The Migration of Birds/Chapter 11
CHAPTER XI
SUMMARY
Migration owes its origin to the potentiality of flight, enabling birds to advantage themselves by extended dispersals, which through heredity become instinctive, regular and periodical. Geological changes, especially the passing away of the glacial epoch, only influenced by opening up new lands for summer colonisation, but climatic conditions prevented these lands from becoming permanent abodes end fostered the habit of periodical migration. Whatever the original home or centre of distribution may have been, the dispersal from it was towards new lands with a retreat towards the food-supply when these lands became untenable. Fluctuating food-supply, love of home, sexual impulses, desire for light, varying temperature, and other factors, all have more or less influence, but the force exerted by any or all depends upon the species operated upon and the locality in which it resides. The present route followed or method of migration is little guide to the history of past migration; during the evolution of present-day migration alterations may have been occasioned by environment and changing conditions. As Seebohm puts it, "The desire to migrate is a hereditary impulse, to which the descendants of migratory birds are subject—a force almost, if not quite, as irresistible as the hereditary impulse to breed in the spring" (44).
The route is simply the course followed between the breeding area and winter quarters; it is more or less restricted by the size of the area in which food is to he found; it is usually the most direct way from one food-base to the next, in a general direction from the seasonal bases. Most birds move between north and south, but migrations are regularly followed in other directions by some species.
Routes may follow coastlines, these providing visible landmarks, and also, for many species, plentiful fond; islands, capes, estuaries and inlets are landmarks, asylums, food~bases, and sites for congregation and departure for cross-sea passages; at these places migration is often specially noticeable. Overland routes may suggest "broad front" migration, when there are no particular restricting influences and the species have no special need for hurry. Migration at great elevations and at high rates of speed is proved, but the highest and quickest possible is as yet unascertained. It may also, under other conditions, be performed at low elevations and very slowly. It is probable that strong air-currents at a high elevation materially assist rapid and lengthened migration. Force not direction of wind influences birds moving at a low elevation.
Birds possess a certain power of orientation, a homing instinct, which need not be called a sixth sense. Brain and eyes assist in the development of this power; birds have an excellent memory. Young birds lose their way more frequently than is generally supposed; variations in routes are explained in many cases by these errors. Young may or may not be guided by experienced adults; orientation is not infallible but develops with age.
There is apparently no truth in the assertion that birds travel by choice against a head wind or in a beam wind; a moderate wind behind, on which they are carried, is most favourable. Leeward drift through contrary winds explains many normal and abnormal routers, and the occurrence of unexpected species in unexpected places. The distance travelled not only varies according to species but in individuals of the some species; the thesis that the most northerly breeder winters furthest south does not always hold good.
Much may be learnt by the careful registration of arrivals and departures of migratory birds, and by the marking of birds. Ornithophænology, the science of migration study, as carried on at present in many countries, would be materially assisted by some better method of international registration and interchange of ideas
In conclusion I would urge the value of the study, citing Herr Herman's reasons put before the International Ornithological Congress in 1905. The solution of the problem is in the interest of science, and therefore of intellectual progress, teaching us the great part which migratory birds play in the scheme of nature. The millions of birds which wander, season after season, from one zone to another, represent an enormous aggregate of labour, by flight and search for food, acting on "the organic life of nature as does the regulator of a steam-engine, at one time accelerating, at another retarding." Full insight into the essence of the work done by birds will give us a correct notion of their usefulness or injuriousness to man, and load us to rational action for their protection
Whilst fully agreeing with Herr Herman I would go further. We live in an age when aerial locomotion has become important, and will be more and more important in the future. Every lesson we can learn from the successes or failures of these most perfect aerial navigators must be of use.
But putting aside economic and utilitarian considerations, there is to some of us a greater stimulus to solve the problems of nature. With the birds, and the insects and plants upon which they feed, we share a common heritage, and the more we learn of the life of these, our fellow workers, the nearer we approach solution of the great riddle of the Universe, the mysterious law-abiding scheme of Nature. The book of knowledge to which we may add some iota, is marred with mystery, superstition and error, but each proved fact cleanses its pages. "Facts," says Laing, "are the spokes of the ladder by which we climb from earth to heaven."