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Travel of Birds
WHY BIRDS TRAVEL
BUT while we may prove that birds possess a sense of direction and may learn all there is to know about when, where, and how they make their great journeys, we still do not know why they make them. That is quite another and much more difficult question to answer.
We can see the Wild Goose on its travels. We know where it is going, where it came from, and when it will arrive. We even think we know how it finds the way. But we do not know why it started. That it should leave the North at the approach of winter is not strange. But why should it leave the bays and lagoons of our southern coasts, with their rich store of food, to follow close upon the heels of retiring winter? So eager is it to return to its summer home that it sometimes is caught by late cold storms and forced to retreat southward.
The Bobolink and many other birds begin their journey to winter quarters before the summer is half over. They stay only long enough to rear their broods and get new suits of feathers; then they are off on the first stages of their fourthousand-mile journey to southern Brazil. Thus we see that they travel eight thousand miles every year to spend only about two months on their nesting ground. What is it that causes them to undertake this remarkable journey with all its many dangers? Why can they not nest in the great campos and marshes of southern Brazil and northern Paraguay just as well as in the meadows of Massachusetts?
When I have been studying birds in April in tropical countries—in Cuba, Yucatan, Colombia, or Trinidad—I have often seen flitting about with the native tropical species, many of our own summer Warblers, Vireos, and Flycatchers. At this time the rainy season was approaching. Trees were blooming, some fruits ripening, insects becoming more numerous. But in spite of this increase in the supply of food, and the fact that many tropical birds were already nesting, day after day the Redstarts, Water-Thrushes, Blackburnian and Canadian Warblers, Red-eyed Vireos, Acadian Flycatchers, Olive-backed Thrushes, and other familiar North American birds were leaving the land of plenty to start on a flight of several thousand miles. Who can say why they go?
Now an instinct is merely a habit of such long standing that we cannot say how it was formed. So when we attempt to explain the origin of this homing instinct we must remember, first, that birds have been migrating for a very long time—how many thousands of years I shall not attempt to say. Second, that during this time there have been far-reaching changes in the climate of the world. Places which have now an Arctic climate, we know once had a warm or subtropical climate.
Thus the discovery of the imprint of magnolia leaves in the rocks of northern Greenland tells us that magnolia trees once grew on the shores of the Arctic Ocean. In a similar way the grooves cut by glaciers in the rocks of Central Park show us that a great ice sheet once spread southward as far as New York City.
If there were magnolia trees in Greenland, it is more than probable that there were also various kinds of birds that we associate with these trees. And if New York was covered with ice, it must have been the home of birds which are now found only in the Far North.
Geologists tell us that in the later history of the earth there have been not one, but several, climatic changes. That is, the climate at one place might be warm, then cold, then warm, and then cold again. When, therefore, we try to explain how these variations in climate acted on the birds which may have lived in a certain place when first it was warm, we set ourselves no easy task.
It is a good rule not to try to answer the whole of a very difficult question at once but to take some little corner of what seems to be the easiest part of it. So I will not now try to tell why birds migrate. I will only attempt to tell why some particular bird migrates.
Following this plan, let us take some simple case, where, so far as we can see, neither climate nor food has anything to do with the matter. The Brown Pelicans of the east coast of Florida will serve our ends admirably.
During the greater part of the year we find these birds scattered up and down the coast. In diagonal files they sail in stately fashion just above the breakers to their fishing grounds, there to plunge recklessly on the menhaden which form their principal fare. At night they gather on some favorite sand bar to sleep. So their days are made up of flying and fishing and sleeping. Then there comes a time when with no change in the seasons and, so far as we know, no decrease in the number of fish, all the Pelicans from the Keys to Georgia, and perhaps farther, have a desire to go to a little mud island about half-way down the Florida coast. This is Pelican Island in the Indian River, opposite Sebastian.
If you were on this island in October you would be surprised by the sudden appearance of a flock of perhaps two or three thousand Pelicans. You might then imagine that they had been traveling together for some distance, if you did not know that until the call came they had been distributed in small companies for a distance of at least four hundred miles. Then, just as though a Pelican king had sent out a wireless command, they all hastened to the island, forming a great flock as they met there. But this command did not come by wireless or from a Pelican king. It came from within each Pelican. What was it? What did it tell them to do?
It told them that the time was at hand for nest-building and egg-laying. In other words, the instinct of reproduction awakened. This, in turn, aroused the homing instinct, which, under the guidance of the sense of direction, draws a bird back to the place of its birth.
