Blue budgerigar mutation

Appearance

The Blue mutation changes the colour of the body feathers, which are light green in the wild-type, to skyblue and the colour of mask and other parts which are yellow in the wild-type, to white. In the domesticated bird this mutation changes the Light Green variety into the Skyblue variety, the Dark Green into the Cobalt and the Olive into the Mauve.

The green colouration of the wild budgerigar is due to the combined effect of a yellow pigment and an interference effect similar to that which gives colour to petrol on water, which in the budgerigar produces a blue colouration. Yellow pigment is present in the outer layer (cortex) of the cells forming the barbs of all feathers of the wild budgerigar with the exception of the cheek patches, although it is very weak in the outermost flight feathers. The distribution of the yellow pigment is clearly shown in the Lutino. The Blue mutation totally inhibits the production of this yellow pigment, and as far as is known, it has no other effect.

The yellow pigment in young budgerigars is paler than in adults, which makes green budgerigars in nest feather appear duller and Lutinos appear paler. A brighter and stronger yellow colouration appears after the first moult.

The chemical nature of the yellow pigment in budgerigars and other psittacidae is unknown, and in the absence of a chemical name George A Smith coined the term "psittacins" to cover the yellow, orange and red pigments found in parrots and parrot-like birds. Psittacins impart a far more constant intensity of colour to feathers than do the more commonly found carotenoid pigments such as xanthophyll, the yellow pigment found in the canary. At each moult the canary extracts xanthophyll directly from its food, and the depth of colouration of the growing feathers is determined by the concentration of xanthophyll in its diet. Budgerigars cannot be colour-fed in this way, because they do not use xanthophyll as a pigment.

The Blue mutation provides a widely accepted division of domesticated budgerigars into two colour classes: the "Green series" and the "Blue series". Birds of the Green series exhibit yellow pigmentation, while birds of the Blue series lack yellow pigmentation. These names can be misleading, since some birds belonging to the Blue series, such as Albinos, are not blue; similarly, Lutinos belong to the Green series, yet are not green.

In combination with the Dark budgerigar mutation the body feathers become deeper shades of blue. A blue budgerigar with a single Dark factor is called a Cobalt, and one with two Dark factors a Mauve. The World Budgerigar Organisation has established precise standards for budgerigar body colours using the Pantone Codes, as shown to the right.

Genetics

The Blue mutation is recessive to its wild-type allele, so a bird possessing a single Blue allele (the heterozygote) is identical in appearance to the wild-type light green. That is, the presence of a single wild-type allele is sufficient to permit the full production of the yellow psittacin pigment. Among the budgerigar fancy such a bird is said to be a Light Green split blue, usually written Light Green/blue. In a bird which has two Blue alleles (the homozygote), the lack of the wild-type allele means the yellow pigment can no longer be produced, and so the body colour is blue — the Skyblue.

The locus of the Blue gene is situated on one of the autosomal chromosomes. The Yellowface Blue I mutation, the Yellowface II mutation form an autosomal co-dominant series of alleles with the Blue mutation .

The loci of the Dark budgerigar mutation and the Blue allelic series are situated on the same autosome, so the Dark mutation is linked to the Blue allelic series (see genetic linkage). The cross-over value (COV) or recombination frequency between the Dark and Blue loci is commonly stated to be about 14%, but some experiments have found much smaller values (see Genetics in the Dark budgerigar mutation).