Читать книгу Colin Clout's Calendar: The Record of a Summer, April-October - Allen Grant - Страница 11

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At last the long-wished-for rain has come in earnest; the ground has drunk in water enough to give it more than a mere surface wetting; and the grass and leaves begin to look themselves again after the long spell of dry and warping weather. We had a few slight showers last week, but they barely sufficed to lay the dust for a couple of hours; and as soon as they had dried up, the east wind blew it about once more, so that even the young green on the hedges and the horse-chestnuts was smothered in a loose coat of greyish grime. Now, however, nature comes out anew after the downpour in its freshest spring colours. The clouds still lower, and the tops of the downs are still lost in slowly shifting mists; so to-day the swallows have left the open meadows and are flitting low above the river, gaping open-mouthed at the water-flies and skimming the surface of the stream with their long blue-black wings. Leaning here on the rough parapet of the old stone bridge, I can see the flies at which they are darting just below me; for swallows are always fearless of man when on the wing, and do not hesitate to approach him flying; though they seem hardly ever to alight anywhere within an easy stone’s-throw when he is by, except of course in their nests. Their ceaseless motion and their curious independence of rest strikingly recall the little hummingbirds whom I have often watched in like manner, whirring past me from flower to flower in tropical gardens; and, strange as it sounds to say so, the swallows and the humming-birds are indeed first cousins to one another, though so very different in outward shape and plumage. Indeed, nowhere else are appearances more deceitful. The humming-birds are not at all related to the sun-birds of India and Africa, which are so like them as to be colloquially called by their name; while they are closely related to the very unlike swallows, being, in fact, American swallows which have never taken to migrating very far north, and have accordingly adapted themselves instead to a continuous tropical or subtropical existence.

Prince Lucien Bonaparte was the first to show that the humming-birds were really most nearly allied to our dingy northern swifts. Of all the swallow family, the swifts are the most ceaselessly active and possess the widest relative stretch of wing. Though a full-grown bird usually weighs scarcely one ounce, it measures eighteen inches from tip to tip of the pinions. No one ever saw a swift perching on a tree or hopping about the ground: except when asleep, it is almost ceaselessly upon the wing. It catches its food flying; it drinks as it skims the surface of the water; it picks up the materials for its nest while sweeping among the meadows close to the ground. Now, if you transfer some of these active, restless, insect-catching swifts to the tropics, what will be the natural result? A large proportion of tropical insects find their food in the large bells or deep tubes of the brilliant equatorial flowers. So the swifts would naturally take to flitting about in the neighbourhood of these blossoms and poising themselves on their powerful wings just in front of their corollas. Those of them which took permanently to such a mode of life would soon adapt their external structure to the new conditions with which they had grown familiar. Tropical swifts with the longest bills and the most extensile tongues would have an advantage over others, because they would best be able to probe the long tubes of the flowers and extract the insects from them, inside the nectary itself. In this way the bill and tongue have gradually grown so long in their descendants, the humming-birds, that all outer resemblance to the parental swallow form has been wholly lost; and the family was, accordingly, classed till quite recently with the externally similar, but genealogically quite distinct, group of sun-birds.

In most other respects, however, the humming-birds continue to resemble the ancestral swifts. The shape of the wing and its proportion to the body is exactly the same; but, above all, the numerous minute anatomical points of similarity settle the question at once for modern biology. Even before evolutionism gave the new key which solves so many of these difficult problems, it was noticed that the humming-birds were very like the swallows in many anatomical particulars, though very unlike them in plumage and in the shape of the bill. Dr. Jerdon, who has spent his life in studying the birds of India, hesitated about ranking the sun-birds by their side because of this structural community between humming-birds and swallows; but he reassured himself when he looked at the general external likeness of the two tropical groups. Now, however, we have learned that such external likenesses are necessarily produced by community of habit and mode of life; while underlying structural resemblance forms the best test of genealogical relationship. Mr. Wallace has shown conclusively that the humming-birds are in reality modified swifts, and that their resemblance to the Oriental sun-birds is wholly due to the similarity of their circumstances.

In fact, the habits of the two races, though much alike in many respects, still bear evident traces of their original derivation. The sun-birds are by origin creepers; and, like other creepers, they have not very large or powerful wings, and their feet are formed for perching, which is not the case with either the swifts or the humming-birds. When a sun-bird wants to suck the honey of a flower, it does not hover in front of it, poised upon swiftly vibrating pinions, like its supposed American allies; but it perches first upon the stalk or branch, and then extracts the nectar at its ease. The humming-birds, on the other hand, being developed insect-eaters, never alight, but catch their food upon the wing, just as their ancestors the swifts were accustomed to do. Moreover, they are not to any great extent honey-suckers; what they seek in the nectary is not so much the honey as the insects which have come to eat it. These they can extract with their long tongues at a single flick, and then they dart away again, just like the swallows, in search of more. Mr. Wallace has shown that young humming-birds starve upon honey, but live and thrive upon insects alone; being, in fact, as he puts it, still in the swift stage of their development.

As for the points of convergence between the humming-birds and the sun-birds, those are easily enough explained. Both races feed upon long-tubed tropical flowers, probing their recesses in search either of honey or flies; and both, consequently, require long bills and extensile tongues. Both races also possess brilliant plumage, with metallic crests or gorgets; and such brilliance is common amongst all flower-feeding and fruit-eating species, such as butterflies, rose-beetles, toucans, parrots, and birds of paradise. The constant association with coloured objects, and the constant search for them as food, seems to arouse a taste for bright colour in the creatures themselves, which is actively exerted in the choice of mates. Why some members of the swift and swallow family should have undergone this change to humming-birds in the western continent and not in the eastern would be a more difficult question to answer offhand; but I fancy the difference may be partly due to two causes. In the first place, the peculiar way in which the Old World is cut up into two distinct regions, hot and cold, by the Mediterranean and the Himalayan range may have favoured extensive migration here; while in America the continuity of land, the warmth of summer, and the general luxuriance of blossoms permit humming-birds to range as far north as Canada; and thus one continent may have favoured only the old open insect-hunting types like the swift, while the other favoured also specialised flower-haunting types like the humming-birds. In the second place, the creepers may already have occupied the field in the small portion of Africa and Asia fitted for the evolution of such a race as the humming-birds, and may thus effectually have prevented the eastern swifts from ever developing in that direction. Of course in any case the specialisation of humming-birds in America must date back to a very remote period, both on account of the profound modifications their form has undergone, and on account of the immense number of genera and species into which they have split up.