12 minute read
Nature
A pair of Scarlet Macaws flying in open woodland. You can see how "flying rainbows" is an apt description!
A Flamingo with plumage rich in carotenoid-derived pink pigment. Very much in good breeding condition! A close view of a male Peacock in full display. Most of his colours, including the blue of his head and neck, are produced by diffraction of light
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Flying
THAT WAS WHAT MY OLD FRIEND DON RISDEN USED TO CALL THE MACAWS THAT REGULARLY FLEW OVER HIS BIRD-GARDEN IN SOMERSET
By Mike George
Mike George is our regular contributor on wildlife and the countryside in France. He is a geologist and naturalist, living in the Jurassic area of the Charente
And startlingly colourful they were. But why should birds – or any animals, come to that – go to the trouble of adding colour to their bodies? For be assured, there must be a reason for it. Colour – any colour – represents an expenditure of valuable energy from the creature’s metabolic quota, and therefore must confer an advantage. What can that be? The natural colour of a bird’s feather is white, and indeed some birds are content to be white and seem able to exist in that state. However, the most common colour we see in birds and indeed most animals is brown. Brown in nearly every instance is provided by a substance called melanin. This is a pigment that occurs in almost all animals to a greater or lesser extent. Humans are particularly aware of its effects. There are several types of melanin, but the two that concern humans the most are eumelanin, which is dark brown to black, and pheomelanin, which imparts yellowish or reddish colours to tissue and hair.
Ebony and ivory ...
Eumelanin is the pigment that makes black people black and whose near-absence makes white people white. In fact, the concentration of melanin in human skin is increased by exposure to ultraviolet tight, which accounts for suntan in whiteskinned races (although black skin tans, too). It is also melanin that gives a brown colour to the iris of the eye – its absence allows blue and grey irises to show those colours. Why and how the split between black and white-skinned humans came about is still something of a mystery. The presence of melanin is generally considered to be a protective mechanism to reduce damage to the exposed tissue, but it also prevents ultraviolet light from producing vital Vitamin D in our bodies, which is the only way humans have of making that vitamin in their own bodies. Of course, in humans, as in other creatures, the concentration and distribution of eumelanin produces varying shades from light brown to black. However, apes (our nearest animal relatives) have light-coloured skin under their abundant hair, so the melanin content of human skin seems to be linked to our abandonment of hair. Eumelanin also accounts for hair-colour in such hair as humans have left. Very low melanin concentration gives blonde hair, while brunettes are more liberally supplied with the pigment. However, the production of melanin in hair decreases with age, which is why we turn grey as we reach later life. Redheads owe their colour to pheomelanin.
Why are there so many little brown birds?
As far as birds are concerned, there seems little point in patterning the feathers with brown, but in fact there are two good evolutionary reasons to do it. One is concealment. A white bird is very visible if it is in most land environments. Those birds that are predominantly white tend to be waterbirds, high-flying birds and birds that live in snow. The rest, those that live in forests, open country, rocky and stony places etc., tend to go for brown to reduce their contrast with their surroundings. Or at least, the females do. It is noticeable that bright colours are in many cases the preserve of the male bird. The females tend to be surprisingly low-key in their choice of colour. The other advantage that melanin confers is strength. A feather coloured dark brown or black by melanin tends to be stronger than a non-coloured feather, and also some 40% more resistant to abrasion. This may well be why, whatever the colour of the rest of the bird, the flight-feathers are often dark brown or black, since they take most stress and move through the dusty, abrasive air most rapidly. Melanin in feathers is apparently nothing new. Of the fossil feathers that are coming out of the fossil-beds of China, which in some cases preserve specimens in incredible detail, scanning microscope studies have revealed traces of structures that look like melanosomes (the structures in cells that produce melanin), and even of melanin granules, in feathers that were attached to birds as far back as the Jurassic period 180 million years ago. The thoughtprovoking thing is that there is evidence that some dinosaurs may have had feathers, and indeed some of the feathers studied may have come from dinosaurs. Does this have implications for their appearance?
Surely there are other colours as well?
Certainly there are. In fact, melanin is by no means the only pigment that can be added to feathers and hair. The class of pigments called carotenoids impart red, pink and yellow colours. Carotenoids are only produced by plants; no animal can make them, but must get them by eating plants (berries, seeds, leaves etc.) or by eating animals that have themselves fed on plants. For example, certain shrimps that feed on carotenoidcontaining algae are in turn eaten by Flamingos and Roseate Spoonbills. These birds transfer the pigment into their feathers. It has been found that parenthood will deprive a female Flamingo of its pink pigment (probably drained as a source of energy) and the Flamingo cannot breed again until she has regained her pink colour, which requires about a year. Male Flamingos are not attracted to white females! Two other groups of pigments are met with in birds: porphyrins and psittacins. These are predominantly red and yellow. Porphyrins occur mainly in the Turaco birds of Africa, whose startling red underwing patches flash through the deepest jungle when the bird takes flight. They are also the only dye family that can produce a green colouration. Psittacins, as the name suggests, are mainly found in the parrot family, and among other things are responsible for the astounding red of the Scarlet Macaw. All of the above are pigments that colour the feather or hair like a dye. Dyes work by
A Broad-billed Hummingbird displays his iridescent breast-feathers.
This Robin looks adorable to us, but he is actually in threatening mode. His red breast is saying, "I'm fully grown up. Come on if you think you're hard enough!"
The Magnificent Bird of Paradise (Diphyllodes magnificus) of Papua New Guinea. This male is fully displaying his iridescent blue tail plumes.
