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Migration

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Miguel Casiri

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In Zoology. In zoology considerable importance attaches to the problems of migration, by which is meant the wandering of living creatures into another, usually distant, locality in order to breed there; this implies a return, and the double phenomenon is annual. All other changes of the abode are either sporadic, epidemic, or fluctuating within lesser limits. Further, migration should not be confounded with " spreading," which proceeds steadily, and in epicycles, with a totally different result. It need not be emphasized that hard and fast lines between these phenomena do not exist; they are often a question of degree. For instance, when the common toad, which is a strictly terrestrial creature, wanders every spring to a frequently distant pool in order to spawn there, this is a true migration. The same applies, strictly speaking, to those insects which hibernate in the ground, at the root of the tree on which they feed and breed. The grey plover breeds in the arctic circle and winters in equatorial countries. To complicate matters further, it is not necessary that the migration be undertaken periodically, more than once, by the same individual. For instance, the common eel ascends the rivers as an elver in its youth; years after it returns to the sea, there to breed and to die, whilst other fishes come and go, year after year. Further, some of the larger birds, for instance swans and cranes, are still immature in their second year, and yet they migrate like their older relations. It seems permissible to use this fact as an indication that the breeding as such is not the prime reason of their wanderings. The fundamental impelling agent must have been the want of food, and what we usually understand by migration cannot suddenly have sprung into existence to its full extent, but is more likely the cumulative effect of the doings of countless generations. The faculty of shifting the abode was of course always there, the necessity of moving further on was also present, and those which went in the wrong direction came to grief, while the others flourished and returned with their progeny. They did not at first cover enormous distances, but just enough to find unoccupied ground. The annual repetiton became an established habit, at last an ineradicable instinct. There can be but little doubt that the prime impulse was want of food. The new growing grass on the prairie or on the veldt attracts every year those creatures which live upon pasture. The inter-tropical belt of the world is so crowded with creatures that there is the keenest competition, whilst in the temperate and cold regions is a long winter quiescence unfit for the support of many creatures, whereas in the summer these same regions are covered with new vegetation, with its concomitant abundance of insects and other invertebrates. The tables are decked again, and these opportunities are not wasted.

The process of migration, in its most striking cases, is now very complicated. Many a bird goes actually to the arctic regions for the shortest of summers, but spends most of the year within the tropics. On the other hand there are many species which do not go so far north, but stop to breed in the intermediate regions. We must not take the extremes when trying to unravel the development of the problem. The periodical migrations of mammals, with their more limited extent and greater leisure, are less perplexing.

It has been argued with some show of reason that the real home of a bird is that country in which it was born, in other words where the species breeds, but this is not in every case a valid conclusion. It applies to most creatures, but it can well bear exceptions if we leave sentiment aside. When it. comes to a question of domicile, the ten weeks' sojourn of the swift, Cypselus, in England are more than weighted down by the nine months or more which these birds spend in southern countries, although we do not know whether they are resident there or roam about. The breeding time is the busiest period of a bird's life; then the numbers of each species are suddenly multiplied, and so is the stress of providing food, and the particular food which is best for the young may not be available in every country. The idea that the arctic circle is the original home of the numerous kinds of birds which breed in it, whence they are now periodically driven away by stress, has been coupled with the glacial epoch, that supposed solution of so many difficulties. We have only to assume that the ol'd, permanent home of these migrants was in the arctic region, that the progressing glaciation drove them away, of course towards the equator, and that, when times improved again, the birds returned to their old home. This sounds very plausible, but it involves huge assumptions. The birds, not the individuals, but the species, are supposed to have inherited such a loving reminiscence of their old home, that after thousands of years - with most of the small birds meaning as many generations - they returned at the first opportunity. It implies that their long continued sojourn in foreign lands, where - under this assumption - thousands of generations must have been bred and have spent all their lives, was not sufficient to naturalize them, so to speak, in other words to supplant the instinctive love of the primary ancient home. That the last glacial epoch has driven the limit of many kinds of animals and plants farther south is as certain as that many have recovered the lost ground after the reversion of the glaciation, but it must have been a very slow and steady process of spreading. It may, and probably does, account for the present annual visitations of arctic lands,. as a phenomenon which has been evolved de novo, which would have come to pass even if no birds had existed in pre-glacial times.

