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.
