One of the most substantial properties of the living organism is its stability and potential to
develop from embryo to maturity. Exceptions to the rule are so-called gynandromorphs which
originate by changes other than normal in the cells of the developing embryo, reflecting in the
adult by an unusual mosaic of color or pattern.
Several diversifications have been reported in fowls as is to be read in a paper published in 1928 by Crew . A fowl has been described with lateral asymmetry resulting in a larger left side and a smaller right side of the body. The right leg was shorter than the left and yellow colored. The left leg was pinky-white. The color of the pinky-white leg has a dominant inheritance and the yellow one has a recessive inheritance.
This bird was an adult F1 individual from a light Sussex hen x Rhode Island Red cock mating. Because the color of the Rhode Island Red cock is based upon a complex of several factors, I will not go into this extensively.
When kept apart from males the bird crowed and courted hens showing typical male behaviour but during the period of observation it failed to fertilise a single egg. Because the bird's plumage structure and behaviour was for the most part male, it was regarded as an abnormal cock. Since the two sides of the body were, on palpation, manifestly of different size, the possibility was considered that this bird was a lateral gynandromorph with the larger left side male in genetic constitution and the smaller right side female.
A gynandromorph is in fact an animal from which one side of the body is genetically male and the other side is genetically female and is the result of the elimination of one Z-chromosome during meiosis. The presence of a W-chromosome in birds is not necessary in itself for developing as a female individual.
After killing the former described fowl, two testis had been found from which the left testis was significantly larger than the right. The lateral assymetry did not affect the skull, comb, brain, and vertebral column unlike the rest of the skeleton. It was presumed that this bird most probably was not a gynandromorph and that the lateral asymmetry could be explained on the assumption that an autosome carrying white had been eliminated during the early cleavage divisions of a male zygote heterozygous for the characters white and yellow.
Another interesting case was described in 1929 . This concerned a mature hen, one half of
whose body was white- and the other half yellow-skinned. The division line of the color
extended the entire length of the body along the medium line.
The most plausible explanation of the origin of such a mosaic is the loss of one chromosome carrying the gene for skin color. White skin in fowls is a simple dominant over yellow skin, the inheritance of these colors being dependent on one pair of genes. White-skinned birds may normally, therefore, be of two types, homozygous W/W or heterozygous W/W+. So birds with a yellow skin are always W+/W+.
This mature hen showing two different skincolors and therefore can be considered as a "halfsider", must have been heterozygous for white skin (W/W+). The non-disjunction must have occured at a very early stage of cell division, possibly at the first cleavage. Such mosaic might also be explained by a gene mutation immediately following the first cell division. If the zygote had been homozygous for yellow (W+/W+) a dominant mutation in one of the pair of genes would also have resulted in a half yellow and half white-skinned bird.
In 1938 a paper was published by Crew and Munro  describing four new cases of color
asymmetry in a canary, a partridge and two more cases in chickens. Also four new cases of
gynandromorphism in birds have been described; two in the Gouldian finch and two in the
The difference in respect of size in fowls were falling into two classes:
1.) that in which the difference between the two sides was in the order of 2-4 percent;
2.) that in which this difference was sometimes even 10-15 percent.
They came to the conclusion that lateral gynandromorphism in birds is always the result of an abbarant chromosome distribution, that it is always associated with lateral size differences and that there are three types:
1.) The sparrow or finch type, in which the plumage characterisation is genetically determined and fallows the aberrant lateral chromosome distribution.
2.) The fowl-type, in which the sex-dimorphic plumage characterisation is subject to hormonal regulation, excluding the type of halfsiders seen in e.g. Budgerigars.
3.) The phaesant-type, in which nearly normal sex type of plumage occurs on one side and an intersexual type on the other.
