Birds and their coloured feathers

The colours observed in bird feathers is made up of keratin, carotenoids, and melanin or a combination of the three.

feather Keratin serves a structural function in the construction of feathers. The keratin differs in implantation, thickness and stiffness of the rachis and barbs, symmetry and curvature of vanes. This structural design plays an important role when the bird combines colour and pattern to produces different signals.

Carotenoids are the pigments responsible for most bright reds, oranges, and yellows. Carotenoid cannot be synthesized by animals, and must be included in the diet, and are often limited in nature. Colours and ornaments that rely on carotenoids are costly to produce. Carotenoid is ingested from the diet at the time the bird is moulting and the colour gets deposited in the feathers.

Melanin pigments are deposited as black or dark brown granules within the epidermal cells. They create colour effects in combination with other pigments. Melanin adds strength and abrasion-resistance to keratins. Melanin is also a predictor of dominance, hormonal balance, foraging abilities, and provides protection from ectoparasites.

The gene responsible for melanin has been identified down to a single locus. This gene is highly variable for different birds and produce a colour ranging from the black, grey, brown to reddish brown and the yellow in chicken chicks.

The persistence of colour variations  in birds from generation to generation shows that feather colour is an inherited trait. Apart from the genetics, the amount of light variation in the environment also controls the variation observed in bird feather colour.

The amount of light in the environment is affected by the environmental features, for example, a forest with trees would exaggerate shadows when contrasted to a treeless environment. Furthermore, the light spectrum changes with increasing altitude.

Bird plumage can be classified into three groups according to the reflecting properties of the light as would be seen by the birds. Type A feathers have strong reflecting properties while type B has no reflective properties; and Type A/B only gives a weak reflection. Melanin colours ranging from pale yellow, chestnut, and brown and, black does not reflect much light in the ultra-violet (UV) range.

 The patterns observed on feathers are created by the scattering of light by ordered layers of melanin granules within the keratin of the barbs. The birds are capable of either repeating exactly from one set of feathers to the next or varying to produce apparent differences between sexes, age group and populations.

The green colour in parrots and other green birds is rarely acquired through the diet, but is a combination of yellow and blue. If the yellow is a Type A then the reflectance will be high, but if the yellow contains melanin then the green will result in Type A/B. CSC_0828

The red coloured feather is most times type A/B.  The type A/B red is mostly observed in birds that inhabit areas with more precipitation where red radiation is weak and allows the bird to blend into a dark background making the bird less conspicuous in the dense canopy of the rainforest.

Many bird species that are involved in predator-prey relationships exhibit colour polymorphism. These birds tend to move continuously, have a generalist habitat requirement and a widespread but patchy distribution. The Owl is a good example.

The adaptative function of melanin type B plumage on the cuckoo brood parasites is related to its life style of stealth.  The dull plumage has no reflectance and therefore is cryptic and conceals the presence of the parasitic bird near host nests. Furthermore, the arrangement of the bars on the feather which is arranged in the broken fashion enables the bird to remain cryptic in the undergrowth of the woodlands and mangrove bush while it searches for possible host.

The Bird of Paradise feathers seem to be the attraction for mate choice. A male with bright colour signals good nutritional condition and could be related to his ability to forage for the high quality food. As a result, bright colours in the male bird have been associated with the ability to provision, disease resistance, dominance and the honest advertisement of sexual prowess. The extra feathers on the Bird of Paradise does not have any aerodynamic significance but developed as a result of selection by females. The choosy female achieves the highest fitness by mating with an individual displaying the feature that signals the best quality.  However such trait often impairs foraging success and increases predation risk, so that only high quality individuals can afford such a trait and only high quality males are chosen by the female.

When human beings begin to understand the importance of feather color to birds, we shall begin to see the implication of our actions of selectively harvesting beautiful feathered birds, or removing bird habitat through logging. Or even the detrimental effects of keeping bids in aviaries as an excuse for conservation.

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