Swallows in Migration
A Every April, along with many other species of birds, the swallow arrives to spend the summer months in northern Europe, in Russia, Iran and parts of Siberia. Here it will breed and raise its young.
B The swallow is well known for several reasons. Firstly, it is very distinctive, with its forked tail and characteristic acrobatic swooping flight. Secondly, it is very common, and, like its near relative the house martin, lives in close proximity to human habitation, at least in rural areas. It is, however, rarely to be encountered in towns or cities.
C For centuries, people have observed swallows, noted their arrival and their patterns of feeding. In several countries, these observations have passed into the language as proverbs or sayings. In England, people comment on unpredictable late spring weather by saying, ‘one swallow does not a summer make’. Similarly, ‘the swallows are flying low’ was held to predict rainy, even stormy weather. There may be some truth in this observation, though it is the insects the swallows feed on that seem to be more susceptible to the fall in barometric pressure that heralds a storm. Insects keep low in these conditions, and so do the swallows that hunt them. At the end of the summer season, when the swallows are about to leave, they frequently flock together in large numbers on convenient high open perches, like roof ridges and telegraph wires. When people remark that ‘the swallows are gathering’, they mean that autumn has arrived.
D At some point in mid-September the swallows leave together, usually all on the same day. One day there are thousands, the next there are none, and none will be seen again until the following spring. For centuries, this was a complete mystery to people. The Hampshire naturalist Gilbert White, writing in the late eighteenth century, believed that the swallows dived into ponds and rivers in autumn and remained in the bottom mud the whole winter, re-emerging the following spring. This idea seems extraordinary to us, but White was not a stupid man: many of his other observations of natural life were informed and accurate. In this case, however, he simply had no means of determining the truth and was forced to make a random guess. The idea that swallows migrate to central or southern Africa would have seemed as fanciful to him as his theory seems to us.
E Although we now know that swallows migrate, there are still unanswered questions. Why do they go so far? Why not stay on the shores of the Mediterranean? The majority continue to equatorial Africa, and some even further south. Also it appears that populations of swallows that have bred in different countries also spend the winter in different areas. Those from France, Germany and much of western Europe have mostly been traced to East Africa, Kenya or Tanzania for example. Above all, how does a bird weighing approximately twenty grams find its way across mountain ranges, ocean and desert to winter in the south, and then return the following year to the very location it was born, in some cases to the very same nest?
F Birds can navigate by the Sun, and are also able to detect the magnetic field of the Earth. Species that migrate at night are also able to navigate by the stars. By these means, they travel long distances. The close navigation that brings them back to the same field or nest appears to be related to memory of local landmarks imprinted on the minds of young birds as they crisscross the area in the weeks before departure.
G Nevertheless, the journey is very dangerous. Long sea crossings, where there is little available food or water, are generally avoided. In western Europe, most swallows cross to Africa via the Straits of Gibraltar, or fly the length of Italy before tackling the relatively short crossing to Tunisia in North Africa. However, in storms they may be blown hundreds of kilometres off course. Exhausted swallows sometimes come to rest on ships way out in the Atlantic Ocean. They have to cross mountain ranges too, where again the weather may be unpredictable and food scarce. Along the coast of North Africa, many young swallows become the prey of Eleonora’s falcons, which time their breeding to coincide with the migration of young birds southwards. But the most dangerous part of the journey is the crossing of the Sahara desert. Here, there is little food or water, sandstorms may delay and exhaust the already weakened birds, and many die. It is estimated that around 50 per cent of adult birds die, and up to 80 per cent of young birds, but enough survive to ensure the continuation of the species.