A new theory proposed by Dr.John Pilkington about the existance of halfsiders has been
published on the INTERNET . His theory is based on the suggestion that the halfsider could
be caused by very slight injury to the embryonic tissues of the neural crest on one side only, at
a most critical stage of development of the melanocytes suggesting that melanocytes are
involved in producing the yellow pigment. He also suggests that the yellow pigment found in
the cortex of Budgerigar feathers belong to the phaeomelanin group and that the yellow
pigmentation found in Budgerigars has never been identified. At this point I disagree with Dr.
The yellow pigment in Budgerigars has actually been investigated [10,11,12] and it was found to be an exogenous pigment not produced by melanocytes at all. It is a fact that the yellow pigment sometimes manifests itself even better in the absence of melanocytes, e.g. in the amelanotic flight feathers in pied Budgerigars unlike the flightfeathers of a lutino being almost white.
A lutino has in fact a normal amount of abnormal functioning melanocytes in the skin and feather follicles in contrast with a dominant pied lacking melanocytes almost completely in amelanotic areas. It appears that the yellow lipochrome in canaries and Budgerigars is in fact a metabolic product distributed throughout the body by the endocrine and neural system. The pineal gland is most possibly involved in this process.
In 1968 a microscopical investigation was done to determine the structure of the pineal gland in Budgerigars . It was found that the pineals of Budgerigars seem to have more distinct structural relations with neuron dendrites than chicken pineals. The pineal gland is situated between both halfs of the brain. If e.g. one half of the brain is malfunctioning in distributing the yellow lipochrome, the opposite half of the body will be excluded from the yellow color and the result will be a halfsider.
Consulted and cited literature:
 Anonymous. Half Sider Feather Duster Hen Proves to be Fertile Budgerigar World Issue 129 (1993); p.p.21  Crew F.A.E., Lamy R. Autosomal Colour Mosaics in the Budgerigar Journal of Genetics Vol.30 no.2 (1935); p.p.233-241  Crew F.A.E., Munro S.S. Gynandromorphism and Lateral Asymmetry in Birds Proc.Roy.Soc.Edinburgh Vol.58 (1938); p.p.114-133  Crew F.A.E. A Case of Lateral Asymmetry in the Domestic Fowl Journ.of Genetics Vol.20 no.2 (1928); p.p.179-187  Desselberger H. Ueber das Lipochrom der Vogelfeder Journal fur Ornith. (1930); p.p.328-380  Driesen H.H. Untersuchungen uber die Einwanderung Diffuser Pigmente in die Federanlage, Insbesondere beim Wellensittich Zeitschr.fur Zellforsch.Vol.39 (1953); p.p.121-151  Hesford C. Comments on "The Halfsider" by Dr. John Pilkington http://ourworld.compuserve.com/homepages/clivehesford/halfside.html  Knox C.W. Color Chimeras in the Domestic Fowl Journ.of Heredity Vol.22 (1931); p.p.133-134  Klören H. Halfzijders ONZE VOGELS no.10 (1990); p.p.471 Lambert W.V. A "Half and Half" Skin-Color Mosaic in the Chicken Journ.of Heredity Vol.20 (1929); p.p.167-169 Pilkington J. The Half-Sider http://www.dolores.oaktree.co.uk/birdroom/genetics/halfside.html Völker O. Ueber den Gelben Federfarbstoff des Wellensittichs (melopsittacus undulatus) Journ.fur Ornithologie Vol.84 no.4 (1936); p.p.618-630 Völker O. Ueber Fluorezierende, Gelbe Federpigmente bei Papageien, eine neue Klasse von Federfarbstoffen Journ.Ornith. (1937); p.p.136-146 Völker O. Gelbes und Rotes Lipochrom im Integrument der Vogel Journ.Ornith.Vol.87 no.4 (1939); p.p.639-643 Völker O. Die Gelben und Roten Federfarbstoffe der Papageien Biol.Zentr.Blatt Vol.62 (1942); p.p.8-13 Völker O. Die Abhangigkeit der Lipochrombildung bei Vogeln von Pflanzlichen Carotinoiden Journ.Fur Ornithologie Vol.82 (1934); p.p.439-450