It has always been clear that swallows disappear from northern parts in winter, but by no means obvious exactly where they go. Some, in former times, favoured the possibility of hibernation. The 16th-century archbishop, Olaus Magnus, of Uppsala, for example, presumably fairly typical of his time, declared that swallows in autumn descend first into the reeds, "and thence into the waters below them, bound mouth to mouth, wing to wing, and foot to foot". Fishermen, it was said, might often draw up a lumpy mass of these coagulated birds, and if the lump were warmed, the swallows would revive to resume, a month or two too soon, their individual lives.
But we, of course, know better. In an annual exodus as old as time itself, the swallows depart to spend the winter in the southern part of Africa. And a hazardous undertaking it is. The 6,000-mile trip takes six weeks, and of the five or six offspring of a typical adult pair of swallows, only one is likely to survive the journey to return the following year. Many of the hazards on the way are meteorological.
Departure begins with large groups taking to the air just after dawn. The first major hazard is a long sea crossing, and the flock will generally wait for fine weather, if necessary, before attempting it. Nonetheless, unexpected gales can claim many victims.
Adult swallows apparently take from three to five days to reach the south of France, while young birds take much longer, and throughout the journey all are at continual risk from predators. Those that cross the Mediterranean at Gibraltar are rewarded with a crossing of a mere eight miles, but those on a more easterly track may have to cope with 300 miles or more; swallows that drift out to the west are in grave danger from the Atlantic storms, and thousands collapse exhausted into the sea to die.
The Sahara desert requires a 900-mile flight, which most attempt non-stop, since there will be few, if any, feeding sites en route; more birds die on this leg of the journey, apparently, than on any other. The grassy plains of the Sahel provide a bountiful, albeit temporary, respite, but the vast rain forests of the Congo, and more particularly the violent tropical thunderstorms that they attract, take a large toll on the remaining swallows. The final irony is that those who accomplish the arduous journey in the shortest time may arrive in southern Africa before the last frosts of the southern winter have occurred - and may then succumb to cold.