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COPYRIGHT 1996
Anthony Olszewski
MUTATIONS IN AVICULTURE
At the chromosome level, little is known concerning the mechanics
of mutation with respect to cage birds. For the avian practitioner, such
knowledge would be desirable but is not needed. Mutations of the feather
colors and the feather structure can be classified into a relatively small
number of categories. An understanding of the different types of
mutations will allow the discerning eye to judge whether a true breeding
strain can be developed from an unusual individual. This understanding of
mutation will also enable the avian practitioner to separate birds with
pathological metabolic disorders and "artificial mutations" from actual
mutations.
As in any form of domestication, mutations are of central
importance to aviculture. Even initial captive breedings may be as much a
matter of the selection of suitable pairs as of husbandry. As soon as
such a first breeding has been accomplished, the aviculturist strives to
produce superior or novel strains. Superior may mean freer breeding,
steadier, resistant to disease, or able to withstand extremes of climactic
conditions. Novelties are selected most often on the basis of size,
feather color, and feather structure.
Mutations are also valuable in the monetary sense. About ten years ago
years ago a lutino Peach Faced Love Bird(Agapornis roseicollis) sold for
one thousand dollars. Such examples of high prices could be listed for
any of the early mutations of exotic birds. Such mutations create, albeit
temporarily, a gold rush atmosphere. The price of even the most drab and
common colors goes up, since interest in and market consumption of the
species increases. Many more people start to keep and breed birds of all
types. Thus it can be seen that mutations serve both to create, improve,
and vary captive strains and to promote aviculture.
The genetics of the mutations in aviculture are relatively simple.
A review of basic genetics will now be given. The patterns of inheritance
that are most important to the production of cage birds will of course be
stressed.
Simple, autosomal, recessives are the largest class of mutations
of exotic birds. It is possible for such recessive traits to be present
in a proportion of the wild population as heterozygotes. Such
heterozygotes exhibit the normal phenotype. The mutant phenotype of
autosomal recessives manifests itself only in the homozygote. Any regime
of inbreeding will quickly bring such mutations to the surface. As soon
as any species becomes established in captivity, such factors are
discovered. This is the most likely explanation behind the phenomenon
noted by Darwin that domestication seems to induce variation.
Domestication does not increase the rate of mutation, but it does increase
the probability of the production of homozygotes. Also such mutant
phenotypes that might be culled by natural selection will be favored by
artificial selection.
The autosomal recessive traits are denoted by lower case symbols.
The corresponding normal, wild gene is given by a plus sign, +.
As a practical matter such mutations may be discovered by
inbreeding. This system is exploited by Dutch breeders of Australian
Parakeets. These clever fanciers seem to turn out mutations on an
assembly line basis.
Dominant autosomal mutations are less common in aviculture. If the
dominant mutated individual appears in captivity from normal parents, the
mutation must have occurred in the germ cells of the parents. The
extremely low rate of mutation explains the paucity of such traits.
Dominant genes are of two types:complete and incomplete. Complete
dominance allows heterozygotes to express the full mutant phenotype. With
incomplete dominance, heterozygotes show a compromise between the normal
and the mutant phenotypes.
The manifestation of the dominant genes is affected by the
phenomena of penetrance and expressivity. Penetrance is defined as "The
percentage of individuals that, carrying a gene in proper combination for
its expression, actually express that genes phenotype." Expressivity is
defined as "the manner in which phenotype is expressed." These terms
clearly describe two aspects of the same situation. Penetrance refers to
the clear cut, discrete, cases where a given trait is or is not expressed.
Expressivty refers to the full range, continuous cases, of intermediate
examples.
Another way of looking at this would to be consider the dominant
trait as not fully suppressing the recessive trait. This may be noted in
the clear dominant white mutation of the canary(Serinus canarius). Rarely
this mutation shows only the faintest hints of lipochrome in the wings.
More often they will have well defined bars of lipochrome color. At the
other end of the spectrum, some show a red or yellow suffusion throughout
the entire plumage.
Expressivity is almost certainly due to the action of modifiers.
Without controlled experiments we are not able to simply discount
environmental influences. Modifiers are secondary genes that mold the
phenotype. "Even when only one principal gene is involved, its expression
is influenced by some extent by countless other genes with individual
effects often so slight that they are very difficult to localize and
analyze." Thus modifiers are by definition a multiple allele phenomena.
