Applying a scientific base to our understanding of Parrot colour morphs; by Terry Martin

The Genetics of Colour in the Budgerigar and other Parrots

This page created 23rd October 2001

Applying a scientific base to our understanding of Parrot colour morphs

by Terry Martin BVSc

There are growing numbers of aviculturists world-wide, with broad knowledge of genetics and varying degrees of scientific training, who are slowly striving to instil more scientific method into our understanding of colour morphs in Parrots. They are, however, handicapped by a significant lack of scientific study of colour morphs in aviculture and only have a handful of sources upon which to base their knowledge.

One of the few geneticists to study and document information on colour morphs is Paul Buckley. He has contributed to two books (Petrak (Ed) 1982, Cooke and Buckley 1987) which are commonly sourced by English speaking aviculturists looking for scientifically based information. However, the systems described have limitations as they are designed for the study of colour morphs in wild species of birds. These shortcomings can be highlighted through a careful assessment of what has been observed and learnt from colour morphs in aviculture. It is vitally important that all aviculturists be aware of Buckley’s system and its flaws, to avoid perpetuating these errors. The following sections are abstracts from papers I have presented on the topic of colour morphs in parrots, dealing with those aspects where Buckley’s system is flawed.

Melanin types

Melanin production is one process that has had significant study through human research as well as that in other species. Two types of melanin are currently recognised: eumelanin, which is black, and phaeomelanin, which is chestnut red/brown. Paul Buckley, who is one author avian veterinarians and aviculturists have had exposure to, through his chapter in Diseases of Cage and Aviary Birds, recognises three forms of melanin defined as follows: eumelanin as grey, phaeomelanin as brown, and erythromelanin as chestnut red. He believes that brown and grey melanins combine together to produce black pigments. In reality melanin is never grey, it only appears that way depending upon its concentration or position within the feather structure.

The differences in Buckley’s system caused problems in terminology for me when I first started expanding my knowledge of colour mutations in birds. For instance, one colour mutation in Zebra Finches (Taeniopygia guttata) is known as a ‘Phaeo’ because it only carries phaeomelanin (all eumelanin being removed). But this pigment is chestnut red and in Buckley’s system would be defined as erythromelanin. I am of the opinion that Buckley’s system is flawed, along with the way he categorises various colour morphs, but to arrive at this conclusion requires study of various species of birds along with the way mutations alter their colouration.

The heart of the matter is whether brown coloured melanin is a melanin type in its own right, or merely a by-product in the process of producing black eumelanin. The answer is that it is produced as one of various intermediate products in the process of eumelanin production. If it were a product in its own right, it would be possible for a mutation to turn off brown melanin production without turning off black (grey) melanin. Buckley categorises a number of mutations in both wild and captive bred species as functioning in this way. However all known examples in captive species are dilution type mutations that still allow brown melanin production and can be combined with ‘brown’ colour morphs to produce ‘lighter brown’ colour morphs. If Buckley were correct, then combining a ‘grey only’ colour morph with a ‘brown only’ colour morph would result in a bird without either brown or grey (or black) pigments. This situation has never been discovered in aviculture.

Classification of colour morphs

There are a number of classification systems in use for colour morphs; some based in the scientific literature and others in use by aviculture. I will briefly discuss the various systems so that we can understand where various names come from.

In mammals a system of categorising colour morphs was proposed by Pearson, Nettleship and Usher (1911), who classified them into the following classes:

total albinism: complete loss of melanin
incomplete albinism: loss of melanin from eyes, skin or feathers, but not all three
imperfect albinism: partial loss of melanin but not total in any area
partial albinism: complete or partial loss of melanin within localised areas

Under this system, all changes to melanin within the feather could be described as albinism. However, because it is a mammalian system, it does not take into account carotenoid pigment or structural aspects of colouration. We also have more knowledge of how various mutations alter melanin production, and see that some do not fit comfortably into these categories.

