During their spring migrations millions of birds cross the skies. They follow a strict ritual, leaving their winter quarters and moving to their summer locations, where they mate and reproduce. The journey can take many days to complete. The European fish eagle, for example, needs an average of 30 - 80 days for its migration from West Africa, the blackpoll warbler travels from North to South America in just 80 - 90 hours; the knot or Icelandic sandpiper, which belongs to the family of stilts, flies thousands of kilometres across the Atlantic from Europe to Greenland, and the cuckoo migrate in winter from Europe to tropical Africa.
Migratory birds almost never deviate from their course. They always depart from the same place in order to arrive in exactly the same location as in the previous year. With admireble precision, they follow the same route again and again, resting mostly at the same places each year. To scientists, the astonishing navigational ability of migratory birds is one of nature's greatest mysteries. We still don't know exactly how animals can travel round the world without getting lost. Nor do we know what navigational aids they use to keep their bearings in ghe boundless space of the skies. What is clear is that migrating birds face the same problems that seafarers, pilots and explorers must solve, knowing simultaneously where they are, which direction to go and how to get there. In other words, the birds must be able to determine their position and choose the correct direction of travel at the same time.
Because migrratory birds have neither compasses nor air-traffic controllers to tell them where to go, how do they orientate themselves? Some keep to their northward course by aligning their flight with the position of the sun. In tests carried out on pigeons scientists have demonstrated the critical importance of the sun.
An Internal Clock.
Carrier pigeons have a kind of inner clock which enables them to find their way back home over many hundreds of kilometres. The decisive factor is the position of the sun. Laboratory tests have shown that it is possible to alter a bird's inner clock by artivicially modifying rhythm of day and night. A number of pigeons were released at noon after their inner clock had been set forward by six hours. The birds perceived the sun to be already at its 6 p.m. position, which is much further in the west. As a result, the birds orientated their flight too much to the east. In another experiment, researchers exposed them to sunlight that had been redirected using a mirror. The pigeons tried to follow the new direction of the light and therefor flew in the wrong direction.
Of course, the sun only plays a role for those birds that are active in the day. Those that migrate at night must use a different point of reference. The stars are a possibility, but the great migrations are not disturbed by the presence of clouds by day or night.
Many researchers favour the theory that the earth's magnetic field plays a role in bird navigation. The North Pole and the South Pole act like the two poles of a huge magnet, creating a field which acts on electrically charged particles from the sun, causing them to deviate. Carrier pigeons in particular are known to use the lines of the earth's magnetic field to orientate themselves. If a magnet is attached to a pigeon's head, the bird becomes completely disorientated if the sky turn cloudy, and is incapable of finding its way back home to its cote. However scientists have to admit that the workings of the internal compass supposedly used by the birds remains a mystery.
A matter Of Smell
Scientists must also tackle the problem of how the birds actually locate their routes. There are probably a variety of ways; some species doubtless possess a good visual memory, while the high resolution capacity from a large distance very clearly. But the visual capacity of other species is inadequate.
According to one Italian scientist, pigeons can partially orientate themselves by their sense of smell. They mentally map the smells around their cote, and estimate their exact position with the help of the wind - which brings known scents closer. Shearwaters, such as the great shearwater (Puffinus gravis), which visits the eastern North Atlantic in summer and autumn, also use their sense of smell to find their way back to their nesting places on the other side of the ocean in the south. This is a promising area of research, but will require closer observation of various bird species.
Mysterious Migration.
Despite years of study, we know little about long-distance migrations of birds. However, learning processes may play an important role. Experiments heve shown conclusive behavioural differences between the young, inexperienced birds and the older ones. On an island in the North Sea, researchers captured a number of hooded crows - both older birds and beginners - when the flock, flying in a southwesterly direction, moved to its annual winter location near the coast of the English Channel. The researchers loaded the captured birds on an aircraft, and flew them to Poland, 600km further east, where they were released. The youngest crows, which were less than a year old and had no navigational experience, immediately took up their southwesterly course, according to their inherent sense of orientation. The experienced, older birds, however, were able to correct their course and flew purposefully towards the northern coast of France.
But what is the reason for bird migration? Contrary to popular belief, the birds do not only fly south in order to warm up in the sun. Most of them are actually quite resistant to cold. However, migration fulfils two vital functions; on the one hand, it safeguards the food supply' on the other, it guarantees the maintenance of the species, as it provides ideal conditions for reproduction. Birds that migrate over long distances and whose departure dates, flight duration and arrival dates tend to be very punctual, are called calender birds.
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