather caused by some interference effects instead. The structure of the object itself causes a different colour to be perceived than what would typically be observed by the pigment. Structural colouration can result in iridescent colours – i.e. colours that are dependent on the viewing angle – or non-iridescent colours, when the colour remains constant regardless of the viewing angle. Examples of iridescent colours are the feathers of a peacock, which are also pigmented brown but appear blue due to the structural colouration, and the setea (or spines) of the sea mouse. The nanostructures of the setea of the sea mouse and peacock feathers are regular and so reflect the light in the same direction. This means that the bright colour is only perceived at a certain angle.

The setea of the sea mouse appear red, green and blue to act as a warning to potential predators. The sea mice in the Grant Museum are some of my favourite specimens in the museum and are often unfortunately overlooked by visitors. Their interesting name likely derives from the fact that they look like drowned mice when washed up on shore, but their Latin name, Aphrodita, comes from the Ancient Greek goddess of love, Aphrodite, supposedly due to their resemblance to female anatomy…

The Sea Mouse specimen in the Grant Museum, G15 (Author’s own photo)

In contrast, the kingfisher’s feathers are an example of non-iridescent structural colouration. The blue stripe appears blue regardless of the angle of the viewer. This is because the structures are randomly oriented and so the reflections of the light are not angled in the same direction. The blue-and-yellow macaw similarly displays bright blue feathers that are due to non-iridescent structural coloration. These feathers also contain the brown-black pigment melanin that is present in those of the kingfisher.

Let that be a lesson that you can never trust your eyes – at least, not when it comes to structural colouration! Next time you visit the Grant Museum, look out for our kingfisher taxidermy specimen, the sea mice and any other brightly coloured creatures that may be cleverly appearing more colourful than their pigments might suggest!

To read more about this phenomenon, check out this paper.

A Physicist’s Guide to Zoology

CatrynWilliams21 February 2017

As any lover of Attenborough will, I’m sure, understand, the idea that someone is not naturally interested in nature and zoology is something that I, as a researcher of primates (specifically, their gut bacteria), had never really considered before. Aware as I am that the fascinating but visually underwhelming (I’m sorry!) sea squirt might take a bit of effort to enthuse people I sort of assumed a general underlying love of at least all the four-legged, big-eyed, furry, woolly things of the world.

This wholly unreasonable assumption of mine was proven wrong during last week’s shift at the Grant Museum by one simple question from a very enthusiastic and lovely retired physicist:

“What would a group of physicists find interesting in a Zoology museum?”

What follow here are just two examples of nature seen through a different lens, which I hope go some way towards enthusing those not naturally curious about zoology.

All that glitters isn’t gold, all that shimmers isn’t green

Most of the green birds you see are pretenders.  Rather than truly being green, they’re a beautiful example of something called structural colouring.

When you use paint to colour a surface, what you are applying are coloured molecules, called pigments.  These produce colour through absorption of different wavelengths of light; to produce green, for example, red and blue light are absorbed whilst green light is reflected into your eyes.

Honeycreeper

The Green Honeycreeper, not a green bird. Photo credit: CC Image courtesy of Lip Kee on Flickr

First observed by Robert Hooke and Sir Isaac Newton and explained by Thomas Young a century later, structural colouring, however, is the production of colour through the interference of white light by microscopic surfaces, rather than absorption of certain wavelengths.  This can work in conjunction with pigments — for example, a peacock feather is pigmented brown, but microscopically structured so that they reflect blue and green light, and also making them iridescent, showing different colours depending on the angle from which you view them.

Structural colouring in animals, particularly birds, can be a big evolutionary advantage.  Creating pigments can be very energy-costly, and often requires rare elements that are difficult to extract from food during digestion, such as metals like cadmium, cobalt or chromium for green pigments.  Structural colouring is an ingenious way to create these brilliant colours through feather shape alone, hugely useful when trying to attract a mate or hide from predators in the trees.

Turacos are the interesting exception to these structural colourists.  Found in forests and woodlands in sub-Saharan Africa, these birds actually produce their own unique red and green pigments, called turacin and turacoverdin respectively, using an unusually high amount of copper.  Just why they make this pigment is still a mystery.  Their habitat coincides with the world’s richest copperbelt, leading some to speculate that this pigment production might’ve evolved to detoxify the large amount of copper these birds ingest through their food.  Whatever the reason, this unique ability to use copper in this way makes turacos some of the only truly green birds.

A truly green Angolan Turaco. Photo credit: C. P. Ewing

A truly green Angolan Turaco. Photo credit: CC Image courtesy of C. P. Ewing on Flickr

There are many examples of structural colouring in the Grant Museum, from the peacock’s feather to the wings of iridescent butterflies and the gold sheen of some beetles.  I highly recommend seeing how many you can spot next time you’re there.

 

A (constructal) theory of everything

 

It might not be the unified theory that Stephen Hawking is searching for, but the Constructal Law is a physics theory that can be used to explain the shapes of all the bones, limbs and preserved animal specimens that you see around you in the Grant Museum.

In its simplest form, Constructal Law states that systems naturally evolve over time to minimise energy waste.  Substitute the word “animals” for “systems”, and you have its application to zoology.  This seems like an obvious benefit; wasting less energy allows animals to get the most out of the food they eat, allowing them to flee from predators faster, spend less time gathering food and more time chatting each other up, and produce better-fed offspring. Where this rule becomes most interesting though is when you consider animal locomotion.

Even though running, flying and swimming have all evolved as separate methods of locomotion, they’re all linked by this simple physics principle.  Despite involving very different body mechanics, it turns out that there is a universal relationship between animals’ mass and speed, as well as the frequency and force of limb or tail movement, whether those are legs, wings or fins.  The relationship between a winged animal’s mass and the frequency of their wing beats shows the same relationship as between mass and rate of swimming in fish, as well as mass and stride frequency in running animals, and has all evolved to move the animal at optimal speed, reducing energy wastage whilst maintaining quick movement.  No other factors, such as type of creature, limb length, wingspan or otherwise, seem to factor in to this, only body mass and limb or tail movement.

Grant Museum

Paddling and running on display at the Grant Museum. Photo credit: CC Image courtesy of Justin Pickard on Flickr

This principle helps determine how animals move around and is a brilliant example of how the great diversity of life still converges to fit fundamental physics principles.  Next time you’re in the Grant Museum, have a think about how all the animals around you have been shaped in part by this universal law.

The physicist I met got me to consider the animal specimens in the museum from a whole new angle, making me think about what different people would find interesting about zoology and, importantly, why, rather than just assuming everyone has an inbuilt love.  Just like the iridescent wings of certain animals, looking at a familiar collection from a different angle can offer a whole new view on zoology.  And seriously, give the sea squirt a chance.