The Half-sider

The HalfsiderFrom time to time, a Budgerigar appears that is described as a “Half-sider”; that is, a Budgerigar which is, in the vertical plane on one side a green coloured bird, and on the other side is blue. The line of delineation that separates the two sides, is the Midline that determines the left and right sides of the body. Such a Half-sider may in fact be Grey-Green on one side and Grey on the other and Half-siders may be Normals, Cinnamons, Opalines Spangles etc. The genetics of the Half-sider have been debated for many years and various theories have been postulated.

In Genetics for Budgerigar Breeders by Prof. T G Taylor, an explanation for the genetic constitution of the Half-sider is given. To summarize Prof. Taylor’s explanation; it is suggested that the original fertilized egg cell starts out as a heterozygous Green/Blue split, and at the first division of this cell into two “daughter” cells, one of the two daughter cells loses the chromosome possessing the green gene. All subsequent cells descending from this cell will have only the blue gene and no opposing chromosome. Further, it is suggested that the daughter cells of the fertilized egg immediately differentiate into the origins of the left and right sides of the body, respectively, at this first cell division. However, there are three obvious flaws in this explanation


  1. If the chromosome carrying the green gene was lost, then many other genes would also be lost. It seems strange that an organism could develop and remain viable without so much genetic information, when it is an acknowledged fact that a single gene can have a major effect on the body.
  2. After the egg has been fertilized by the sperm, the cell divides into 2 new cells, which in turn divide into 2 new cells each, and so on to form a ball of cells called the Blastula. During the first few cell divisions, no differentiation into future body parts takes place in the Blastula, and so the right side or the left side of the body does not originate from a specific single cell.
  3. If Prof Taylor’s theory were correct, then one would expect to see Half-siders of many different sorts, such as; half-Opaline and half-Normal or half-Spangle and half-Normal etc. As far as I am aware such birds have not been seen.

In considering the conundrum that the Half-sider poses, it would appear, that to date, it has been assumed that the puzzle has always had a genetic explanation. Is this really so? Can there be other explanations?

Half-sider is Congenital

Since the Half-sider is born this way the condition is congenital, but, not all congenital conditions are of a primary genetic (and therefore heritable) origin. Most are developmental problems of varying degrees, that have occurred somewhere between the fertilization of the egg and birth, but of insufficient severity to prevent further development.

In this article, I wish to propose that the Half-sider is in fact, displaying such a developmental failure which was caused at a very early stage of development within the egg, and that the condition is not hereditary.

I recall that, when I was a boy, I knew a woman who had a white patch in her hair. This was at the back of her head, and had been the result of being hit on this spot by a tennis ball when she was 8 years old. Afterwards the hair always grew white. The white patch, clearly, cannot have a genetic connection, because the hair originally grew brown and the permanent change of colour was as a result of injury to the hair producing cells. In this case, the cells producing hair were permanently damaged and could no longer produce melanin pigment, yet functioned perfectly in every other way. It is therefore possible that minor injury can permanently affect melanin pigment production.

A Type Of Pied

In the widest descriptive sense, the Half-sider is a Pied Budgerigar, but not of the Dominant Pied or Recessive Pied varieties, which are the result of failure to produce melanin pigment in the medulla of the feathers in certain parts of the body, and has a known and provable genetic origin. The recognized Pied varieties breed according to genetic theory, and produce offspring that conform to the expected ratios of colour production. However, in the Dominant and Recessive Pied varieties, the markings are bilateral and are not delineated at the Midline.

The most obvious demarcation of the midline in any of the higher forms of life is shown by the nervous system. The classic example that displays the absolute difference between the nervous system to the left and right sides of the body, is when a person has a “stroke” and paralysis of one side of the body only, occurs demarked at the Midline.

The whole of the body is supplied with a very intricate nerve supply, that can be categorized into two major groups, the Cranial nerves and the Spinal nerves.

The whole of the Nervous System can be further divided into two other major groups; the Sympathetic and the Parasympathetic nervous systems. The Sympathetic nervous system generally works in a way that “speeds up” the body and prepares the body for “fight and flight”, whereas the Parasympathetic nervous system works when the body is slowed down (digestion and temperature control).

The Sympathetic nervous system works in conjunction with the endocrine system. In particular with the hormone, Adrenaline, which we all know prepares the body for “fight and flight”. The point that I am making here, is the association between the hormone Adrenaline and the nerve supply to one half of the body that is demarked at the Midline. This is exactly the same area that lacks the distribution of the Yellow pigment in the Half-sider.

