- feather growth, barbs and barbules, blue structure and psittacine pigments-
Feathers are wonderful instruments. Feathers have several functions. They protect the body against injuries, rain, sun, they help to maintain the optimum body temperature, also when the external temperature changes. Last but not least they make flying possible. The quality of the plumage is very important. Bird-fanciers have to control the developing of the feathers. And they are responsible for the conditions like nourishment, a clean and well ventilated nest at a proper place. There is a lot to learn about the development process.
Feathers grow out of the skin. At an early stage, after six days of development of the embryo, follicles are formed, little increases in the skin. They are laying in rows. At the base of these follicles, a little papilla is formed, like a cone. From every follicle one feather is growing. The base is very important because here is decided what kind of feather will develop. This is proven by experiment. Transplantation of little pieces of skin from a white chick, to the skin of chicks of fowls with various colours, produced pure white feathers in every case. This means that heredity plays a dominant role in controlling feather qualities, like form, function, colour. The place where this is happening is the base of the follicle.
A global sketch can give an impression: You see a part of the skin with one follicle, the increase in the skin. Here a feather grows in the form of a cone. Such a cone has three layers:
1. Exterior layer. A very thin layer of cells. When the growth of the feather is completed this horny grey layer burst and the rolled up feather comes out. This is happening in the nest. The tail feathers are short. Afterwards they grow further. Sometimes the feather does not come out of this layer .
2. Middle layer. This much thicker layer starts as a kind of round collar, from where the feather shaft and the feather barbs are growing. And from this barbs the barbules and hooks are growing. In this collar the production cells of the melanin are working.
3. Internal layer. In the kernel of the papilla we see the little blood vessels, which are feeding the growing feather. The blood cells bring the colour pigments on the right time, in the right amount, on the exact place. It is a miracle of coordination and timing of the distribution and the formation processes. This delicate inner part is protected by a thin internal layer.
4. The zone of differentiation. There is a lot of difference between feathers, in colour, colour pattern, form and function of the feather. Which kind of feather is growing out of the papilla is decided in this zone of differentiation. This zone lays directly under the collar from which the feathers grow. In this zone of differentiation the genetic code is active. This code is specific for every sort of birds and every colour variety.
When the feather is ready the process stops. The remnants in the inner layer are absorbed in the blood. In every feather there is a hole in the shaft, the entrance for the blood vessels. The down part of the shaft is hollow and not coloured. The upper part of the shaft is darkened by the eumelanin. The shaft, barbs, barbules and hooks become hard, horny material, under influence of an albumen, named keratin. Then the external layer burst and the feather unrolls like a flag. Form and function cannot be changed any more. Only wearing off takes place. The follicle becomes inactive till the time of moult. The old feather is shed and a new feather develops.
From every follicle a feather is growing. Follicles are arranged in rows. Feathers that are growing out of the follicles are arranged in feather tracts. We see the developing feathers when we observe the growing young in the nest. All feathers grow in the direction of the tail.
The greater contour feathers we find in the wing and the tail. The smaller feathers are contour feathers we find in the belly and breast and the back. The smaller contour feathers are the coverts, they protect the quill of the larger contour feathers and are laying in rows like tiles on a roof. Together they form the surface of the plumage.
The picture shows a part of a feather of the tail, a contour feather. 1. tiny opening 2. quill ( first hollow part of the shaft) with a white fluffy part, 3. barbs, 4 vane, with a blue outer web and a brown inner web, 5. central shaft.
The feather has two vanes. In this picture we see a feather of the Bourke. One vane is blue, one van is brown. The vanes are build up from barbs, laying parallel (3). Barbules and hooks we see only with the microscope.
A closer look to the barbs and barbules we see in this design. The shaft (1), how the barbs are fixed on the shaft(2). The barbules (3) and the hooks (4) They are forming a very flexible but strong structure.
