MIGRATION. The term migration is often used very loosely in popular writings about animals, so that it seems wise to define it as limited in the present article to: (1) The annual change of residence by a species with the change of seasons from winter to summer or the reverse; (2) irregular mass-movements of a species under pressure of famine, over-population of a locality, or some more obscure influence. While these classes have been enumerated in the order of their prominence to our eyes, especially in the behavior of birds, it will be well to consider the second sort of migrations first, as these sporadic cases may throw light on the more regular phenomena, and how they came to be habitual.
Insects.— An eminent entomologist tells us that certain butterflies, as our milkweed frittilary (Amosia plexippus) and the cotton moth (Aletia argillacea), pass northward in the United States for hundreds of miles in spring, and again in huge swarms southward in autumn; but whether the individuals are the same is not determined. (See Milkweed Butterfly). Among other butterflies periodical migrations occur, as in movements of vast columns across the Isthmus of Panama out to sea, and flights miles in breadth have been observed to cross Ceylon, the individuals occupying several continuous days in their passage. Wallace observed the swarming of pierid butterflies in the Indian Ocean, and Clark in Venezuela, the vast throng composed of males moving steadily eastward for several days in the face of the trade winds.
The late Dr. A. S. Packard, whose special studies of the habits of the Rocky Mountain locust about 1880 were so valuable, reported that that destructive insect is migratory in certain seasons favorable to the species when overproduction occurs; the young on hatching, after having devoured every green thing at hand, are forced, when becoming winged, to rise in enormous swarms and sail on the wings of the wind for hundreds of miles to other regions where they lay their eggs. The next year's brood sometimes returns to the original spawning ground to lay their eggs. The same thing is characteristic of similar locusts in Syria and central Africa.
Crustacea.— The members of several families of crabs, mostly tropical, have acquired the power of living out of water, and even of wandering extensively inland, but regularly return to the sea, sometimes in marching hordes, to deposit their eggs in the water, after which they go back to the highlands. See Land Crabs.
Fishes.— Many kinds of fishes are regular migrants; the anadromous families, such as those of the shad, herring and salmon, annually ascend the rivers to spawn, whence in some cases they return to the sea, in others never get back, but their young, after the succeeding winter, go back to salt water. Certain fishes retire to the deeper or warmer parts of the ocean during the winter, but in early summer travel toward the shore-shallows, or to the cool north, in vast swarms; and the same is true of a large variety of other marine creatures, including some of the humblest and most minute forms, in which cases the direction of the mass-movements are largely determined by the ocean currents. Moreover there occurs in the ocean a regular movement of deep-sea forms toward the surface in the night, the animals sinking again as daylight approaches. Fishermen in the north Atlantic and on the coast of Norway are familiar with the vast influx in the spring of such fishes as herrings, cod, plaice and capelan. The eminent Norwegian naturalist, G. O. Sars, concluded that some of these fish-migrations were undertaken in order to obtain food, and others for the purpose of reproduction. “When the capelan gathers in millions on the coast-banks of Finmark or Labrador, when countless numbers of cod approach the banks of Lofoten, and when the herrings flock to western Norway, they migrate to spawn. The fat-herring collecting off the coast of Nordland, and the cod gathering around the shoals of capelan in the Barents Sea, are examples of feeding-migrations.”
Sea-turtles have a similar history, going regularly toward shore in the breeding season to deposit eggs in the beach sand.
Mammals.— The reader will have perceived that most of the foregoing cases are not examples of true migration because the element of habitual return is absent, or at best a very few survive to return; or else the movement, when seasonal and regular, is purely local, such as going to the nearest shore for spawning; or is merely the pursuit of traveling prey. In this class must be put most of the so-called migrations of mammals. From time to time certain small animals, as lemmings, field-mice, squirrels and the like, develop enormous numbers in some region (or formerly did so, before civilization was so worldwide) and overflow in great “armies” into neighboring parts of the country, where they gradually expire. In the plains regions of the world a lack of good pasture in one place will often cause movement of antelopes, bisons, etc., to some better district in great herds; and in other situations the wild animals are accustomed to go up into the hills in summer and come down to the shelter of the valleys in winter, but these are local movements. The only examples of real migration afforded by the mammals are the case of certain bats that regularly journey every year between the tropics and more northern climes, and the case of the caribou and, to a less extent, the reindeer. These deer do make a real fall migration from the barren Arctic coast to the margin of the forested region southward and go back in the spring. It is to the birds, then, that we must turn for a study of migration in the stricter sense of the term, and even here it is only partial as regards many species and groups.
Migration of Birds.— This subject is so large that we can give no more than a superficial sketch, following in general the lines of investigation conducted by the late Wells W. Cooke, of the United States Biological Survey, who devoted almost his whole life to a study of this phase of ornithology as exhibited especially in North America.