The pigment Melanin is produced in the body by cells known as Melanocyles, which are formed in the Neural Crest during vertebrate embryonic development. With the exception of the retinal melanocytes, (eye pigmentation cells), melanocytes migrate outwards from the Neural Crest to cover the various pigmented sections of the body. The pigment Melanin, is responsible for most mammalian coat colouration, and is widely found in many other living creatures including birds, insects and moulds etc.

Within the melanocytes there are enzymes capable of bringing about the production of Melanin from the amino acid, Tyrosine, in a series of chemical reactions. In the first of these chemical reactions, the enzyme Tyrosinase slowly oxidizes Tyrosine to Dihydroxyphenylalanine better known as D.O.RA. It so happens that D.O.RA. can be synthesized into both Melanin and Adrenaline. Therefore there is a common link between pigmentation within the body, nerve distribution and a hormone. It takes two chemical reactions to convert D.O.RA. into Adrenaline, but many more to convert D.O.RA. into Melanin. Each of the chemical reactions requires a specific enzyme, and each of the different enzymes is the product of specific genes.

The Metabolism
of Melanoproteins and Andrenaline

Melanin Is A Polymer

The chemical Melanin is in fact a Polymer; that is; a chemical compound composed of many different sub-units all of the same Chemical structure linked together. In the case of Melanin, the sub-units are Indole Quinone. The polymer of Melanin, once formed, combines with various proteins to form Melanoprotein.

There are many different melanoproteins, and this accounts for the variability in body colour seen in many animal life forms. Much work has been done by geneticists on mice, which display many differences of coat colour, and it has been found that there are several different genes that have the effect of altering the different chemical reactions, at varying stages between D.O.RA. and Melanoprotein formation. Each different gene resulting in an altered chemical structure of Melanin, producing an enormous variety of coat colours and patterns. In mammals, there are two basic forms of Melanin; a brown-black form known as Eumelanin, and a yellow-reddish form known as Phaeomelanin. The two Melanins, Eumelanin and Phaeomelanin, are of course, groups of Melanoproteins of many different chemical compositions, giving rise to the great differences in coat colour, from black to white.

In the absence of any evidence to the contrary, I would suggest that the Melanin granules found in the medulla of Budgerigar feathers, belongs to the Eumelanin group, and, that the Yellow pigment found in the cortex of Budgerigar feathers belongs to the Phaeomelanin group. The yellow pigmentation found in Budgerigars has never been identified. It would seem reasonable to me, to assume that the yellow pigment is of the Phaeomelanin group, since the biochemical pathway for the synthesis of both Eumelanin and Phaeomelanin will be the same except for the last few chemical reactions. The production of Yellow pigment within the feathers of the Budgerigar is under the control of just one gene; as shown by the fact that the dominant wild Green gene has mutated to produce the recessive blue gene, which stops the production of all yellow feather pigment.

Eyes Can Be Different Colours

Referring back to the origins of the retinal melanocytes; the eyes are one of the few normally pigmented nervous tissues in the body. Therefore, in this tissue, the link between nervous tissue and pigmentation is obvious. We are all familiar with the fact that most people’s eyes are either shades of blue or brown, and the two eyes are usually very similar in colour. However. there are a few people whose two eyes are of different colours. Probably the most famous example being the pop-star, David Bowie. This condition can therefore, be considered to be similar to the Half-sider. Brown eyes are caused by Melanin pigment in the Iris of the eye, and blue eyes are the result of there being no pigment in the Iris. The blue colouration is the result of the effect of the fluid within the eyeball being visible. The brown or blue colour in the eye is the result of just one gene system. The dominant gene produces brown eyes and the homozygous recessive gene produces blue eyes. There are, of course, other shades of eye colour; grey, green and hazel, which are all produced by lesser amounts of melanin pigment being present in the Iris than that found in brown eyes. Such shades are caused by modifying genes which act in conjunction with the Brown/Blue gene, and if present, have an effect upon the formation of the healthy eye.

