Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/785

Rh MIGRATION.] BIRDS 767 until the movement which began in the far north is com municated to the individuals occupying the extreme south ern range of the species at that season ; though, but for such an intrusion, these List might be content to stay some time longer in the enjoyment of their existing quarters. This seems satisfactorily to explain the southward move ment of all migrating Birds in the northern hemisphere ; ward but when we consider the return movement which takes neiit. place sume six mouths later, doubt may be entertained whether scarcity of food can be assigned as its sole or suf ficient 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 wa ; ting, 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, illace Mr Wallace in some remarks on the subject (Nature, ?inof x p_ 459) ingeniously suggests the manner in which the tory habit of Migration has come to be adopted : 2 &quot; 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 influ ence. 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 under stand 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 grada tion still exists in various parts of the world, from a complete coincidence to a complete separation of the breeding and the sub sistence 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.&quot; A few more particulars respecting Migration are all that can here be given, and it is doubtful whether much can be ifmale peated observation that in the spring-movement of most into. S p ec i es O f 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 1 If the relative proportion of land to water in the Southern Hemi sphere 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 tro pical 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 south ward 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 now-a-days exists can exert but little influence, and its effects may be practically disregarded. In principle Captain Hutton had already foreshadowed the same theory. (Trans. New Zeal. List. 1872, p. 235.) prolonged throughout some 50 or 60 of latitude, the stronger individuals should outstrip the weaker by a very perceptible distance, and it can hardly be doubted that in most species the males are stouter, as they are bigger than the females. Some observers assert that the same thing takes place in the return-journey in autumn, but on this point others are not so sure, which is not surprising when we consider that the majority of observations have been made towards what is the northern limit of the range of the Passeres, to which the remark is especially applicable in the British Islands, France, North Germany, and the Russian Empire for it is plain that at the beginning of the journey any inequality in the speed of travelling will not have become so very manifest. There is also another matter to be noticed. It has been suspected that where Connec- there is any difference in the size of birds of the same tio11 of l species, particularly in the dimensions of their wings, the * an f W1 individuals that perform the most extensive journeys are win naturally those with the longest and broadest remiges, and in support of this view it certainly appears that in some of the smaller migrants such as the Wheatear (Saxicola oenanthe) and Willow-Wren (Phylloscopus Irochilus) the examples which reach the extreme north of Europe and there pass the summer possess greater mechanical powers of flight than those of the same species which stop short on the shores of the Mediterranean. It may perhaps be also inferred, though precise evidence is wanting, that these same individuals push further to the southward in winter than do those which are less favoured in this re spect. It is pretty nearly certain that such is the case with some species, and it may well be so with individuals. Canon Tristram has remarked (Ibis. 1865, p. 77) that, in many genera of Birds, &quot;those species which have the most extended northerly have also the most extended southerly range; and that those which resort to the highest latitudes for nidification also pass further than others to the south ward in winter,&quot; fortifying his opinion by examples adduced from the genera Turdus, Fringilla, Cypselus, and Turtur. But supposing this to be true for many Birds, it may fairly be doubted whether it is so for all, and whether in some species certain individuals do not always occupy the most northern portion of the range and others always keep to the most southern, no matter what the season of the year may be, or over what countries the range may ex tend. On this point therefore it will be advisable to await further investigation. For many years past a large number of persons in dif- Presum ferent countries have occupied and amused themselves by effects c carefully registering the dates on which various migratory Birds first make their appearance, and certain publications abound with the records so compiled. 3 Some of the observers have been men of high scientific repute, others of less note but of not inferior capabilities for this especial object. Still it does not seem that they have been able to determine what connection, if any, exists between the arrival of birds and the state of the weather. This is not very wonderful, for the movements of the migrants, if governed at all by meteorological forces, must be influenced by their action in the places whence the travellers have come, and therefore to establish any direct relation of cause and effect corresponding observations ought equally to be made in such places, which has seldom been done. 4 3 These are far too numerous to mention here. Perhaps the most remarkable series of them is that carried on from 173G to 1810 and again from 1836 to 1874 by four generations of the Marsham family at Stratton-Strawless and Eippon near Norwich, of which an account is given by Mr Southwell (Trans. Norf. and Norw. Nat. Soc. ii. p. 81). 4 To a limited extent it must be admitted that the popular belief as to certain Birds being the harbingers of severe weather is justifiable. Cold comes from the north, and when it is accompanied, as is most gene-
 * r re- built upon them. It has now been ascertained by re-