Even though multiple alleles may not be analyzed by simple Punnet
squares, we, as aviculturists, can still control them. Simply and
drastically, we can cull out any bird that we deem undesirable. More
subtly we can utilize as stock birds specimens that are extreme examples
of the expression of a trait. By blending these together, we hope to take
a middle path and so obtain a few outstanding birds.
The symbol for dominant genes is a capital letter. The
corresponding wild, normal allele is again given by a plus sign.
Sex linkage is a very important pattern of inheritance in
aviculture. The sex chromosomes are of two types:X and Y. The X
chromosome is relatively large and contains many genes. The Y chromosome
is much smaller and, in the case of cage birds, no traits are known to be
located on it. Male birds have X chromosomes in pairs. In females, the
sex chromosome pair consists of one X and one Y. The X chromosome is
sometimes designated as Z and the Y as W. The Z-W notation is used in
most genetic literature to signify the difference between birds and
mammals. The sex linkage system in mammals is the inverse of that in
birds. Avicultural literature universally uses the X-Y notation.
All sex linked mutations so far described in exotic cage birds are
recessive. One reason for this, as for autosomal recessives, such sex
linked recessives can build up hidden in the wild gene pool. Dominant sex
linked traits are certainly possible, in fact, Levi describes many common
ones in the domestic Pigeon(Columba livia domestica).
For the male bird to exhibit a sex linked recessive phenotype, he
must posses to such factors, be homozygous for the mutant gene. A hen
will exhibit the mutant gene with only one factor. This is explained by
the fact that the X chromosome has no corresponding allele on the Y
chromosome to dominate the normally recessive mutation. Such hens are
called hemizygous. Some traits seem to act differently in the homozygous
and the hemizygous configurations. Two examples are the Pearl mutation in
the Cockatiel(Nymphicus hollandicus) and the Pastel mutation in the
Canary(Serinus canarius). More research is needed to confirm these
observations. An alternate explanation is a different phenotype is due to
hormonal differences.
In some organisms a few genes are located on the Y Chromsome.
None have been described for cage birds.
Crossing over and linkage must be taken into consideration when
any traits that are to be combined are located on the same chromosome
pairs. This is obviously the case for any double mutations involving sex
linked traits. This is also true for autosomal chromosome pairs. An
example is the traits for blue, s, and the dark factor, D, in the
budgerigar(Melopsittacus undulatus). The breeder attempting to get a
combination of the dark and blue mutations is in for a surprise if he
starts by mating an olive, a two factor green bird, with a sky blue. This
mating will produce all one dark factor green birds that are heterozygous
with respect to, split for, blue. Realizing that blue and the dark factor
are autosomal, not sex linked, he computes the expected frequencies of the
progeny. Mating a green carrier of blue to a blue bird yields 50% blue
and 50% green carriers of blue. Pairing a dark factor bird to a no dark
factor bird gives 50% one dark factor birds and 50% normal, no dark
factor, birds. By simple multiplication(50% blue X 50% one dark factor),
this fancier expects 25% of the young to be cobalts, one dark factor blue
birds.
If only a few pairs are being used it is very likely that no
cobalts will result. If he manages to breed one hundred young from this
sort of pairing, on the average, only seven cobalts will be obtained.
Why is there this gap between theory and practice?
Our fancier assumed segregation of the traits. These genes do not
follow Mendel's law of independent assortment because they are not
independent. Both traits, blue and the dark factor, are located on the
same chromosome. Genes located on the same chromosome are said to be
linked or in linkage.
During prophase 1 of meiosos homologous chromosomes, chromosome
pairs, are situated in close proximity to each other. Sometimes points of
contact and breakage are formed. These discrete points are visible under
magnification and are called chiasma, plural chiasmata. At these points,
single strands, chromatids, of each chromosome may break and switch
chromosomes. This whole process is known as crossing over. It is through
crossing over that linked factors may undergo recombination.
There are two basic plans by which linked traits may become
involved in crossing over:in coupling and in repulsion. Genes are linked
in coupling when mutant traits are located on the same chromosome. They
are linked in repulsion when mutant genes are located on opposing
chromosomes. See figures 1 and 2.
Figure 1 s+ /+D
Figure 2 sD/++
Figure 1 symbolizes traits linked in repulsion. In figure two
they are in coupling. Linkage in coupling is also known as cis linkage.
Linkage in repulsion is also known as trans linkage.