At all stages of development and life, the eyes are exceptionally vulnerable to damage. Disease conditions or injury, affecting the surface of one may cause colour changes to the affected eye, either by increasing Melanin production, making the damaged eye appear darker or, by preventing Melanin production, making the Iris appear lighter; (similar to the woman hit by the tennis ball). This will produce dissimilar coloured eyes. Obviously, injury to Melanin-producing cells, may permanently modify the function of the cells. If a person has dissimilar eyes at birth, it is quite possible that the two eyes were subjected to different pressures within the womb, since the eyes only have the eyelids for protection. Babies are commonly born with pressure marks of many different kinds, from being so tightly-folded within the womb, and these are often referred to as “birth marks”. The eye, like any other of the delicate foetal tissues, could display such birth marks in the form of dissimilar eye colour if the melanocytes in the Iris were slightly damaged at a critical period of development.

It is well known that once incubation of bird eggs has begun, the eggs need to be turned on a regular basis, otherwise the embryo will end up dead-in-shell. Embryo death can occur at any stage before hatching, and it is doubtful if any surviving chick has had the perfect development throughout it’s 17 or so days within the shell.

Living Creatures Are Not Perfect

Minor structural defects must be present in all living creatures, that is one reason why we are all slightly different to each other; even identical twins.

It has been my experience that of all Budgerigar eggs that I have been able to confirm as fertile, some 50% or so, subsequently fail to hatch. Since the egg cell is fertilized before the shell is made around it, and it is only possible to identify a fertile egg by handling on the third or fourth day at the earliest, the death of embryonic Budgerigars must be greater than the numbers that hatch. However, I have only observed one live chick that had a structural defect at birth, namely, an under-developed wing. It would seem that Mother Nature has a very high standard of quality control,in rejecting sub-standard biological entities. Perhaps there is a lesson here for Budgerigar fancy, in attempting to create a “new” version of a tried and tested product.

It is my suggestion that the Halfsider could be caused by very slight injury to the very delicate embryonic tissues of the Neural crest on one side only, at a most critical stage of development of the melanocytes. Once the developing melanocyte cell has been permanently damaged, then all subsequent direct descendants of this cell would also display the same dysfunction; this is the reason that scars permanently display sites of previous injury. This injury could be caused by the very slightest local chilling of part of the embryonic tissue within the egg, or very slight pressure differences within the egg. Such injury could be caused by the hen not sitting properly, absence from the eggs for too long a period, or a knock on the egg.

Whilst we have been considering the relationship between pigmentation and the nervous system, I would propose that this direct relationship may account for many of the unique patterns of markings seen in the animal kingdom, especially when the perimeter of specific markings are so clearly defined. The Budgerigar being an excellent example of many different markings which are strictly defined. To demonstrate the possible link between the nerve supply to the body, and local pigmentation, I will attempt to explain the head and face markings of the Budgerigar.

of the three branches of the Trigeminal NerveI am unaware of the specific nerve supply to the head of the Budgerigar. I will assume that it is similar to most other animal forms, which display an exceptional similarity to each other, because they have evolved from the same ancestral roots. If we consider the sensory nerve supply to the human face and head, then a map of the nerve supply can be drawn.

The nerve in question is the 5th Cranial Nerve, the Trigeminal Nerve. Birds do have the same cranial nerves as do other creatures. This nerve leaves the brain, and later divides into 3 separate nerves, each having specific, clearly defined areas of sensitivity as shown in the diagram. There are in fact, two Trigeminal nerves: left and right. There is very little overlap where the two nerve supplies meet along the Midline. If we now assume that the nerve supply to the Budgerigar face and head is the same Trigeminal nerve, and superimpose a similar nerve distribution map over the face of a Normal Budgerigar, the clearly defined nerve supply shows exceptionally close correlation to the different types of feather and markings that appear on the head and face.

Likewise, the markings on the back of the head, neck, wings and back in the Normal Budgerigar could be associated with the nerve supply to these areas; i.e., the cervical and thoracic spinal nerves. The primary flight feather markings to the nerve supply equivalent to the human hand, and so on.

Many species of the Parrot family, display similar elaborate, clearly defined patterned markings, similar to those which the Budgerigar displays, perhaps associated with different nerve distributions in each species.

I have considered all of the many different colour patterns that can be seen in the exhibition Budgerigar, and it is possible to account for every single variety of colour pattern, in direct relationship to the pigmentation patterns seen, and the maps of nerve distribution served by the Cranial nerves and the Spinal nerves; however, this will be the subject of a future article.

Original text Copyright 1996, Dr John Pilkington

Photo: Copyright 1997, Mick Freakley

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