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Migrating birds use the sun and stars to navigate their miraculous journeys

Posted in Biology, Birds, Nature, Wildlife on Wednesday, 27 June 2012

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This edited article about bird migration originally appeared in Look and Learn issue number 741 published on 27 March 1976.

Migrating swallows, picture, image, illustration

Migrating swallows by A Oxenham

The radar operator stared in horror at the screen in front of him. The ‘blips’, or echoes of light on the radar, indicated a vast number of enemy planes heading straight for Britain, possibly on a massive bombing raid!

An air-raid warning was immediately given, and thousands of war-time Britons muttered unkind things about the Germans before hurrying to their shelters or into the Underground stations.

Yet no bombs fell that night, no aircraft were picked up by the probing searchlights, and before long, the “all-clear” siren filled the air with its welcome wail.

These false alarms happened many times before it was realised that the mysterious aircraft were in fact birds.

Since then, radar has provided much valuable information about the heights birds fly at, their course and their speed, adding to our knowledge about the annual movement of birds that takes place every year without fail and which has been taking place every year since man has been around to observe it.

But although we know where the birds go and we know why they go, we still do not fully understand how they are able to find their way hundreds, even thousands, of miles across strange lands and seas until they reach their destination. And we certainly do not know how the birds recognise this destination when they reach it.

Take the swallows, for instance. During the spring and summer we see them wheeling and darting above our houses and fields, ever on the look-out for insects. But as autumn approaches, they turn their faces south.

Over the Channel they head, right across France, throughout the length of Italy and over the Mediterranean. Still they travel on, down the Nile into the heart of Africa until they reach the very southern tip, Cape Town.

But as the sun begins to gather strength in Britain, as the first buds begin to appear on the trees, the swallows make the return journey, often going back to the very same farm, the very same barn, where they had nested the previous year.

Then there is the Arctic Tern which travels from near the North Pole to near the South Pole every year. Or the Greater Sheerwater which ranges the whole of the Atlantic but returns unerringly to the tiny island of Tristan da Cunha in the South Atlantic to breed.

We can only guess at how the birds achieve this miracle of navigation, and the best guess we can make is that they rely on the oldest navigational aids known to man, the sun and the stars. We all know that the sun rises above the horizon in the east, that it is due south at 12 o’clock noon, and sets in the west in the evening. It seems that these facts are also ‘known’ to the birds.

Some years ago in Germany, experiments were carried out with starlings in an aviary during the migration season. The birds were placed in the aviary, which was enclosed by opaque glass walls and clear windows above, and they immediately gathered in the north-west section, the direction in which they would normally migrate. Then the aviary was rotated slightly, and the birds promptly moved until they were in the north-west area of the aviary again. This happened every time the aviary was rotated.

Next, shutters were placed on the windows, and large mirrors fixed in such a position that the sun was reflected in them, but appearing to the birds to be in a different position to its actual one.

Immediately the birds moved into a new section of the aviary, this time north-west in relation to the apparent position of the sun. And every time the mirrors were moved, so, too, did the starlings, always to a position north-west as it appeared to them in relation to the sun.

But if we accept that the birds use the sun as an aid to navigation then it would seem natural that migration would come to a stop at dusk. Yet this is not so.

It is true that swifts and swallows migrate only by day; so, too, do birds of prey, who need the warm up-currents to soar from area to area in much the same way as a glider pilot makes a cross-country flight.

But many species, such as fieldfares, blackbirds, starlings, redwings and song thrushes migrate by day or night. And warblers, flycatchers and chats seldom set off until the sun has sunk below the horizon.

We can rule out navigation by the moon, for migration takes place when there is no moon at all. But what about the stars? The constellations are there all the time, even though they change their position during the night and during the season.

This led the scientists to carry out further experiments, with shutters completely closing the aviary by day and being opened only at night. The tests showed that during the migration season the birds still always faced the direction in which they would normally fly!

So at the moment, sun and star navigation seems to be the most likely explanation of how birds find their way.

However, we can be more definite about where the birds go. Bird watching and bird ringing (the attaching of identification rings to birds’ legs) are international occupations and so we have a very clear picture of migratory habits.

Bird ringing has been carried out for over 60 years. The modern ring is a little strip of light-weight aluminium alloy that cannot harm the bird’s leg. It carries a serial number and an address to which the finder can report his discovery.

Why do birds go on these long journeys? The simple answer is to find warmth and to visit lands where there is an ample supply of food. And what is cold to one type is warm to another.

Scandinavian birds, for instance, start leaving their homelands as their winter draws near. During October we see floods of these northern visitors arriving in Britain, waves of ducks, geese, woodcock, gulls, thrushes, starlings and so on.

Yet even as they are arriving, the swallows, flycatchers, warblers and many others have already left Britain and are winging their way south to Africa to find the warmth and conditions that they need.

Of course, it would be natural to assume that each species behaved in roughly the same way, that they migrated at about the same time, in approximately the same direction. Unfortunately, this is not the case.

We can lay down general rules, but it does not mean that the birds will follow them. One group of song thrushes, for example, will leave Britain to spend the winter in Spain and Portugal; another group will head for Ireland; and a third group will not migrate at all.

And we still cannot explain how a young bird, with no previous experience, knows when it is time to leave its surroundings, where it is to go and how it knows when it gets there. It somehow realises these things from information passed on from generation to generation over millions of years. In other words, it is an instinct, something no scientist can explain but only watch and wonder at. For truly it is one of the marvels of our universe.

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