How do birds manage to find their way, thousands and thousands of miles across land and water? This question has been extolled as a mystery of mysteries. It has been stated that the old birds show the way to the young, a speculation which does not apply to those many cases in which old and young notoriously travel at different times. It has been assumed that they travel by sight, taking advantage of certain landmarks; another untenable idea, since - experience having to be excluded in a flock of birds which made the journey for the first time - it implies that the young must have inherited the reminiscence of those landmarks! Others have likened the bird to a kind of compass, because in eastern Siberia E. von Middendorff found some migration routes to coincide with the direction of the magnetic pole. The whole question reduces itself to a sense of direction, a faculty which is possessed by nearly all animals; in some it is present to an astonishing extent; but the manifestations of this sense vary only in degree. The cat which escapes out of the bag finds its way back, directly or after many adventures. The bee, after having loaded itself with pollen, returns by the proverbial line to the hive which may be a mile away, but, move the small entrance hole in the meantime an inch to the right or left, and the bee will knock its head against the hive and blunder about; move the hive a few yards and bee after bee returning will be puzzled to find its hive again. They, maybe with the help of landmarks, have accustomed themselves to steer a course. Such instances need not be multiplied. The principle is the same whether 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 on 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 red breast, 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.' 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 springmovement of most species of the northern hemisphere the cockbirds 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. 2 In principle F. W. Hutton had already foreshadowed the same theory ( Trans. New Zeal. Inst., 1872, p. 235).

should outstrip the weaker by a very perceptible distance, and it can hardly be doubted that in most species the males are stronger, as they are bigger than the females. Some observers assert that the same thing takes place in the return journey in autumnSeebohm, for instance, says that, from Europe, first go the young, then the males, having finished their moult of autumn, and lastly the females - 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 there is any difference in the size of birds of the same species, particularly in the dimensions of their wings, the individuals that perform the most extensive journeys would be 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 trochilus) - 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 respect. It is pretty nearly certain that such is the case with some species, and it may well be so with individuals. H. B. Tristram has remarked ( Ibis, 1865, p. 77) that, in many genera'of birds, " 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 southward in winter," fortifying his opinion by examples adduced from the genera Turdus, Fringilla, Cypselus and Turtur. For many years past a large number of persons in different countries have occupied and amused themselves by carefully registering the dates on which various migratory birds first make their appearance, and there is now an abundance of records so compiled. Still it does not seem that they have been able to determine what connexion, if any, exists between the arrival of birds and the weather; in most cases no corresponding observations have been made about the weather in the places whence the travellers are supposed to have come. As a rule it would seem as though birds were not dependent on the weather to any great degree. Occasionally the return of the swallow or the nightingale may be somewhat delayed, but most sea-fowls may be trusted, it is said, as the almanac itself. Foul weather or fair, heat or cold, the puffins ( Fratercula arctica ) repair to some of their stations punctually on a given day as if their movements were regulated by clockwork. Whether they have come from far or from near we know not, but other birds certainly come from a great distance, and yet make their appearance with scarcely less exactness. Nor is the regularity with which certain species disappear much inferior; every observer knows how abundant the swift (Cypselus apus ) is up to the time of its leaving its summer-home - in most parts of England, the first days of August - and how rarely it is seen after that time is past.

It must be allowed, however, that, with few exceptions, the mass of statistics above spoken of has never been worked up and digested so as to allow proper inferences to be made from it, and therefore it would be premature to say that little would come of it, but the result of those exceptions is not very encouraging. E. von Middendorff carefully collated the records of the arrival of migratory birds throughout the Russian Empire, but the insight into the question afforded by his published labours is not very great. His chief object has been to trace what he has termed the isepipteses (IQos = aequalis, brcrrnacs = advolatus ) or the lines of simultaneous arrival, and in the case of seven species these are laid down on the maps which accompany his treatise. The lines are found by taking the average date of arrival of each species at each place in the Russian dominions where observations have been regularly made, and connecting those places where the dates are the same for each species by lines on the map. The curves thus drawn indicate the inequality of progress made by the species in different longitudes, and assuming that the advance is directly across the isepiptesial lines, or rather the belts defined by each pair of them, the whole course of the migration is thus most accurately made known. In the case of his seven sample species the maps show their progressive advance at intervals of a few days, and the issue of the whole investigation, according to him, proves that in the middle of Siberia the general direction of the usual migrants is almost due north, in the east of Siberia from south-east to northwest, and in European Russia from south-west to north-east. Thus nearly all the migrants of the Russian empire tend to converge upon the most northern part of the continent, the Taimyr peninsula, but it is' almost needless to say that few of them reach anything like so far, since the country in those high latitudes is utterly unfit to support the majority. With the exception of some details this treatise fails to show more. The routes followed by migratory birds have been the subject of a very exhaustive memoir by J. A. PaImen, but it would be beyond our limits to do more than mention his results concisely. He enters very fully into this part of the inquiry and lays down with much apparent probability the chief roads taken by the most migratory birds of the palaearctic region in their return autumnal journey, further asserting that in the spaces between these lines of flight such birds do not usually occur. Broadly speaking, the birds of Europe, Russia and Western Siberia go for the winter to Africa, those of middle Siberia to Mongolia, and those of Siberia east of the Lena go towards Japan.

