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return along the eastern coasts of the United States, and often fly from point to point across bays and inlets. They are then liable to be blown out to sea by storms, which are prevalent at this season; and it is almost always at this time of year that their occurrence has been noted on the shores of Europe. It may, however, be doubted whether this is not an altogether modern phenomenon, dependent on the number of vessels constantly on the Atlantic which afford resting-places to the wanderers; as it is hardly conceivable that such birds as titlarks, cuckoos, wrens, warblers, and rails, could remain on the wing without food or rest, the time requisite to pass over 2,000 miles of ocean. It is somewhat remarkable that no European birds reach the American coast but a few which pass by way of Iceland and Greenland; whereas a considerable number do reach the Azores, fully half way across; so that their absence can hardly be due to the prevailing winds being westerly. The case of the Azores is, however, an argument for the unassisted passage of birds for that distance; since two of the finches are peculiar 'species,' but closely allied to European forms, so that their progenitors must, probably, have reached the islands before the Atlantic was a commercial highway.
Barriers to the Dispersal of Birds.-We have seen that, as a rule, wide oceans are an almost absolute barrier to the passage of most birds from one continent to another; but much narrower seas and straits are also very effectual barriers where the habits of the birds are such as to preserve them from being carried away by storms. All birds which frequent thickets and forests, and which feed near or on the ground, are secure from such accidents; and they are also restricted in their range by the extent of the forests they inhabit. In South America a large number of the birds have their ranges determined by the extent of the forest country, while others are equally limited to the open plains. Such species are also bounded by mountain ranges whenever these rise above the woody region. Great rivers, such as the Amazon, also limit the range of many birds, even when there would seem to be no difficulty in their crossing them. The supply of food, and the kind of vegetation, soil, and climate
best suited to a bird's habits, are probably the causes which mark out the exact limits of the range of each species; to which must be added the prevalence of enemies of either the parent birds, the eggs, or the young. In the Malay Archipelago pigeons abound most where monkeys do not occur; and in South America the same birds are comparatively scarce in the forest plains where monkeys are very abundant, while they are plentiful on the open plains and campos, and on the mountain plateaux, where these nest-hunting quadrupeds are rarely found. Some birds are confined to swamps, others to mountains; some can only live on rocky streams, others on deserts or grassy plains.
The Phenomena of Migration.—The term “migration” is often applied to the periodical or irregular movements of all animals ; but it may be questioned whether there are any regular migrants but birds and fishes. The annual or periodical movements of mammalia are of a different class. Monkeys ascend the Himalayas in summer to a height of 10,000 to 12,000 feet, and descend again in winter. Wolves everywhere descend from the mountains to the lowlands in severe weather. In dry seasons great herds of antelopes move southwards towards the Cape of Good Hope. The well-known lemmings, in severe winters, at long intervals, move down from the mountains of Scandinavia in immense numbers, crossing lakes and rivers, eating their way through haystacks, and surmounting every obstacle till they reach the sea, whence very few return. The alpine hare, the arctic fox, and many other animals, exhibit similar phenomena on a smaller scale; and generally it may be said, that whenever a favourable succession of seasons has led to a great multiplication of any species, it must on the pressure of hunger seek food in fresh localities. For such movements as these we have no special term. The summer and winter movements best correspond to true migration, but they are always on a small scale, and of limited extent; the other movements are rather temporary incursions than true migrations.
The annual movements of many fishes are more strictly analogous to the migration of birds, since they take place in large bodies and often to considerable distances, and are
immediately connected with the process of reproduction. Some, as the salmon, enter rivers; others, as the herring and mackerel, approach the coast in the breeding season; but the exact course of their migrations is unknown, and owing to our complete ignorance of the area each species occupies in the ocean, and the absence of such barriers and of such physical diversities as occur on the land, they are of far less interest and less connected with our present study than the movements of birds, to which we shall now confine ourselves.
Migrations of Birds.-In all the temperate parts of the globe there are a considerable number of birds which reside only a part of the year, regularly arriving and leaving at tolerably fixed epochs. In our own country many northern birds visit us in winter, such as the fieldfare, redwing, snow-bunting, turnstone, and numerous ducks and waders; with a few, like the black redstart, and (according to Rev. C. A. Johns) some of the woodcocks from the south. In the summer a host of birds appear-the cuckoo, the swifts and swallows, and numerous warblers, being the most familiar,—which stay to build their nests and rear their young, and then leave us again. These are true migrants; but a number of other birds visit us occasionally, like the waxwing, the oriole, and the bee-eater, and can only be classed as stragglers, which, perhaps from too rapid multiplication one year and want of food the next, are driven to extend their ordinary range of migration to an unusual degree. We will now endeavour to sketch the chief phenomena of migration in different countries.
