DICHRO'MATISM IN BIRDS (ak It-. di-, double + xpCiga, ehroma, color). The name given to that peculiar occurrence of two phases of color in the plumage of a species of bird, not I hie to age. sex, or health, which is so well illustrated by the common screeeh-owl of the l'nited States ( ilegaseops asio). some individuals of which are gray and some rusty red. This phe is not infrequent among birds, but it is not always easy to determine what constitutes a clear case of it. Thus some ornithologists regard the various species of flicker (Calaptes) as di. or rather tri-chromatic forms of a single species. while others do not regard this as the same sort of a phenomenon as the color phases of the sereech-owl. There are at least three distinct forms of dioeh•omatism, known as atbinism, me and a rythrism. The tirst is that form in A{ h it'll I Me of t hi phase. is white., and a minibe• mt occur among the heron-, as well as in other grump- of birds, Oise of t lie be,..1 _known rases is if t hhie heron rflea Iu nth 11 of whieli are pure white, while others are slaty-bore. It was at one time supposed that the former were the young, the latter the adults; loll careful study on their Iffeeding g•Ottink has shown that the differences are it t as...elated in any way with age or sex. eV )1:IS been produced to show that geographical conditions may have something to do with the matter, for the white bird- are said to be most common on the .\ tlantie coast of Florida. the colored phase is most abundant on the Gulf coast. Alelanisin is a form of dichro mati-in in which one color phase be-otties very dirk. almost or quite black. Examples of this
among the large hawks of the genus niter). It should he stated that in these the color phases are not sharply off front each other, as in the herons, and the melanism is closely asso •iated with distribution. Erythrisin is the form shown by the sereceh-owls, already referred to, in which one phase tends to become red.
A satisfactory explanation of diehromatisin is still to be offered, but recent investigations have thrown some light upon it. In the scree•h-owl, the red phase i- due to a 'quantitative. difference' in the distribution and relative amounts of pig ment, and gray individuals may and do become red. In other eases, such as the herons, it is more than possible that we have examples of species in the process of formation, and that at ...one future day the slaty-blue individuals of the little blue heron, for exaninle. will breed only with slaty-blue individuals and produce only slats-blue offspring. while the white birds will breed only with white birds and produce while We would then have just such a condi tion as now exists in the ibises of the geniis teaura. The white ibis (Giitt•a iti/m) and the scarlet ibis (l'htitra •ohrat are exactly alike, ex cept for the striking difference in color, but they never breed together and are therefore regarded as two distinct speeies. and not as dichromatic forms of one. 'Hie whole question is one of ex ceptional interest. and -es-111.; to be closely bound up with that of the origin of species.