The motive or cause of the periodical migration of birds has excited inquiry since ancient times, and at present two different methods of explaining it are in vogue. The opinion is general that in Pleistocene times, just previous to the advance of the cold climate and finally to the great accumulations of ice and snow over the northern parts of the world, the whole of the northern hemisphere possessed a mild climate, and birds of every sort dwelt comfortably all the year round throughout virtually its whole extent. The coming of the Glacial Period so affected the north, as to limit more and more the residence in winter of birds there, although in summer they might venture somewhat toward it, when vegetation and insect life annually revived. As the ice advanced very gradually, now and then receding, but on the whole enlarging itself, these enforced northward and southward movements of the birds increased both in distance and duration, until migration became a fixed habit with all birds whose life was affected by the change of climatic conditions. Finally most northern birds were restricted all the year to middle America and the Mediterranean region and southern Asia. But the habit of migration had been formed, and when the glacial ice began to retreat toward its present position, the birds annually followed its receding margin, until at last they had established their present long and diversified migration routes.
Thus far all theorists are in substantial agreement. The divergence is as to the prevailing motive. One school argues that a longing to continue their inherited habit of residence in the north, and individually to return to their birthplaces, is the incentive that compels them to leave the tropics and make a journey, often of surprising length, every spring. The other school maintains that "the birds' real home is in the Southland"; that that region becomes overcrowded, and the birds in annually flying northward are seeking a region where there is less crowding and less competition for food. The truth perhaps lies in a combination of these influences, varying in intensity with different kinds of birds. It is an important circumstance, especially with reference to the second theory, that no similar migration occurs southward from the tropics to Bolivia and Argentina, whose plains and mountains offer a poor supply of bird-food, and not much more from the equatorial to South-African districts. “The conclusion is inevitable,” Cooke believes, "that the advantages of the United States and Canada as a summer home, and the superb conditions of climate and food for successful rearing of a nestful of voracious young, far overbalanced the hazards and disasters of the journey thither. It must be remembered too that the migratory species have acquired various adaptations relating to their migrating habits that tend to fit them more and more to endure the exertion and danger required; also that the regular routes followed by each species are the products of thousands of generations of experience, and presumably represent the easiest way in each case.
Phenomena of Bird-Migration.— In the restricted space of this article it is impossible to go into detail as to the general subject, and attention must be confined mainly to what appears in North America. Australasia and the South Pacific islands share to some extent in the annual movements of continental species, but have an inter-insular migratory system of their own. “In Europe,” says a recent reviewer, “and central Asia there are numerous routes, at least nine, according to Palmen. Of these one begins on the Siberian shores of the Polar Sea, Nova Zembla, and the north of Russia, and passes down the western coast of Norway to the North Sea and the British Isles; another arising in Spitzbergen follows much the same course, but is prolonged past France and Spain to the west coast of Africa. Many migrants wintering in North Africa (Algeria, etc.) have flown there from northern Russia, by way of the Baltic Sea, Holland, passing up the Rhine Valley, and crossing to the Rhone, the column splitting on reaching the Mediterranean, one line of migration passing along western Italy and Sicily, a second crossing by way of Corsica and Sardinia, the third by southern France and eastern Spain. Egypt receives its winter visitors from the Russian river-valleys of the Obi and Volga, the line crossing the Black, Bosporus and Ægean seas to the Nile Valley. One important migration-route is to and from India along the Danube Valley and across Persia.”
An important fact to consider at first is that the migratory habit is possessed in its completeness by comparatively few birds, and these belong almost wholly to a single order, that of the Passeres, or insect-eating song-birds. The exceptions are mainly seafowl and certain water-birds. Even in these groups two classes are to be found, one of species that are resident the year round in the regions they severally occupy; and the other whose migrations are of slight extent. Even in the northern half of the United States many birds are present in winter, some of which are those that retreat from the North only so far as driven by deep snow and excessive cold; while the Southern States have a longer list of resident birds, supplemented in winter by many kinds that moved only a little way southward to escape the dearth of food at that season in the snowy parts of the country. This shows that in respect to distance, migration varies from a distance almost as great as the breadth of the globe (the Arctic tern passes annually from Patagonia to Alaska, and back again) to no change of residence at all, even on the Arctic coast and islands.
Northward Movement in Spring.— Let us now consider the actions of the real migrants, who have been spending the winter in tropical America, on the arrival of spring. Nothing that we can see compels them to move, yet they abandon the delights of their winter home and proceed northward as soon as the proper time comes. This “proper time” seems to have no relation to the weather or to food-conditions there in the tropics, which are almost changeless, but is determined by the weather and food conditions the bird will find when it arrives at its northern destination, if the season there be an average one. This varies with the requirements of different birds, so that some start much earlier than others. Thus the ducks and geese, which ask only that the rivers and ponds in the north shall be free from ice, come to us much earlier than do the warblers and flycatchers that must wait until flowers are in bloom and insects numerous. As the last are the most numerous they come in crowds soon after the leaving-out of the northern woods and orchards.
