Classifying and defining Fallow colour morphs
in Parrots

by Terry Martin BVSc

Fallow is a name brought to the avicultural world by the Budgerigar Fancy. It is now popularly used in many species of parrots, so it should have a strong definition. I always thought it did and have been guilty of being critical of those who did not seem to know it. Yet, as I have learnt more about colour morphs called Fallow in different species, it is clear that the definition is not as clear as it should be. Or more precisely, that there is more than one type of Fallow and some types are only loosely similar to others.

If we go to the dictionary for a definition, we find that fallow is not a colour at all. So instead, we look at its use in Budgerigars for a definition. Taylor and Warner describe the three forms of fallow (English, Scottish and German) as having melanin converted to brown, giving an overall reduction in plumage colour from light green towards yellow and distinctive red eye colouration in adults. This seems to differ from current day descriptions of English Fallow, where breeders describe the foreground melanin as grey-brown and microscopic studies have shown all melanin to be black, although reduced in granule size (work by Inte Onsman). Therefore I feel it is best to start at the beginning and define fallow as thoroughly and accurately as possible.

Categories of colour morphs

The mutations that cause colour morphs can be divided into four main types

The mutations that create fallow phenotypes clearly belong to the melanin altering group which also includes many other common colour morphs including lutino, cinnamon and various dilutes. We can then divide melanin altering mutations into three subgroups, which correlate to how they achieve the alteration to melanin in the plumage.

Albinistic mutations correlate with mutant alleles of loci involved in the process of melanin production including metabolism and the construction of melanosomes. The hallmark of an albinistic mutation, compared to the other two groups, is the reduction in melanin within soft tissue and not just feathers. In other words, they lose melanin from eyes, skin, legs, beak and toenails. Some of these colour morphs have dark eyes when mature, but all show signs of reduced eye colour upon hatching. Melanin granules produced by these colour morphs are altered in structure, the change varies depending upon the function of the locus involved, but all show qualitative melanin changes.

Dilution mutations correlate with mutant alleles of loci controlling deposition of melanin from the melanocytes into the melanosomes of the feather. They are able to produce normal melanin granules, but do not transfer normal quantities into feathers. They are said to have quantitative changes to their melanin levels. Soft tissues are unaffected because the melanin deposition process in the eyes, skin and other tissue is different to the feathers. Put more simply, the dilute loci have no function in skin melanin deposition.

Leucistic mutations correlate to mutant alleles of loci that control melanocyte life functions, in particular migration of melanoblasts (melanocyte precursors) from the neural crest in the embryo. True leucistic mutations are uncommon, the black-eyed clear phenotype being the true appearance. But partial leucism is very common. These are Pied birds; those that have some regions devoid of melanin and yet other regions fully coloured. The hallmark of leucistic colour morphs is that melanin is either totally absent or fully retained. There are never qualitative nor quantitative changes to melanin in these colour morphs. Many mutant alleles with leucistic characteristics also show evidence of pleiotrophic behaviour. This means they have effects beyond the basic effect on melanin production and is not surprising for loci controlling complex functions like melanoblast migration.

In summary, there are basic three ways we can alter the melanin content of a feather. Damage the cells responsible for its production resulting in leucism, damage the metabolic process of melanin production resulting in albinism, or damage the transfer process for melanin granules from the melanocyte into target areas in the feather resulting in dilution.

Distinguishing albinistic colour morphs

Fallow colour morphs clearly belong to the albinism subgroup, along with the classical colours of lutino and cinnamon. As mentioned above, members of this group are all involved in the process of melanin metabolism. Most loci code for an enzyme with specific function in melanin metabolism, damage to the wildtype allele for a locus results in loss of enzyme function or sometimes partial loss of function. We can identify each locus through a combination of gene action and mode of inheritance.

Two albinistic loci are sex-linked. They are the cinnamon locus and the sex-linked ino locus. There are currently no other sex-linked loci with melanin altering behaviour. Both known loci are well documented in many species, and not just restricted to birds. Whilst many dominant and co-dominant loci are known to produce colour morphs, most are responsible for control of feather structure. Some with melanin altering behaviour are known, but none so far have features of albinism.

That leaves only autosomal recessive loci and here we find the greatest number of melanin altering mutations. But if we eliminate those with leucistic or dilution traits, we are finally left with a smaller group of albinistic colour morphs that include the fallow phenotypes.

The ‘darkest’ albinistic phenotype has been given the designation ‘Faded’. These alleles produce only very slight reduction in melanin compared with normal phenotypes. Chicks have slight reduction of melanin when they hatch, but have dark eyes upon fledging. Only a few species have examples of this phenotype amongst their colour morphs.

