The seas which surround the British Isles and the small extent of the islands prevent the migration of land animals which is such a feature of continental Europe and North America where some species, most notably the Reindeer (Rangifer tarandus), or Caribou as it is known in America are constantly on the move. Most noticeable are the vast numbers of birds which migrate annually to and from or pass through the British Isles. Less noticeable are the many species of insects such as moths and butterflies and even dragonflies which migrate to our shores every spring. The annual cycle of the season also brings fish and other marine species as visitors to our shores.
One of the most spectacular migrations amongst the birds is made by the Arctic Tern (Sterna paradisaea), some 15 inches long and with a wingspan of 31 inches. Most Arctic Terns nest within the Arctic Circle and a few have nested on the Isles of Scilly, the southern extremity of their breeding range in the north. They spend the northern winter, quite literally, at the other end of the world, in the Antarctic. During the summers, the sun never sets above the Arctic and below the Antarctic circles, thus the Artic Tern spends most of its life in daylight.
It is likely that this incredible migration route was established by the species during the ice ages when the distances involved were much smaller.
Some of these migrations, such as those of the Atlantic Mackerel (Scomber scombrus) are of great economic importance.
Typical of summer visitors to the British Isles is the Swallow (Hirundo rustica). Breeding in the British Isles during the spring and summer, very few individuals overwinter in Britain in the milder regions of the South West such as Cornwall. They spend the winter months in South Africa.
many of the bird species which visit the British Isles in the summer to breed spend the winter south of the Sahara desert.
Migrating species can be devastated by changing environmental conditions, particularly changes in climate. Drought increasing the exent of the Sahara desert in 1969 caused a crash in the populations of the Whitethroat (Sylvia communis), which has since recovered.
The summers are warmer and the winters colder than in the British Isles on continental Europe while, further south, warm winters can be guaranteed in north and central Africa. Animals, particularly birds which nest in the British Isles can take advantage of spring and summer warmth and food and longer days (the difference between the length of the day and the length of the night become smaller towards the equator). In the autumn, as food becomes scarce and the weather cold, many species fly south for the winter to take advantage of warmth and food.
Some species which nest in the Arctic regions, Scandinavia and Eastern Europe escape the harsh winters there where temperatures can fall several tens of degrees celcius below freezing by migrating to the British Isles to over-winter in the milder climate.
Many species of birds pass through the British Isles in spring and summer en-route to and from their summer and winter territories. Our estuaries and wetlands form important "refuelling" stops where large numbers of waders stop for rest and food as they pass through the British Isles twice every year. Seabirds such as the Arctic Tern, Auks and Shearwaters visit coastal areas as they pass through along their migration routes.
"Vagrants" is the term used to describe those animals, particularly birds and insects which have strayed or been diverted from their normal migratory courses to arrive at places they would not normally visit.
Many young birds, making the annual migration for the first time without the strong sense of direction developed by more experienced birds, stray to places where their species is not normally found.
Some bird and insect migrants southwards along the eastern seaboard of North America are blown eastwards over the North Atlantic by autumn gales. Many are caught in deep depressions and carried across the ocean to Europe, while many more perish en-route. Some "hitch" passage on ships on which they land to rest. The west coast, and particularly the Isles of Scilly and Cornwall are excellent places to find such vagrant individuals.
The world's oceans cover almost seventy per cent of the earth's surface and the oceans over sixty per cent of the planet's suface are a mile or more deep. The marine environment contains many more animals than the land both in terms of numbers and of biomass (the combined mass of the individual animals).
Sunlight can only penetrate the ocean waters to depths measured in only hundred of metres, thus the photosynthesis on which marine plants and the vast majority of marine animals rely is only possible in the top few hundred metres of the oceans. As the sun sets, vast numbers of marine animals of all sizes make their way to the surface waters to feed on the plankton, to descend to the dark depths again at sunrise and await the night again.
Changes in climate cause both animals and plants to extent their range where conditions beyond the edge of their range become favourable and, conversely, retreat before adverse conditions. A succession of such gradual migrations populated northern Europe and the British Isles at the end of the last ice age. This northward migration included Stone Age man himself. The gradual warming of the British Isles in the last decades of the twentieth century and into the 21st century which have resulted in many species extend their range northwards and some disappearing from areas they had previously occupied. These changes have affected the marine environment as well with species normally associated with the coast of Portugal and the Mediterannean being found in British waters.
Human migrations have also happened, on greater and lesser scales, throughout human history and into modern times. While agriculture ties farmers to their land, localised seasonal human migration are very common in pastoral societies as the human population moves its livestock to suitable pastures from season to season.
Modern transport, particularly the volume and speed of air transport has facilitated the rapid migration of bacteria and viruses which cause diseases. One of the earliest examples is the infamous plague or "black death" which arrived in Europe along trade routes via the Middle East from the Orient. More recently, virulent strains of influenza and AIDS.
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|1969||Drought causes the extension of the Sahara desert in North Africa|
The extension of the desert had effects on migrating bird species such as the Whitethroat (Sylvia communis) whose populations crashed
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