Most avicultural literature, in particular budgerigar literature,
refers to linkage in repulsion as type 1 linkage, type 2 designating
linkage in coupling. This is an undesirable terminology for several
reasons. Firstly, no genetic text books ever use these terms. Secondly,
some authors switch type 1 with type 2. They designate type 1 for
coupling and type 2 for repulsion. This leads to confusion. Also some
authors use type 1 and type 2 to label a host of unrelated
concepts:homozygous versus heterozygous, separate forms of the yellow face
factor in the budgerigar, and various genotypes of specific phenotype,
e.g.the fallow phenotype in the budgerigar. Thus we should follow the
example of the geneticists.
The concepts of coupling and of repulsion are of both practical
and of theoretical importance. The percentages of the genotypes and the
phenotypes produced are most often different, since the percentages of
young produced from crossing over is most often different.
This difference may be best observed by the frequencies of young
produced from the cross of a one dark factor green budgerigar that is
split for blue with a sky blue bird. Both blue and green are autosomal,
non sex linked traits. These factors are located on the same chromosome
and are thus linked.
The first example will be for in repulsion
D - dark factor
olive green + D /+D X s+ /s+ sky blue
The symbols graphically represent how the traits, both mutant and
normal, are located opposite to each other on the chromosome.
This bird will produce the following gametes:+D, s+, ++, and sD.
The last two gametes are due to crossing over. Pairing this bird with a
sky blue will give the young described in the accompanying chart.
Percentages are according to Hart.
+ D/s+ X s+/s+
+ D /s+ 43% dark green/blue
An alternate method to get a dark green carrier of blue is to mate
a mauve, a two dark factor blue budgerigar, with a light green.
mauve sD/sD X + + /++ light green .
This is an example of traits in coupling. Though of the same
phenotype as the preceding example, the genotype differs by the
arrangement of the factors on the chromosomes. The mutant genes are on
one chromosome and the normal genes are on the other chromosome of the
homologous pair. Pairing this bird with a sky blue gives the same
genotypes and phenotypes as the case in repulsion, put the frequencies are
modified.
+ + /sD X + s /+s
+ + /+s 43% light green/blue
Percentages are again according to Hart.
Thus the position of the genes determines the manner in which
crossing over occurs. In the first example, linkage in repulsion, the
gametes ++ and sD are only obtained through a crossover. This is the
explanation for the low frequencies of cobalt and light green/blue young.
In the second case, the example of linkage in coupling, the gametes +s and
+D are obtained by means of the process of crossing over. Therefore the
sky blue and the dark green/blue young are of the lowest frequencies.
The probability of crossing over and the implied frequencies of
the progeny may only be inferred from breeding results. The closer the
two linked traits are located to each other, the less likely is the chance
of breakage and the subsequent crossing over. Genes located at a great
distance from each other have a much greater probability of crossing over.
This probability is expressed as a percentage and varies from 0% to 50%.
At the rate of 0% there is, for all practical purposes, no chance of
crossing over. The percentage of 50% implies that, since the chance of
breakage and recombination is so high, in practice, the traits may be
considered as independent. The probability of crossing over for any two
specific traits, as it is a function of the location of the gene locus, is
nearly constant. To calculate this probability, the percentage of
crossing
over may be expressed by the following formula:
PERCENTAGE OF CROSSING OVER =( NUMBER OF CROSSING OVER INDIVIDUALS/
TOTAL NUMBER OF YOUNG) X 100
No complete chromosome maps have been constructed for any exotic
birds. There are many problems concerning the formulation of such maps in
birds. All birds have microchromosomes. These microchromosomes are
extremely small, less than one micron in diameter. Even under
magnification these are very small, in fact dot like, and hard to
distinguish. Because of the difficulty of viewing these structures, the
exact counts of even the well researched species is not known. The
accepted counts of the domestic pigeon(Coloumba livia domestica), the
budgerigar(Melopsittacus undulatus), and the canary(Serinus canarius) are
+ _80, + _58, and + _80 respectively. The plus or minus notation is used
to stress that the exact count is not known. For the majority of cage birds,
no attempt at a karyotype has been made. Very few traits have been
documented in ornamental birds. Once we possess a more comprehensive
understanding of the genetics of avicultural subjects, we will be in a
position to deductively construct chromosome maps.
We all learned as children that the whole equals the sum of the
parts. This very basic mathematical concept is so obvious that we accept
it as common sense. Common sense does not carry much weight in genetics.
Here the whole, the phenotype, might be more or less than the sum of the
parts, the genotype.
The most important form of genetic interaction is epistasis.
