The Scottish Fallow budgerigar mutation is one of approximately 30 mutations affecting the colour of budgerigars. At least three types of Fallow, the German, English and Scottish, all named after their country of origin, have been established, although none of these types is common. They are superficially similar, but adult birds may be distinguished by examining the eye. All have red eyes, but the German Fallow shows the usual white iris ring, the eye of the English Fallow is a solid red with a barely discernible iris and the iris of the Scottish Fallow is pink.
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Appearance
In most respects English, Scottish and German Fallows are very similar. All resemble Cinnamons, but differ in having a much weaker body colour, which results in a rather attractive mustard-yellow breast shading to green on the rump (blue in the blue series). The depth of the green or blue suffusion varies in individual birds, but is always more intense towards the vent and on the rump. The throat spots, head and neck striations, and wing markings are a medium brown on a yellowish ground. The cheek patches are a lighter and duller shade of violet than normal. Cocks have a greyish-purple rather than the usual blue cere. The feet and legs are pink and the beak orange.
The most obvious distinction from Cinnamons is the red eye, which in the Scottish Fallow is a clear bright red rather like an English Fallow but with a pinkish iris ring.
The Dark mutation deepens the body colour of the Fallow, but the difference between the Light Green, Dark Green and Olive Fallow is far less than that between the normal Light Green, Dark Green and Olive. The Olive Fallow is "a beautiful rich golden orange shade, and the chest is deep yellow olive - a truly lovely colouring", says Cyril Rogers in The Fallows.
Fallow Greys and Grey-Greens generally have darker wing markings. Opaline by itself lightens the body colour of Fallows and in combination with Cinnamon produces a bird very similar in appearance to a Lacewing (i.e. a Cinnamon Ino), with virtually no body suffusion.
Genetics
The Scottish Fallow is an autosomal mutation causing recessive changes to the form of the melanin pigment. There is no universally accepted genetic symbol for either the locus or mutant allele, so the simple symbol fs+ will be adopted here for the wild-type allele at this locus, and the symbol fs for the Scottish Fallow mutant allele, in keeping with the most widely used name in budgerigar circles.
In its visual effect, the Scottish Fallow mutation is recessive to its wild-type allele, so a bird possessing a single Scottish Fallow allele (the heterozygote, fs+/fs) 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 and normal distribution of the black melanin pigment. Among the budgerigar fancy such a bird is said to be a Light Green split Scottish fallow, usually written Light Green/Scottish fallow.
In a bird which has two Scottish Fallow alleles (the homozygote, fs/fs), the lack of the wild-type allele means that normal black melanin pigment cannot be produced. Instead a pigment giving a brown appearance is substituted, resulting in brown markings where black would appear in the Normal.