July 2015

On Springwatch the other night, Chris Packham, referring to the blue plumage of something like a swallow, came out with the statement ‘There are no blues in nature’.   A considered response to that statement may be ‘OK Blue-eyes, come out from behind those delphiniums and bluebells and explain yourself!’   What he was referring to, of course, is that there are no natural blue pigments in nature – or are there?  In fact, blue is a relatively uncommon hue in nature, with less than 10% of all flowering plants showing any shade of blue.  Nature is very good at reds and yellows, greens and browns, but nature has almost never evolved a true blue pigment.  Blue pigment, chemically speaking, is actually a fairly recent discovery.  When John Constable painted ‘The Hay Wain’ in 1821, he painted a cloudy sky – not because he was good at clouds, which he clearly was – but because blue pigment was fiercely expensive in those days.  It was made by grinding up the semi-precious stone lapis lazuli.

Salvia - 'Deep Blue'

Chris Packham was wrong regarding plants though, because there is at least one blue pigment in nature – indigo.  Though it is now produced synthetically, indigo dye was originally produced by extracting it from the leaves of the tropical American plant añil (Indigofera suffruticosa).  Indigo dye is most famously used to dye the cotton woven into blue jeans.  Most plants, like bluebells, etc. make their colour using anthocyanins, which though normally red will go blue in a suitably alkaline environment.

But when it comes to the blues in animals, Chris Packham is right.  That doesn’t mean that the bluetit, the swallow, the peacock’s tail, and the common blue butterfly are all suddenly going monochrome, because they have a different trick up their sleeves.  They use what is called structural colour.  Normally when you see blue colour, the blue object has reflected blue light back at you and absorbed light of other colours, and that’s all done at the atomic level with photons of light being absorbed by electrons or not as the case may be. 

Australian Fairy Wren

But a bird’s feather is made mostly of keratin which is mainly a pale brown colour at the atomic scale.  At nanoscale, however, things are different.  (Nanoscale just means things measured in nanometres which are very small – one millionth of a millimetre in fact.)  Nanoscale is way bigger than atomic scale, but still way smaller than you can see with the best optical microscope.  The reason that things are different at nanoscale is that the wavelength of light is also measured in nanometres.  With structural colour there are surface features on the feather that correspond to the wavelength of blue light so that only blue light reflects back.  In the peacocks tail and in the scales on some butterfly’s wings there is a slightly different thing going on, and the surface features are transparent and only half the wavelength of light thick so that when blue light is reflected from both the front and back surface, it effectively doubles the intensity of the light and produces iridescence.  The surface features are normally parallel ridges and this means that the iridescence can only be seen at certain angles.   This is best seen in the blue morph butterfly which is a tropical species not normally seen in Pett, but if you take a close look at the ‘eyes’ on the wings of a peacock butterfly, or the ‘eyes’ on an eyed hawkmoth, you can see a flash of that iridescence.

Eyed Hawkmoth

Peacock Butterfly

So when the baby bluetits have all hatched and they’ve grown their adult plumage, just think how much physics they needed to know to produce it.  Or better still, just marvel at how lucky we are to have such beautiful creatures so commonplace.