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page.The Mystery of Migration
Birds are divided into the following basic groups, depending on whether they remain in their breeding grounds throughout the year or leave for the winter:
Resident birds - that stay close to the gener-al area of their nesting grounds
Migratory birds - that leave their nesting grounds each year in the autumn or late sum-mer, fly to warmer quarters for the winter, and return again in spring.
Transient migrants or dispersive birds - that range far afield, often hundreds of kilometres, in all directions from their nesting grounds after the breeding season, depending on the weather and available food supply.
There may, however, be various transitional stages between these three basic groups and, sometimes, contrary classification for members of one and the same species. For example, some birds that nest in northern Europe, such as the peregrine falcon and kestrel, are migratory, whereas those of western and central Europe are resident. In other species, some members are migratory and others resident. There are also species which, in general, are classed as migra-tory but some of their members remain in their nesting grounds for the winter. Sometimes birds that are otherwise resident will suddenly set out in large flocks on a long journey south or southwest; these are called invasion migrations. One example is the nutcracker, normally a resident or dispersive bird, which sometimes, however, travels in large flocks to central Europe from the north or northeast. There are also the birds which are regular winter visitors, migrants who have left their breeding grounds in the far north to spend the winter in central or Western Europe.
The answer to why some birds migrate and others stay the winter is not as simple as it seems. Ornithologists have studied the phenomenon for many years and still have not come up with a clear-cut explanation.
In some species the primary reason is a short supply of food; typical insectivorous birds would not find enough to subsist on in northern and central Europe during the winter. Swallows and martins depart when insects start becoming scarce. Why, however, do some birds, such as the common swift, leave as early as the end of July, when insects are still plentiful, remaining in their nesting grounds a mere three months? Then there are the birds that were originally migratory, e. g. the European blackbirds, but have since become resident, increasingly settling in the vicinity of man.
What causes migratory birds to fly south or some other direction, depending on the species, at a certain time of the year? One factor is almost certainly the changing hours of daylight, which bring about an alteration in the bird's hormonal balance. For example, in autumn the rapidly shortening days affect the activity of the sex organs which triggers the migratory instinct. In the more northerly parts of Europe there are fourteen to eighteen hours of daylight during the nesting period and birds have plenty of time to forage for food for their offspring. However, their broods are reared just as successfully by birds that breed in the tropics, where there are only twelve hours of daylight. And the crossbill even nests in Europe during the winter season when there is even far less daylight, the deter-mining factor, apparently, being the abundance of food. As one can see there can be no clear-cut answer to all the many questions. One thing that is certain, however, is that migration in various species of birds is triggered by various combina-tions of external events.
Another theory about bird migration is based on the presumption that migrant birds originally lived only in tropical and subtropical regions. Here they bred and raised their offspring and when they over-multiplied they set out for more northern parts in search of food for their families, returning south again when the young were fully-grown. This, however, may be the case with only a few species of birds native to the tropics, such as the golden oriole and European bee-eater.
Exactly the opposite is the substance of still another theory according to which birds inha-bited the entire northern hemisphere before the Ice Age but were pushed farther south by the advancing ice sheet, being forced even to aban-don their breeding grounds in cruel winters. After the' ice sheet receded the birds again made their way north to their original homes and over the millennia migration became an established phenomenon. Why, however, do birds in other parts of the world untouched by the ice sheet migrate? It would seem that bird migration is a phenomenon that existed before the onset of the Ice Age.
These are only a few of the several theories about migration but none is able to provide a fully satisfactory and exhaustive explanation. There were doubtless a number of causes under-lying the origin of this phenomenon which may have been different not only in the various parts of the world but also for various species of birds so that one must look to each of the many theories for an answer to the questions of how and why.
More is known about where and how birds migrate, where to and by what routes. Answers to many of these questions are now possible with the aid of high-powered radar equipment, air-craft and short-wave transmitters. Today there are ringing stations in practically all parts of the world which mark individual birds, both young and adults, by putting a ring round the bird's leg with a number and the address of the respective country's ringing scheme. Some hundreds of thousands of birds are ringed annually in Europe alone; some of those that are recovered help to make it possible to determine the route, speed of flight and winter destination of some species. Recovered rings are sent to the respective 'ring-ing scheme's headquarters which then plots maps of migrations. These are added to and made more precise every year on the basis of information received from other ringing schemes throughout the world.
Ringing also provides some answers to other questions, such as whether birds remain together for life when they have paired, whether they return to the same places from their winter quarters, whether only the young migrate, etc.
