Glimpses of Indian Birds :: VIII BIRDS IN WHITE

VIII BIRDS IN WHITE

Almost every species of bird and beast throws off an occasional albinistic variation or sport, which tends to breed true. Such sports are of two kinds—complete and incomplete albinos. In the former, the organism is totally devoid of external pigment, so that the eye looks red, there being no colouring matter in the iris to mask the small blood vessels in it. In the incomplete albinistic form the iris retains the pigment, so that the eye colour is normal. True albinos have very poor sight, hence when such sports occur in a species in a state of nature they soon perish in the struggle for existence. The white varieties with pigmented eyes are not handicapped by bad eyesight, but their whiteness makes them conspicuous to the creatures that prey upon them; so that, unless they are well able to defend themselves or unless they dwell in a region of everlasting snow, they tend to be eliminated by natural selection.

If protective colouring were as important to the welfare of birds as Wallaceians and modern Darwinians assert, all the birds of the Polar regions would be white and not a single white species would be found in the temperate zones or in the Tropics. That coloured species occur in the Arctic regions and white species in the Tropics is conclusive proof that in those particular cases, at any rate, it is not of paramount importance to the species that they be protectively coloured.

Finn and I have shown in The Making of Species that the ice-bound Arctic and Antarctic regions are not inhabited, as popular works on zoology would have us believe, by a snow-white fauna. We have shown that in the Polar countries the coloured species of birds outnumber the white species. I will, therefore, not dilate further upon this subject. It will suffice to repeat that in the area of eternal snow the white forms are at an advantage in the struggle for existence, as their whiteness tends to render them difficult to see, while, in regions where snow is unknown, such organisms labour under a disadvantage because of their conspicuousness, and, other things being equal, they ought not to be able to hold their own against less showy rivals.

The fact that white birds exist in the plains of India must mean that their colour is not a matter of great importance, that a conspicuous organism can survive in the fight for life provided it be otherwise well equipped for the contest. From this it follows that it is incorrect to speak of the whiteness of such organisms as the direct product of natural selection.

Let us take a brief survey of those birds of India of which the plumage is largely white, and try to discover how it is that each of them is able to hold its own in the struggle for existence, notwithstanding its showy plumage. These birds are the spoonbill, the egrets, the black-winged stilt, the avocet, the white ibis, the flamingo, adult cock paradise flycatcher, and certain of the gulls, terns, pelicans and storks, including the open-bill. With many of these every one is familiar. Accordingly, it will not be necessary to describe the sea gulls, the pelicans or the flamingo.

The spoonbill (Platalea leucorodia) is a bird larger than a kite with very long black legs and a bill of the same hue which is flat and expanded at the end like a spoon, hence the popular name of the bird. Perhaps another name for the bird—Banjo-bill—still better describes its beak. Spoonbills dwell on the fringe of water and feed much as ducks do.

The white ibis (Ibis melanocephala) is another wading bird, rather smaller than the spoonbill and with considerably shorter legs. All its plumage is white, but the legs, bill, and featherless head and upper neck are black. The bill is long and curved like that of the curlew. The stilt (Himantopus candidus) may be described as a sandpiper on red stilts. It is a white bird with dark wings and back which spends its days wading in shallow water. The avocet (Recurvirostra avocetta) is perhaps the most elegant of all wading birds. It is slightly bigger than the stilt but with shorter legs. Its body is white picked out with black. Its most characteristic feature is a long, slender bill which curves upwards. Like the species already mentioned, it feeds in shallow water, and I have seen it on the Cooum.

The open-bill (Anastomus oscitans) looks like a shabby specimen of the common white stork. It is characterised by a peculiar beak, of which the mandibles do not meet in the middle and look as though they had been bent in an attempt to crack a hard nut. The egrets, of which there are several species in India, are snow-white, heron-like birds. The most familiar is the cattle egret (Bubulcus coromandus), which Finn characterises as one of the most picturesque birds in the East. This is the bird that struts along beside a cow or buffalo and seizes the grasshoppers disturbed by the motion of the quadruped. It is the least aquatic of all the egrets, most of which are true waders.

Terns may be described as very graceful and slenderly built gulls. Their feet are webbed, so that they can swim after the manner of ducks and sea gulls, but they spend most of their time on their powerful pinions and so elegant is their flight that they have been called sea-swallows. The adult cock paradise flycatcher (Terpsiphone paradisi) is one of the most beautiful birds in the world. As he is described in another essay it is only necessary for me to state in this place that he is a white bird with a black-crested head. He is not much larger than a sparrow, but his two median tail feathers are twenty inches in length and float behind him like streamers of white satin as he flits from tree to tree.

It will be observed that of the above list of Indian birds that are mainly white, only the paradise flycatcher belongs to the great Order of Passeres; moreover, with this exception, all are wading or aquatic birds. These are significant facts if we can interpret them aright. I interpret them in the following manner. It may be taken as a fact that every species throws off occasionally white mutations or sports, which breed true, so that, if allowed to persist, they form the starting point for new varieties and species. As most passerine birds are small and preyed upon by the raptores, white varieties among them usually perish at an early age on account of their conspicuousness. Thus there are very few white passerine birds. The paradise flycatcher lives amid thick foliage, and so is comparatively immune from the attacks of birds of prey; but even here it is note-worthy that the hens are not white but chestnut in colour throughout life, and the cocks have chestnut-coloured plumage until they are two years old. As the cock shares in the duties of incubation equally with the hen, her failure to acquire white plumage cannot be accounted for by supposing her to have a greater need of protection. Finn has suggested that the whiteness of the cock is a senile character; that it is the livery of old age.

The majority of the non-passerine birds that are altogether or mainly white are large and able to fight well, so that they are comparatively immune from the attacks of raptorial birds. The gulls and terns, although small, fly so powerfully as to be equally safe. In the case of birds which secure their food in the water, whiteness is probably useful in rendering them less conspicuous to organisms living in the liquid medium than they would be were they coloured.

Further, whiteness of feather seems to be correlated in some way with the power to resist cold and damp.

It should be noted that not one of the larger fruit-eating birds is white. The reason of this would seem to be that in the case of non-aquatic birds such white species possess no advantage in the struggle for existence, but, on the contrary, the whiteness of their plumage is perhaps correlated with weakness of constitution. This, of course, is a heavier handicap to a large bird than being conspicuous is.

The correlation or interdependence of various characteristics and organs is a subject full of interest, but one which has hitherto attracted comparatively little attention. Close study of this phenomenon may eventually revolutionise zoological thought. Whether this surmise prove right or wrong, one thing is certain, and that is there is more in the philosophy of whiteness than the old-fashioned evolutionist dreams of.

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