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Country: Nature Note

IN THE past few days, swallows and house-martins have been lining up in rows on telephone cables - a sure sign that they are about to start their migration.

As late as the 18th century, scientific observers believed that swallows spent the winter hibernating under water.

Even that great savant, Dr Johnson, thought that "a number of them conglobulate by flying round and round and then, all in a heap, throw themselves under water and lie in the bed of a river".

Ringing has shown that they fly several thousand miles to southern Africa: it has been estimated that after a good breeding season in Europe, some 220 million of them will head for the far south. British swallows seem to favour the area round Johannesburg.

Scientists are still not certain what triggers their annual departure, but one key factor appears to be the shortening of daylight hours.

Nor do experts agree on how the birds navigate: they appear to steer partly by the sun, partly by landmarks, and partly by responding to the earth's magnetic field.

Swallows are specialists at in-flight refuelling: they eat as they go, catching insects on the wing, swooping low over lakes or rivers to drink, and pitching into reed-beds to take a rest at night.