Not all, however, pursue the same route, although each species keeps to its traditional path until it arrives in the district suitable to it, when it scatters. The configuration of continents, narrowing into a mere isthmus between North and South America, permits only a very narrow land-path for the migrants between their winter and summer resorts, yet most of those who winter south of Panama crowd along this narrow neck. Certain shorebirds, confident of their strength, strike straight north from Brazil to Nova Scotia, and a few species follow the line of the Antilles from Venezuela to Florida. Birds whose destination is California and northward follow the western coast of Mexico, and those aiming at summer homes in the Rocky Mountain region pass straight north through central Mexico and across the desert, or skirt the eastern coast of Texas and the plains. All these are habitual routes for certain species. The great body of migrants, whose songs are later to be heard in the eastern United States and Canada, fly straight across the Gulf of Mexico from Yucatan to landings on the northern shore all the way from eastern Texas to Florida. Arrived there — and as a rule this is about daylight, the journey from Yucatan having taken but one night — they spread northward along two main channels, one up the coast eastward of the Alleghanian Mountains (diverging into these uplands at each river valley); and two up the Mississippi Valley, separating into bands that ascend every tributary, and rapidly cover the whole country, while those bold species, surprisingly many in number, who are content only in the subarctic zone, hasten on to Alaska and down the Mackenzie Valley. The point to keep in mind in this general sketch is that every species pursues the same route every year, and sometimes it is far from the most direct one. The speed of travel is not great as a rule. The birds must feed by the way, and this food must be found as a rule by daylight; therefore, most species travel during the night, and are often delayed by foggy, rainy nights or by cold storms. The swallows, swifts, nightjars, hawks and some others that capture their prey on the wing feed as they go and probably rest at night. Those bound for the far north must and do move more rapidly than the southern breeders; and Cooke has gathered some very interesting statistics on this matter.
Autumnal Migration.— The autumnal return of the birds presents some very different features. It begins in the far north before any change in weather or food suggests the necessity of departure, and is led by male birds, after whom the mothers and young follow as soon as they have strength to travel. The movement is far less direct and hurried than the spring flight. They make a long stage by night, flying sometimes a mile or more above the earth, and drop down at dawn to feed and rest, then loaf along. The members of each species gather gradually into the regular route, which in many cases is quite different from that followed northward in May; and those that are inclined to flocking at this season form large companies that go on together, striking out at last from the Gulf shore to cross in a night that space of dark water, or disappearing in the Mexican forests.
How do Birds in Migration find Their Way?— We have seen that they follow definite routes; also that these in places lead across wide spaces of water; also that the routes in some cases differ according to season; and had we been able to give more details it would appear that often these routes are very eccentric. Moreover, they travel mostly at night; and finally certain species cross areas of ocean hundreds of miles wide, and far from land, as when golden plovers fly from Nova Scotia to hundreds of miles wide, and far from land, as in the case of the curlews that migrate between Australia and New Zealand. It is also pleasingly evident that birds return year after year to the same grove, dooryard and nesting-place. How do they do it?
It was formerly taught that they followed landmarks, such as coast lines, ranges of mountains and large river-courses, which are visible even on clear nights from a great height, and doubtless these are aids to the day-fliers and when the sky is clear. But many lines of migration cut across such landmarks, instead of follow them, and others stretch across wide plains and vast water-spaces. Other theories, as of magnetic influences, etc., are without value. It appears plain that birds are guided by an innate sense of direction. This need not be esteemed miraculous, far in a lesser degree it is possessed not only by various other animals, but by wild men, especially those who dwell in a forested region, where in following game they would become lost daily had they not a faculty for orientation. The direct testimony to such a faculty in savage mankind, and comparison of the evident ability of many animals in this direction, makes its presence in the minds of birds easily credible, the more so as the sense is by no means infallible, since birds sometimes become completely bewildered when buffeted about at sea by high winds. Add the elements of observation and memory and a sufficient explanation is at hand of how migratory birds find their way.
Bibliography.— An early treatise on this subject of much value is by Alfred Newton in his ‘Dictionary of Birds’ (London and New York 1893-96). A more recent review is a part of the introduction of Dr. Frank M. Chapman's ‘Handbook of Birds’ (2d ed., New York 1912), which includes an extensive bibliography. The latest and most important results of study are in the writings of W. W. Cooke, summarized in Bulletin 185 of the United States Department of Agriculture (Washington 1915).