Some clarity on Melanism (photo shows redwing (turdus iliacus) dark phenotype present in breeding.
PLUMAGE COLORATION AND THE CAUSES OF THE ABNORMALITY
The polychromatic appearance of the birds is due to the presence of special substances that are deposited in the feather structure during its growth.
The main pigments (organic coloring products of purely metabolic origin) present in the structure of the plumage are: melanins (substances responsible for black, gray, brown, tawny and dark-yellow coloration) and carotenoids (which give yellow, orange, red and, very rarely, blue-violet).
The blue and azure hues are normally produced by “optical,” not bio-chemical means; in practice, the presence of these colors depends on the particular way in which light is reflected from the surface of feathers and feathers. This is also how the numerous iridescences and “metallic” and “bronze” reflections arise.
Finally, the manifold variety of shades depends on how the different dyes and reflective areas are distributed in the right doses and in the right places.
In some cases this natural pigment arrangement does not occur according to traditional patterns and then, occasional individuals exhibit abnormal, but often aesthetically pleasing liveries, at least to human eyes.
These specimens exhibit their characteristic aberrant appearance from birth; this happens because the real cause of the coloration abnormalities is directly related to the erroneous development of the growing chick, still inside the egg.
In practice, the organism produces the coloring substances in quantities either too small or too abundant compared with those present in the metabolism of “normal” individuals; the excess or deficient synthesis of these substances is the essential difference that allows the distinction of the various main types of chromatic aberration.
The origin of the hypo- or hyper-production of pigment (which generates the abnormality) is inherent, in turn, in a mutant genetic code, whose random variations at the chromosomal or amino acid level, are reflected in bizarre development and, generally, abnormal metabolism (this abnormality in the bio-chemical functioning of the organism can be so far-reaching as to become lethal; this is why many “mutants” are aborted before birth).
CHROMATIC ABERRATIONS: THE “BASIC” TYPES
Coloration abnormalities fall into two broad categories: if a specimen’s organism produces the coloring substances in smaller amounts than usual we will have (depending on how small the actual amount of pigment produced is) Isabellism, Leucism or Albinism; if, on the contrary, too many substances are accumulated in the pens we will have (depending on the type of pigment overflow) Melanism or Erythrism
Sometimes the abnormality is uniform over the whole body, other times it occurs in scattered patches. In addition, the parts not covered by feathers (beak, legs, eyes, and any patches of bare skin) show the same aberrations exhibited by the plumage, as they contain the same coloring substances present in the feathers.
Melanism and Erythrism
Unlike the cases previously seen, Birds affected by these genetic abnormalities possess an exaggeratedly abundant amount of dark pigment, which is deposited in the structure of the most superficial tissues.
Melanic specimens appear uniformly dark (partial melanism is rarer than complete melanism) even where, in typical individuals, light, white, grayish or yellowish hues normally predominate.
Erythrism consists simply of the predominance, over other coloring substances, of darker carotenoids, which, together with melanins, generate a plumage that is (usually) uniformly and exaggeratedly reddish, brown or, even, purplish. Among the various forms of chromatic aberration, erythrism is certainly one of the rarest and most peculiar.
