Art. 6.-BIRDS AND THEIR COLOURS.*

1. Concealing-Coloration in the Animal Kingdom. An exposition of the laws of disguise through colour and pattern. By G. H. Thayer. Illustrated, 4to. New York: the Macmillan Company, 1909.

2. Farbenphotographie durch Körperfarben und mechanische Farbenanpassung in der Natur. Von O. Wiener. 'Annalen der Physik.' Vol. 55. Leipzig, 1895.

3. Evolution of the Colours of North American Land Birds. By Chas. A. Keeler. With 19 plates. With 19 plates. San Francisco:

California Academy of Sciences, 1893.

4. Ueber die Farben der Vogelfedern. By V. Haecker. Archiv fuer mikroscopische Anatomie, xxxv, pp. 68–87. Bonn: F. Cohen, 1890.

5. Ueber die Wirkung organischer Farbstoffe auf das Gefieder der Voegel bei stomachaler Darreichung. By C. Sauermann. Archiv f. Anatomie u. Physiologie, pp. 543–549. Leipzig: Veit and Co., 1889.

6. The Development of Colour in the Definitive Feather. By R. M. Strong. Bulletin of the Museum of Comparative Zoology, Harvard, pp. 147-184, 9 plates. Cambridge, Mass., for the Museum, 1902.

7. The Value of Colour in the Struggle for Life. By E. B. Poulton. Article xv in Darwin and Modern Science.' Edited by A. C. Seward. Cambridge: University Press, 1909.

MANY readers of the 'Quarterly Review' have probably seen the simple and yet marvellously effective models of a couple of ducks, exhibited in various museums, by which Mr Abbott H. Thayer was the first to explain that the white colour of the under surface of so many birds and fishes turns the unavoidable dark shadows into an

invisible grey. That was a discovery so far-reaching and obvious that everybody wondered why he had not thought of it before. Mr Thayer, being an artist and

*This article contains the substance of a course of lectures delivered in the autumn of 1908 at the Lowell Institute of Boston, Mass. The coloration of birds covers, however, such a wide field that only some of its aspects are here dealt with, others, of great interest, such as the colours of the nestling plumage, seasonal changes male ornamental plumage, and above all, the protective coloration in the usually restricted sense, being scarcely touched upon.

an enthusiastic naturalist, has carried his investigations further, and these have now been published by his son in a volume sumptuously illustrated. His main theses are the following.

Concealing-coloration means coloration that matches the background. Beyond a certain distance all objects show mainly by their silhouette or outlines. The pattern and the bold colours cut up the silhouette and thus make the animal less conspicuous. The general principle of obliterative shading and of picture pattern has been well, perhaps best, expressed in some of his previous writings, when he said that the total abstract effect of the lights and shadows and colours of the surroundings is stamped upon the animal's coat.

'Animals are conspicuous when in the wrong place, or, what comes to the same, when looked at from the wrong point of view, the right being that in which the creature appears at the crucial moment, when on the verge of catching or being caught.' This principle, applicable beyond doubt in many cases, has been illustrated by many surprising photographs and coloured drawingswitness the Blue Jays placed over sunlit snow, on plate vi. But it is a pity that the authors should press this idea too far, even to the verge of ridicule. Nothing will, for instance, obliterate a Scarlet Ibis or a resplendent White Egret; at least that is our experience, who have had the delight of watching such beauties in their haunts, and we must be permitted to doubt whether a red and white cloudy sunset sky is the proper background and crucial moment in the Roseate Spoonbill's life, cf. plate viii.

Concealment is important, but it is not everything in coloration, the wherewithal of which are the colours. To understand how they are produced and how they behave, it is necessary to mention some technical detail by way of introduction.

White is due either to the total reflection of light or to its multiple refraction by small particles which are themselves colourless but possess strong refractive power. Feathers are composed of countless cells with particles of various density within them. The opaque white of the pith of a feather is due to the innumerable air spaces in the pith. If these interstices and air spaces could be done

away with, for instance by compression, or if they were filled with some fluid the refractive index of which is more like that of the ceratine cell-walls, the whole feather would appear quite as colourless and transparent as is the quill or spool. All feathers would be white provided they possessed no pigment, but most organisms produce some pigment or other, and in birds most of this is formed, or at least deposited, in the epidermal structures. The only birds quite devoid of such colouring matter are the albinos; they do not possess even black where it is wanted, namely for the black screen of the retina; and this is the real criterion of a true albino. The difficulty of ridding a bird's organism of the inherited habit of depositing pigment in its feathers is shown by the fact that those species, the final dress of which is white, almost all start with a coloured first plumage, generally tints of brown or grey, mottled as the case may be, and they have to undergo several moults before they succeed in assuming a pure white plumage, and even then some coloured feathers are liable to crop up, spoiling the effect. Familiar examples are Swans, Egrets, Snowy Owls, and certain Gulls.

