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Migration

by Steve Portugal

By now, most our visiting summer birds will have left us and headed for sunnier pastures. Replacing them, as winter draws near are millions of birds joining us from their breeding grounds in Northern Europe and the Arctic. These tortuous journeys are made worthwhile by the advantages of summertime habitat with an abundant food source plus similar advantages on wintering grounds.

Bird migration is a topic that has fascinated mankind for centuries and it is only relatively recently that we have begun to understand more about migration and in particular, migration routes and wintering grounds. Although a lot remains to be discovered we've come along way since thinking that Swallow's winter at the bottom of lakes and Barnacle Geese (left) summer in shellfish of the same name.

Birds put a lot of effort into preparing for their journeys, as timing is critical. Leave too early and you may not have the reserves to last the journey, but leave it too late and you may encounter harsh weather and too strong an opposing wind. Navigation is possibly the most important factor of a successful journey. Birds use three main compasses to ensure they fly in the right direction for the correct length of time; the sun, the stars and the earth's magnetic field. Which they use often depends on the time of day the bird is travelling, another factor they have to consider.

Birds use a huge variety of techniques when migrating, often dependent on the terrain they are crossing or the distances they are travelling. The chicks of many seabird species have not yet learnt to fly when they fledge, so, accompanied by their moulting parents, swim from their breeding grounds to their chosen wintering quarters. A lot of songbirds double their body weight in autumn enabling them to migrate long distances without having to keep stopping to refuel. The Sedge Warbler, for example, averages an additional fat load equal to 100 per cent of its normal weight, with which it can probably cover more than 3,000 km in one non-stop flight of perhaps three or four days. Large birds such as Storks, Cranes and Ibis's have to rely on hot wind thermals to carry them, as they cannot rely on flapping their wings as that would not be energy efficient. Nor can they land on water so this collection of birds have to take a migration route that avoids large bodies of water and seek out the kinds of landforms that generate rising thermal currents. This often results in large numbers of birds being channelled through an area relatively small, which fulfils their requirements. On such place is the straight of Gibraltar, which provides adequate thermals and the shorting crossing distance across the Mediterranean Sea.

Some migrations undertaken by birds truly are awe-inspiring. Perhaps the most renowned migrator in the bird kingdom is the Arctic Tern (right). These birds breed in the Northern Hemisphere, from temperate latitudes to the most northerly land in the world, then winter on the edge of the Antarctic pack ice. The complete round trip for a single Arctic Tern may be as much as 40,000 km, an astonishing feat. Another feature of the Arctic Terns phenomenal travels is their record of experiencing more hours of daylight than any other living thing. Birds breeding in the north of the arctic circle experience the 24 daylight that occurs there in the summer months, and then, by travelling south of the Antarctic circle, find the same conditions during the southern summer.

The Quail and Corncrake are lost distance migrants, something which can come as a surprise considering how reluctant they are to break cover during the breeding season. However these skulking birds cross large expanses of sea and desert during their twice-yearly migration. Both species favour the south west of Africa to winter, and most British breeding birds would appear to cross the Mediterranean via Gibraltar.

Red Backed Shrikes that nest in Western Europe take a lengthy route to their wintering grounds in the southernmost parts of South Africa. All breeding birds head over the eastern Mediterranean and down through the savannah areas of the Great Rift Valley. One advantage the shrikes have over most other migrant birds is that they are able to prey on their fellow migrants, so they always have a food supply.

A lot of birds that don't breed in the United Kingdom still use the country as a stop over during migration. Many wading birds exhibit this behaviour. Birds such as Knot, Curlew Sandpiper and Little Stint that breed in the high Arctic and winter in South Africa stop over in Britain en route to take advantage of the rich food source to be found in our estuaries and marshes. These birds are not taking the obvious route to their African wintering grounds so it really must be worth their wile migrating via Britain to re-fuel.

Many birds less characteristic of Britain regularly over shoot their usual breeding grounds and appear in spring and autumn. Birds such as Wrynecks, Hoopoes and Bee-Eaters typically appear at these times of year. They are often blown west off their usual migration course, particularly in spring and on occasions have stayed and bred.

The natural phenomenon of migration truly is a wonder, how birds manage to navigate and locate themselves, often in the case of young birds with no prior knowledge, and how they can time their journey to take advantage of good weather conditions as their bodies reach an optimum condition.

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