Migration of Animals. An animal is usually described as 'migrating' when it shifts its quarters at particular periods of the year. This, in the northern hemisphere, takes place in spring and autumn, the directions at these seasons being respectively north and south; in the opposite hemisphere the course taken by the migrants is reversed, while in most warm and temperate regions herbivorous mammals, to escape insects and to obtain fresher vegetation (and following them the predatory carnivora), ascend in summer to higher altitudes, descending again when the snow covers their grazing ground. Examples are afforded by the deer in North America and Scandinavia, the alpine hares in Scotland, the Himalayan monkeys, &c. But, just as in Hibernation (q.v.) there is every gradation between ordinary sleep and the long-continued dormancy so designated, so it is possible to trace numerous steps connecting the ordinary roaming about of an animal in search of food with the persistent flight or march in one definite direction at a date so determinate that it may be reckoned upon to within a few days. There are, however, in addition, migrations of an irregular character, stimulated by causes still imperfectly understood, though in the majority of cases the necessity of seeking more abundant food-supplies—the primum mobile of all roaming—is at the bottom of what seems at first sight a whimsical movement.
Birds are the most marked migrants. The phenomena exhibited by them in this respect have been sufficiently described in Vol. II. p. 172. Various species (the robin and the song thrush, for example) which stay in Great Britain during winter are in various continental countries migrants. Again, all winter there is a drifting over from the mainland to the British shores of species which are not recognised as birds of passage, this shifting of quarters, in a manner akin to migration, except that it is indeterminate as to season, being probably due to a lack of food when continental Europe is covered with snow. This reason may also be assigned for those irregular appearances of the wax-wing and the nutcracker in the British Isles, and for those sudden arrivals from central Asia in Europe, on to the western limits of Great Britain, of the Pallas sand-grouse, which were the ornithological events of 1859, 1863, and 1888.
Mammals are less migratory, their movements being restricted by the impossibility of crossing the ocean. The reindeer, however, in many parts of the Arctic continent, moves north at the beginning of summer and south at the approach of winter, and in the Hudson Bay region the Arctic fox—the young more especially—often retreats southward in October, returning northward in spring, though in Greenland, Spitzbergen, and the Arctic regions generally both animals are winter residents, the fox shifting its quarters very little, and the reindeer only from one valley deeply covered with snow to another in which the lichens and the withered vegetation can be reached more easily. The polar and other seals and cetacea make regular migrations, the latter following the melting ice-fields, while each species of the former reaches the coast at different dates, though their winter haunts are still problematical. But most northern mammals are hibernators when they cannot find sufficient food-supply during winter. The irregular migrants are the most remarkable. Among these the Lemming (q.v., Myodes lemmus) is the best known. These animals, at intervals of five, twenty, or even a greater number of years, suddenly start from their home in Northern Scandinavia, in vast droves, followed by myriads of predatory birds and mammals, and steadily pursue a southward course until they meet the sea, when they boldly plunge in and are often drowned in enormous numbers. None ever return. The North American gray squirrel (Sciurus migratorius) will often remain for years in one district, and then suddenly migrate in millions. These two cases—and the lemmings of North America (M. torquatus, var. hudsonius) exhibit traits similar to their European congeners—may, like the corresponding influx of rats and mice in various districts, of the Hesperomys in Brazil, and of the flying foxes (gigantic bats of the genus Pteropus) in parts of Australia, be ascribed to the food-supplies in a particular area having suddenly failed or become unequal to the demands upon them; for it is hard to credit the theory that the southward incursions of the lemmings are stimulated by their instinctive eagerness to reach a long vanished 'Atlantis,' to which, in former days, their ancestors periodically resorted, and of the existence of which they possess an inherited though inaccurate remembrance. The American bison was also in the habit of migrating southward and northward after the fashion of the reindeer and Asiatic wild-ass (Asinus Onager), and no doubt for identical reasons. But the irregular migrations of the quagga and the South African antelopes, followed by herds of lions and other carnivora, are, or were, due to a scarcity of water and naturally of food also. The occasional inroads of bears and wolves on settled districts are not migrations proper, for their boldness is stimulated solely by hunger through the scarcity of food during more than usually severe winters; and to an allied cause may be traced the long excursions of various monkeys (Entellus and Rhesus) at irregular periods.
Many fishes are regular migrants. The salmon, the shad, some smelts (the North-west American oolachan, Osmerus pacificus, for example), eels, the sea lamprey, the sturgeon, the river trout, &c., all migrate from the sea up rivers at the spawning season, while the roach and the ide of Scandinavia migrate from lakes into the tributary streams for the purpose of depositing their ova. The herring, sprat, pilchard, and other Clupeidæ move from deep water in spring to shallower places more inshore, in order that the greater warmth of the water may hatch the spawn; while other species (e.g. the sudden arrival of the Temnodon saltator off the southern coast of Morocco in 1887, and succeeding seasons, after an absence for twenty-seven years) appear and disappear periodically, though in all probability most fishes are more or less migratory.
