Colour in the Parrots

Part 2 - The Elements of Colour

The feather of a bird is a wonderfully intricate structure, combining aesthetic delight with utilitarian purpose. The perfect product of predetermined and precisely regulated growth. Each is but one of many; performing its own particular function in protection, flight, and decoration.

A feather is composed mainly of an almost colourless translucent material known as keratin. From the central shaft, or rachis, a large number of barbs branch out on opposite sides to form an almost two dimensional structure. Each barb grows to a length determined by the overall outline of the feather and, in most cases, carries on each side a row of barbules which engage with the barbules on adjacent barbs. The barbules have a hooklike shape so that the series of adjacent rows lock together much in the manner of a zip fastener (a zipper), giving the whole feather a remarkable degree of strength and rigidity.

The barbs are not solid; rather, there is an outer skin known as the cortex which encloses an inner core having a cellular (or honeycomb) structure which we call the cloudy zone or spongy layer. This structure serves both to minimize the weight of the feather and, when called upon, to form the basis of one of the elements (or constituents) of colour in the feather.

Mother Nature, having solved the weight saving problem, must have been delighted that she was so fortuitously presented with further opportunity to display her virtuosity. In fact the feather, superficially so simple, is capable of such manipulation as to endow birds with an almost limitless display of colours, textures, and effects; besides their principal role of bestowing protection and the power of flight.

Feathers grow from follicles in much the same manner as the hairs of mammals. Feather follicles and the surrounding tissue have, however, evolved to perform more specialised functions than hair follicles. Not only are feathers far more complex and varied in their structure than hairs, but also their movement and control mechanisms are of a different order. Mammals are able to raise their hair to provide better insulation or in response to such emotions as fear, aggression, excitement, or sexual display. But, although mammals possess erector muscles within tissue around the follicle in order to achieve this movement, they rely on the natural elasticity of the tissue to return the hair to its normal position. In evolving the power of flight, the skin tissues of birds have of necessity acquired extra mechanisms to control the movement and positioning of feathers in response to the forces encountered in flight. In particular, they have developed depressor muscles with which the feather can be actively held in position against the body rather than relying merely on the elasticity of tissue. (See abstracts from work of Dominique G. Homberger at Louisiana State University, Abstract 1, and with K. N. de Silva Abstract 2) Feather follicles are not haphazardly placed. They are arranged along more or less parallel lines or tracts in specific areas of the body. [These paras. added 22nd April, 2001]

Whiteness

When a feather contains no pigment or other colour element it appears white. This results from the myriad small reflecting surfaces of the barbs, which scatter and reflect back the incident white light which falls on them with little absorption of any wavelengths. When the reflecting surfaces are arranged randomly the resulting whiteness has a matt appearance, whereas larger and/or more regularly arranged surfaces convey an impression that is better described as pearly or silvery.

In this context it is interesting to note that in the Albino budgerigar (a composite variety showing the effects of two distinct colour genes) the cheek flash stands out, with a silvery appearance against the matt white of the rest of the body, because of the relatively large reflecting surfaces of these particular feathers. Similarly, and for a reason we shall appreciate better later in this section, the Albino budgerigar in certain lights shows distinct ‘shadow’ markings where the real markings would be if its colouring were normal.

It is upon this backcloth of white that the elements of colour work their own frequently dazzling brand of magic; not only of colour, but of pattern too. Many feathers, particularly those of the flights and tail, individually show almost abstract patterns of colour which vary just slightly from those of neighbouring feathers. Yet in the outstretched wing or the underside of the fanned tail all these patterns merge together in a bold and striking display. What mysterious processes or intelligence informs each emerging feather of the part it must play in the unfolding drama it can never see?

Introducing colour

When considering colour in warm-blooded animals we generally think of pigments and, in almost all cases, those pigments are the melanins. Traditionally, although there is little basis in biochemistry for such distinction, the melanins have been divided into two categories; the eumelanins which produce black and dark brown colouring, and the phaeomelanins which produce paler brown ranging through to yellowish colouration. In almost all animals melanin in its various forms is the only colouring agent. Birds, in marked contrast, show a much broader and brighter range of colours. It comes as no surprise to find that a wider range of colouring agents, or elements, is required to produce this array of colours.

The two forms of melanin also occur widely in birds. In the parrots, however, only eumelanin is thought to occur and to be responsible for all the black, grey, and brown colours which these birds display. Additionally, and as unlikely as it might seem, they also play a crucial role in producing the greens, blues, and violets, which parrots frequently use to such stunning effect.

The only other pigments occurring commonly in parrots are those responsible for the yellows, oranges, and reds. Although such colours are frequently produced in other birds by the class of pigments known as carotenoids, the parrot yet again confounds us by apparently having its own class of pigments. These have so far not yielded up their secrets to scientists, nor been given a generic scientific name. In the absence of input from this source aviculturalists have coined the word psittacin to describe this class of pigment unique to parrots.

