by Grant Allen
Birds, however, offer the best evidence of all. It will be desirable to mark briefly the most conspicuous instances, and then to give the minor cases in detail.
The birds of prey — eagles, hawks, and falcons — and the carrion birds — vultures, condors, and (since we are speaking of habits only) ravens or adjutant storks — are all dull greyish or blackish birds, and their colouring may be compared to that of the flies and crocodiles. Nocturnal birds, again, such as owls and goat-suckers, show considerable analogies to moths and other night-flying insects. In fact, all the raptores, of whatever habit, and almost all birds of similar habit elsewhere, are nearly or quite destitute of decorative colouring.
On the other hand, among the Insessores we find an immense number of the most brilliant of all organic creatures. Especially remarkable are the humming-birds and the sun-birds. Now, Prince Lucien Bonaparte has abundantly shown that the former family are really the allies of our dingy northern swifts, and that the sun-birds are not at all connected with them genetically. But both families feed upon the mixed nectar and insects which they catch in bright-coloured entomophilous flowers, and both are equally noticeable for their exquisite metallic gloss, their varied hues, and the profusion of their decorative devices, such as crests, ruffs, feather lappets, and long tail-plumes. Is it not a significant fact that these two families, one in the western hemisphere and the other in the eastern, separately developed from dingy ancestors, should have acquired exactly the same exquisite plumage under exactly like conditions of food? We can hardly resist the inference that a taste for colour has been aroused in their constant search after flowers, and that this taste has reacted through sexual selection upon their own appearance.
Next in importance to these two families come the parrot group. These are either fruit-eaters, or else, as in the case of the lories, they feed upon nectar. And here I venture to borrow Mr. Wallace’s words. “No group of birds,” says he— “perhaps no other group of animals — exhibits within the same limited number of genera and species so wide a range and such an endless variety of colour. As a rule parrots may be termed green birds, the majority of the species having this colour as the basis of their plumage, relieved by caps, gorgets, bands, and wing-spots of other and brighter hues. Yet this general green tint sometimes changes into light or deep blue, as in some macaws; into pure yellow or rich orange, as in some of the American macaw-parrots (Conurus); into purple, grey, or dove-colour, as in some American, African, and Indian species; into the purest crimson, as in some of the lories; into rosy-white and pure white, as in the cockatoos; and into a deep purple, ashy or black, as in several Papuan, Australian, and Mascarene species. There is in fact hardly a single distinct and definable colour that cannot be fairly matched among the 390 species of known parrots. Their habits, too, are such as to bring them prominently before the eye. They usually feed in flocks; they are noisy, and so attract attention; they love gardens, orchards, and open sunny places; they wander about far in search of food, and towards sunset return homewards in noisy flocks, or in constant pairs. Their forms and motions are often beautiful and attractive. The immensely long tails of the macaws, and the more slender tails of the Indian parraquets; the fine crest of the cockatoos; the swift flight of many of the smaller species, and the graceful motions of the little love-birds and allied forms; together with their affectionate natures, aptitude for domestication, and powers of mimicry — combine to render them at once the most conspicuous and the most attractive of all the specially tropical forms of bird-life.”
Even the minor variations of these three great groups — the humming-birds, the sun-birds, and the parrots — show us a like result. For there is one sub-family of the former group — the Phaëthornidæ — which have not taken to flower haunting, but which catch minute insects on exposed situations; and these are described by Mr. Wallace, not in that language of sapphire, ruby, and amethyst which is lavished on their congeners, but simply as “small brown humming-birds.” “The members of all these genera,” says Mr. Gould in his magnificent work on the Trochilidæ, “are remarkable for being destitute of metallic brilliancy, and, as their trivial name of ‘hermits’ implies, for affecting dark and gloomy situations. They constitute perhaps the only group of the great family of humming-birds which frequent the interior of the forests, and there obtain their insect food — some from the under-side of the leaves of the great trees, while others assiduously explore their stems in search of such lurking insects as may be concealed in the crevices of the bark. It has been said that spiders constitute the food of many species of this group.” And he adds significantly a few lines further down, “in the colouration of their plumage both sexes are generally alike.” Then, again, we learn of the Arachnotheræ, or spider-hunters, “which are sun-birds without any metallic or other brilliant colouring,” that they hunt for food among the anemophilous and uncoloured blossoms of the palm-trees. So, too, among the sombre vegetation of New Zealand an anomalous night-parrot (Strigaps habroptilus) is found, which lives in crevices of the Ground, or in rocks and tree-roots, only coming out after dark, and its colour is spoken of as “dull yellowish green.” Here one may feel almost certain that the primitive bright hue has become faded and dingy owing to the altered habits of the bird, which would effectually prevent the action of sexual selection.
Scarcely less interesting are the group of pigeons, which fall under two principal heads, so far as our present purpose is concerned, the fruit-pigeons and the ground-pigeons. The former class are extremely brilliant in their colouring, comprising a large number of the most beautiful known birds; while the latter almost always display sombre dove-colours, slates, and browns. Now, the fruit-pigeons are “especially arboreal in their habits,” and “their nourishment consists for the most part of fruits:” while the ground-pigeons feed almost entirely on seeds.