Epistasis is defined as "the suppression of the expression of a gene or
genes by other genes not allelic to the genes suppressed. Similar to
dominance but involving the interaction of non-allelic genes." Epistasis
is sometimes referred to as genetic masking for it may disguise or
camouflage the genotype. Epistasis implies hypostasis in the same way
that dominant implies recessive. The gene that is doing the masking is
said to be epistatic to the other trait. The trait that is being masked
is said to be hypostatic to the first factor.
Related to the concept of epistasis is the phenomenon of
complementary genes. With complementary genes two or more traits must all
be present, in the proper dosages, for a given phenotype to be expressed.
The crest factor in the budgerigar is a perfect example.
Lethal traits so disrupt the metabolism that they cause the death
of the individual. Dominant lethals are clearly self deleting. Recessive
and incomplete dominant genes are perpetuated. In cage birds, very few
lethal traits have been posited:the crest, hard feather, and dominant
white traits in the canary. These traits are all incomplete dominants.
In one factor, in the heterozygous state, these genes produce an unusual
phenotype, desirable to the fancier. In two factors, in the homozygous
configuration, they cause death. Penetrance and expressivty may also come
into play.
Mutations, though often of an essentially simple genetic nature,
tend to become confused in aviculture. Unusual specimens fetch high
prices and bring prestige to their owners. This fact generates one of two
responses from the person lucky enough to spot something different in the
nest or in a consignment of wild caught birds. The egotistical fancier
informs all that he possesses a new variety. The aviculturist more noted
for business acumen will keep his new type a secret and build up his
stock. In this way the market is cornered and a handsome profit may be
realized at the time of sale of the novelties. All give poetical and
fanciful names to mutations.
On occasion the same mutation arises in two or more locations at
about the same time and greatly confuses the issue. Most every country
regulates the import and export of birds. In Australia, an important
country both ornithologically and aviculturally, trafficking in birds is
all but outlawed. It may be years before the proper test matings are
performed to determine the true nature of the new mutations. Without test
matings, only comparisons from photographs or, even less reliably, from
memory can be used. These comparisons, even if perfect pictures are
available, are only the roughest of guides. The same mutation may be
drastically affected by modifiers or by environmental factors.
Conversely, identical phenotypes may be expressed by completely different
mutant genes. For example, there were originally both a sex linked
recessive and an autosomal recessive ino factor in the budgerigar. Sadly,
the latter has been lost.
The term sport is sometimes used with a wide range of meanings.
The broadest definition is of any different and unusual appearance. This
would include extremes of phenotype caused by both genetic and non genetic
factors. We here restrict the definition to include only oddities that
owe their unusual appearance to environmental or to pathological factors.
Of course, such phenotypes will not be passed on the future generations.
Sports are not unknown in birds. Many unusual colors are caused
by metabolic disorders or by injury to the skin or growing feather. Some
very striking color combinations, half siders and other mosaics, though of
a genetic nature, are also not inheritable.
Hybrids, inter-specific crosses, are common in aviculture. Mule
breeding, the production of mostly sterile hybrids involving the canary
and various finches is very popular in Europe. The society finch(Lonchura
domestica) is possible a free breeding blend of several Mannikin species.
With species of waterfowl, it takes real effort to not get hybrids.
Hybrids are very interesting genetic subjects. Mutant genes have been
transferred to the canary and some love birds from closely related species
by means of hybrids.
Unfortunately, the haphazard production of hybrids has often
become a liability in aviculture For example, it took many years for the
American stocks of Love Birds to become sorted out after various species
had become mixed up. Sometimes a hybrid might be mistaken for a new
species or for a mutation.
At best, the heredity of birds is not easy to study. The shortest
generations are several months. Many birds take years to mature. Some
species insist on choosing their own mates. Others are difficult to keep
alive in captivity, let alone rear. For the altricial species, some are
poor parents. In these cases, the breeder must attempt to foster or to
hand raise the young. This is not a perfect solution, for imprinting and
other unnatural forms of socialization complicate further breeding. Even
relatively simple mutations may be lost. The London fancy color variety
of the canary was lost through ignorance. At the turn of the century this
simple recessive melanin diluting gene was confused with variegation.
Through pairings with variegated birds, the rare recessive was lost. We
are very lucky that any mutations have been established for the more
temperamental species.
The great majority of mutations in aviculture affect the color of
the plumage. A basic knowledge of the mechanics of feather color is
needed. The appearance of color in the feather is due to two
mechanisms:chemical coloration and structural coloration. Biochromes,
compounds actually present in the feather, cause chemical coloration.
