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Animal Migrations

Updated on April 21, 2014

Crossing Borders Without Passports!

In many countries of the world, the issue of migration has increasingly come to the fore of public discussion especially as concerns the alleged problems posed by such migrants particularly those designated as illegal migrants and all manner of theory, some plain absurd and others just bigoted, have been advanced as to how to deal with this menace. Yet, a cursory look at the historical and archaeological record plainly informs that man has always been a highly mobile species and that migration is an integral part of the human story. It remains to be seen whether the ranting of populist minded politicians and would-be demagogues will provide a solution to the problem as they define it.

Yet although man is, as pointed out above, a highly mobile species, when humans are placed in comparison with some of the other species with which they share planet Earth, then they rank as just plain novices in the migration stakes; and these other migratory species require no passports and pay absolutely no attention to lines drawn on pieces of paper by humans and designated as national borders!

Take, for instance, the shining bronze cuckoo, chrysococcyx lucidus, the world’s smallest cuckoos. These birds nest and breed in New Zealand, arriving between August and October, but as soon as the chicks are hatched the parent flock, in most un-parently fashion, desert the hatchlings, take to the skies and return to their winter residences as far away as the Bismarck Archipelago, a group of islands that lie off Papua-New Guinea. This trip, by way of Australia, covers a distance of more than 2000 kilometers, most of it over open seas. Impressive, yes? Well, there’s something more to add.You remember the hatchlings, don’t you? Well, sometime around March, those hatchlings, now grown, without even a solitary adult from the parent generation to act as guide, take flight and, following the path taken by their parents months earlier, unerringly navigate their way to the Bismarck Archipelago to join up with their mums and dads! Keep in mind that in undertaking this journey, the shining bronze cuckoo has no margin for error. As we have noted, most of the journey takes place over open seas; any mistake by a bird is a fatal mistake: because the shining bronze cuckoo cannot swim, a fallen bird is a dead bird!

If 2000 kilometers seems a long distance to travel (and it is, in fact), then what does one say when one considers the journey of the Arctic Tern, sterna paradisaea? This sun-lover literally flies from the top of the world to the bottom of the world and back again. The Arctic Tern breeds in the almost unending daylight of the Arctic summer then, as summer comes to an end, the bird flies to feed in the equally unending daylight of the Antarctic summer, a round trip of some 35,000 kilometers and the longest known animal migratory journey known. Taking off from Canada, Iceland and Greenland, the birds travel down by way of the African coast or the American Pacific coast on the journey to Antarctica, and they are extremely fast travelers. One bird, caught and ringed in Northumberland, England on 25th June 1982 had reached Melbourne, Australia, a distance of 17,600 kilometers, just 115 days later: the bird had covered an average of 160 kilometers ever day of its journey! Amazing.

For untold generations, the brown locust, locustana pardalina, has played an extremely important role in human affairs on account of its ability to destroy every single trace of vegetation it comes across when it is swarming. A typical swarm, which can be up to 50 or so kilometers in length, 8 kilometers wide, contain more than 500 billion insects and weigh more than 80,000 tonnes, can eat each and every day the weight of food that is required to feed up to 20 million people! (A 1958 locust plague in Somalia covered over 1000 square kilometers!). Given these dire statistics, the appearance of these insects were and remain a matter of great importance to man. Yet for generations no one had a clue as to where these insects came from or how they came to be. Not surprisingly, all kinds of myths were woven surrounding them. Amongst the Igbo speaking peoples of Nigeria, for instance, it was postulated that the insects were bred in deep caves by a race of dwarves who allowed them out periodically to wreak the havoc as well as to provide a rare delicacy for humans. The truth, discovered by the British scientist, Sir Boris Uvarov (1889 – 1970) in 1921, is somewhat more prosaic. Locusts are actually merely an altered form of a green grasshopper, arcridididae, commonly found in Africa and Asia. The change from green grasshopper to brown locust comes about as a result of increasing population which puts increasing pressure on existing food supplies. When available food supplies diminish to a certain point, it triggers a metamorphosis whereby the grasshopper becomes the locust and the locusts swarm in search of new sources of food. Once the crisis of habitability is over, either by the swarm finding new areas with adequate food supplies or by population in the original habitat falling, as a result of migrations, to a level where the food resources are once more adequate, a reverse metamorphosis occurs; the locusts vanish and the ordinary green grasshopper returns.

Butterflies are amongst the most beautiful creatures that exist on earth, but even amongst such attractive insects, the American Monarch butterfly, danaus plexippus, is outstanding for its beauty. But, apart from its beauty, this butterfly is also one of the world’s great migrants and, in its case, it migrates both in space and time! The life span of an American Monarch butterfly averages just about 6 weeks so that during the summer in America’s east coast some three or four generations are born. As the North American summer comes to an end, these lovers of warm climes leave eastern North America to travel to their summer habitat over 3000 kilometers south and 3 kilometers up in the mountains of Mexico. Bear in mind, however, that these migrants are the grand and great-grand descendants of the last set of migrants who had come up north from Mexico; they, the present set of migrants have never been to Mexico and neither did their parents. Yet, like the shining bronze cuckoo, their migratory instinct is completely unerring; the current migrants go to precisely the same places that their direct ancestors had wintered in 3 or 4 generations ago. Come spring, and these beautiful insects head back north. A northern summer of rapid breeding; 3 or 4 generations; then it is time once again to head south to the mountains of Mexico.