The commonest pigment is black, melanine, always appearing in the shape of tiny granules, sometimes rodshaped, often conglomerated, never in solution. It is insoluble in water, alcohol, acids and ether, but it can be dissolved and destroyed in caustic potash and when treated with chlorides. Owing to the difficulty of getting this pigment pure without admixture of other cell-contents its exact composition is still unknown; suffice it to say that, its principal constituents being carbon, hydrogen, and oxygen, it belongs to the great carbo-hydrate group, probably allied to the lipochromes but in a more advanced state of oxidisation.

This black pigment is not carried from the general system into the feathers, but is produced in loco, in specialised members of the ectoderm cells, the so-called chromatophores. These have the faculty of attracting into their plasma certain materials which then condense into extremely small corpuscles, and, by some subtle physico-chemical change, assume colour. They are therefore called chromogenes. The chromatophores, large cells with amoeboid protoplasmatic processes, are as a rule situated in the deeper layers of the malpighian stratum,

never in the cutis or mesodermal portion of the skin. Whilst the feather is being built by the proliferating and cornifying cells, the chromatophores send processes towards them and squeeze some of the pigment towards the cells, and some kind of osmosis induces the chromogenes to enter the cells to be coloured. At first the feather cells are still juicy and soft, but later the contents of each cell are quite shut off from the rest of the world. No new matter can be taken in and nothing can be taken out, but this hermetical seclusion of the cell contents does not prevent the enclosed chromogenes, or pigments, from undergoing subsequent changes. Thus it can come to pass that changes of colour without moult, i.e. within a so-called finished feather, take place; not only a deepening or a bleaching of the deposited pigment, but even the appearance of a new colour, or of colour at all if the chromogenes had hitherto been white.

An important group of colours are red, orange and yellow. Since these are due to a direct pigment and possess a fatty basis they are called lipochromes. They are true carbo-hydrates, containing on an average 70 per cent. carbon, 10 per cent. hydrogen, and the rest oxygen; they differ from all the bile pigments by the complete absence of nitrogen. Hence we understand why the colours of the egg-shells have no correlation whatever with those of the plumage. Both owe their colours to totally different sources.

The lipochromes are easily extracted, being soluble in alcohol, chloroform and ether. Quite a number of terms have been invented for their many shades between red, yellow and brown. Their spectral differences may depend upon the admixture of an infinitely small amount of some other body, or the differences in colour may be due to a slightly more or less advanced state of oxidisation. Krukenberg, one of the few workers in this field, came to the conclusion that the fat of birds contains a kind of fundamental or mother-substance of colouring matter, which he called corio-sulfurin. Analogous is the lipochrin of Amphibia. This stuff courses in the vascular system whence it is distributed to the tissues of the skin, feathers and scales, where then, in loco, metabolism into the pigments proper takes place, by action of the chromogenes mentioned before.

There are no green and no blue pigments in the Vertebrates. Blue without exception is a so-called structural colour. The principle involved is that a turbid medium has the peculiarity of reflecting more of the short than of the longer or reddish waves of light, the latter being less refrangible, and, if the turbid medium is too dense, like good milk, it appears white, since all the rays are reflected, therefore unable to penetrate the medium.

In the blue skin of Amphibians and Reptiles the turbid medium is represented by mineral deposits, either waste products allied to urates, or carbonates of lime, stowed away in a stratum of large box-like cells beneath the horny, colourless surface layer of the skin. This mineral-impregnated layer, if not too dense, lets pass and emits yellowish brown or reddish, i.e. impure orange light, but if this light, which spoils the phenomenon, is absorbed by an underlying layer of black pigment, then only the reflected blue will reach our eye under direct light. In feathers such an infiltration with mineral matters does not occur, but the little crystals and granules of the Amphibian box-cells are, in blue feathers, represented by the equally colourless contents of the so-called marrow or pith-cells, which in blue feathers, and only in these, form one continuous stratum of enlarged polygonal cells just beneath the surface layer, and beneath them are spread and closely packed dark coloured pigmented cells of irregular shape. The size of the box-cells averages in height from 0·011 to 0.019 mm., i.e. from to of a millimetre, equal to about twenty to forty wave-lengths of orange light. If a blue feather be well soaked it looks dull grey or brown, nor does it look blue if examined under transmitted light. Further, if we crush such a blue feather, so as to destroy the box-cells, the blue vanishes. Indeed the blue

* The most exceptional of all pigments is the turacin, so called because it colours the wing feathers of the Touracoes, or Plantain-eaters (an African family of Cuckoos), a deep purple-red. When such a bird gets soaked with rain the red colour comes off when rubbed between the fingers, and it colours the water. This pigment is composed of C,H,O and 5 to 8 per cent. of metallic copper. Its spectrum is almost identical with that of oxyhæmoglobin, which colours our blood, only with the difference that it contains copper instead of iron. Curiously enough the green feathers of these birds, and only these, owe their colour to a pigment which contains rather much iron instead of copper.