The barbs of the feather can have a specific micro structure. This micro structure is very interesting, because it forms a keratin structure, that gives strengths but also a contribution to the colour. In the first half of the twentieth century a lot of research is done about micro structure. One of the researchers was Hans Steiner (1935), a Swiss biologist and bird fancier. He studied the belly-. back-, and rump feathers of full grown budgerigars in different colour varieties, green, yellow and blue. He used an electron microscope. His detailed report is very interesting to read.
The origin of the green colour of the budgerigar was already discovered in 1914. A feather was prepared with spirit or paraffin wax. The prepared feather was cut in slices of three micron. Such a cross-section can be studied, sketched or photographed. Steiner used this method studying the budgerigar. He compared the micro structure of different colour varieties of the budgerigar, green (wildtype), olive, yellow and blue variety of the budgerigar. This fundamental work learned us a lot about the structure of barbs of different coloured feathers.
Beckmann, member of the Technical Committee of the N.B.v.V, studied the micro feather structure of the Neophema and the Bourke (1973). He also made cross sections of barbs. He distinguished two types of barbs: barbs of the common type (barbs without a micro blue structure) and barbs of the structural type ( barbs with a micro blue structure). Most of the feathers of the Bourke have barbs of the common type. This feathers are brown. I copied some of his designs to give you an impression.
Cross sections of barbs
The difference between the common type and the structural type types is very clear. In the barbs of the common type, the eumelanin granules are distributed randomly. In the blue barbs of the structural type, there is a keratin layer with very fine tubes under the cortex, named the sponge zone. The eumelanin granules are grouped around the cells in the middle.
The sketch shows four cross sections of barbs, from the left to the right:
1. A barb of the common type of the Bourke with a reduced amount of eumelanin granules. It is a barb of a light brown wing covert of a fallow Bourke
2. A barb of the common type of the Bourke with a normal amount of eumelanin granules. It is a barb of a brown wing covert of the wildtype Bourke. In the wildtype the concentration of the brown eumelanin is high.
3. A barb of the structural type with a sponge layer, but without grouping of the eumelanin granules. The sponge zone, a keratin layer, we see as fine stripes under the cortex in the direction of the middle cells. Beckmann used a wing covert with a blue and a brown vane. This is a cross section a barb of the brown vane of the wing covert.
4. A barb of the structural type with a sponge layer and grouping of the eumelanin around the middle cells of the barb. This is a cross section of a barb of a blue vane of the same wing covert. The sponge zone reflects only the light of a short wave length, blue and violet. The grouping of the melanin around the middle cells is needed to absorb the light with a wavelengths longer than the wavelengths of blue light. Only this part of the feather, with barbs with a sponge zone and with grouping of the eumelanin around the middle cells can show the blue colour.
5. The sketch of a contour feather shows the direction of cutting the barbs.
It is very valuable that Beckmann made his research so early, in 1972. At that time only a few mutations of the Bourke exists. The "isabel", fallow and pastel. At this time most fanciers bred only the wildtype Bourke.
Research with the scanning electron microscope
Nowadays it is possible to study the structure of the barbs with a scanning electron microscope. This gives a much better picture of the three dimensional structure of the keratin layer that causes the blue colour. One of the most recent studies is the study of Jan Dyck. He gives a new explanation of the blue structure. Thus fare they explained the blue colour as the scattering of light reflected by a vertical structure of little channels within the barb, smaller than the wavelength of red light, or less than 0,6 micron. Particles of dust in the sky make that the sunlight is refracted and reflected. They are producing the blue sky. The phenomenon was named the Tyndall scattering. Because they thought this layer is in a same way producing the blue colour they named this layer the cloudy layer. Dyck found that the structure of the layer of keratin is much more complex. With the scan the layer is shown from all sides. He discovered little bent keratin cylinders, filled with air, laying in this colour producing zone. The keratin looks like a sponge. This sponge zone is refracting and reflecting light in such a way that certain wavelengths are mutually reinforced and intensified while others are nullified by mutual interference. Today we speak about a sponge layer.
