It is an annual event as old as time itself. Every autumn, its numbers augmented by a new generation, a large proportion of the bird population begins a southward trek. It has been calculated that at the end of summer every year as many as five million birds of more than 200 species leave the northern regions of Europe to travel south to Africa, to avoid the rigours of a European winter.
It is not primarily, however, the cold winter weather that the migrants seek to leave behind: it is rather its consequences for their food supplies. Even some of the smallest migrant birds are very hardy creatures, provided that they have an adequate supply of food. But during the winter season in the north plants stop growing, many insects disappear, and many other potential sources of essential nourishment are rendered unobtainable for long periods by a layer of ice or snow.
Autumn migration begins as early as July for some, but November may have arrived before the last tardy stragglers leave for the south. For many of the migrants, the weather they experience en route is a matter of life or death, and an unexpected storm may spell disaster. For obvious reasons, the travellers prefer a following wind, and favour cloud-free skies to help them with their navigation.
Migratory birds en route have three main compasses: the stars, the sun and the earth's magnetic field. To use the sun as a direction-finder, they have to take into account the time of day - but their "body-clocks" appear to cope. They are sufficiently familiar with the way in which the constellations seem to rotate around polaris to be able to orientate themselves, if needs be, using only a small window in the night sky. And evidence also suggests that some birds can detect local variations in the earth's magnetic field with sufficient accuracy to be able to use them as landmarks to recognise familiar areas; pigeons, for example, are believed to be able to detect changes in the magnetic field of less than 0.2 per cent.
Autumnal southbound flights often begin in the aftermath of a deep depression, so that the birds can take advantage of the northwesterly flow of air behind the low. The north-westerly is frequently succeeded by a brief ridge of high pressure before the arrival of the next depression, and this too provides very favourable conditions. Indeed, in retrospect for a particular season, it is often possible to associate major movements of the migrants, as recorded by birdwatchers, with the irregular pattern of the depressions of the North Atlantic.