Structural coloration, on the other hand, produces an optical illusion by
means of anatomical elements in the feather. These elements might manage
to give the appearance of a blend of chemical colors. Orange, red, and
yellow are most often caused by carotenoids being deposited in the
feather. These compounds are called lipochromes in the avicultural
literature. These chemicals are metabolized from plant and animal matter.
They are not synthesized by the bird.
Species that possess carotenoid colors will often exhibit
variation in color due to changes in the diet. The red of the male
Virginia Cardinal(Richmondena cardinalis) fades in the North East United
States during the Winter when fruits, berries, greens, insects, and other
small animals that are part of the bird's diet become scarce. The
Venezuelan Black Hooded Red Siskin(Carduelis cuccullata) will lose its
natural red color, turning yellow, if not offered a source of carotenoids.
Yellow ground birds can obtain a sufficient supply of carotenoids
from a diet of seed. This is not true for red or orange ground birds. In
captivity the most common practice is to supply these birds with a source
of synthetic carotenoids. The most commonly used substances are
beta-carotene, apo-carotenol, and canthaxanthin. Canthaxanthin gives the
brightest scarlet red. Beta-carotene and apo-carotenol allow the birds to
develop golden, orange shades. Beta-carotene is of limited usefulness for
much is metabolized as vitamin A, which lacks color, according to Doctor
Adams, technical director of Hoffman La Roche. All these chemicals may be
used together, for their action is complimentary.
In parrot type birds orange, red, and yellow are probably not
caused by carotenoids. George Smith has posited a new class of chemical
compounds. He has named these substances psittacins. In parrot types,
color is not clearly a function of the diet. Ramon Noegel states that
birds raised in captivity do show a greater extent of red and orange
color. His believes that this increased degree of color is related to a
diet rich in carotenoids. It must be noted that an improperly fed scarlet
macaw or chattering lory, or any other scarlet parrot, might be near death
from malnutrition, but still have fire engine red plumage .
Unique to touracos, musophagiformes, is green caused by the
deposition of a biochrome, turacin, in the feathers. Since there are, to
my knowledge, no mutations of touracos in aviculture, all cases of green
plumage may be considered as structural colors.
The blacks, browns, and grey colors are from melanin pigments in
the feather. These pigments are synthesized by the birds from amino
acids. The exact color and shade is due to the size, density, and shape
of the melanin granules. Mutations that affect melanin coloration most
often affect either the size or density of the granules.
White is a structural color. Here all micro structures in the
feather are transparent, including the covering cuticle. Since the whole
spectrum of light passes through, our eyes register the color as white.
This is seen, literally, in any color less chemical. An single sugar
granule is transparent. A teaspoon of sugar is white.
Slate blue, as in the canary, is given by a combination of a
transparent cuticle and underlying melanin cells. The transparent cuticle
is called white ground in aviculture. The bright blue, as in the blue
jay(Cyanocitta cristata) is due to a refractive layer of polyhedral cells
situated between a transparent cuticle and a refractive melanin base. A
green effect occurs when yellow carotenoids or psittacins are spread
through the cuticle. Avicultural writers call this yellow ground. If the
carotenoids or psittacins of the cuticle are primarily of a red nature, a
red ground bird is the result. If no melanins are present, a red or
orange hue is here observed. Red biochromes do not readily interact with
melanins to form structural colors. Red is most often obscured by
melanin.
Iridescence is given by spectral colors due to light interference.
This interference is caused by twisted and broadened, melanin containing,
barbules or by spherical granules of melanin in close proximity to the
cuticle.
Since feather color is governed by only two phenomena, all color
mutations may be divided into two classes:mutations of the biochromes or
of the micro structure of the feather.
The most common mutations are of the chemical colors that affect
the melanin granules:inos, cinnamons, fallows, pieds, and yellows.
The inos are the most distinctive colors. All the melanin is
deleted from the entire bird. Here the ability to synthesize melanin is
completely disrupted. A pinkish color is seen in the eyes, beak, skin,
and nails. This is actually the red blood circulating below the
transparent tissues. Carotenoids or psittacins are not affected. Thus an
ino love bird is yellow with red-orange markings. A bird that is of a
predominantly white ground is an albino. A yellow one is a lutino. A red
orange ground bird is called a rosino. Unfortunately, the term rosino is
often misused. The rose bourkes neophema(Neophema bourkii) and the rosino
canary are not inos. These mutations do not delete all the melanin. The
canary does have a true ino mutation, possibly transferred from the
European Greenfinch(Chloris chloris), the satinette variety.
