Probing the Mysteries of Migration

BY AWAKE! CORRESPONDENT IN SPAIN

THERE is an old song that tells of the swallows’ return to the old Mission San Juan Capistrano in San Juan Capistrano, California, U.S.A. It is said that without fail, on March 19th every year, they return to their nests there.

European swallows follow a similar timetable. A Spanish saying predicts that by March 15th the song of the swallow will be heard once more.

In the Northern Hemisphere, countryfolk have always greeted the return of the swallow, a traditional harbinger of spring. But some curious ones also wondered where they had been during their winter absence. Some thought they had hibernated. Others suggested that they had gone to the moon​—someone calculated that they could fly there in two months. A 16th-​century Swedish archbishop claimed that swallows spent the winter underwater, huddled together at the bottom of lakes and marshes. His treatise even contained an illustration that depicted fishermen hauling in a net full of swallows. Odd as these ideas now seem, the truth turned out to be nearly as strange as fiction.

During this century ornithologists have ringed thousands of swallows. A small, but significant, percentage of these ringed birds were located in their winter quarters. Incredible though it seems, swallows from Britain and Russia were found wintering together thousands of miles away from home​—in the extreme southeastern tip of Africa. Some of their North American counterparts fly as far south as Argentina or Chile. And swallows are not the only birds to make such epic journeys. Hundreds of millions of birds from the Northern Hemisphere winter in the Southern Hemisphere.

Ornithologists were amazed to discover that a bird as small as a swallow could make a round-​trip of 14,000 miles [22,500 km] before returning to the same nest the following spring. Knowing where the swallows had gone only raised more perplexing questions.

“Swallow, Why Do You Leave Your Nest?”

What makes a bird journey to the other end of the globe? Or, as a Spanish saying puts it, “Swallow, why do you leave your nest?” Because of the cold or in order to find food? Doubtless, their need for a reliable food supply is the answer rather than the onset of wintry weather, since many small birds that have difficulty surviving cold winters do not migrate. But bird migration is not just a wandering in search of food. Unlike human migrants, birds do not wait until times are bad before moving on.

Scientists have discovered that it is the shorter day that triggers the migratory urge. In the autumn captive birds get restless when the daylight hours decrease. This is so even when the effect is produced artificially and when the birds have been reared by investigators. The caged bird even faces in the direction it instinctively knows it should take during its migratory flight. Evidently, the urge to migrate at a specific time of the year and in a certain direction is inborn.

How do birds navigate successfully over large distances? Many migrate over featureless oceans and deserts, and they do so both by day and by night. In some species the young birds travel on their own without the help of experienced adults. Somehow they stick to their route despite storms or side winds.

Navigation​—especially across vast oceans or deserts—​is by no means easy. It took man thousands of years to master it. Doubtless, Christopher Columbus would never have ventured so far across the ocean without navigational aids such as the astrolabe and the magnetic compass.a Even so, toward the end of his first voyage, it was the birds that showed him the way to the Bahamas. Following the custom of ancient mariners, he changed his course to the southwest when he spotted migrating land birds flying in that direction.

Successful navigation requires a system for maintaining a steady course and also a means of determining position. Simply put, you need to know where you are in relation to your destination and what direction to take in order to get there. We humans are not equipped to handle such a task without tools​—but birds evidently are. Patiently, scientists have pieced together data that throws light on how birds determine the right direction in which to fly.

Some of the Answers

Homing pigeons are the “guinea pigs” of choice for scientists bent on unraveling the mysteries of bird navigation. Long-​suffering pigeons have been fitted with frosted glass “spectacles” so that they could not see specific landmarks. Others have been equipped with magnetic backpacks to prevent them from using the earth’s magnetic field for guidance. Some were even drugged while traveling to their point of release, to ensure that they had no way of knowing the outward route. The resourceful pigeons overcame each handicap separately, although a combination of certain obstacles did prevent them from homing successfully. Clearly, birds are not dependent on just one navigation system. What methods do they use?