Although only two types of pigment are found in parrots there is a third, and final, element which enables these birds to display virtually every colour of the spectrum and to embellish this display with a variety of optical effects. These effects are known collectively as structural colours and are produced by the very construction of the feather. The most important is the blue colouration, conjured up by the cloudy zone, which, mingling with yellow ground colour, produces the brilliant greens which must have been vital to the survival and eventual proliferation of the early parrots.

The three basic elements (or components) of colour briefly outlined above largely, but not completely, coincide with the three layers of colour considered in the previous section (Describing colouration). We are now in a position to look at these in more detail.

Melanin

The melanins are a class of pigments occuring throughout the animal and other kingdoms. Their exact chemical structures are still not known although most of those found in animals are indolic polymers divided into two types known as eumelanins and phaeomelanins. The classification of melanin into these two broad types relies not on any defining difference in chemical structure but, rather, on the colours they produce; differences in their solubility in acids and alkalis; and the slight inflorescence of phaeomelanin in ultraviolet light. Also, from the point of view of the interested aviculturalist, what can be learned from the way they are affected by quite a large number of different genes; sometimes in tandem, sometimes separately. Whilst many bird genera, like mammals, synthesize both eumelanins and phaeomelanins this is not so in the parrots which as remarked on in a previous paragraph are generally recognised to produce only eumelanin.

Where eumelanin is present it takes the form of granules, roughly oval in shape, and large enough to be seen quite readily under the microscope. The concentrations of melanin granules present, their size, and their colour, together with any accompanying colour elements, determine the patterns and colours we see and admire in the bird before us. Various mutant colour genes affect these properties in different ways, besides sometimes altering their distribution, to produce the many novel colour varieties we seek to cultivate.

The main characteristics of eumelanin distribution are readily appreciated by visualising the Grey colour variety of the budgerigar, which has lost the ability to show either blue or yellow (and hence green) colouration. The black areas (known as the markings) have the same distribution as in the normal light green, or wild-type, and are caused by heavy melanin deposits in the cortex of the feathers. The lighter grey areas, which largely coincide with the body colour area (green in the wild-type), are caused by lighter deposits of melanin within the barbs of the feathers in the central region known as the medulla.

I have long called these two types of deposits foreground melanin and background melanin respectively, as an aid to describing the effects of budgerigar colour genes. This is not to imply that the two types are necessarily different either chemically or physically. The description background melanin derives from the fact that it is not directly visible (as grey) in most varieties and yet forms the essential background against which the blue colour (green when modified by the presence of yellow) produced by the action of the cloudy layer is displayed.

It has also been found that in some budgerigar colour varieties the relative concentrations of foreground and background melanin vary independently of each other, which further reinforces the decision to treat them as sub-elements which can be subject to separate genetic control. This would not be obvious in those species which do not carry prominent black markings

Psittacin pigment

Brief placeholder: Psittacin is the name given to the yellow, orange, and red pigments found in parrot plumage. It has long been known that these pigments are not the same as the similarly coloured carotenoids (lipochromes) found in, for instance, the passerines. Where present, these pigments are finely diffused in the keratin forming the cortex of the feather barbs. The depth of colour depends upon their concentration and will vary in different parts of the bird. They can also be diluted or eliminated altogether by different mutant genes. Generally, this produces a blue or parblue colour morph or variety. Recent scientific research has partially identified the pigments, which are found to belong to the group known as polyenals [see Research Paper - PDF].

Structural colour

Brief placeholder: Structural colour is produced, as the name suggests, by the very structure of the feather barbs. The structure is often known by the names 'cloudy layer' or 'spongy zone' and lies within the outer layer called the cortex. Usually the colour produced is a lightish blue (skyblue) but different mutant genes modify the structure to produce different colours within the blue to violet range. For these colours to be seen visually, there must be an underlying layer of background melanin to absorb all the other colours of the spectrum and only allow the reflection of the blues and violets. This is why, when the structural element is completely disrupted by other mutant genes, a greygreen or grey bird is produced. Until recently structural colour was thought to be due to the 'Tyndal effect' but, again, recent research suggests instead that it is due to 'constructive interference'.

To produce the green of the typical parrot all three elements of colour must be present - background melanin, the spongy zone (structural), and psittacin. Overlying this there may be foreground melanin which may deepen the colour (typically in the flights and tail) and/or form patterns or 'markings' as in the budgerigar, rosellas, etc. If any element is absent or modified, either naturally or due to mutant genes, other colours will be seen in those areas affected.

Each of the three elements of colour (treating background and foreground melanin as one) are produced by quite distinct and separate biological or biochemical pathways. And each of these pathways is under separate and distinct genetic control. Each mutant colour gene therefore (although there are certain exceptions) affects only one of the three elements of colour. (The Opaline is one such exception.)

Copyright: Clive Hesford, January 1998 - 2002

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