American Monarch Butterfly
American Monarch Butterfly

Some animal migrations, according to current scientific knowledge, are just plain impossible; yet they take place not as a one-off phenomenon, but with a regularity that rivals the rising and setting of celestial bodies. Take the ruby-throated humming bird, archilochus colubris. This extremely tiny bird (it weighs in at just about 3.5 grams!) flies non-stop each autumn from North America to South America and again in spring flies non-stop from South America to North America, a journey of some 800 kilometers each way. Current scientific thinking suggest that this trip is an impossibility because the bird is just too tine to store enough energy in its body to complete the journey; yet, of course, the birds make the journey year-in and year-out. Unfailingly!

Then, of course, we have the lemmings, those vole-like Norwegian rodents which, from time to time, set off gigantic groups to plunge off the cliff tops into rivers and even into the sea in seemingly uncontrollable orgies of self-destruction. Why? Just like the locusts, hunger occasioned by overpopulation. In normal times, adult females will have 2 litters a year, each litter having about 5 or fewer young. From time to time, however, there are exceedingly good times with an overabundance of food. Then females may deliver up to 4 litters a year, each litter containing up to 8 young! The final result is inevitable; in a short time, the time of plenty becomes a famine time as more and more lemmings fight for increasingly depleting resources. So, as hunger becomes more and more pressing, the lemmings in northern Europe, just like the locusts in tropical Africa and Asia, set out in search of a meal. The great majority of them will die during this search for a meal, but then the balance will be restored, just as for the locusts, and everything goes on smoothly until the next time.

The list of migrants is an extremely long one, but it would seem that food and climate are the most important factors that underlie these regular mass migrations; more or less the same factors that historically and presently cause human migrations. Swallows, for instance, breed in Europe during the spring; however they must be out of Europe and settled in North Africa before the European winter sets in. failure so to death simply means death. The cold European winter will kill both the birds as well as the insects on which they feed. But not all migratory habits are so easily explained. The reasons why the Arctic Tern, for instance, makes its fantastic trans-world odyssey is still shrouded in mystery.

It has been suggested that some migratory behavior may be the result of long-ingrained habit which the migrant species is unable to discard although the circumstances that led to the development of the habit in the first place no longer exist. Thus, it has been suggested that the migratory habits of some sea-migrants like salmons and eels may have originated in the dim recesses of history at a time when the continental masses were still joined together or were much closer one to the other than they are today. As continental drift has, ever so slowly, increased the distances between the land masses, the migrants have had to adapt to the increasing distances so that each new generation travels just that bit farther than the preceding generations.

On the surface, this proposition does not seem unreasonable, but a closer look suggests some difficulties. The time spans involved in continental drift are extremely great and nothing in the geological record suggests that any of the sea-migrants was in existence at the times when the continental masses were close enough to one another to justify this conclusion. However, there may be a solution to this objection. As we saw in the case of the American Monarch butterfly and shining bronze cuckoo it is possible to transmit migratory information over generations without requiring the actual presence of a previous migrant. So, if the migratory behavior actually started amongst the evolutionary forebears of today’s sea-migrants, could that information have been transmitted through every step of the evolutionary process to today’s migrants? Perhaps.

Then there is the question as to how the migrants know where it is they are going to, and this is of particular importance when we consider migratory flyers. Some have suggested that the pectin, a comb like structure found in the retina of birds, may operate in conjunction with the sun to serve as a navigational device of sorts. This may very well be so, although concrete evidence is lacking, but it does not explain how such birds find their way on cloudy days or when flying through the night. Somewhat more plausible, perhaps, is the suggestion that such migrants are somehow able to navigate by aligning themselves with the earth’s magnetic field. This suggestion has been strengthened by the discovery in 1977 by the State University of New York neurobiologist, Charles Walcott, that certain migratory birds have deposits of magnetic iron in their brains. But the ability to navigate by aligning oneself to the Earth’s magnetic field still does not answer one question: how does the shining bronze cuckoo or the Monarch butterfly find its way to the correct site when it has never before visited that site? Perhaps, as usually is the case, the simplest answer is the correct one: perhaps each migrant is equipped with a Google-like search program, infinitely more sophisticated of course, put in by God at creation: if the shining bronze cuckoo, for example, inputs the proper search parameters, he or she arrives safely at the Bismarck Archipelago; if, however, the bird inputs the wrong parameters, he or she falls out of the sky and, not being a swimmer, drowns in the unforgiving sea.


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