Sometimes rings of different colours are used to enable observation of individual birds through a field glass to determine the size of the nesting territory, courtship, etc.
European birds migrate roughly in three main directions: south-west from northern and north-eastern Europe across western Europe and the Iberian peninsula, straight south from the north across Italy and Sicily, and south-east across the Balkan peninsula and Asia Minor. Various species of birds fly in different directions, as may individuals of the same species but of different populations (nesting in a different place). Birds do not follow specific routes as was originally believed; their migration encompasses a broad front, though in some places it narrows, for instance in the case of some barrier such as a range of high mountains when the birds seek more convenient routes such as mountain pas-ses, etc., or when flying across the open sea then over islands where the birds can rest.
Not all birds follow one of the three southerly routes. Some, such as starlings from northwestern Europe, fly west to the British Isles. In winter tens of thousands of these birds forage for food in the vicinity of as well as in London's parks flying into the very heart of the city to roost on the cornices of high buildings for the night. An interesting example of such westerly migration from the Continent to the British Isles are the many small birds which in adverse condi-tions literally `hitch a ride' on regular ships during their flight across the English Channel. Finding a suitable spot on the boat's topside they `ride' for several hours in comfort until within sight of the British shores, taking to the air and covering only the last several hundred metres under their own power.
Many European birds winter no farther than southern or south-western Europe, others win-ter in North Africa, and still others journey as far as the tropics or even South Africa. Such a journey, however, is not made non-stop with-out a break, as many people once believed. Some birds, such as waders, move about 100 kilometres a day, birds that hold the record cover as much as 300 to 600 kilometres a day. On the way they may stop to rest and, what is more important, obtain food. Species, such as warblers and shrikes, that feed on insects spend several hours foraging for food. In bad weather, especially if it is foggy, they may even remain for several days in one place and thus the journey to, say, tropical Africa may take as much as several months. Their stay in their winter quar-ters is confined to only a few weeks after which they set out again on the return trip to their nesting grounds. The journey to southern Europe sometimes takes only a few days, de-pending on the birds' point of departure.
The return trip to the nesting grounds is usual-ly faster, sometimes by as much as a third, because the birds are urged on by the breeding instinct. The spring migration may be unfavour-ably affected by external factors such as a sud-den drop in temperature or snowfall.
The speed of a bird on migratory flights is less than that of which it is capable over short dis-tances and varies according to the given species - the rook flies at about 50 kilometres per hour, the chaffinch 52 kilometres, the jackdaw up to 60 kilometres and the sparrow up to 74 kilometres per hour. Some species, such as starlings and finches, can often be observed migrating in flocks. Most small birds, however, make the flight alone or in small groups that pass unnoticed. Some, such as swallows, corvine birds and finches, migrate by day, others, such as warblers and flycatchers, travel mostly by night. Thrushes and wagtails fly both by day and at night.
Species that migrate alone include the Euro-pean cuckoo, wryneck, and certain raptors. Of the birds that migrate in flocks some do so in their own special formations, e. g. in V-forma-tions or slanting lines, the ones in front breaking the air for those behind, the birds sometimes taking turns as leader.
Birds also travel at various altitudes. The rook and the jackdaw, for instance, at a height of 30 to 100 metres, the starling 50 to 100 metres, small birds usually up to 100 metres and most raptors at a height of about 100 metres. Some birds, however, have been known to fly much higher, for instance when they had to pass over a high mountain range.
It is amazing that birds nearly always find their way back to their nesting ground after travelling from as far away as South Africa. Though in many cases they fly in flocks together with older birds that know the way, having made the journey before, some undertake such a jour-ney alone and for the first time and yet do not go astray. It would seem, then, that birds must have an innate sense of long-distance orientation. But how do birds navigate and what influences this trait that leads them almost unerringly to their goal? This problem has been studied by many scientists for years. One theory states that birds are directly influenced by the earth's magnetic field whereby they are able to distinguish the various points of the compass. Complex experi-ments, mostly with homing pigeons, did not prove the truth of this theory. Modern instru-ments revealed that the earth's magnetic field has only a slight influence on animals.