A male Pheasant in full breeding plumage displays before a rather less colourful (but better camouflaged) female.
The startling blue panels on the European Magpie's wing owe their colour to light interference
Sparrows rely on their pigment colouring for camouflage (using counter-shading - dark above, pale below) and to strengthen their plumage
This Australian Rainbow Lorikeet is replenishing his supply of red and yellow pigment by eating flower buds containing carotenoids
absorbing certain wavelengths of light and reflecting others, thereby colouring the feather. The alert among you will have noticed something missing. I have not mentioned blue. That is because there is no dye available that can be metabolised to produce a blue colour in either a bird or an animal. “What?”, I hear you cry. “There is blue everywhere in the bird world. The Magpie’s wing is blue, so is the Jay’s. Even the little top-knot on the Blue-tit that pinches my milk is blue. And then there’s the Kingfisher. You’re having a laugh!”
Structural colour
The thing about blue, which I agree is abundant in the bird world, is that it is produced by a totally different process to the other colours. Pigments absorb all but a few wavelengths of light, which they reflect. Structural colours destroy the wavelengths they do not want and refract out only a single wavelength, though that may vary. The effect is produced by structures in the feathers that are on a size-scale that is hard to imagine. They are of the order of the size of the wavelength of light itself, and are measured in nanometres (ηm), which are one thousand millionth of a metre (10-⁹ m). They cannot be seen, but their effects can. At that size-scale they can interfere with the light, causing one wavelength to be reflected in phase and thus strengthened, while the other wavelengths interfere with each other and are destroyed. They do this either by having layers of different refractive power in a sandwich which acts like an oil-layer on water reflecting colour (layer-colour), or they can be tiny particles of different optical power arranged in patterns that scatter the wavelengths of light, rather like an opal does (scatter-colour). These structural colours are recognisable as they become more intense the more strongly the light shines. In a dark environment like a jungle, structural colours tend to be muted, but pigmented colours can still be seen clearly. For this reason, the Birds of Paradise of the islands around Borneo and New Guinea have to find sunlit spots to perform their displays, so that their colours, most of which are structural, may be seen to best advantage. Indeed, so finely-tuned are the colourzones on the plumage of these birds that some can direct the colours towards their paramour and away from prying eyes elsewhere. By careful arrangement, many different colours can be produced by the structural technique (as in any good opal) but for the most part, they are used to replace the missing blue – and, of course, green, which is made by shining the blue structural colour through a yellow pigment –probably the most complex way of producing a colour in the animal kingdom! This even works in humans. If the iris of your eye is blue, that is because it is unpigmented and a structural effect is producing blue. Other iris colours owe their presence to melanin, swamping or altering the blue colour Insects tend to use these structural colours to good effect. The tiny blue butterflies that charm us throughout the summer months owe their colour to structures in their scales, as do the great Blue Morpho butterflies of South America. The iridescent green of the Rose Chafer beetle is due to a yellow dye colour through which shines a blue structural colour. In fact, there have been instances of fossil beetles found that have retained their iridescence over millions of years.
What about Woad?
Strangely, as far as is known no vertebrate creature has a pigmented blue colouration. Where it seems to appear it is usually in fact a form of grey contrasting with another colour. The Ancient Britons are reputed to have coloured themselves with a preparation of the plant Woad (Isatis tinctoria) which started off yellow-green and oxidised to indigo-blue on exposure to the air. No less an authority than Julius Caesar mentions this in his account of the invasion of Britain in 55 BC. The woad dye is a strong anti-inflammatory, and may have been intended as a protection against the pain and infection of wounds. These days a blue colouration is a serious dangersignal, as it indicates that oxygen is not reaching the blueing tissue.
But what is all this colour for?
As we can see, the energy expenditure in producing colour can be considerable, and in birds, generally speaking it is the male who makes the greater expenditure. Where striking colour and display are concerned the idea is simply to prove to one or more females that he is the strongest and, genetically, the best choice. His colours are best, his stamina is high and he will produce the best chicks. After mating, his job is often finished, so if his colourful display leads to his falling victim to a predator, it is not a loss as far as his continued line is concerned. The female, who is to raise the brood, is protected by her relative drabness. Where the two parents are involved in raising the brood, and the absence of either will be detrimental, both tend to be lower-key in appearance. Sometimes the colours are a warning. The red breast on Robins, coloured by carotenoids derived from berries, marks them out as aggressive defenders of their rights and territory. The size of the red patch increases with age, showing newcomers that this is a successful and an experienced old-stager. Among invertebrates, bright warning colours can indicate a species that is toxic or otherwise harmful. Once a predator has learned the reason for the warning, he will avoid the prey. One clever trick that other less harmful species can try is to mimic the warning in the hope of borrowing the safety. But of course, colour is only of use to you if you and your fellow-creatures can see it. Birds, and many invertebrates, have a very strong colour-vision – in many cases covering an even greater range of wavelengths than can our eyes – so colour is very important to them for sending messages, creating confusion, defending themselves and so forth. Mammals, generally speaking, have poorer colourvision, so colour is less important to them. Humans and some apes have evolved good colour vision, possibly because it proved an advantage in recognising ripe fruit among the leaves, and this enables us to enjoy, as a bonus, the colours used by other creatures. This is a complicated subject, and I have only skimmed the surface. Much is not fully understood, and a great number of questions remain unanswered. A lot of good work is done by amateur observation of behaviour. So enjoy the beauty, and keep your eyes peeled.