But lay down the paths of migratory birds, observe their comings and goings, or strive to account for the impulse which urges them forward as we will, there still remains for consideration the most marvellous thing of all - how do the birds find their way so unerringly from such immense distances? This seems to be by far the most inexplicable part of the matter. Year after year the migratory wagtail will build her nest in the accustomed spot, and year after year the migratory cuckoo will deposit her eggs in that nest, and yet in each interval of time the former may have passed some months on the shores of the Mediterranean, and the latter, absent for a still longer period, may have wandered into the heart of Africa. That particular form of bluethroat which yearly repairs to breed upon the mosses of the subalpine and northern parts of Scandinavia (Cyanecula suecica ) is hardly ever seen in Europe south of the Baltic. Throughout Germany it may be said to be quite unknown, being replaced by a conspicuously different form (C. leucocyana ), and as it is a bird in which the collectors of that country, a numerous and well-instructed body, have long taken great interest, we are in a position to declare that it is not known to stop in its transit from its winter haunts, which we know to be Egypt and the valley of the Upper Nile, to its breeding-quarters. Other instances, though none so crucial as this, could be cited from among European birds were there room here for them. In New Zealand there are two cuckoos which are annual visitors: one, a species of Chrysococcyx, is supposed to come from Australia, the other, Eudynamis taitensis is widely spread throughout Polynesia, yet both these birds yearly make two voyages over the enormous waste of waters that surrounds the country to which they resort to breed. But space would utterly fail us were we to attempt to recount all the examples of these wonderful flights. Yet it seems impossible that the sense of sight should be the faculty whereby they are so guided to their destination, any more than in the case of those which travel in the dark. J. A. Palmen asserted ( op. cit. p. 195) that migrants are led by the older and stronger individuals among them, and, observing that most of those which stray from their right course are yearlings that have never before taken the journey, he ascribed the due performance of the flight to " experience." There are, many birds which cannot be said to migrate in company. While swallows, to take a sufficiently evident example, conspicuously congregate in vast flocks and so leave our shores in large companies, the majority of our summer-visitors slip away almost unobserved, each apparently without concert with others. Experience here can only signify the result of knowledge acquired on former occasions and obtained by sight. Now it was stated by C. J. Temminck ( Manuel d'ornithologie, III. Introd., 1820) many years ago, and so far as would appear the statement has not been invalidated, that among migrants the young and the old always journey apart and most generally by different routes. The former can have no " experience," and yet the greater number of them safely arrive at the haven where they would be. The sense of sight, essential to a knowledge of landmarks, is utterly insufficient to account for the success that attends birds which travel by night, or in a single flight span oceans or continents. Yet without it the idea of " experience " cannot be substantiated. We may admit that inherited but unconscious experience, which is really all that can be meant by instinct, is a factor in the whole matter - certainly, as Wallace seems to have proved, in originating the migratory impulse, but yet every aspect of the question is fraught with difficulty.