Cinnamon-inos are special cases. Ino mutations do not completely
delete brown melanin. Daniels has shown that it is not unusual for birds
of this genotype to evince a laced phenotype. This is seen in the brown
satinette canary and in one form of the lacewing budgerigar. Similar
cinnamon-ino colors are being researched in peach face love birds and the
cockatiel.
Many species have patches of different ground colors. The most
notable example is the cockatiel. These patches are almost completely
hidden by melanin in the normal bird. Once the melanin is removed, the
previously hidden psittacin is revealed.
In pieds the melanin mutation is removed in patches. These
sections may be almost completely random, like the variegation in the
canary, or may be very definite, as is the European clear flight mutation
in the budgerigar. Most pieds are neither completely restricted to
certain areas, nor are they random. The Australian banded pied mutation
in the budgerigar affects mostly the feathers of the lower wings, belly,
and tail, often giving a circle or band of clear feathers about the
waist. The harlequin mutation of the budgerigar affects mostly the head,
upper wings, and chest. The exact delineation of the areas from which
melanin will be deleted is random.
Birds may be completely pied. In budgerigars such examples are
called black eyed clears. I will use this term for all species. Here all
melanin is removed from the skin, nails, beak, and feathers. Melanin is
retained in the eyes. Black eyed clears may be obtained from selective
breeding of a single pied mutation. For example, in the American pied
peach faced love bird light strains exist. These are extremely pied birds
that produce a percentage of black eyed clears. Variable penetrance is
very common among pied mutations.
Black eyed clears may also result form a combination of distinct
pied mutations. The black eyed clear budgerigar is derived from a cross
of the European clearflight and the recessive pied. In the canary black
eyed clears, most often called simply clears or lipochrome birds, are
birds that possess two factors for an incomplete dominant, V, the
variegation gene. Individuals of the genotype +/+ are the normal melanin
forming birds. Those of the genotype V/+ are variegated, pied.
Cinnamon(brown) and fallow mutations change the shape and size of
the melanin granules. The melanin is changed from black to brown and the
size of the granules may also be reduced. Cinnamons often have red or
plum colored eyes as nestlings and juveniles. In fallows this red eye is
retained in the adults.
Strangely enough, despite the profusion of melanin diluting and
restricting mutations, traits that increase melanin distribution are very
rare in aviculture, at least in the exotic species. The only one is the
black breasted zebra finch(Poephila eastanotis). All other black forms
have so far proven to be sports.
Yellows result from genes that affect the quantity and
distribution of melanin granules. Yellow mutations reduce the amount of
melanin in the feather. A lighter, diluted, but not clear appearance is
the result. Yellow might not change all the feathers. The clearwing
mutation of the budgerigar most effectively reduces the melanin in the
wing, but does not reduce the body striations. Yellow type genes can also
restrict the melanin distribution to certain parts of the feather or
body. The lizard mutation in the canary deletes the melanin only from the
edge of the feather, but does not change the granules elsewhere. The
terminology yellow is most descriptive in yellow ground birds. In white
ground birds the term white may be used, as it is for a budgerigar
mutation. It is important to keep in mind that here we are discussing a
whole class of melanin affecting mutations.
A few categories of mutations affect the ground colors.
As has been explained, the ground colors are due to the presence or
absence of carotenoids or psittacins in the plumage. The only true
mutations of ground colors are those that reduce or delete the pigment,
producing a white ground or, like the ivory factor in the canary, reducing
the deep original shade of the ground color.
The deletion of carotenoids or psittacins from the feather is very
common. This is seen in the blue canary and the blue budgerigar. The
phenotype is blue for the melanins are unaffected by this group of
mutations. The combination of this sort of mutation and an ino factor,
thus deleting all biochromes, is an albino. Albino double mutations have
been produced in the canary, budgerigar, and cockatiel.
At this point a brief digression concerning feather anatomy is
appropriate. The central shaft or quill of the feather is called the
rachis. The visible rays of the feather that run perpendicular to the
rachis are called barbs. Very small, but just visible to the naked eye,
are structures called barbules that run perpendicular to the barb. Under
magnification, hooklets, or barbicels, may be observed. These hooklets
catch adjoining barbules, holding the feather in a continuous sheet. The
importance of these hooklets to mutations will soon become apparent.
The ivory factor in the canary is somewhat more interesting and
also more complicated. This sex linked recessive manifests itself in a
ground color one shade lighter than normal. Yellow ground ivory canaries
have the light yellow bone ivory color of old piano keys. Red ground
ivories appear rose or pink. This pale ground color occurs because
carotenoids are deleted only from the hooklets. The pigmentation in the
other parts of the feather is unchanged.
