Читать книгу Colin Clout's Calendar: The Record of a Summer, April-October - Allen Grant - Страница 4

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Last week’s showers, much longed for and anxiously expected after the apparently endless spell of bitter east winds, have brought out the meadows at last into the full fresh green of early spring. The buds upon the horse-chestnuts, which stood idle and half-open for so many days, have now finally burst forth into delicate sprays of five-fingered foliage; and the young larches among the hillside hangers are revelling in the exquisite and tender freshness of verdure which larches alone can exhibit, and even they only for two short weeks of April weather. As for the hedgerows, I really think I can never recollect anything to equal them. The innumerable pecks of March dust from which we have been suffering seem to have brought forth gold enough in the celandines and crowfoots for many royal ransoms; and the masses of primroses on the sunny banks are both thicker in tufts of bloom and with larger individual blossoms than I ever before remember to have seen them. The copses on Wootton Hill are carpeted with daffodils, wood-anemones, and hyacinths, in great patches of yellow, blue, and white; and it is no wonder that to-day I should have seen the swallows, enticed back from their winter quarters in Algeria by the sun and the flowers, flying low above the gorse and the violet-beds in the undercliff, where they may now catch hundreds of small insects on the wing around the honey-bearing blossoms which attract them out of their cocoons upon these warmer and brighter mornings.

What marvellous complexity of interaction and mutual relations between all the parts of nature and organic life this familiar fact of the swallows’ yearly return implies for us! Hard-billed seed-eating and berry-eating birds, or mixed seed-eaters and insect-eaters, can manage to find food for themselves in England all the year round. Nay, even those species which live mainly upon worms, slugs, and other hardy small deer, can pick up a living somehow or other through our northern winters. But pure fly-catchers, like the swallows, must starve during the five months when winged insects are almost wholly lacking in temperate climates. Thus it becomes a matter of necessity with them to move south at the beginning of autumn, towards the orange groves of Italy and the palms of Africa. Before they can return, there must be insects in the north; and these insects must have been hatched from the egg, and re-hatched from the chrysalis stage, before they are fitted to become food for swallows, since swallows feed only on the wing. Accordingly, it is not until the spring flowers are well out, and the winged insects have begun to suck their honey, that the various species of the swallow family make their appearance.

The true swallows come first, and, taking one year with another, the second week of April may be taken as the average date of their return to the south-western counties of England; but this year the spring, in spite of its early promise, has hung fire a little in a curious half-hesitating way: and so I have not seen the first swallow till this morning. The swifts, larger and stronger birds, which fly even more incessantly than their cousins and therefore require a more abundant food-supply, do not usually come northward till the beginning of May, when the flowers and insects are in full force; and they leave us again in August, while the swallows linger on till the late autumn. Both kinds fly low and open-mouthed over the most flowery meadows, where they catch honey-sucking insects in abundance: or over the ponds and rivers, where they meet with innumerable mayflies and other winged species, whose larvæ live as caddis-worms or the like under water, while the perfect insects hover above it to lay their eggs upon the surface.

The question as to the supposed instinctive feelings which drive the swallows north or south at the proper season is an extremely interesting one: and perhaps only very recent views as to the nature of climatic changes and zones can enable us in time to give the true explanation. Hitherto it has been usual to think of the differences of climate between Europe and Africa as though they had always been permanent, and so to raise unnecessary difficulties in the way of a rational solution to the problem. If England had always had a cold winter, while Algeria always had a warm one, and if a double belt of sea had always separated us from the two continents, it would indeed be hard to understand how an English bird could first bethink itself of moving southward in winter, or how an Algerian bird could ever be seized with an original impulse to go northward in the spring-time. It is not surprising, therefore, that early naturalists should have taken refuge in the hypothesis of a special instinct implanted in the swallows, independently of experience, and prompting them to seek the appropriate climate by some unknown ‘sense of direction’ at the proper times of year. But, with our existing knowledge as to the past history of European geography and meteorology, no such cutting of the Gordian knot is now necessary.

We know that the climate of England in comparatively recent times was apparently as warm as that of North Africa; and we know that at the same period the beds of the Mediterranean and the English Channel were dry land. Hence it was then at least as easy for the swifts and swallows to range from Scotland to Sahara as it now is in America for the hardier humming-birds to range from Canada to Mexico. But when the change of ‘cosmical weather’ made England by slow degrees too cold in winter for flowers and midges to flourish all the year round, the swallows would begin gradually to fly a little to the south, as each autumn came on, and remove a little to the north again as spring returned. At first, no doubt, they would only have to shift their quarters very slightly in search of more plentiful food, without themselves being conscious of any special migration. In course of time, however, as the difference in climate became more and more marked, the birds would have to fly further and further south with each successive autumn, and would be enticed further and further north again to their original homes with each successive spring. Thus at last the practice of migration would become engrained in the nervous system, and would grow into what we ordinarily call an instinct—that is to say, an untaught habit. This is the stage at which the migratory custom has always remained in America, where broad stretches of land extend from the Arctic region to the tropical forests, unbroken by any intermediate zone of severing sea.

In Europe, however, special circumstances have added another and more complicated element to the problem—the element of discontinuity. The Mediterranean, the English Channel, and the Baltic practically cut off the various parts of the swallows’ summer hunting-grounds from their African wintering-places. To get from England to Algiers, many swallows fly over wide expanses of sea, far too broad to see across, and therefore quite destitute of landmarks. It is simple enough to find one’s way by land from Canada to Mexico; but it is quite another thing to find one’s way across the sea, without a compass, from Algeria to Marseilles: yet this is the route annually taken by one large body of northward-bound swallows. Dr. Weismann, however, has suggested an ingenious and fairly satisfactory explanation of the difficulty. He points out that the lines taken by the swallows and other migratory birds correspond on the whole with the shallowest parts of the Mediterranean, where it is most intersected by peninsulas and islands. When the Mediterranean valley began to sink below the sea-level it must at first have produced two or three large lakes in the deepest portions of its bed; and between these lakes there must have been connecting belts of land, now marked respectively by Sicily and Italy, by Sardinia and Corsica, and by Gibraltar and Tangiers, with their uniting submarine banks. Of these the Spanish belt is still almost entire, and it offers no special difficulty: the others are now broken up into peninsulas or islands. Dr. Weismann supposes that various flocks of birds grew accustomed to proceed north or south along one such connecting belt, while the land was still in process of subsiding: and that their descendants still continue to follow the same lines till they reach the final headlands, and then fly straight over sea in a definite direction till they sight the opposite land. The younger birds follow their elders: while the elders themselves have learned the proper landmarks and directions from similarly following their own predecessors, and gradually take the lead in their turn as the seniors drop off one by one. Thus, if we may believe so plausible a theory, by a sort of unconscious hereditary teaching the memory of the lost land-connections has been handed down from one generation to another since pre-glacial times. Were Corsica and Sardinia now to sink slowly beneath the waves, it is not difficult to conceive that the swallows might still gather yearly upon the hills at Mentone, and fly southward across the blank space to Tunis under guidance of their most experienced elders.