Among reptiles, turtles are the only known migrants, moving on shore at fixed periods to deposit their eggs in the sand, and disporting themselves in the sea during the rest of the year.
Among Invertebrata there are no regular migrants; but many of them perform irregular migrations of considerable interest. The destructive flights of various species of Locusts (q.v.) and Grasshoppers (q.v.) in search of green food, the occasional flights of butterflies, and the quasi-marches of the termites-ants are cases in point. In July 1890, near Romershof, in the Riga district, there was witnessed a flight of small beetles. The mass in motion was about 2 miles long, mile broad, and 7 yards thick. It moved in a northerly direction, obscuring the sun, and at times settling on the fields. The flights of the various species of locust (especially Pachytylus migratorius) are, however, not always impelled by hunger, for though they generally breed in a sandy district when food is scarce, and are therefore compelled to seek another where it is more plentiful, the reverse course is sometimes taken, as if the species was seeking the barren home of its ancestors. Temperature may also have something to do with locustal migrations, and there is a tendency for the flights to take particular directions. The nearest approach to periodical migration known is the curious movement of butterflies across the Isthmus of Panama and seaward in June and the beginning of July many years in succession. But it is not known where they go to, though butterfly showers have been met with in the Pacific several miles from land, and other flights have crossed Ceylon of such an extent that, though miles in breadth, they occupied several continuous days in their passage. It may be taken for granted that all marine animals, like all terrestrial ones, shift about in search of food, when not permanently fixed in their adult condition. Yet, with the exceptions mentioned, it is difficult to point to any which are migrants in the true sense of the term. A curious inroad of crustaceans—a species of Sesarma—has been noticed off Cuba, where, at Cape San Antonio, they invaded the houses and the light-tower. Land-crabs are sometimes migratory. The violet species of the West Indies (Gecarcinus ruricola) lives several miles inland, returning to the sea once a year to deposit its spawn. Other species are found at heights of 4500 feet, in the Deccan, though they do not appear to visit the sea, being fresh-water spawners. But in Ascension land-crabs climb up Green Mountain and steal rabbits from their holes, and in Japan the crab Telphusa is found on the tops of mountains several thousand feet high, and in Greece and Italy by streams far inland.
Migration is, therefore, in the vast majority of cases, due (1) to the necessity of searching for food and (2), as in the case of fishes, crustacea, reptiles, &c., of finding a suitable place for depositing their eggs. The causes which impel birds to migrate are more complicated, though the two mentioned are the leading ones. With mammals, in all likelihood, this trait is of an origin so remote that it preceded the present distribution of land and water. This is shown by the range of various extinct groups. Thus, true horses of the genus Equus are of older-Pliocene date in Europe, but of Post-Pliocene, or even of later appearance, in America, into which they have migrated, though on that continent they expired comparatively soon. But tapirs, though now essentially an American group, being more abundant there than in Asia, are in the Old World Lower Miocene, while in the New World they do not extend further back than the Post-Pliocene epoch. Lastly—and the list might be largely extended—camelidæ, though now confined to Asia and South America, are really an American group, having been largely developed in the Miocene age of the northern part of the continent, whereas the true camels, according to Wallace, seem to have passed into Asia and the llamas into South America. We may, indeed, infer with confidence that in the most remote periods of the world's history migration existed; indeed, if we accept (though few palæontologists do) the conclusions of Barrande, they lend countenance to the belief that at a date incalculably distant by our methods of measuring time this animal trait was established among the trilobites and other palæozoic invertebrates. It is thus, in some instances, permissible to suggest that the habit is a 'survival,' though it is difficult to point to a case in which it is at present practised without the animal benefiting thereby. For just as hibernation, by enabling an animal to live within its area, serves an important purpose in the struggle for existence, so migration, by enabling it to escape vicissitudes of climate and the revolutions of the globe which would be fatal to it or to its offspring, renders the species more likely to survive longer than it would otherwise be capable of doing.
See De Serres, Des Causes des Migrations des Animaux (1842); Brown, Short Studies from Nature, and Our Earth; Palmén, Om Föglarnes flyttningsvägar (1874), with bibliography of subject so far as birds are concerned; Harting, Our Summer Migrants (1877); Duns, Science for All; Reports of the British Association Committee on the Migration of Birds (1883 seq.); Richardson, Fauna Boreali-Americana; Von Wrangel, Expedition to the Polar Sea; Eschricht, Forhandl. Skandinaviska Naturhistoriska Forening (1847); Rink, Danish Greenland (1876); C. Dixon, The Migration of British Birds (1892 and 1895); Gätke, Helioland as an Ornithological Observatory (1895); Von Ihring, in Kosmos (1885); Challenger Report, vol. i. p. 927; Nicholson and Lydekker, Manual of Palæontology (1889); and the article GEOGRAPHICAL DISTRIBUTION.