Fascinating Feathers – looking at the birds in our own back gardens

Structural colouration – the orange on the chest of the kingfisher is due to a pigment, but the colours on the back and tail are due to optical effects (credit: Timo Schlüter from Pixabay)

Why Dorset birds may not be the colour you think

Science feature by Professor Glenn Patrick

OVER the last weekend of January, the 42nd Big Garden Birdwatch took place. This is an annual citizen science project organised by the RSPB when we are asked to record the birds visiting our gardens in one hour.

I have to say, I am usually a bit disappointed with my results as the wildlife always seems to suddenly desert my garden during my chosen hour.

No matter, even for a novice like me, Dorset offers a wonderful array of birdlife to observe through the rest of the year – from large birds of prey down to the tiny goldcrest.

Living by both coast and countryside and with the Seaton Wetlands on our doorstep, there is even the opportunity to spot rare species migrating from distant shores.

One of the striking features about our local birds is the variety of colours on display, ranging from red-breasted robins to the vibrant multi-colours of the kingfisher. This raises the question of how are the colours made and why do they vary so much?

The shades and tones that birds and animals display on their fur, skin or feathers are often due to the food that they eat and the pigments they contain.

For example, flamingos get their pink colour from eating shrimps, which contain a pink pigment called canthaxanthin. Zoos even stop flamingos from turning grey in captivity by adding a synthetic version of the pigment to their diet.

Other birds and animals can produce shades of brown, black, or grey using their own pigments called melanin. This is the way golden eagles and buzzards achieve their colouration.

The lobsters found in Lyme Bay are naturally a black/dark blue or sometimes a muddy brown colour, but after cooking – hey presto – they turn to that orange/red colour when they arrive on our plate.

This is due to a reddish pigment called astaxanthin, which is bound to a protein called crustacyanin inside the lobster giving it that dark colour. When a lobster is cooked, the astaxanthin is released from the protein and the crustacean magically displays an orange colour.

Blue is my favourite colour and it seems I am not alone as shades of blue often appear at the top of polls around the planet. Yet, the animal kingdom seems to struggle to produce the colour blue.

Whereas pigments explain many bird colourations, nature lacks a pure blue pigment. This means that plants must undergo some gymnastics by mixing other pigments – a bit like mixing paint – to achieve blue leaves.

Birds though must rely on some physics wizardry if they wish to display a blue plumage to the world.

Kingfishers are, of course, known for their striking plumage. If you are lucky, you may see one near the River Lim – a vibrant flash of blue, orange, and cyan as they hunt for food.

The orange feathers on the chest contain small pigment granules. These pigments absorb the short blue wavelengths of light and scatter the longer red wavelengths and explain the orange colour of the chest feathers.

However, what we perceive as blue and cyan plumage on the back and tail of the bird is really brown! How can this be? Well, it is all due to an effect called structural colouration which was first observed by the physicists Isaac Newton and Robert Hooke in the 17th century.

They were particularly interested in the striking colours of peacocks and the various shades and tones that appeared depending on the viewing position and angle.

Electron microscopes have revealed that, although the cyan and blue barbs of kingfisher feathers contain no pigment, they do contain spongy nanostructures. Light waves scatter off these tiny internal structures and the colours that we observe depend on how these light waves interfere.

The structure of the feathers forms a diffraction grating. It is like the way a rainbow pattern can be seen flashing from the grooves of a CD or DVD.

Take a close look at a starling and you will see a variety of iridescent colours amongst the black plumage (photo by Glenn Patrick)

Starlings might seem to be ordinary birds as we watch them strutting about our gardens or congregating in an evening murmuration. It may surprise you to discover that there are 113 species around the world – some of them very colourful.

From a distance, the common starling appears black, but take a closer look and notice how its feathers can shimmer with different colours depending on the angle of the light.

This is an example of iridescence which means that the surface appears to shift in colour when seen from different angles. It is the layered structure of the feathers – not a pigment – which causes iridescence.

Iridescence is also observed in the beautiful tail feathers of peacocks. When a peacock is wet its natural colour is brown! However, when the bird is dry and fans its tail, we get that display of multi-coloured patterns in the form of “eyes”.

These are caused by a complex structure of indentations in the keratin forming the barbs and fibres of the feathers. When light is reflected from these periodic structures, we see a variety of optical effects due to the interference of light waves from different layers of feathers.

The emitted colours shift and shimmer with viewing angle – just like we observe in soap bubbles or oil spills.

blue tit
The yellow on a blue tit is due to a pigment but the blue crown, wing and tail is caused by structural colouration (photo by Glenn Patrick)

At this time of year, blue tits add a splash of colour to our gardens as they start to build their nests. Their yellow breast and flank are caused by the carotenoid pigment acquired through their food.

Their bright blue features – including the distinctive crown – are due to light scattering off tiny tubes in the keratin making up the feathers. The crown even glows with ultraviolet (UV) light – which the birds can see, but human vision cannot detect.

It is thought that the UV brightness of the crown plays a part in finding a mate – after all spring is in the air!

Glenn Patrick is a particle physicist and science communicator who explores the quantum world of sub-atomic particles (including at the Large Hadron Collider) and now lives in Lyme Regis.

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