BIRD MIGRATION

Where do Swallows go in winter? Just over two hundred years ago, this was the subject of lively debate between those who believed they migrated and those who believed they hibernated. In his Natural History of Selborne Gilbert White includes his correspondence about this; although some of his correspondents were clearly watching the migration of birds through Gibraltar, he was personally rather sympathetic to the view that they hibernated, though not going as far as the contemporary Swedish theory that they did so in the mud at the bottom of ponds.

Now, of course, with the aid of more modern techniques such as bird ringing we know that British Swallows fraternise with Russian Swallows in the very southernmost part of South Africa, while central European Swallows go not quite so far south. While not quite the longest migration on record, it is still a prodigious distance for a small bird.

The migration of birds must be one of the most fascinating phenomena of the natural world. The Arctic Tern may spend its breeding season in the Arctic and then, while the arctic is frozen by winter, enjoy the austral summer in Antarctica. A Manx Shearwater from the UK has even been found in Australia. Yet, a bird such as the Golden Pheasant may spend the whole of its life within a couple of kilometres at most from where it was born. What is it that drives some birds to make such long and potentially hazardous journeys?

Perhaps the most familiar migration is seasonal migration in response to temporal changes in the abundance of resources. In the UK, this manifests itself as summer visitors, such as the familiar Swallows and the Cuckoo, which take advantage of abundant insect life to breed here but which have to migrate to Africa for the winter, and winter visitors, such as many species of wildfowl and thrushes, which breed further north and are forced by the harshness of winter to seek milder conditions here. Although migration may be hazardous, it is clearly less hazardous than the alternative.

There are, however, several other types of migration. Some species may only occasionally be faced with a shortage of food and irrupt to seek new sources: Crossbills famously do this from time to time when the cone crop fails. Most wildfowl become flightless for a period when moulting and potentially vulnerable to predators; to reduce this risk, Shelducks from many parts of Europe congregate on the rich feeding grounds of the Knechtsand in the German Bight. In a similar fashion, many Canada Geese from all over the UK gather to moult in the Beauly Firth. In some species, some populations are migratory while others are not; Stock Doves in the UK, for example, are sedentary, while those from Scandinavia leap-frog them to winter further south. Sometimes only a part of a population will move; it is often true that younger birds and females will move further than older birds and males.

Migration makes significant energetic demands. Some birds, such as the larger birds of prey and storks, seek to minimise their use of energy by avoiding sea crossings wherever possible and utilising thermals to gain height before gliding on to the next thermal. Others, faced with unavoidable sea or desert crossings, have no choice but to load up with sufficient fuel in the form of fat deposits. Indeed, a small bird like a Whitethroat will almost double its weight before setting off on its trans-saharan journey. Some species migrate by day, and those that depend on thermals have to, but more migrate by night, when lower temperatures reduce energy demands. They will also reach heights at which the winds are more favourable and assist them on their way.

Birds' navigational abilities are clearly well developed. The small warbler that comes back to the same garden in the UK to breed for several summers may return to virtually the same bush in sub-saharan Africa each winter, and may even do so stopping off en route at the same places it stopped off in previous years. These feats of site fidelity require not only a directional sense, but also a map sense. Much must be innate (how else could a young Cuckoo that has never known its parents make its way successfully to its winter quarters?), but some is learned. In some famous displacement experiments, migrating Starlings were caught and released some way away. Young birds on their first migration carried on in the direction they had been originally going for roughly the same distance as they would have gone, but older birds, which had already migrated at least once, re-oriented, changed direction and returned to the winter quarters with which they were familiar.

The mechanisms involved in their navigation are still not fully understood. We do know that they use visual clues, particularly when close to home, that they can use the sun and stars, and that they are sensitive to the earth's magnetic fields. Even the sense of smell has been shown to have a role. Just imagine how human beings would struggle to undertake such journeys without maps, and, these days, GPS.

The tools with which we can study migration have come a long way since the days of Gilbert White. We are no longer restricted to visual observation, but can now watch mass movements by night, and by day when birds are beyond visual range, on radar. At the level of the movement of the individual bird, ringing has provided us with a wealth of information, now being supplemented by real-time satellite tracking and data-logging which can reveal whether a bird is flying or resting and even what its heartbeat is. We can experiment with their directional sense using captive birds in planetaria and chemical analysis using stable isotopes can reveal where a bird's feathers were grown. Even so, many mysteries remain to be resolved. The winter quarters of such a common bird as House Martin are still unknown; there is only one sub-saharan recovery of a British ringed bird, in Nigeria, and a few more for continental birds.

Although many migration patterns must have been established since the retreat of the ice at the end of the last ice ages, migration is a dynamic phenomenon, responding to both climatic and genetic changes. When Blackcaps started to winter in the UK, it was at first thought that they were our native ones not needing to go further south; in fact it turns out that they come from central Europe and get back to their breeding grounds earlier than those that have gone south for the winter. This gives them a selective advantage, so they have more young, which in turn winter in the UK and so the cycle is reinforced.

It is of course not only birds that migrate, but many other animals as well, from the mammals that follow the rains in Africa's grasslands, to the salmon that return to the river where they were spawned and even insects, such as the Monarch butterfly which can fly from Canada to winter in Mexico, and has, on occasion, even managed to cross the Atlantic.

For those who would like to learn more about the movements of Britain's birds, The Migration Atlas, available from the British Trust for Ornithology and booksellers, summarises the results of ninety years of ringing, and the arrival patterns of our common summer migrants can be seen on www.bto.org/birdtrack.

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