In the wild, not all colouration is as it appears to the eye, says Paul Donavan. Here he explains the science behind our colourful domestic chicken breeds.
As we know, not all chickens are brown. The red jungle fowl Gallus gallus from which the domestic chicken originated from, is a dull reddish brown colour; males marginally more colourful. However, the whites, blacks, blues, silvers and so forth that we see in the domesticated ornamental varieties which so many people keep nowadays, have been selectively bred over the course of the chicken’s domestication to attain those colours. Many of these colours, of course, have been carefully bred to enhance that particular breed, and may often be accompanied by expressive amounts of plumage, particularly in the tail, as can be seen in the Red Pyle or Black breasted red, for instance.
The environment for which the chicken was originally domesticated from, the jungle, placed constraints upon the colouration of the bird which helped protect it from predators. If we were to release many of the colour variations which we have bred into chickens since we began to domesticate it, back into the wild, they would not survive for very long before being eaten by predators. I will touch on this again!.
Many of the fancy breeds of chickens which we keep, are as far removed from their original forerunners as they could possibly be. With selective breeding, we have manipulated into those breeds certain traits which have now become atypical of that species. Examples being enhanced tail plumage, colouration, and to some extent, size. These traits have now become Poultry Association standards around the world, by which the breed is judged.
The interesting thing here, is that as we look at a chicken, the colours we see with our eyes may not necessarily be the representative colour of the feathers themselves. Just because one of the fancy breeds is exhibiting bright plumage that we take delight in admiring, it does not necessarily mean that the feathers themselves are red, white, brown, black, or whatever. But why not?
To try and explain it simply, the colouration shown by any birds’ feathers, is created in one of two ways; through pigmentation or keratin. Within pigmentation, there are three key factors which influence the feathers’ colour.
One is melanin, which is responsible for the dark colours we see such as black or brown. The second are carotenoids which are responsible for the vivid colours we see such as yellows, reds and oranges etc. Finally we have Porphyrins. These are responsible for all the other remaining colours. Let us briefly look at these pigmentations in a bit more depth to try and understand them a bit more.
Most dark feather pigmentation is derived from melanin which is synthesised through the oxidation of the amino acid tyrosine, and is the most common pigmentation in the animal world. Melanin is a biological polymer and arises from several forms, the two most common of which are:
1) Eumelanin. This is the most widespread type as it forms the foundation for black, brown and grey pigmentation.
2) Phaeomelanin. This pigment is what gives rise to a reddish or beige tinge.
Although there may be defined differences in terms of colour variation between the two, both eumelanin and phaeomelanin are usually present in dark feathers. Feathers containing melanin pigmentation are much more abrasive resistant than those without it.
Carotenoids were first discovered in carrots in 1831 (orange coloured fruits and vegetables have the highest carotenoid levels), from which a compound called ‘beta-carotene’ was isolated. These are naturally occurring fat-soluble pigments present in both animals and plants, and have important physiological functions (certainly in animals) as they act as antioxidants. Carotenoids and are found in various regions of the body, notably the skin, liver, fat, the egg yolk, and of course the feathers. Some carotenoids, such as beta-carotene, are termed pro-vitamin A carotenoids, as they can be converted into vitamin A.
Birds are unable to synthesise carotenoids, and so obtain it from their diet, either by eating a plant/food source rich in them, such as red berries, or something which has eaten it, such as an insect.
Carotenoids can show the heath of the bird, as a vibrant red, yellow or orange colour will indicate a healthy bird that has been able to locate good sources of carotenoids. In stressed birds, or those where carotenoids have been lacking in the diet, reduced vibrancy in pigmentation is caused by the withdrawal of carotenoids from the bloodstream which manifest in paler pigmentation.
Some recent research into feather pigmentation suggested that the conversion of carotenoid pigments occurs directly at the follicle level during the feathers growth, with synthesis taking place in the liver and not the skin as was first thought, where it then travels to the marginal tissue via the bloodstream.
Porphyrins are a class of water-soluble nitrogenous, biological pigments produced by modifying amino acids. Although the precise chemical structure of each porphyrin differs, they do share a common peculiarity, in that they fluoresce red when subjected to ultraviolet light. Porphyrins manufacture a range of colours, the most common of which is brown, followed by pinks and reds. As many birds are able to see ultraviolet light, they will perceive these colours differently to the way we see them. This may be an important mechanism for species recognition.
A further key player in pigmentation is keratin. Keratin is a strong protein – it is what the bird’s claws, leg scales and beak are made from, as well as the feathers. Keratin is actually an opaque protein which reflects light at a particular wavelength. Therefore, feather colouration not made-up from pigmentation such as blues and greens, actually comes about as a result of light reflecting off this layer of keratin.
