the journey be one of a few yards or of many miles. Given
the sense of direction, it is no more difficult to steer a course
due north than it is to lay one south-east by east, provided
always the impetus to be on the move. There is no mystery,
except that we, the most intellectual of mankind, should so
well nigh have lost this sense, and even this fact is simply an
instance of the loss of a faculty through long-continued disuse.
Birds.—(The following account is to a great extent based upon A. Newton’s article “Birds” in Ency. Brit., 9th ed.)
In almost all countries there are some species which arrive in spring, remain to breed, and depart in autumn; others which arrive in autumn, stop for the winter and depart in spring; and others again—and these are strictly the “birds of passage”—which show themselves but twice a year, passing through the country without staying long in it, and their transient visits take place about spring and autumn. These three apparently different categories of migrants are all acted upon by the same impulse in spite of the at first sight dissimilar nature of their movements. The species which resort to Britain and to other temperate countries in winter are simply those which have their breeding quarters much nearer the poles, and in returning to them on the approach of spring are but doing exactly as do those species which, having their winter abode nearer the equator, come to us with the spring. The birds-of-passage proper, like our winter visitants, have their breeding quarters nearer the pole, but like our summer visitants, they seek their winter abode nearer the equator, and thus perform a somewhat larger migration. As H. Seebohm puts it (Geograph. Distrib. of the family Charadriidae, London):—
“They all represent birds which breed in the north and winter in the south. Every migratory bird wintering in England goes north to breed, and every migratory bird breeding in England goes south to winter. It is a rule without exception in the northern hemisphere that each bird breeds in the extreme north point of its migrations. To make the rule apply to the southern hemisphere as well it must be modified as follows: each bird breeds in the coldest climate which it visits on its migrations. . . . It is a remarkable fact that whilst there are many birds breeding in the northern hemisphere and wintering in the southern, it is not known that any land-bird breeds in. the southern and habitually winters in the northern! This is probably owing to the difference in the distribution of the land, there being no antarctic breeding grounds. . . . Birds breeding in the tropics are always resident, except when they breed on mountains, where the climate causes them to descend into the valleys for the winter.”
In many countries we find that while there are some species, such as in England the swallow or the fieldfare, of which every individual disappears at one period of the year or another, there are other species, such as the pied-wagtail or the woodcock, of which only the majority of individuals vanish—a few being always present—and these species form the so-called “partial migrants.” In England the song-thrushes receive in the autumn a considerable accession in numbers from the birds which arrive from the north, though the migration is by no means so well marked as it is on the continent, where the arrival of the strangers sets all the fowlers at work. In most localities in Britain the newcomers depart after a short sojourn, and are accompanied by so many of the homebred birds that in some parts of the island it may be safely declared that not a single song-thrush can be found from the end of November to the end of January, while in others examples can always be seen. Much the same may be said of the redbreast. Undeniably resident as a species, attentive scrutiny will reveal the fact that its numbers are subject to very considerable variation, according to the season of the year. At no time do our redbreasts collect in bands, but towards the end of summer they may be seen in the south of England successively passing onward, the travellers being mostly—if not wholly—young birds of the year; and so the great majority disappear, departing it may be safely presumed for more southern countries, since a few weeks later the markets of most towns, first in France and then in Italy, are well supplied with this species. But the migratory influence affects, though in a less degree, many if not most of the redbreasts that remain with us. Every bird of the northern hemisphere is to a greater or less degree migratory in some part or other of its range.
Want of food, and perhaps of the special, proper kind during the breeding season, seems to be the most obvious cause of migration, and none can wonder that those animals which possess the power of removing themselves from a place of scarcity should avail themselves of it, while it is unquestionable that birds possess this faculty in the greatest degree. Even among those species which we commonly speak of as sedentary it is only the adults which maintain their ground throughout the year. It has long been known that birds-of-prey customarily drive away their offspring from their own haunts so soon as the young are able to shift for themselves. The reason generally, and no doubt truly, given for this behaviour, which at first sight appears so unnatural, is the impossibility of both parents and progeny getting a livelihood in the same vicinity. The practice, however, is not limited to the birds-of-prey alone, but is much more universal. We find it to obtain with the redbreast, and if we watch our feathered neighbours closely we shall perceive that most of them indulge in it. The period of expulsion, it is true, is in some birds deferred from the end of summer or the autumn, in which it is usually performed, until the following spring, when indeed from the maturity of the young it must be regarded as much in the light of a voluntary secession on their part as in that of an act of parental compulsion, but the effect is ultimately the same.
