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Apr 26

Big Question: Why and How do Birds Migrate?

The swallow. A quintessential summer bird if ever there was one. It typically arrives in the UK around now (late April), spends the summer raising its young, and then leaves our shores again in September. The swallows travel back to sub-Saharan Africa and other warm regions to spend the winter. This periodic movement of animals to and from different areas is known as migration.

So what prompts a bird to migrate? It might be to find a better location in which to lay eggs or rear young; perhaps to avoid the lack of food in one area and exploit the abundance of food in another; or maybe to avoid adverse weather conditions. Often, it is more than one of the above that triggers migration.

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Barn Swallow Photo: Martin Mecnarowski

To make a long migration, birds must make use of almost every sense that we know of – and probably a few we have yet to discover. Some may follow the geographical features of the land – coastlines, mountain range etc. – and this gives them a mental map by which they steer. In this way, many birds travelling to Europe from Africa ensure that they cross the Mediterranean Sea at its narrowest point, the Straits of Gibraltar in the far west. They may also navigate using the sun, the stars, the Earth’s magnetism, or a combination of all of these.

Some birds, such as swans and cranes, undoubtedly learn the predetermined route by following their parents on the journey for the first time. Others, such as the cuckoo, abandoned by its mother even before it hatched, must surely inherit a mental map instead.

Mysterious Migration

Today, thanks to ringing and tracking technology, we know a great deal about where migrating birds go when they leave our shores, but a few hundred years ago people were stumped as to where birds such as swallows spent the winter. There were several theories, most of which were none too sensible.

‘They hibernate at the bottom of ponds, packed into the mud like sardines,’ some people declared confidently.

‘Nonsense! They fly to the moon and back!’ others argued.

The mystery was only solved little over a century ago, in 1912. A swallow was ringed at a nest in Staffordshire by a man name John Masefield and the same bird was later found in Natal, South Africa, in December of that year. It proved that the swallows made mammoth journeys like some people suggested – perhaps not quite to the moon, but impressive nonetheless.

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A few examples of long distance bird migration routes. Image: L. Shyamal

But why do swallows even bother coming to Europe to breed at all? Why can’t they stay in Africa and breed? To answer that, a small trip back in time – some 12,000 years or so – is required. During this time period, the Ice Age had gripped the northern hemisphere and huge glaciers stretched down into the middle of Europe. Birds that lived in Africa soon learned that just across the nearby Mediterranean Sea was a cool land that, during the summer at least, was very rich in insect food, which is excellent protein for growing chicks. So the birds – swallows included – just popped over the Mediterranean once a year to raise a family, and then flew back to Africa when the harsh winter returned.

Eventually, the Ice Age ended. The glaciers retreated. The habit of flying north, however, still persisted, if only to take advantage of the superabundance of food there. Indeed, this migratory behaviour has remained to this day, despite the fact that the journey to the summer feeding and breeding grounds is no longer a few miles away, but a few hundred.

Some birds have taken migration to the extreme. The longest migration of any bird – or indeed any animal – is undertaken by the arctic tern, which travels all the way from the Arctic to the Antarctic, and then back again. But not all birds are such avid travellers; Clark’s nutcracker, a member of the crow family, embarks on what is probably the shortest migration in the world, a mere few hundred metres down the mountains as winter approaches.

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