Another system is reported by Buckley and is outlined as follows:

albinism: total loss of pigment from all body parts
leucism: total loss of pigment from feathers but not other body tissue (this would be called incomplete albinism in the above system)
partial leucism: total loss of pigment from localised areas of plumage with no alteration in others (this would be called partial albinism in the above system)
dilution: reduction in the amount of pigment deposited, but not total loss of pigment (this would be imperfect albinism in the above system)
schizochroism: the absence of one pigment type without alteration to others (this is not classified in the older system)

This system is used to classify types of colour morphs in wild birds (Paul Buckley is an Avian ecological geneticist). Because of the nature of these morphs (occurring in wild species) it is not possible to fully explore the nature of their changes. However, in captive species we can determine much more information about a colour morph and as a result a number of flaws have been identified in Buckley’s system.

Buckley does take into account the presence of more than one pigment type, hence the category known as schizochroism. Total loss of melanin in an animal carrying only melanin would be defined as albinism in this system, but in a species naturally having carotenoid pigment as well, the same genetic change would be classified as schizochroism.

Partial leucism which will be recognised by many as any one of the numerous ‘Pied’ mutations, excludes the large number of ‘Pieds’ that do remove pigment from feet and legs.

Dilution type colour morphs are common, but like albinism, since carotenoid and melanin pigments are under separate genetic control, no mutation fits properly into this category as they only reduce one pigment or the other.

Under schizochroism, Buckley includes mutations that in his view remove one melanin type but not others. As mentioned in the section above, discussing melanin pigments, his system appoints a separate category for ‘brown’ eumelanin and he then tries to identify colour morphs that remove separately either ‘grey’ melanin or ‘brown’ melanin. As discussed earlier, brown melanin is merely a metabolic stage in the production of black melanin and Buckley’s fawn ‘schizochroism’ is the result of a defect in melanin metabolism preventing the final stages of melanin oxidation. It is not a schizochroic by the definition used by Buckley. And the colour morphs identified by Buckley as grey ‘schizochroism’ are, in fact, all forms of dilution.

Another flaw in this system is that the genetic relationship of various colour morphs is not recognised and they can be classified as totally different types of mutations. For instance, the blue locus has more than one allele. Those that totally prevent carotenoid production (blue alleles) would be classified as schizochroism, whilst the partially effective alleles (parblues) would be defined as a carotenoid dilute. And for the sex-linked ino locus, the ino allele (causing total loss of melanin) would be either albinism or schizochroism, whilst the partially effective alleles (par-ino or lime) would be melanin dilution.

The final deficiency is the lack of categories for mutations altering structural colouration. These colour morphs are created by alteration to various aspects of the feather barb microstructure and are well known in parrot species.

Aviculturists have created their own system of naming and identifying colour mutations. The precedence for naming was set for parrots by the Budgerigar fraternity and initially this naming systems was adhered to. However, due to insufficient communication within the avicultural community regarding the correct usage of names, many more recently occurring colour morphs have been incorrectly named.

Useful aspects of Buckley's work

Whilst I maintain that there are significant deficiencies in the information presented in Paul Buckley's books, there is still value in considering his classification system for colour morphs in greater detail. Whilst it lacks consideration of psittacin pigment and structural altering colour morphs, it does provide three useful categories for melanin altering colour morphs. The separation of melanin altering colour morphs into albinism, dilution and leucism roughly correlates to genes that control melanin metabolism, melanin deposition, and melanocyte cellular functions. Careful observation of mutant phenotypes for particular features allows them to be placed into one of these categories and thereby define basic differences between these colour morphs.

References:

Cooke, F and Buckley, P.A., 1987. Avian Genetics: A Population and Ecological Approach, Academic Press, ISBN 0-12-187570-9

Pearson K, Nettleship E., Usher CH. A Monography on Albinism in Man. Draper’s Company Research Memoires Biometric Series 6,8,9: Parts 1,2,4 London Dulau (1911-1913)

Petrak, Margaret L. (Ed) 1982. Diseases of Cage and Aviary Birds, Chapter 4: Avian Genetics by Paul A. Buckley, 2nd edition, Lea & Febiger, ISBN 8121-0187-1