Color in Birds

The third class of colors, Subjective structural, prismatic or metallic colours, change according to the position of the light and the eye of the observer, and they always change in the order of those in the rainbow. They are restricted, as a rule, to the radii without cilia, and moreover to those parts of the feathers which are not covered by others.  The metallic portions of the radii are composed of one row of compartments, which often partly overlap each other like curved tiles.  In the inside black or blackish-brown pigment is collected; and each compartment is covered with a transparent colourless layer of extreme thinness, e.g. 0.0008 mm. in Sturnus. The surface of this coat is either smooth and polished as in Nectarinia, or exhibits very fine longitudinal wavy ridges when the feather is violet, or numerous small dot-like irregularities as in Galbula.

Diagram of positions for observing the colour of Metallic Feathers.

The coating seems to act like a number of prisms.  All metallic feathers appear black when their surface is parallel to the rays of the light in the same level with the eye and the light.  To the eye of the observer at A, in the lower part of the diagram above, the metallic collar of Ptilorhis magnifica will appear absolutely black; the eye at B will see it bright coppery red, and at C rich green; the metallic feathers of the sides of the breast in the same bird will change from black to green at B, and to blue at C.  The beautiful Pharomacrus mocinno changes from greenish bronze through golden green, green, and indigo to violet.  Oreotrochilus chimborazo in position B exhibits the whole solar spectrum, namely, violet and red on the head, followed by orange and green on the back, blue, violet, and lastly purple on the long tail feathers. The red colours of the spectrum lie nearer towards the position A, the blue colours towards C. The colours always appear in the same order: no feathers are known, which when looked at from B towards A, change from the red towards the blue end of the spectrum. In case two or more of these spectra (of which we imagine the horny coating to be composed) overlap each other, only a limited number of colours are able to reach the eye of the observer. Thus in the theoretical case illustrated in the diagram red only will be visible besides black.

A peculiar case is that of Artamia bicolor; the pure white feathers of the underparts have no metallic gloss, but nevertheless they seem to be prismatic, because in position A the underparts appear bluish-white, in B delicately pale blue, and in position C pale grey.

Deviation from the normal coloration is more or less pathological, and can be conveniently expressed by the term Heterochrosis.  The following are the chief cases:-

Albinism, caused by the pathological absence of the black pigment, and often locally produced by a lesion of the pulp of the growing feather; extreme instances are white Ravens and Blackbirds.

Melanism produced by the superabundance of black pigment, mostly causing the feathers to assume a darker or more sooty colour.  Melanistic specimens have been described of many birds, such as Bullfinch, Skylark, and in particular of the common Snipe, which in this phase has by some been regarded as a distinct species, Sclopax saibinii.

Xanthochroism, mostly in originally red or orange feathers; when the feathers are yellow instead of green, this may possibly be a reversional step or a case of arrested development because of the absence of the green-making superstructure.

Erythrism, the abnormal occurrence of red, mostly confined to originally yellow or orange feathers, occasionally produced by abnormal food, like cayenne pepper, or directly by the colouring matter of Rubia tinctoria, one of the madder-worts. A certain correlation between green and red is exhibited by the intensely green adult males of Eclectus polychlorus, the females being bright red and the young of both sexes being reddish, without any indication of green in the young male.