Blue birds are exalted throughout the world. From ancient Chinese mythology to modern-day films, blue birds symbolise happiness and good fortune for some, or lost loves and ruined lives to others. So, it’s something of a shock to find out they don’t really exist…
OK, let’s rephrase that a little. There are thousands of bird species who have blue feathers, in the sense that when we look at them, we see that they are blue. This isn’t an existentialist thing, however; they don’t cease to be birds when we look away, but they do cease to be blue birds. It all comes down to light.
Mr. Blue Sky
Cast your minds back to those confusing days of science in school. We are told that the light from the sun that gives us our days is called white light, and this is composed of all of the colours, which we can see when the light is scattered through water droplets in the sky – a rainbow.
It all comes down to light.
White light travels through space in waves, and the colours in that white light have their own unique length to the wave pattern. Red has the longest wavelength, and at the other end of the spectrum, blue has the shortest. When these wavelengths hit an object, they will interact with that object in various ways: they can be reflected back like a mirror, or they can be absorbed by the object, or they can be refracted, scattered and deflected in different directions.
Before sunlight reaches us here on Earth, it has to travel through our atmosphere first and that layer of mostly nitrogen and oxygen particles refracts the white light in all directions. Shorter blue wavelengths are scattered much more than the longer ones by these particles, which is what makes the sky look blue. When we look at a sunrise or sunset, we are viewing that white light at a different angle than during the full day, which gives us the range of reds, oranges and yellows depending on any other particles in between us and the sun. Famously, green flashes lasting a few seconds can be seen but only very rarely as conditions have to be just right – when the light from the sun hits the horizon through clear and stable air, more of the light reaches the observer without being scattered.
The blue wavelengths are reflected back to our eyes, and this is called the Tyndall Effect.
This blue sky colour was first explained by physicist John Tyndall in 1892, who incidentally, during his work on glacier movements, was the first to understand and explain thermal radiation on glaciers, the science behind what we know today as the greenhouse effect.
We know that bird plumage is composed of individual feathers, made of the protein keratin, and those feathers are composed of a single shaft with barbs coming off either side, and then even smaller barbules coming off from those barbs. Pigments that occur naturally in the bird like melanin or are absorbed into the bird’s bloodstream via plant food like carotenoids are deposited into those barbs and barbules, dictating the plumage colours, giving us all of those vibrant reds, flaming oranges, dusky greys or jet blacks. The only known green pigment made in birds is found in turacos, a bird group endemic to sub-Saharan Africa. But what about blue?
Blue rarely occurs naturally. The most common pigment in plants is chlorophyll, which is constructed in such a way that it reflects back the green wavelength of white light. The only known existing blue pigment in the animal world is produced in a handful of butterfly species who live in South America. But there are no true blue pigments in plants: so, birds can’t turn their feathers blue through food.
But they are blue, right? Yes, they are, and no they’re not. There is no blue pigment in the feathers of a kingfisher, or the blue jay, or the fairy wren. But there is air, and there is light.
Somewhere in between
The structure of feathers is the key. Structural colouration is when light hits an object that is made in such a specific way that it only reflects back a certain colour, whilst the rest of the colours are absorbed. If you were to view a cross section of a red feather under a microscope, you would see that the surface is transparent and colourless but the underlying structures are filled with red pigment granules whose composition reflect back the red light. When you look at a blue feather, the surface layer of keratin is there, but there is also an underlying layer of dark melanin granules.
Structural colouration can also result in what’s known as iridescent colours.
The surface layer of the blue feather appears cloudy, unlike the transparency of red feathers. This is because of air cavities in that layer. As a blue feather grows, the keratin molecules stretch like string and separate from water in the cell, like oil from vinegar. When the cell dies, this water dries up leaving only air. The resulting structure is a feather made of keratin protein dappled either with air pockets, like beer foam at the head of a pint, or a network of air channels, like a sponge.
When white light strikes this keratin pattern, the red and yellow wavelengths are absorbed by the underlying melanin but the blue wavelengths are reflected back to our eyes, and this is called the Tyndall Effect. Different shapes, lengths and sizes of these air pockets and keratin make the many different shades of blue we can see in birds.
You can easily see the difference between this structural colouration and pigment colours by performing a simple experiment. If you were to find a blue feather, and a feather of any other colour like red or yellow, grind them up with a pestle and mortar. Because structural colour is entirely dependent upon reflective surfaces, a blue feather becomes dark when it is ground into a powder as it is only keratin and melanin, but the red or yellow feather will stay red or yellow in powder form, as the pigment has not been damaged.
All that glitters
Structural colouration can also result in what’s known as iridescent colours, colours that are dependent on the viewing angle. Examples are the feathers of a hummingbird, a peacock, in magpies and on the head of a male mallard duck. The structures of these feathers are not irregular patterns like the beer foam or the sponge, but are aligned in the same direction, so that when the light hits them the colour is reflected back in only one direction. This means that the bright colour is only perceived at a certain angle – iridescence. The rest of the time we just see the dark of melanin underneath.
We hope you’ve enjoyed today’s science lesson, and next time you see one, remember that blue birds do exist, but only when we look at them.