The toucans form another group in which like adaptations occur. They live nearly altogether upon fruits, though they also devour birds’ eggs, fish, reptiles, and insects, to a slight extent. The exquisite colours which adorn their large bills, besides the varied black, white, green, red, and yellow of their plumage, are well enough known to call for no further detail.
Several other families, allied to one or other of the preceding groups, are almost equally noticeable for their magnificent colouration. First on the list may come the Australian honeysuckers, and the plantain-eaters of Africa, whose name sufficiently proclaims their habits. Next, we may place the allied genus of Touracos, “generally of a green colour, with the quill feathers of the wing and tail violet or red.” The birds-of-paradise, too well known to need description, feed on fruits, though some species are flower-suckers. The barbets, known by such expressive names as golden-throated, blue-throated, crimson-breasted, and so forth, live almost exclusively on fruits and the buds of flowers. The ornithological reader will have no difficulty in filling in other cases for himself.
Again, many of the tropical birds, less brilliant on the whole than these pure vegetable-feeders, yet still remarkable for beautiful colouration, live upon a mixed diet of fruits, tropical insects, eggs, lizards, and other bright-hued foods. “Owing to the prevalence of forests and the abundance of flowers, fruits, and insects,” says Mr. Wallace, “tropical, and especially equatorial, birds have become largely adapted to these kinds of food; while the seed-eaters, which abound in temperate lands, where grasses cover much of the surface, are proportionately scarce.” To this cause, I believe, we may trace the general brilliancy of tropical as compared with temperate birds, especially among the great groups of Passeres and Picariæ.
Among other instances, which I can only note briefly, may be mentioned the trogons, which are in all cases conspicuous for their varied beauty; but those which inhabit America are more gorgeous than the Indian species; and fruit forms part of the diet among the former, while that of the latter is mainly composed of insects. Mr. Gould mentions that the stomach of T. collaris contained on dissection “fruits and caterpillars.” The swallows of te
mperate climates are plainly coloured, as might be expected from the nature of their food; but their allies, the rollers, decked in gorgeous violet, blue, and green, live upon “insects and fruits” in sub-tropical countries: and with them may be included the exquisite todies, with their green plumage and gay scarlet breasts. The lovely motmots of tropical America and the West Indies feed upon insects, fruits, and lizards. The diet of the Eurylaimus is described as consisting of “insects, at times berries and fruits.” The omnivorous crow family are dingy creatures, usually with no colouring save black and white; but their near relations, the jays, have a much more vegetal diet, and are often decorated with very striking colours. In summer they “visit gardens, tempted by the cultivated fruits,” and they also feed on eggs and insects. The hornbills live almost entirely on fruit and eggs; and their large beaks are coloured somewhat after the same fashion as the toucans. But I shall not extend this list, which might be easily enlarged with numberless other instances. I shall ask the reader instead to glance over any ornithological work, and to notice the universal coincidence of coloured food and coloured plumage for himself.
On the other hand, most of the seed-eating and omnivorous birds are dull brown, black, or otherwise dingy in their plumage. All our own small field-birds may be roughly included under this generalisation. Such are the starlings, finches, sparrows, larks, thrushes, ouzels, wagtails, titmice, nightingales, swallows, and martins. Many of these live upon seeds and grains; others search for larvæ, insects, mollusca, and other small animals, often amid dung or like refuse. Even amongst these, we find instances which bear out our general theory. Thus the thrushes as a rule are very modest in their plumage; but the fruit-feeding orioles have comparatively bright hues. So, too, while most of the finches are plainly clad, the forestine rose-finches have exquisite tints of pink and crimson. Again, among our own birds, the few brighter-coloured species point somewhat in the same direction: for the bullfinch, the most notable of them, has very arboreal habits, feeds upon various berries, and attacks flower-buds in gardens to such an extent that it has become a perfect nuisance to gardeners: and I find the food of the Bohemian chatterer specified as consisting of “the berries of the mountain ash, hawthorn, and ivy,” together with the fruit of the juniper.
Intermediate between these two classes of brilliant and dingy birds come a number of moderately-beautiful tribes, some of which nearly equal the parrots and humming-birds, while others scarcely rise above the level of European song-birds. The birds of this division are more or less forestine in their habits, live amid a very varied environment, and feed upon insects (often very brilliant) or vegetal matters, including seeds, fruits, bulbs, flower-buds, and leaves. Besides the rollers, todies, and motmots already mentioned, we may class here the cuckoos, wood-peckers, pastors, gross-beaks, manakins, pachycephalinæ, fly-catchers, hoopoes, bee-eaters, jacamars, and kingfishers. It would take up too much space to specify all instances in full; but I have satisfied myself that a general connection may be traced amongst them all, on the average, between bright food and pretty colouring.