As the light hits the feather, the keratin layer reflects the short wavelengths of light, such as blues, greens and violets. The longer wavelength is absorbed by the darker underlying layer of melanin; melanin is present in the feathers as tiny granules. As a result, the corresponding blues or greens we see, are not the feather colours themselves but, coloured light refraction. In other words, green plumage arises through yellow pigmentation and light refraction. Conversely, dull olive- greens will have an element of melanin involved as well. As with the combination of yellow, it will absorb some of the refracted light.
This may sound a rather easy explanation, but in fact the different effects keratin produces, are determined by its structure. For example, where keratin is made-up with pockets of air spaces, the blues and greens are produced by what is termed light ‘scattering’. Conversely, where it is formed from a series of layers, blue, green and metallic are caused by ‘iridescence’.
The tail plumage of a Cream light brown is a good example of this. In the right light, the feathers glisten with a shimmering blue/purple sheen. Of course, a combination of light refraction from the keratin, coupled with pigmentation, can also contribute to the feathers’ colours. An example being, a Fawn Silver Duckwing
or Lavender, gets its colour as a result of a blend of white light ‘scattering’, and dark melanin pigmentation in the feather.
Specific pigmentation can also contribute to the feathers’ strength depending on where the feather is on the body. Dark melanin gives the feathers greater strength and is particularly common in wing feathers. It not
only reduces their ‘wear-rate’ quite considerably, but also allows them to better cope with the continual rubbing together. Conversely, white feathers are less robust, and therefore have a higher wear rate. You may be asking then “what if the feather is both black and white?”. The same holds true here; the white proportion will have a higher wear rate than that of the black.
So, it is with the amalgamation of all these different types of pigmentation that results in the colours our birds’ display. There are of course throw backs with pigmentation, in which abnormalities are all too common. By this I mean, a bird which does not exhibit its ‘normal’ range of plumage, but something quite different. This is widely seen in reptiles in which colour ‘morphs’ are considered more desirable than their atypical colouration. Or, the occasional throwback is hatched which deviates from the norm. And this also holds true with birds. Some typical ‘anomalies’ can include;
Albinism; Typically, albinism is the total lack of any pigmentation in the feathers, skin and eyes giving the bird a snowy-white appearance. True albinos have complete lack of pigmentation in the eyes, exhibiting pink eyes, pink skin and pink beaks.
A pure white bird, but without the pink eyes is not a true albino. Generally, in the wild, albino birds would not survive for very long, as they become more conspicuous to predators. In the jungle, a brown bird would be well camouflaged. If that bird was white, it would draw attention to itself. Also, the loss of eye pigmentation makes the eyes much more sensitive to light, and therefore can affect the individuals’ vision. As a consequence, the individual may completely alter its behaviour.
Eye colouration in animals is dictated by the amount of melanin present. Those with black or brown eyes have higher amounts of melanin that those with lighter coloured eyes such as blue. Albinos are totally lacking in melanin.
Albino birds may have also lost some of the insulation properties of the feathers, and in cold climates, such birds would suffer more from the cold, than a darker bird would.
Leucism; This can often be confused for albinism, as the feathers take on a washed-out appearance, but not quite to the extent of appearing pure white. Also, it may not cover the entire body, and only be evident in patches; such individuals are often referred to as being pied, or piebald. Such birds retain normal eye and leg colouration, and do not develop the pink as seen in true albinos. Leucism is not such an extreme colour variation as witnessed in albinism, and such individuals tend not to be as vulnerable as albinos. However, as with all white birds, the feathers are less robust and wear out more quickly.
One other aspect with leucistic birds, is that colour plays an important role in courtship, and any deviation from what the ‘normal’ colour is, may impact on that bird actually being able to find a mate.
Melanism; Classically, an excess of brown or black pigmentation in the feathers results in the bird taking on a more uniform dark colouration. The eyes are their natural colour, though sometimes the beak and legs can also be black. In the wild, such individuals would have the same survival rate as the atypically coloured individuals in terms of predation, though may suffer from heat related problems, as black absorbs heat, whereas a lighter colours reflect it.
STATE OF HEALTH
The state of the feathers’ colours can indicate the health of a bird. Vibrant colours, and the bird is showing good health. Whereas feathers that suddenly become dull and listless looking, can often be an indication of a metabolic problem. An example being where white feathers turn yellow. This is usually a sign of liver disease caused through fatty liver syndrome (Hepatic lipidosis). Poor diet, or nutritional changes can also influence changes in feather colouration, as can some viruses. Any changes in the colour of the feathers, whether over the long term or short term, should be checked out by a vet.
Of course, by selectively breeding birds with very similar colour traits over the course of several generations, we can breed into these birds that particular colour attribute. As times goes on, that colour will then become the ‘normal’ for that bird, and its original colour will become regressive.
Nature is a wonderful thing, and it is interesting how ‘tricks of light’ can play on our perception of what we think we see, as opposed to what we actually do see. So, the next time you sit there looking at your chicken, just deliberate over the complicated way in which its colours are brought about.
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