Once we remove those colours from our consideration, we have left a large group of colour morphs with one feature in common - all have red eyes as adult birds. (This does not mean every bird with red eyes is a fallow, it is possible for eye colour alone to be changed, without alteration to plumage colour. These birds are not fallow colour morphs, although they would be defined as ocular albinism.) Apart from the obvious change to eye colour, all colour morphs with a ‘Fallow’ phenotype have alteration to melanin quality. But here they differ tremendously. Some have the classical ‘Brown’ change to their melanin as described by Taylor and Warner, whilst other alleles produce weakened melanin deposits that appear grey in colour instead of the wildtype black. Are they all the same mutation? It seems unlikely.

In Budgerigars we know there are at least three loci that produce birds of this general phenotype. In Peachface Lovebirds there are also two distinctly different loci that have the typical traits of a fallow phenotype, yet are distinctly different. Many other species also have more than one fallow colour morph, but only one has been given the name fallow, with the second being labelled with a different name through error.

Identifying specific fallow loci and their distinctive variations

I believe it is time that we try to recognise and categorise all the various fallow colour morphs across different species. To do so presents a challenge and we must start by documenting the differences between the various alleles and look for consistency across species.

To begin with, one fallow allele in the Budgerigar retains the white iris ring produced by lipid deposition in the eye, whilst the other two forms lose this feature. This could be a useful feature to distinguish one locus, except that most species of parrots do not normally have white iris rings. But in those that do we will be able to use it to confirm affiliation.

The Indian Ringneck is one species that does naturally carry the white iris ring and the two fallow colour morphs in this species (see table below) vary in this feature. One retains the white iris ring, therefore correlating to the German Fallow Budgerigar, whilst the other loses its iris ring and shares this feature with the English Fallow Budgerigar. A new fallow colour morph in the Moustache Parrot also loses the white iris ring as well. With time we will have a useful list of colour morphs in different species demonstrating variation in this feature.

Another feature of some fallow alleles is the possible genetic relationship with the NSL (non sex-linked) ino locus. Inte Onsman has theorised that certain fallow alleles may be in effect autosomal ‘par-ino’ alleles. The difficulty in testing this theory is that few species have both fallow colour and NSL lutino colour morphs. One species that does is the Pacific Parrotlet, however evidence suggests the fallow allele in this species is not allelic with NSL ino. Therefore at this stage, this feature remains to be proven, but could offer a means to positively separate certain fallow colour morphs in the future.

But at this point in time, the basic distinction between alleles is in the colour of their melanin deposits and their relative depth of colour. There are ‘light’ fallow and ‘darker’ fallow colours, as well as ‘grey’ fallow, ‘grey-brown’ fallow and ‘brown’ fallow. The following table attempts to catalogue the various fallow alleles in different parrot species into four main categories.

Each of the four categories requires a distinctive name. Dun fallow has been suggested by Inte Onsman for ‘grey-brown’ fallow phenotypes, Bronze fallow for the ‘brown’ fallow phenotypes, and Pale fallow for the ‘pale brown’ phenotypes. Beige fallow may be an alternative for the for the ‘pale brown’ phenotypes. Clive Hesford has suggested Smokey fallow or Ashen fallow for the ‘grey’ phenotypes.

Please note that I have included the ‘Australian’ fallow Budgerigar in the list. Most Budgerigar breeders believe it is the same allele as the German fallow. Yet whilst modern day breeders describe the German fallow as having grey-brown melanin, the fallow Budgerigar in Australia has pale brown melanin markings, that Australian breeders agree is never ‘grey-brown’. It could in fact be an allele of the German fallow locus or a distinct locus of its own (Inte Onsman believes both are possibly alleles of the NSL ino locus). Another consideration is that the combination of the grey allele with fallow makes the melanin seem darker and more grey than it would otherwise appear. Clearly it deserves further investigation and study and when possible in the future, test mated to the other fallow alleles in this species. Also please note that the third recognised fallow allele in Budgerigars – the Scottish fallow – is believed to be extinct and is not included in my table.

Finally, it is important to realise that there are a growing number of colour morphs in different species being incorrectly called fallow. The ‘fallow’ Indian Ringneck (also known by the equally poor choices of Isabel and Dominant Cinnamon in the USA) is a dilution mutation of dominant inheritance, therefore clearly does not fit the fallow definition. Similarly the ‘fallow’ Green-cheeked Conure is incorrectly named. The mode of inheritance for this colour morph is sex-linked recessive, therefore it is either a cinnamon allele or a par-ino allele (multiple allele of the sex-linked ino locus) with the later appearing more likely.

© Terry Martin, January 2001

More articles by Terry Martin:

The nature of the Opaline locus

The nature of the Opaline locus - an Update

The Pied (or Olive-Pied) Redrump Parrot

Applying a scientific base to our understanding of Parrot colour morphs

Top of Page   •  Other sites index   •  Articles index   •  Books   •  Please take me Home