Experiments using artificial suns or night skies demonstrate that birds can navigate by the sun during the day and by the stars at night. What if the sky is overcast? Birds can also fix a route using the earth’s magnetic field, as if they had a built-​in compass. In order to return to the same nest or loft, they must also be able to recognize familiar landmarks. Furthermore, researchers have found that birds are much more sensitive than humans to sounds and smells​—although they don’t know to what extent this ability is used for navigation.

The Mystery of the “Avian Map”

Although all this research has gone a long way toward establishing how birds can fly in a fixed direction, a baffling problem still remains. It is one thing to have a reliable compass, but to get home, you also need a map​—first to determine where you are and then to plot the best route.

What “avian maps” do birds use? How do they know where they are after being taken to an unknown place hundreds of miles from home? How do they calculate the best route, when to all outward appearances they have no maps or signposts to guide them?

Biologist James L. Gould says that a bird’s “map sense seems likely to retain its status as the most elusive and intriguing mystery in animal behavior.”

The Mind Behind the Mystery

What is abundantly clear is that migration is instinctive behavior. Many species of birds are genetically programmed to migrate at certain times of the year, and they are born with the skills and senses needed to navigate successfully. Where did that instinctive ability come from?

Reasonably, this instinctive wisdom could come only from a wise Creator, who could “program” the birds’ genetic code. God pointedly asked the patriarch Job: “Does a hawk learn from you how to fly when it spreads its wings toward the south?”​—Job 39:26, Today’s English Version.

After a hundred years of intense research on bird migration, scientists have come to respect the birds’ tiny brains. After plotting the principal migratory routes, scientists can but marvel at the incredible distances some birds travel. Generation after generation, in spring and autumn, millions of migratory birds traverse the globe. They navigate by the sun during the day or by the stars at night. In cloudy weather they use the earth’s magnetic field, and they quickly learn to recognize familiar landscapes. Possibly they even orient themselves by smell or infrasonic waves.

How they “map” their journeys remains a mystery. We know where all the swallows go; we don’t know how they get there. Nevertheless, when we see the swallows flocking together in the fall, we can but pause to marvel at the wisdom of God who made their migration possible.

[Footnotes]

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World Champion Migrants

Distance. In the northern summer of 1966, an arctic tern was ringed in North Wales, Great Britain. In December of that same year, it turned up​—appropriately enough—​in New South Wales, Australia. It had flown more than 11,000 miles [18,000 km] in six months. Such a feat is probably quite normal for arctic terns. In the course of a year, some of these birds regularly circumnavigate the globe.

Speed. American golden plovers are perhaps the fastest migrants. Some of these birds have crossed the 2,000 miles [3,200 km] of ocean separating Hawaii from the Aleutian Islands, Alaska, in just 35 hours​—at an average speed of 57 miles per hour [91 k/​hr]!

Endurance. Blackpoll warblers of North America, which weigh only three quarters of an ounce [20 gm], are the greatest marathon fliers. On their journey to South America, they fly 2,300 miles [3,700 km] nonstop across the Atlantic in just three and a half days. This extraordinary endurance test has been compared to a man running 1,200 four-​minute miles [1,900 km] without stopping. The flight is also a weight-​watcher’s dream​—the warbler burns up nearly half its body weight.

Punctuality. Apart from the swallow, the stork (shown above) also has a reputation for punctuality. The prophet Jeremiah described the stork as a bird that “well knows its appointed times” and its time for “coming in.” (Jeremiah 8:7) Nearly half a million storks still pass through Israel every spring.

Navigation skills. For Manx shearwaters there is no place like home. A female bird taken from her nest in Great Britain was released some 3,000 miles [5,000 km] away in Boston, U.S.A. She crossed the Atlantic in 12 1/2 days and arrived home before the airmail letter that gave details of her release. The achievement was all the more amazing because these birds never traverse the North Atlantic in their migratory journeys.

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The stork punctually returns each year to its nest

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Migrating cranes in a typical V-formation

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Photo: Caja Salamanca y Soria