The latest theory is the one that believes birds navigate by means of light, or rather by the position of the sun as well as the position of the moon and stars. This theory was substantiated by experiments with many species of birds. In the spring and autumn migration periods captive starlings placed in a round aviary flew in the same direction as when migrating, being able to orientate themselves only by the heavens, which they could see from their cage. The birds orien-tated themselves precisely according to the posi-tion of the sun and at night by the position of the stars. When the apparent position of the sun was altered by a suitable arrangement of mirrors the starlings made corresponding changes in their position. Observations with radar equipment al-so showed that many birds migrate according to the position of the stars. Nowadays we know that other animals, such as sea turtles, likewise navigate primarily according to the position of the sun and stars. Birds can orientate themselves by the position of the sun even when the sky is overcast, but not in very foggy conditions.
Though the light theory was substantiated in many birds, in others it may be presumed that they can orientate themselves in part also by the earth's magnetic field, etc. This, however, would have to be substantiated by further complex experiments.
At the nesting site and in the surrounding neighbourhood birds find they're way mainly by their memory of various landmarks. Many birds move in regular circuits, somewhat like mam-mals along certain trails, and these are firmly fixed in their memory. When they have fledged, young birds usually remain for a time with their parents thus learning to know the surroundings of their nest. Birds that become independent shortly after fledging get to know the coun-tryside through their own observations. This, however, does not fully explain the problem of orientation. Birds captured by their nests and released several hundred kilometres away re-turned to their home site, yet they couldn't possibly know the countryside such a distance from their nest. One starling taken by plane 341 kilometres north of its nest returned within sev-eral days; captured once again it was taken 500 kilometres south of its nest and was back once more within five days. The same thing happens in the case of many other birds. It is evident from these experiments that long-distance orientation is not limited to migration but comes into play also during the nesting period. It is even developed in birds that are resident and remain in their nesting grounds throughout the year. In experiments with swallows, birds taken thirty-five kilometres from the nest returned within 2.5 to 8 hours. The reason it took them so long to travel a distance which could easily have been covered in an hour was because they no longer knew the landmarks that far away from their home base and had to orientate them-selves approximately by the position of the sun. They probably did not fly a direct course at the start but only after they reached ground with familiar landmarks.
The distances some birds travel when migrat-ing are almost unbelievable. Many European birds such as the swallow, cuckoo, nightjar and hobby, fly as far as southern Africa, a distance of about ten thousand kilometres which they travel twice a year, in spring and autumn. The record journeys, however, are those made by the arctic tern which breeds from the coast of Germany and England north as far as the arctic regions of Europe, Asia and North America. Birds from northern Europe travel along the coast of west-ern Europe and are joined by individuals from Greenland and the eastern nesting grounds of North America. They continue along the west coast of Africa to the tip of South Africa whence many individuals continue on to circle the Ant-arctic, returning again to the tip of South Afri-ca. When the time comes they fly back to their European and other nesting grounds. It is unbe-lievable that such a small bird, weighing only 90 to 120 grams, flies a distance of 36,000 kilometres to its winter quarters and back every year. Long journeys are also made by the ful-mars, shear waters and petrels. The fulmar, for example, is both a dispersive as well as a migra-tory bird, some individuals flying as far as the shores of Brazil and Argentina south of the equator. One bird ringed in Stockholm was re-covered even as far away as the coast of south-ern Australia.
Flight is the general method of travel in the bird kingdom but some birds travel long dis-tances by swimming. These, of course, include penguins, which have lost the power of flight. Some birds, however, even though able to fly, swim part of the way when migrating. The young of the gannet, for example, abandon the nest at the age of about 75 days and leap into the sea, swimming in the direction of migration until they are able to fly, which is at the age of 95 to 107 days. The eider, as well as some divers and Brunnich's guillemot sometimes journey hun-dreds of kilometres in this manner.
Only very occasionally are birds known to migrate in part by walking. In one instance thousands of American coots (Fulica americana) were observed walking for three days in the direction of their winter quarters.
The migration of birds of the Southern hemi-sphere is naturally in the opposite direction, that is from the south northward to the equator.
Click on any of the other bird links, these are Geese
Barnacle Goose : Brent Goose : Canada Goose : Greylag Goose
Or on the Ducks:
Gadwall : Goldeneye : Goosander : Mallard : Pintail : Pochard : Red Breasted Merganser : Red Crested Pochard : Shellduck : Shoveler : Teal : Tufted Duck : Wigeon
And the rest, in no real order of importance:
Common Sandpiper : Coot. : Corncrake : Curlew. : Dunlin : Greenshank : Lapwing : Moorhen : Oystercatcher : Redshank : Ringed Plover : Snipe : Spotted Crake : Stone Curlew : Turnstone : Whimbrel : Woodcock.
Problems Caused by Certain Sea Birds
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