Nevertheless, all birds are not migratory; many species remain during their whole lives in the locality where they were hatched, straying but little distance from their birth-place. The majority of those which migrate perform their journeys annually and with great regularity; a few irregularly and accidentally; that is, they are caused by necessity, or by atmospheric influences, to change their residence; and it is no unusual sight on such occasions to see numerous flocks of birds assembling under the leadership of a chief, and taking their flight in perfect order, traversing seas, and passing from one continent to another, with astonishing rapidity. On the 22nd of September, 1771, White, of Selborne, witnessed the flight of a flock of Swallows which had rendezvoused the night before in a neighbour's walnut-tree. "At dawn of what was a very foggy day, they arose all together in infinite numbers, occasioning such a rustling from the strokes of their wings against the hazy atmosphere that the sound might be heard at a considerable distance." In the Old World, choosing a time when the winds are favourable, most migratory birds direct their flight towards the south-west in the autumn, and the north-east in spring. In America the migratory birds take a south-east direction in autumn, and the reverse in spring. These aërial travellers instinctively direct their flight to the same regions--often to the same district; and there are good grounds to believe that the same pair frequently find their way year after year to the same nest. The duration of the life of birds in a state of nature is one of those subjects on which little is known. Some ancient authors--Hesiod and Pliny, for example--give to the Crow nine times the length of life allotted to man, and to the Raven three times that period; in other words, the Carrion Crow, according to these authors, attains to seven hundred and twenty years, and the Raven two hundred and forty. The Swan, on the same authority, lives two hundred years. This longevity is more than doubtful. Paroquets, however, are known to have reached more than a hundred. Goldfinches, Chaffinches, and Nightingales unquestionably, even in the confinement of a cage, have lived four-and-twenty years. A Heron, Girardin tells us, lived fifty-two years, which was testified by the ring which he bore on one of his legs, and even then he lost his life by an accident, while in full vigour. A couple of Storks, moreover, have been known to nestle in the same place for more than forty years. All that we can affirm is that birds live much longer than the Mammalia.