It would be wrong to believe that these migrations occur in their season, without any definite choice of days, and at indeterminate epochs. We ourselves have been able to observe, on the contrary, the exact and lucid decision which regulates them; not an hour too soon or too late.
When living at Nantes, in October 1851, the season being still exceptionally fine, the insects numerous, and the feeding-ground of the swallows plentifully provided, it was our happy chance to catch sight of the sage republic, convoked in one immense and noisy assembly, deliberating on the roof of the church of St. Felix, which dominates over the Erdre, and looks across the Loire. Why was the meeting held on this particular day, at this hour more than at any other? We did not know; soon afterwards we were able to understand it.
Bright was the morning sky, but the wind blew from La Vendée. My pines bewailed their fate, and from my afflicted cedar issued a low deep voice of mourning. The ground was strewn with fruit, which we all set to work to gather. Gradually the weather grew cloudy, the sky assumed a dull leaden gray, the wind sank, all was death-like. It was then, at about four o'clock, that simultaneously arrived, from all points, from the wood, from the Erdre, from the city, from the Loire, from the Sèvre, infinite legions, darkening the day, which settled on the church roof, with a myriad voices, a myriad cries, debates, discussions. Though ignorant of their language, it was not difficult for us to perceive that they differed among themselves. It may be that the youngest, beguiled by the warm breath of autumn, would fain have lingered longer. But the wiser and more experienced travellers insisted upon departure. They prevailed; the black masses, moving all at once like a huge cloud, winged their flight towards the south-east, probably towards Italy. They had scarcely accomplished three hundred leagues (four or five hours' flight) before all the cataracts of heaven were let loose to deluge the earth; for a moment we thought it was a Flood. Sheltered in our house, which shook with the furious blast, we admired the wisdom of the winged soothsayers, which had so prudently anticipated the annual epoch of migration.
Clearly it was not hunger that had driven them. With a beautiful and still abundant nature around them, they had perceived and seized upon the precise hour, without antedating it. The morrow would have been too late. The insects, beaten down by the tempest of rain, would have been undiscoverable; all the life on which they subsisted would have taken refuge in the earth.
Moreover, it is not famine alone, or the forewarning of famine, that decides the movements of the migrating species. If those birds which live on insects are constrained to depart, those which feed on wild berries might certainly remain. What impels them? Is it the cold? Most of them could readily endure it. To these special reasons we must add another, of a loftier and more general character—it is the need of light.
Even as the plant unalterably follows the day and the sun, even as the mollusc (to use a previous illustration) rises towards and prefers to live in the brighter regions—even so the bird, with its sensitive eye, grows melancholy in the shortened days and gathering mists of autumn. That decline of light, which is sometimes dear to us for moral causes, is for the bird a grief, a death. Light! more light! Let us rather die than see the day no more! This is the true purport of its last autumnal strain, its last cry on its departure in October. I comprehended it in their farewells.
Their resolution is truly bold and courageous, when one thinks on the tremendous journey they must achieve, twice every year, over mountains, and seas, and deserts, under such diverse climates, by variable winds, through many perils, and such tragical adventures. For the light and hardy voiliers, for the church-martin, for the keen swallow which defies the falcon, the enterprise perhaps is trivial. But other tribes have neither their strength nor their wings; most of them are at this time heavy with abundant food; they have passed through the glowing time of love and maternity; the female has finished that grand work of nature—has given birth to, and brought up her callow brood; her mate, how he has spent his vigour in song! These two, then, have consummated life; a virtue has gone out from them; an age already separates them from the fresh energy of their spring.
Many would remain, but a goad impels them forward. The slowest are the most ardent. The French quail will traverse the Mediterranean, will cross the range of Atlas; sweeping over the Sahara, it will plunge into the kingdoms of the negro; these, too, it will leave behind; and, finally, if it pauses at the Cape, it is because there the infinite Austral ocean commences, which promises it no nearer shelter than the icy wastes of the Pole, and the very winter which exiled it from Egypt.