The other traits that are thought to affect the ground color are
actually mutations of the structure of the feather.
The most commonly noted change in the feather structure is soft
feather or buff. This is seen particularly in canaries, but also in
budgerigars. Soft feather birds have defective hooklets on the barbules.
The feathers are in this way all slightly raised. With the canary, a
white frosting from the defective hooklets may be observed without
magnification. This sort of plumage gives the bird a larger appearance.
Soft feather reduces the intensity of all colors.
In the budgerigar, the feather duster, an abnormally long
feathered bird, thus the name, is said to be a genetic abberation - analogous
to Down's syndrome in the human. Feather dusters generally die soon after
leaving the nest.
The dark factor of the budgerigar is due to changes in the feather
structure. This gene reduces the layer of cells on the barbs that scatter
and reflect light. Due to this less efficient structure, a dark factor
budgerigar is of a deeper color. Because the reflecting layer is thinner,
more color escapes. This trait reduces the layer of cells by about one
third for each dose of the gene present. In a dark green the layer is
about two-thirds of normal. In an olive, a two dark factor budgerigar,
the layer is about one-third of normal. Similar acting dark factors are
seen in the peach faced love bird and in the Indian ringneck parakeet.
The only remaining types of mutations are the long flight in the
budgerigar, the frill in the canary, and the crest in the canary,
budgerigar, society, and zebra finch. All other mutations of exotic and
ornamental birds may be classified according to the previous discussion.
Modifiers are very important in all domesticated species. The
very changed size, shape, plumage, and posture of the Norwich, Belgian,
Yorkshire, and Scotch Fancy canaries are due to modifiers. Fanciers,
through slow, selective breeding and the artful combining of breeds,
derived these varieties.
Distinctive strains of may other species exist in aviculture.
Some lines of Lady Gouldian finches(Poephila gouldia) are very free
breeding but require foster parents to rear the young. The most commonly
used foster parents are society finches. Other strains of Gouldians are
known to be good parents. Some American fanciers are consistently rearing
Lady Gouldians about fifty percent larger than usual, thus creating a
modified phenotype. Families of pied peach faced love birds and pied
cockatiels that are very light also exist. Primrose, extremely yellow,
cockatiels also are available as distinctive strains.
Splashed or variegated birds are the most common sports. These
are relatively common in Indian ringneck parakeets(Psittacula krameri
manillensis) and budgerigars. Other parrot types sometimes spontaneously
develop maroon patches. Some lutino cockatiels suddenly get a deeper
yellow color. Black canaries have been produced but have never been
reproduced.
No bird should be dismissed as a sport without careful test
matings. Any individual that was born with a normal phenotype but
develops an unusual color must be suspect. A careful examination by a
qualified veterinarian is certainly in order.
Half siders are birds that have a dual phenotype. The plumage,
size, color, sometimes even the gender, differs from the left and right
sides. It seems as if two halves of different birds have been glued
together, which is not far from the truth. Half siders are one facet of
the larger phenomenon of mosaicism Mosaics are organisms of patchwork
phenotype and/or genotype. In aviculture, half siders are most frequently
encountered in budgerigars, though they have been reported in canaries and
Lady Gouldians.
Hollander has discussed half siders in many species of birds.
Though of a genetic origin, half siders can not be intentionally bred.
Half siders are the result of cytological accidents and are the inverse of
twins. The half sider phenotype is not inheritable. A strain of half
siders can not be developed.
Hybridization can be a valuable technique in aviculture. Fanciers
have always delighted in the production of novel forms. The progressive
aviculturist uses hybrids to achieve specific results. The red factor
canary was produced by a cross of the Venezuelan red hooded
siskin(Carduelis cucullata) and the canary. The satinette in the canary,
actually an ino factor, as described above, might have been the result of
a cross of the canary with the lutino European green finch(Chloris chloris
var.) the yellow gene was transferred to the Fischer's love
bird(Agapornis fischerii) from the masked(Agapornis personata).
The haphazard production of hybrids must be descried. The clumsy
breeding of mules and intergrades can not be tolerated. All birds used in
a hybrid breeding scheme must be closed banded. Closed bands are seamless
metal rings. They can only be placed on a young bird within about two
weeks after hatching. These bands are coded and allow positive
identification. Any birds that can not be identified should be destroyed.
