In the Palaearctic,
Sterna hirundo ranges from Europe (
hirundo), Turkestan (
turkestanica), Mongolia (
minussensis) to Siberia (
longipennis).
The western races have red bill and legs and pink breast, the eastern having black bill and legs and greyish breast, and
minusseusis intergrades to the western races or the eastern race on both sides of its range (Stegmann, '36).
The young of European race has orangish bill and legs, and the author found, by examining the specimens in Yamashina Museum, that the young of
longipennis have also orangish bill and legs, while the one he obtained had the webs of feet mottled with orange and brown. These facts suggest that the ancestral form of
S, hirundo would have had orangish bill and legs, and red color became predominant in the western and black color in the eastern population.
The anthor attempts to explain this genetically by considering that, concerning the color of bill and legs, all the populations of this tern have a basic gene for orange, as well as the red and black-modifiers. Under European climatic conditions, the former (red) dominates the latter (black) giving also a pinkish tinge on the breast (which is the case in some European birds), while in Siberia the reverse takes place. This color change mechanism by change of dominance of modifiers under climatic factors might further be applied to explain the Gloger's rule, and is also considered to be affected by bird's seasonal physical condition (hormones), as in the change from summer to winter plumase. In
hirundo the red bill becomes black in winter.
The winter plumage often resembles that of the juvenile, which would be the primitive coloration based on a series of genes,
An. Then, a primitive form can be expressed by
An An. The adult plumage would be the result of mutations during the phylogenetical history and this mutant series of genes can be expressed by
Bn. The present forms of birds, therefore, have the genes,
An Bn, and
An is considered to be dominant during the young stage (
An bn), while as the bird matures, the change of dominance would, occur under physical (hormones) control (
an Bn).
In
S. hirundo, for example, the orange-gene corresponds
An, and with the growth, the European birds obtain the color
Bn1 affected by red-modifier and the Siberian birds
Bn2 by black-modifier. The spotted webs in the authors specimen would show a partial irregular change of dominance from
An to
Bn2.
In the case when sexes are different,
Bn can be divided into
Fn (_??_) and
Mn (_??_), and
Fn is often similar to
An.
Further complicated cases can be explained by such an idea, but future experimental proof is needed.
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