Never were the birds so gentle and confiding as they were during that spring. A female redstart took up her residence in my yard for fully a week, flitting about in the trees and grape-arbor, seeking for nits and worms; and you are to remember that I live in town (though in the outskirts), with many houses and people about, and an electric car whirling along the street every few minutes. A dainty bay-breasted warbler—little witch!—kept the redstart company, letting me stand beneath the trees on whose lower branches she tilted, and watch her agile movements; yet one of my bird books declares that the bay-breasted warblers remain in the highest tree-tops of the woods! Both these birds occasionally uttered a trill.
The goldfinches, too, were very familiar. They came with the procession as far north as my neighborhood, but stopped here for the summer, instead of continuing their pilgrimage. Some of their brothers and sisters remained with me all winter. Within a few feet of my rear door stands a small apple-tree, in whose branches these feathered gold-flakes flashed about, and sang their childlike ditties, and one little madam fluttered in the leafy crotches of the twigs, fitting her body into them as if trying to see if they would make good nesting-sites; the while Sir Goldfinch sang and sang at the top of his voice. Several white-crowned sparrows also came to eat seeds thrown out into the back yard. These handsome sparrows were not shy, but perched on the fence or the trees, and trilled their sweet refrains.
VI.
WINGED VOYAGERS.
The subject of bird migration is one of absorbing interest, presenting many a perplexing problem to the student who cares to go into the philosophy of things. Why do the birds make these wonderful semi-annual pilgrimages, and whence came the original impulse, are questions often asked. With my limited opportunities for observation I cannot hope to shed much, if any, new light on the subject; yet it seems to me that some persons are disposed to invest it with more of an air of mystery than is really necessary. There are several patent, if not wholly satisfactory, reasons that may be assigned for the migrating impulse.
As this is not a scientific treatise, the writer will not be over-methodical in presenting these reasons, but will mention them in the order in which they occur to him. If we keep in mind the invariable succession of the seasons, and that this annual rotation has continued for ages, and if we also remember that all animals are dowered by their Creator with as much intelligence as is necessary for their well-being, much of the difficulty attaching to this subject will at once disappear. Birds, like their human kinsmen, learn by experience and tutelage, and are also gifted with a sure instinct that amounts in many cases almost to reason. Take, for instance, this one fact. As the sun creeps northward in the spring, it pours a more and more intense heat upon the northern portions of the tropical and sub-tropical regions. The heat would soon become intolerable to certain birds, which have doubtless tried the experiment of spending the summer in equatorial countries; or if individuals now living have not tried it, perhaps some of their more or less remote ancestors have. That birds do make experiments is proved by the fact that several pets of mine will carefully “sample” a new kind of food offered them, and if they do not find it to their taste, will let it severely alone; nor is it any the less evident that young birds receive instruction from their elders. Thus the necessity of leaving the torrid regions as summer approaches may have been impressed on the migrating species from time immemorial.
Again, as spring advances, insect and vegetable life is revived in regions farther north, and this certainly must act as a magnet upon the birds, drawing them from point to point as the supply of food becomes scarce in the more southern localities. Then, let us suppose for a moment that all the birds did remain in the south through the summer; there would sooner or later be a bird famine in the land, for the supply of seeds and insects would soon be exhausted. Our feathered folk are simply obliged, on account of the exigencies of food, to scatter themselves over a larger extent of country. They solve the problem of food supply and demand by these annual pilgrimages to the boreal lands of plenty.
To go a little more to the root of the matter, we may easily imagine how the migrating spirit got its first impulse and gradually became evolved into a habit of something like scientific precision. If the first birds lived in tropical climates, as was probably the case, some of them, as the food supply became exhausted, would be crowded northward, or would go of their own accord, and wherever they went they would find well-filled natural larders. Having once discovered that spring replenished the north with food, they would soon learn the desirability of making periodical journeys to that part of the globe. With this constant quest for food must also be coupled the instinctive desire of most birds for seclusion during the season of reproduction,—an instinct that would naturally drive them northward into the less thickly tenanted districts. But it may be objected that many species make long aerial voyages, passing over vast tracts of country to reach their chosen summer habitats in various parts of the north; and it is well known that the same individuals will return again and again, on the recurrence of spring, to the same locality. How are these facts to be accounted for?
If we accept the glacial theory—a hypothesis pretty well established now among scientific men—we may readily conceive that, as the sun melted the ice at a greater distance in both directions from the equator, the habitable area of the earth’s surface would gradually become enlarged. For the sake of vividness let us fancy ourselves living at that period of the world’s history. Let us select a point north of the equator where a given pair of birds can live in summer. They find plenty of food there, and are comparatively undisturbed by other birds, and they therefore become attached to the place, most feathered folk having a strong “homing instinct.” When winter comes, they and their progeny are forced to retire to the south; but they do not forget their pleasant summer haunt, their Mecca in the north, and therefore, at the approach of the following spring, they obey the home impulse and hie by easy stages to the beloved spot. Some of their number doubtless find it possible from time to time to push farther northward, and thus other breeding-haunts are selected. As the glacial accumulations melt away, the whole temperate region and a large part of the frigid zone become habitable. All this takes place by a very gradual process, requiring thousands of years, thus giving ample time for heredity to infuse the migratory habit into the nature of the birds. Every new generation would learn the route and other needful details from their predecessors, and thus the process would go on in an unending circuit year by year.
After the foregoing was written, my attention was called to the following quotation from Dr. J. A. Allen’s valuable paper on the “Origin of the Instinct of Migration in Birds.” The extract is taken from an article by Frank M. Chapman, published in “The Auk” for January, 1894: “Nothing is doubtless more thoroughly established than that a warm temperate or sub-tropical climate prevailed down to the close of the Tertiary epoch, nearly to the Northern Pole, and that climate was previously everywhere so far equable that the necessity for migration can hardly be supposed to have existed. With the later refrigeration of the northern regions, bird life must have been crowded thence toward the tropics, and the struggle for life thereby greatly intensified. The less yielding forms may have become extinct; those less sensitive to climatic change would seek to extend the boundaries of their range by a slight removal northward during the milder intervals of summer, only, however, to be forced back again by the recurrence of winter. Such migration must have been at first ‘incipient and gradual,’ extending and strengthening as the cold wave receded, and opened up a wider area within which existence in summer became possible. What was at first a forced migration would become habitual, and through the heredity of habit give rise to that wonderful faculty which we term the instinct of migration.” The reader’s attention is also directed to Mr. Chapman’s own article in the number of “The Auk” indicated.
It may be asked why some species remain in torrid and temperate climates, while others wing their way to the far north, even beyond the boundary of the Arctic Circle. My answer is, There is some Power that has wisely arranged all these matters, either by gradual development or by an original creative fiat. Every species is made to fit with nice precision into its peculiar niche in the creation. Perhaps Bryant suggests the true explanation in his poem entitled “To a Waterfowl”:—