Europe.--It is well ascertained that most of the birds that spend their spring and summer in the temperate parts of Europe pass the winter in North Africa and Western Asia. The winter visitants, on the other hand, pass the summer in the extreme north of Europe and Asia, many of them having been found to breed in Lapland. The arrival of migratory birds from the south is very constant as to date, seldom varying more than a week or two, without any regard to the weather at the time; but the departure is less constant, and more dependent on the weather. Thus the swallow always comes to us about the middle
of April, however cold it may be, while its departure may take place from the end of September to late in October, and is said by Forster to occur on the first N. or N.E. wind after the 20th of September.
Almost all the migratory birds of Europe go southward to the Mediterranean, move along its coasts east or west, and cross over in three places only; either from the south of Spain, in the neighbourhood of Gibraltar, from Sicily over Malta, or to the east by Greece and Cyprus. They are thus always in sight of land. The passage of most small birds (and many of the larger ones too) takes place at night; and they only cross the Mediterranean when the wind is steady from near the east or west, and when there is moonlight.
It is a curious fact, but one that seems to be well authenticated, that the males often leave before the females, and both before the young birds, which in considerable numbers migrate later and alone. These latter, however, seldom go so far as the old ones; and numbers of young birds do not cross the Mediterranean, but stay in the south of Europe. The same rule applies to the northward migration; the young birds stopping short of the extreme arctic regions, to which the old birds migrate. When old and young go together, however, the old birds take the lead. In the south of Europe few of the migratory birds stay to breed, but pass on to more temperate zones; thus, in the south of France, out of 350 species only 60 breed there. The same species is often sedentary in one part of Europe and migratory in another; thus, the chaffinch is a constant resident in England, Germany, and the middle of France; but a migrant in the south of France and in Holland : the rook visits the south of France in winter only: the Falco tinnunculus is both a resident and a migrant in the south of France, according to M. Marcel de Serres, there being two regular passages every year, while a certain number always remain.
i Marcel de Serres states this as a general fact for wading and swimming birds. He says that the old birds arrive in the extreme north almost alone, the young remaining on the shores of the Baltic, or on the lakes of Austria, Hungary, and Russia. See his prize essay, Des Causes des Migrations, &c. 2nd ed., Paris, 1845, p. 121.
We see, then, that migration is governed by certain intelligible laws; and that it varies in many of its details, even in the same species, according to changed conditions. It may be looked upon as an exaggeration of a habit common to all locomotive animals, of moving about in search of food. This habit is greatly restricted in quadrupeds by their inability to cross the sea or even to pass through the highly-cultivated valleys of such countries as Europe ; but the power of flight in birds enables them to cross every kind of country, and even moderate widths of sea; and as they mostly travel at night and high in the air, their movements are difficult to observe, and are supposed to be more mysterious than they perhaps are. In the tropics birds move about to different districts according as certain fruits become ripe, certain insects abundant, or as flooded tracts dry up.
On the borders of the tropics and the temperate zone extends a belt of country of a more or less arid character, and liable to be parched at the summer solstice. In winter and early spring its northern margin is verdant, but it soon becomes burnt up, and most of its birds necessarily migrate to the more fertile regions to the north of them. They thus follow the spring or summer as it advances from the south towards the pole, feeding on the young flower buds, the abundance of juicy larvæ, and on the ripening fruits; and as soon as these become scarce they retrace their steps homewards to pass the winter. Others whose home is nearer the pole are driven south by cold, hunger, and darkness, to more hospitable climes, returning northward in the early summer. As a typical example of a migratory bird, let us take the nightingale. During the winter this bird inhabits almost all North Africa, Asia Minor, and the Jordan Valley. Early in April it passes into Europe by the three routes already mentioned, and spreads over France, Britain, Denmark, and the south of Sweden, which it reaches by the beginning of May. It does not enter Brittany, the Channel Islands, or the western part of England, never visiting Wales, except the extreme south of Glamorganshire, and rarely extending farther north than Yorkshire. It spreads over Central Europe, through Austria and Hungary to Southern Russia and the warner parts of Siberia,