There are other blue producing structures in the word of birds. Pigeons, herons, and cranes have a blue-grey colour because of the cortex of the barbs and barbules is build out of a lot of little transparent sheets. This forms a cloudy medium. Blue white or milk white. With a dark melanin layer behind. It brings the characteristic blue colour of the pigeon. (Spöttel,1914).
Another kind of blue structure we find in the canary. Here the specific structure of the barbs causes the blue colour. Kop, (1986) made this visible by a special microscopic technique. Kop explains the blue light as a result of the Tyndall scattering. Most of the barbs contain medial cells filled with air. But there also are places where much of the air filled cells are missing. Here we see only a massive granular keratin structure with some marrow cells. The granules are reflecting the blue light. Feathers or part of this feathers produce in this way a kind of blue structure. The length of this structure, in the wild canary, is about 2,3 mm. The single blue structure is 2 to 4,5 mm. The double blue structure is 4,5 mm. and more By selective breeding the maximum length of this structure is 6,2 mm. This kind of blue structure is very different compared with the blue structure of parrot feathers. The blue colour of the canary is another kind of blue colour, slate or steel blue.
The Bourke's Parakeet has two kind of pigments: the brown eumelanin and the red and yellow psittacine. Most we know about the formation of the eumelanin. The eumelanin is deposed in the middle layer of the papilla, brought into the barbs and the barbules and feather shaft. Sometimes we find eumelanin also in the cortex. We discuss this in a later page.
Colours are genetically specified for each kind of bird. The transformation of elements of the diet, the transport to the place where the feather is growing (the follicle in the skin), the timing of the deposition are influenced by specific enzymes. Much has to be learned about this process.
Distribution of the red and yellow pigments. In the cross section the psittacine pigments are found in the cortex. The wild type Bourke has this colour pigments in the crown and hind neck, the throat, the chest and the belly. The chest and the belly are brown with dark rose, the belly is lighter than the chest and is more red. The head and neck are brownish rose. This rose is a mix of a lot of red and a little bit of yellow pigment and a lot of eumelanin. In the Bourke the influence of the brown eumelanin is always visible. The mixed colour is not red but rose.
Influence of domestication on the red and yellow colour
In the original wild type Bourke the red and yellow pigments are restricted to the front side of the plumage. The rose colour is functional as part of the camouflage. Only a little bit of yellow is seen in the margins of the wing- and back covers. These are yellowish white (Forshaw). Underneath the eumelanin in the back, the wings and the tail feathers almost no psittacine pigments are found. The Bourke is an aviary bird in Europe since 1870. In the domestication process the yellow and red psittacine was extended and intensified in the whole back side. And this you can see today in the margins of the back coverts and the wing coverts of the wildtype and in the colour varieties in the yellow and red series. To understand the situation today we discuss the distribution of red and yellow pigments. Some examples of new colour varieties will be helpful to explore this extension and intensifying of the psittacine pigments.
The first yellow and rose colour varieties
Because there is a great amount of eumelanin in the plumage of the wildtype Bourke, we can have the best impression of the yellow and red pigment in colour varieties, where reduction of the eumelanin took place. There are two early examples: the yellow pastel (cream) and the rose opaline (rosy)
The first eumelanin mutations came forth about seventy years after the first import of the Bourke in Europe. The first pastel mutation (cream) was bred in Holland, by Van de Brink, in 1957. He named this yellow. Later the name yellow pastel was introduced. The inheritance is recessive. This factor gives a reduction of the eumelanin in the whole plumage. In this way the psittacine pigments became more visible. Rose in the front side, yellow in the backside of the plumage. The fanciers loved the yellow colour. This yellow colour variety became very popular in Holland. A lot of people worked on it, and selected the most yellow specimen. Selection is going on till now. Today the yellow pigment is much more spread and the colour is more bright and intensive yellow than in the past. Selection is possible because the inheritance of the psittacine pigments is intermediate (Beckmann)
In 1972 the rose opaline mutation appeared. This opaline shows a reduction of the eumelanin in the back, the wing coverts, the saddle, the head and the tail. The reduction took place in specific feather fields, not in the whole plumage. The mutation factor inherits sex-linked. It is a feather field mutation. In the opaline colour variety also selection was possible. Selection points are: The spreading of the red pigment in the back and tail, the amount of psittacine pigment and the reduction of eumelanin. Yet, after years there are specimen that are red and not rose. This means that the eumelanin diminished. Rose is a mix of red and brown pigment. The feathers are red when the eumelanin is gone. When we see the very yellow pastels and the red opaline, and we compare this with breeding results in the beginning, it is clear that only a strong selection could give this beautiful results.