The mode in which the want of sustenance produces migration may best be illustrated by confining ourselves to the unquestionably migrant birds of our own northern hemisphere. As food grows scarce towards the end of summer in the most northern limits of the range of a species, the individuals affected thereby seek it elsewhere. Thus doing, they press upon the haunt of other individuals: these in like manner upon that of yet others, and so on, until the movement which began in the far north is communicated to the individuals occupying the extreme southern range of the species at that season; though, but for such an intrusion, these last might be content to stay some time longer in the enjoyment of their existing quarters.
This seems satisfactorily to explain the southward movement of all migrating birds in the northern hemisphere; but when we consider the return movement which takes place some six months later, doubt may be entertained whether scarcity of food can be assigned as its sole or sufficient cause, and perhaps it would be safest not to come to any decision on this point. On one side it may be urged that the more equatorial regions which in winter are crowded with emigrants from the north, though well fitted for the resort of so great a population at that season are deficient in certain necessaries for the nursery. Nor does it seem too violent an assumption to suppose that even if such necessaries are not absolutely wanting, yet that the regions in question would not supply sufficient food for both parents and offspring—the latter being at the lowest computation twice as numerous as the former—unless the numbers of both were diminished by the casualties of travel.[1] But on the other hand we must remember what has above been advanced in regard to the pertinacity with which birds return to their accustomed breeding-places, and the force of this passionate fondness for the old home cannot but be taken into account, even if we do not allow that in it lies the whole stimulus to undertake the perilous voyage.
A. R. Wallace in some remarks on the subject (Nature, x. 459) ingeniously suggests the manner in which the habit of migration has come to be adopted[2]:—
“It appears to me probable that here, as in so many other cases, ‘survival of the fittest’ will be found to have had a powerful influence. Let us suppose that in any species of migratory bird, breeding can as a rule be only safely accomplished in a given area; and further, that during a great part of the rest of the year sufficient food cannot be obtained in that area. It will follow that those birds which do not leave the breeding area at the proper season will suffer, and ultimately become extinct; which will also be the fate of those which do not leave the feeding area at the proper time. Now, if we suppose that the two areas were (for some remote ancestor of the existing species) coincident, but by geological and climatic changes gradually diverged from each other, we can easily understand how the habit of incipient and partial migration at the proper seasons would at last become hereditary, and so fixed as to be what we term an instinct. It will probably be found that every gradation still exists in various parts of the world, from a complete coincidence to a complete separation of the breeding and the subsistence areas; and when the natural history of a sufficient number of species is thoroughly worked out we may find every link between species which never leave a restricted area in which they breed and live the whole year round, to those other cases in which the two areas are absolutely separated.”
A few more particulars respecting migration are all that can here be given, and it is doubtful whether much can be built upon them. It has been ascertained by repeated observation that in the spring-movement of most species of the northern hemisphere the cock-birds are always in the van of the advancing army, and that they appear some days, or perhaps weeks, before the hens. It is not difficult to imagine that, in the course of a journey prolonged throughout some 50° or 60° of latitude, the stronger individuals
- ↑ If the relative proportion of land to water in the southern hemisphere were at all such as it is in the northern, we should no doubt find the birds of southern continents beginning to press upon the tropical and equatorial regions of the globe at the season when they were thronged with the emigrants from the north, and in such a case it would be only reasonable that the latter should be acted upon, by the force of the former, according to the explanation given of the southward movement of northern migrants. But, though we know almost nothing of the migration of birds of the other hemisphere, yet, when we regard the comparative deficiency of land in southern latitudes all round the world, it is obvious that the feathered population of such as nowadays exists can exert but little influence, and its effects may be practically disregarded.
- ↑ In principle F. W. Hutton had already foreshadowed the same theory (Trans. New Zeal. Inst., 1872, p. 235).
