DICHROMATISM. The designation of a phenomenon among birds defined by Chapman as ((the existence of two phases of color in the same species?' Some writers have included under this term, albinism, or lack of pigment; melanism, or excess of dark pigment, producing an extensive or complete blackening of the stir face (common in mammals, but rare in birds) and erythrism, or an excessive reddening of the plumage; but it seems best to learn these outside of the present subject, and also the matter of normal differences in color between male and female and adult and young individ uals, for dichromatism implies the two phases existing independently of age, sex, or season* (Beebe, 'The Bird,' N. Y. 1906). The subject has been most thoroughly considered by Leonard Stejneger, but neither he nor any one else has been able to offer an adequate explanation of the condition.
The best known example is our little screech owl (Stops Asio), known over almost the whole continent. Most commonly its beautifully mottled plumage is decidedly red or deep red dish-brown in general tone; but many specimens are purely gray. The size, special ornaments, voice and habits of both are alike; but all the early American ornithologists considered them as two separate species, or else as young and old respectively— but some said the red ones were the adults and others the gray. In 1853, however, Dr. J. P. Hoy announced that he had taken both red and gray young from the same nest; and now it is understood that both varieties often occur in the same brood and in any part of the country, although on the Pacific coast the gray phase predominates. The same tendency to be of two colors exists in several other owls, as the brown owl of Europe, and the California pigmy owl (Glatscidium), as mentioned by Cones.
Dichromatism is also displayed by many other kinds of birds, notably those of the heron and family, and by certain sea-birds. One of the *ger-gulls appears in two different styles, one white on the under side, the other sooty black all over. The reddish egret (Dichroman assa rufescens) of Central America has a white phase long named separately as °Peale's' egret; and °Wurdemann's heron' is now considered merely a color-phase of the great white heron of Florida. The common little blue affords another example.
The only noticeable explanation ever offered for dichromatism is that it may indicate the beginning of a divergent, or °nascent° species, which might subsequently become fixed by birds of the same phase mating; but experiments with the screech-owl have shown that red and gray young will appear °whether both parents are red, or both gray, or one red and the other gray.' Barrows states ('Birds of Michigan,' Lansing 1912) that it °has been shown that in captivity the gray bird can be converted into a red one by feeding regularly with liver, and by withholding this food afterward the bird has eventually resumed the gray plumage. This would seem to indicate that the color of the plumage may be largely influenced by the char acter of the food, yet it is difficult to see how this fact can be used to explain the conditions actually found in nature.' Consult Steineger, (Birds) (Standard Na tional History, Vol. IV, Boston 1885) ; Allen, American Naturalist, Vol. II, 1889, p. 327.