Frauds are not unknown in aviculture. South American Indians have
many techniques for treating the growing feathers of parrots to get
bizarre and beautiful colors. Dyed finches are seen in quarantine
stations. The most common fraud is the "double yellow head" conure. The
cheap green conure becomes an expensive peroxide blond and is passed off
the unsuspecting Yankee tourist as a juvenile Mexican double yellow head Amazon
parrot. Similar combinations of dyes and bleaches must always be looked
out for upon the announcement of any new and high priced mutation.
A more subtle form of deception also takes place. Many mutations
in aviculture are sex linked recessives. Clearly, hens can not be split
for these traits. Sometimes the developer of such a trait reports the
mutation to be an autosomal recessive. This way he can sell normal hens
as high priced splits. This occurred with the rosy variety of the
Bourke's neophema.
IN CONCLUSION
This paper shows that the overwhelming majority of mutations in
aviculture can be broken down into a limited number of patterns, in all
species of birds. The underlying genetic phenomena show an amazing degree
of similarity.
All bird breeders must be extremely selective. In all
probability, many species will soon become extinct as viable wild
populations. The only hope for these birds is aviculture. We must decide
what sort of population or populations that we want to maintain. If the
hope is for eventual re-introduction, the wild type, both in appearance
and in behavior must be used as a model and as an ideal. Some feel that
the wild type is superior on strictly aesthetic grounds. I hope that the
normal type of most captive birds will not be lost. I see no reason why
different breeds, as have been produced in all domestic plants and
animals, should not be developed. Selection can not be held in abeyance.
It can be either used to improve and vary a species, or ignored with
penalty.
Most every country regulates the import and export of birds. This
will greatly affect the ability of aviculturists to work with wild
species. It will also become increasingly difficult to make test matings
between mutations occurring in different parts of the world and to thus
determine if two birds of similar description are the same or different
mutations.
We pontificate concerning the uninformed and poverty stricken
third world peoples that catch and sell wild birds. The Western
aviculturist is noted for his pious sermons of "saving from extinction."
Unfortunately all too few breeders of the larger parrots deserve to be called aviculturists. Instead of
developing any captive strains - despite
their knowledge of the reality of rain forest destruction and their relative degree
of financial comfort - they sell all the young parrots produced as pets. I
hope that there is a special place in hell for these hypocrites. Perhaps
a waste land that was once a rain forest?
Anom., 1981, MOULT AND FEATHER COLOR, Bird World, Aug-Sept
Daniels, Trevor, 1981, UNDERSTANDING CINNAMON INOS, Cage and Aviary Birds,
Jan. 17
Darwin Charles, 1872, THE ORIGIN OF SPECIES, 1958 ed., The New American
Library, NY, NY
Hart, 1978, BUDGERIGAR HANDBOOK, TFH Publications, Neptune, NJ
Hollander, W.F., 1944, MOSAIC EFFECTS IN DOMESTIC BIRDS, Quarterly Review
of Biology,19:285-307
Keeton, W.T., 1980, BIOLOGICAL SCIENCE, W.W. Norton, NY, NY
Levi, THE PIGEON, Levi publishing
Merrel, 1975, AN INTRODUCTION TO GENETICS, W.W. Norton, NY, NY
Ohno, S., 1970, EVOLUTION BY GENE DUPLICATION, Springer-Verlag, NY
Smith, G.A., 1980, MUTATION COLOURS IN PARROTS, The Magazine of the Parrot
Society, vol XIV, #9, Sept. Nov, 220-222
Smith, G.A., 1981, COLOUR MUTATIONS, The Magazine of the Parrot Society,
vol. XV, #11, Nov., 307-310
Smith, G.A., 1982, FEATHER DUSTERS BUDGERIGARS:HOW THEY OCCUR AND WHY
THEY
DIE, Cage and Aviary Birds, Jan. 23
Mutation has been defined as "a heritable change in the base
sequence sequence of a cistron." These disrupted base sequences are
caused by mistakes in DNA replication. Due to the structure of the DNA
molecule, and due to proofreading by repair enzymes, such errors are very
rare. Mutation may occur at a universal rate of 1 X 10-7 per base pair
per generation under normal conditions. Certain chemicals, radiation, and
even high temperatures, can contribute to an increase in the probability
of such mistakes.
s - blue
The young will all be: + D /s+ Dark green/blue
s+/s+ 43% sky blue
sD/s+ 7% cobalt
+ +/ s+ 7% light green/blue
The young will all have the phenotype of ++/sD
sD/+s 43% cobalt
s+ /s+ 7% sky blue< br> +D/s+ 7% dark green/blue
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