I present the precise description of the adult male Bourke of Joseph Forshaw, writer of the magnificent volume: Australian Parrots,1972, pg. 243. He gives a careful description of an Australian wild-type Bourke. The names of the feather fields, used in the description, you will find further on this page together with a photo of a Bourke's hen.
New colour varieties do not come out of the blue. They are dependent of the possession of colouring elements of the wildtype. A description is very useful because the wild type should be the starting point for every breeder of colour varieties. When you don't know this colour varieties I recommend to look at the pictures in this site or postpone the reading of my comments until later.
Quote: Forehead and line above eyes blue, crown and hind neck slightly tinged with dark pink; lores and eye region dusky white; cheeks dull brown, tinged with ash white
The extension of the bluestructure on the head (sometimes totally blue) in the blue opaline, is perhaps developed as an extension of the blue frontal band and the blue line above the eye. Crown and hind neck have a dark pink colour. In the yellow pastel and lutino colour variety we see that crown and hind neck have a lot of red pigment. There is a sharp border-line between the rosy hind neck and the yellow upper back. There is a dusky white eye region and ash white cheeks in the wildtype. In the yellow pastel and the lutino colour variety there are specimen with a white head. Sometimes bird-fanciers named them white face.
Quote: The back and wings are earth brown This dark brown colour is due to an excessive amount of eumelanin. When this brown colour is deleted or partial disappeared, we find that this brown pigment was covering several colours, red or yellow , the blue colour, and the combination of blue structure and yellow pigment in the green colour.
Quote: The feathers of throat and chest are brownish, margined with pink; abdomen rose-pink, deepest in centre This means that there is a lot of red pigment in the throat and chest, but especially in the belly. We see this red pigment in all mutations of the eumelanin, when the eumelanin is reduced. The light cinnamon , the yellow pastel , the rose opaline , the lutino , the rubino and so on have this rose or red belly. There are about eight eumelanin mutations. In most eumelanin mutations the front side of the Bourke did not changed colour. The only exception comes with the blue mutation. I had in my strain Bourke's birds with a white, a grey or a blue front. Thus fare the names of the colour varieties of the Bourke are mostly referring to the back of the Bourke.
Quote: under tail coverts, vent, thighs, flanks and sides of upper tail coverts are pale blue We see in the blue rump colour variety and the blue opaline Bourke that this area becomes fully sky-blue or cobalt blue.
Quote: rump and upper tail coverts are dark earth brown This brown eumelanin is found in the feather barbs but also in the barbules. This covers the psittacine pigments. When the eumelanin is reduced we see what colours are hiding. In the yellow pastel we see the yellow pigment, in the opaline we see the red pigments. In the blue opaline Bourke we see a blue rump and tail coverts. Beckmann made cross-section of the feather barbs and studied them under the electron microscope and found the eumelanin around the cells in the middle and in the cortex.
Quote: middle and upper wing coverts are dark earth brown margined with yellowish white This margins are very important to determine what colour pigment is hiding under the eumelanin in the wings and back. When they are white, there is no red or yellow pigment in this area When they are yellow, the yellow pigment is extended in the back, but hidden by the brown eumelanin. When they are blue a alteration of the micro-structure has taken place, the feathers of the common type changed to feathers of the structural type. In the original wild-type there was no psittacine pigment or blue structure in this area.
Quote:Under wing coverts and bend of the wing are pale blue So we know that this region has feathers with a blue structure. When the eumelanin is gone, for instance in the lutino , this region is not white but is shining silver white.
Secondaries and outer web of the primaries violet blue One of the differences between male and female Bourke is the different kind of blue in this areas. This blue colour of the cock is violet ( Forshaw) or ultramarine blue (Beckmann), this blue colour of the hen is sky blue. In the lutino and rubino colour varieties we see that this area is white, because the eumelanin got lost. In the rose opaline there is a white triangle seen in this area. Red colour and blue colour from the feather microstructure cannot be combined. This is the reason that the extension of the red pigment in the rubino probably will never reach the edge of the feather .This area shall be white.
Quote: Central four tail feathers blackish- brown, tinged with blue, lateral (outer) feathers blue margined and tipped with white The two middle tail feathers are important for the mutation breeder. In the different colour varieties this should have different colours. White, yellow, red, blue and green are possible colours. The three feathers of the outer part of the tail have a white tip.
Quote: Bill greyish horn, iris brown, legs greyish brown. The beak of the Bourke with a dark factor is black just as the toenails. The legs are very dark grey. The beak of the lutino is horn coloured. In the toenails of the lutino we see the red blood vessel. The colour is altered by mutation factors of the eumelanin.
Conclusion: The Bourke has a dull earth brown plumage. But on second thoughts there is much more to see and to appreciate. The brown eumelanin is dominating the plumage, but the little colour accents in the plumage are interesting. The colours are pale because of the mixture of colour pigment and brown eumelanin, so became red, rose and blue, pale blue. The full spectrum of red and yellow colour possibilities becomes visible when the eumelanin is reduced or totally gone. It is really surprising how many beautiful different colour varieties came out of this wildtype Bourke.