The cases so far examined belong to the great central group of birds which composes the common orders Columbæ, Scansores, and Passeres: we may glance briefly at the more divergent orders, whose habits have produced a very different structure. The swimming birds, with webbed feet, are seldom conspicuous for their colouring. The marine species (as penguins, auks, puffins, grebes, gulls, and albatrosses) are more or less whitish, with a toning-down of black or grey. But the ducks and flamingoes, which live amid a much more varied environment, and feed off more diversified food, are often adorned with conspicuous colours. These, I confess, cannot always be explained by our present principle.
The wading birds (snipes, storks, cranes, herons) display for the most part only cinereous or other dingy plumage. Psophia, however, a brilliant South American crane, is a tropical forestine bird, and feeds on fruits and grains. The scarlet ibis of the Amazons has also the common brilliancy of its country, which may be equally noticed in the roseate spoonbill. Here, as elsewhere, I must ask the reader to take each order or family in the mass, omitting such aberrant cases as do not readily admit of explanation.
The running birds (ostriches, emus, cassowaries, apteryx) have little beauty of colouring. The most brilliant among them, the cassowary, whose naked head and neck are tinted with red and blue, is also the most frugivorous of its order, feeding upon fruits, herbage, and seeds; while the dingiest of all, the apteryx, has purely nocturnal habits, and feeds upon insects.
The only real difficulty is presented by the gallinaceous birds, and these must be allowed at first sight to offer a great obstacle to our theory. But even here a little consideration considerably modifies the opinion we form at a cursory glance. To begin with, most species, like the grouse, partridges, quails, and guinea-fowls, are by no means remarkable for the purity or intensity of their hues. There is really only one family, that of the Phasianidæ or pheasants, including the turkeys and peacocks, which can lay claim to much beauty on the score of colour. Even among the pheasants themselves many species are far from brilliant; and when we come to compare the whole family with that of the parrots or the humming-birds, we shall find that the peacock alone can fairly come into competition with the typical fruit-eaters and flower-feeders. Moreover, the pheasants as a group are thoroughly forestine birds; they pass their life in the midst of brilliant objects, and many of these serve them as food. Turkeys in the wild state feed on grain, berries, fruits, grass, and insects, being especially fond of locusts and grasshoppers. The diet of the true pheasants comprises the same varied items, and Yarrell mentions blackberries, sloes, haws, and acorns as among their favourite viands in English copses. The forests of the Himalayas and of the Malay Archipelago, with their great brilliant fruits and flowers, and their exquisite insects, form the haunts of the most beautiful species of pheasants. The peacock in the wild state is also a jungle-frequenter, and feeds upon grain, fruits, and insects. On the whole, I think it may be fairly said that the gallinaceous birds, though not strong supports for our general theory, may be regarded as friendly neutrals at least. The dingy species are skulkers in underbrush, who feed off small grain, seeds, bulbs, and insects; while the brilliant species are tropical or sub-tropical forestine birds, whose food comprises many bright-coloured objects, both animal and vegetal.
Passing on to the Mammalia, we find facts of the same sort presented on every side. The whole lower series, whether among marsupials, pachyderms, cetacea, ruminants, Carnivora, or Insectivora, show us almost uniformly tints of black, brown, grey, or dingy yellow. It is true that many animals, like the zebras, tigers, spotted deer, and giraffes, have very noticeable alternations of light and dark shades, but they do not yield us pure spots of green, blue, red, or yellow. When we come to the essentially arboreal mammals, however, the tree rodents and the quadrumana, we get many comparatively brilliant species. The squirrels are often remarkable for their beautiful colours, and the so-called flying-squirrels call for special notice in this respect. The contrast between these pretty little creatures and their allies, the mice, rats, beavers, and water-voles, strongly brings out the peculiarity of their hues. So, too, the purely frugivorous monkeys give us a variety of colour which we find nowhere else among the Mammalia: and in the scarlet faces of many among them, or most remarkably of all in the bright red and blue of the mandrill, we have the only pure tints which are to be found in the whole class. Compared with the nocturnal bats, we see at once the action of sexual selection.
Lastly, I should like to add that while I attribute special importance to the nature of the food, I do not deny that the whole environment must have a considerable modifying influence upon the tastes, and therefore upon the colouration of each species; and accordingly I freely allow the general truth of my friend Mr. Romanes’ theory upon this subject, though I cannot agree with him in setting down the growth of tastes to mere association.
I am aware that this long catalogue, by mixing up the
more certain with the less certain instances, has presented the evidence in its weakest light. To redress the balance, then, let us recapitulate the main facts from another point of view.
Flowers are the most brightly-coloured of all vegetal productions. Among the creatures which find their food in flowers may be mentioned butterflies, the most brightly-coloured of all insects, besides humming-birds and sun-birds, the most brightly-coloured of all vertebrates. Other flower-haunting insects are the rose-beetles, loveliest of all the Coleoptera, and many more of the handsomest species. Other flower-haunting birds are the lories, far the most beautiful among the parrot tribe, and some of the birds-of-paradise. The pretty little barbets also feed in part on flowers. The butterflies, rose-beetles, humming-birds, sun-birds, and lories have been highly modified in adaptation to the blossoms in which they find their food.