But what awakened the sense of reproduction? What makes an apple tree bloom? It is true that Pelicans and apple trees are not much alike; still both are subject to the same laws of nature. A Pelican could not lay eggs, hatch them, and care for young Pelicans throughout the year any more than an apple tree could bear several crops of apples in a year. To develop either eggs or apples takes strength, and the continued use of one’s strength means that one becomes tired and must rest.
So when the crop of Pelicans, or of apples, is ripe, the parent Pelicans as well as the parent apple trees, rest. The Pelicans shed their feathers, and since they could not live without them, get a new set at once. The apple tree sheds its leaves and the new set comes later. Then fall and winter follow, and in both bird and tree the instinct of reproduction rests. The return of warm weather sets the sap flowing in the trees, the buds begin to swell, blossoms open, leaves unfold, and, in due time, the fruit ripens.
With most birds, also, the coming of spring, if it does not actually start the sap flowing, sets new forces in action. These are the reproductive forces. They produce not buds, blossoms, and fruit, but eggs, and the eggs when incubated give birth to birds. So we see that in reality birds, as well as trees, bloom; that both have their regular season of blooming and of reproduction, and this season is generally in the spring when increasing warmth sets the sap flowing.
It is true that the Pelicans of Pelican Island begin to nest when the weather is becoming colder instead of warmer. Why they should do so no one knows. On the west coast of Florida the same kind of Pelicans do not nest until April. This is doubtless the proper nesting time, but for some as yet unknown reason the birds of eastern Florida have chosen a time of their own.
The important fact here is that they all go at the same time and for the same purpose. In everything, therefore, but length, their journey to Pelican Island is as much a migration as is that of a bird which flies from the tropics to the Arctic regions. Both go each year at a certain season; both go to nest; both are prompted to start by the awakening of the nesting instinct with its desire to go to a proper place in which to rear the young; and when this task is finished the birds leave the nesting ground.
“But,” you may ask, “if the Brown Pelican goes only as far north as Pelican Island, why does his cousin, the White Pelican, go all the way to Great Slave Lake in British America?” You might ask the same question about many other bird travelers that winter in the South and nest in the Far North.
We have every reason to believe that when last the Arctic regions were warm, White Pelicans fished along the borders of the Arctic Ocean. When the climate began to change and became cooler and cooler, they, of course, had to retreat slowly southward. Finally, we know that the great ice sheet reached as far south as the central part of the United States. At that time the White Pelican must have lived on the Gulf of Mexico and probably farther south.
Then as the climate began to grow warm again, the ice slowly melted; each year the great sheet grew smaller until at last the land was free as it is today. As the ice disappeared, the White Pelicans gradually returned to the country from which they had been driven. Now, as we have seen, they have actually gone as far back as Great Slave Lake. But every winter when the water freezes they go South, only to return as soon as the ice thaws in the spring. So through many centuries they slowly formed the habit of making a journey which gradually grew longer and longer.
So then we may
think of the marvelous travels of birds as due, first of all, to those changes
in climate which turned a warm Arctic to a cold Arctic. As the ice gradually
receded, the homing instinct led the feathered exiles back to the land they had
been forced to desert. The “blooming” or reproductive instinct tells them when
to go, and the sense of direction guides them on their way.
SUGGESTIONS FOR STUDY
Have you ever seen Wild Geese migrating? Describe their flock formation. Can you suggest any reason for the birds’ journey? Do our summer birds leave their winter quarters in the tropics because of any change in the climate there? Because of any failure in the supply of food? If you have ever hunted hens’ eggs, have you noticed that the birds often hide their nests in places where they never go except to lay? Mention several places of this kind. Why do you suppose the hens select them? Why do many kinds of birds visit remote and inaccessible islands on which to nest? Does not the difference between the habits of hens and sea birds in nesting in out-of-the-way places consist largely in the difference of the length of their journeys? What prompts the hen to return to her nest? What prompts the sea birds to go back to their islets? Compare a year of a bird’s life with that of a tree. What evidence have we for the belief that a much warmer Climate once existed in the Arctic region? How do we know that the greater part of North America was once covered with ice? How far south did the ice sheet come? Describe the change in climate due to the gradual freezing of the North, and the invasion and retreat of the ice sheet. How is it believed to have affected the migration of birds?