Olaus Magnus, the 16th century Archbishop of Uppsala, knew precisely where the swallows went in wintertime. He was of the view that in the autumn the swallows descended 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. We know that the swallows depart to spend the winter in the southern part of Africa. And a hazardous undertaking it turns out to be.
The 6,000-mile trip takes six weeks, and of the five or six off-spring of a typical adult pair of swallows, only one of them is likely to survive the journey to return the following year.
But migration, on balance, seems to be a safer strategy than to stay at home. It is not especially the cold that makes wintertime so difficult for birds; the high mortality rate is more a consequence of the effect of the winter weather on the food supply.
Even the smallest birds are hardy creatures, provided they have an adequate amount to eat; during the winter season, however, plants stop growing, many insects disappear, and many other potential sources of essential nourishment may be rendered inaccessible for extended periods by a layer of ice or snow upon the ground. The risks of staying put to brave the winter are apparent from the relative breeding strategies of different species. Resident songbirds, apparently, produce several broods each year, each containing as many as a dozen eggs; migratory species, by contrast, in general lay fewer eggs and breed only the once.
The reason for the difference is assumed to be that the resident species have a higher mortality rate in winter than that experienced by their travelling cousins on their lengthy journeys.
The travellers have three main compasses: en route: 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 familiar enough 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 small local variations in the Earth's magnetic field with sufficient accuracy to be able to use them as landmarks by which to recognise familiar areas.