The red and yellow pigment factors (P-factors) are inheriting intermediate. The factor that is reducing the melanin in the pastel is inheriting recessive and the factor that counts for the opaline is sex-linked (M-factor).
Many breeders are so used to the combination: yellow and pastel, that they did not appreciate an other combination. There were also beautiful red pastels. A friend of mine, Besters, bred them in the seventies. People did not like them so much, so the variety disappeared. But the variety is back again. The combination of rose opaline and yellow pastel gave not only rose opaline-pastel (pink) but also rose pastel. I had, some years ago, a combination of a beautiful rose coloured opaline-pastel (pink) hen and an intensive yellow coloured pastel cock (cream), split for opaline. The result was a rose pastel hen. This hen is definitely a pastel. Not an opaline-pastel. I controlled the plumage after all the possible characteristics of opaline. There was none. A hen can not be split for opaline! This proved the independency of red psittacine factor and opaline factor.
Most breeders of the rose opaline think that the opaline colour variety is always rose coloured. But when I suppose that there is no dependency between the red pigment factor (P-factor) and the opaline factor (M-factor), there should be also yellow opaline. I know, from personal contact, that the first fancier, who bred the opaline Bourke in 1972, Goossens, was trying for years to reach a rose Bourke. He had luck. The first rose hen was born in his aviaries. A yellow opaline Bourke is possible too. I bred a beautiful yellow opaline cock. The yellow pigment we find in the whole backside of the plumage . This bird has the opaline characteristics, but he is yellow. I want to develop a strain of this yellow opaline, but I am not so far yet. Maybe other fanciers have them? Also a pure yellow opaline-pastel hen is possible. I bred them this year for the first time. There is no red pigment in the plumage of the backside of the birds. Also the head is yellow.
My conclusion is that the factors are inheriting independent. The first combinations with psittacine: yellow- pastel and rose-opaline happened accidental. The opaline inherits sex-linked, the pastel inherits recessive, both are inheriting different, compared with the psittacine factors. The intermediate inheritance of the psittacine factor makes that selection and mutation breeding are the main factors in the process of domestication. And this process started before the mutation arrived.
What is the explanation of spreading and augmentation of the pigments?. Steiner used the term hyper function, when the pigment formation is stronger than normal, and the term hypo function, when the formation is less than normal. In the wildtype there is a natural variation in the amount of red and yellow pigments. The wildtype in our aviaries is not original any more. We find wild types with yellow margins and red margins. Wild types with white margins and colour varieties with a white back are seldom found. I think that the early conclusion of Beckmann, that domestication will be the most important factor in the development of the psittacine pigments, is very adequate. It is nice that he studied the pigments of Neophema and Bourke already in 1972, before the enormous development of colour varieties took place.
We don't know yet the exact chemical structure of the red and yellow colour pigments. In the past people called them carotenoïd pigments. But it is proven that this pigments do not belong to the group of carotenoids. The breeder knows that it is not possible to intensify the colours of parakeets with chemical expedients, the so-called red food for canaries or feeding with vegetables with a lot of carotene like carrots.
The colour pigments of parrots and parakeets are a total new kind, is the meaning of researchers. There is recent research done in Italy (Prodi, et all). In biochemical analysis of the feathers of the Red Macaw they found four new pigment components, the polyenals. But much research has to be done.