Less than nothing is known about the speed at which birds fly during their long stretches of migration. Gaetke, in his otherwise very interesting book, has startled ornithologists by various statements, but his calculations were based upon such crude observations that the results are ridiculous. For instance, he proved to his satisfaction that the grey or hooded crow, Corvus cornix, which notoriously is not a fast bird, flies from Heligoland to the coast of Lincolnshire in England at the rate of one hundred and twenty miles an hour. To the little bluethroat he assigned a velocity of two hundred and forty miles an hour, a statement as silly as that made by some fanciful observer in Portugal who convinced himself that " Turtle-doves leaving Kent or Surrey at dawn might easily be the very birds that a few hours later were skimming over the Portuguese pine forests on their way to Central Africa." Fifty miles an hour would be a high average speed for most migratory birds, and there are no reliable data to tell how long such birds can continue their flight without interruption. All we seem to know is that not a few kinds manage, in various parts of the world, to cross enormous distances without the chance of a break. It was Gaetke's notion that migration was for the most part carried on at such a height in the air as to be beyond our ken, and that what comes to our perception consists chiefly of the abortive or unsuccessful attempts, when birds are checked in their course, and being unable to proceed present themselves to our sight and hearing. Now for obvious reasons birds could not well fly at very great heights in very thin air, as experiments with pigeons released from balloons have shown, and the condor soaring far above the tops of the Andes is a myth. The few trustworthy instances in which birds have been observed through a telescope passing across the face of the moon have naturally yielded but vague calculations as to distance and height. W. E. D. Scott ( Bull. Nuttall Orn. Club, vi. 97-100), computed heights varying from I to 2 m. F. M. Chapman's observations ( Auk, 1888, pp. 37-39) resulted in a height of from 1500 to 15,000 ft.; average, say, i m. If the sky is:' clouded and the birds fly above the clouds the migration proceeds beyond our ken, and if for some reason or other they are below the clouds the phenomenon becomes to us very noticeable. It is well known " that on clear and bright nights birds are rarely heard passing overhead, while on nights that are overcast, misty and dark, especially if slight rain be falling, flocks may often be heard almost continuously." It is in such,weather, continues Newton, that birds while migrating are most vociferous, doubtless with the result that thereby the company of fellow-travellers is kept together.

There yet remain a few words to be said on what may be termed Exceptional Migration, that is when from some cause or other the ordinary practice is broken through. The erratic movements of the various species of crossbill (Loxia ) and some allied forms afford perhaps the best-known examples. In England no one can say in what part of the country or at what season of the year he may not fall in with a company of the common crossbill (L. curvirostra ), and the like may be said of many other lands. The food of these birds consists mainly of the seeds of conifers, and as its supply in any one locality is intermittent or precarious, we may not unreasonably guess that they shift from place to place in its quest, and may thus find an easy way of accounting for their uncertain appearance. The great band of nutcrackers (Nucifraga caryocatactes ) which in the autumn of 1844 pervaded western and central Europe (Bull. Acad. Bruxelles, xi. 298), may also have been actuated by the same motive, but we can hardly explain the roaming of all other birds so plausibly. The inroads of the waxwing ( Ampelis garrulus) have been the subject of interest for more than 300 years, and by persons prone to superstitious auguries were regarded as the forerunners of dire calamity. Sometimes years have passed without the bird being seen in central, western or southern Europe, and then perhaps for two or three seasons in succession vast flocks have suddenly appeared. Later observation has shown that this species is as inconstant in the choice of its summer as of its winter-quarters. One of the most extraordinary events known to ornithologists is the irruption into Europe in 1863 of Pallas's sand-grouse ( Syrrhaptes paradoxus). Of this bird, hitherto known only as an inhabitant of the Tatar steppes, a single specimen was obtained at Sarepta on the Volga in the winter of 1848. In May 1859 a pair is said to have been killed in the government of Vilna, on the western borders of the Russian empire, and a few weeks later five examples were procured, and a few others seen, in western Europe - one in Jutland, one in Holland, two in England and one in Wales. In 1860 another was obtained at Sarepta; but in May and June 1863 a horde computed to consist of at least 700 individuals overran Europe - reaching Sweden, Norway, the Faeroes and Ireland in the northwest, and in the south extending to Sicily and almost to the frontiers of Spain. On the sandhills of Jutland and Holland some of these birds bred, but they were all killed off. A much greater visitation took place in 1888, which met with the same fate. The number of birds was quite incalculable, the ware extending from Norway to southern Spain.

In comparison with the periodic annual migrations of so very many birds, those of other creatures are scarce and insignificant, excepting fishes.

Mammals

Few trustworthy observations have been recorded. The most regular and least limited migrations seem to be those of the eared seals. The walrus also goes each year to the north in the summer, further south in the winter. Delphinapterus leucas, one of the Cetacea, ascends the Amoor regularly on the breaking of the ice, a distance of 400 m. up the stream, Some bats are supposed to migrate. The American bison used to roam north and south, according to the season, in search of pasture; and similar periodic wanderings have often been recorded of various kinds of game on the South African veldt. They are all obviously a mere matter of commissariat and have little to do with the breeding, except in the case of seals.

In one way the lemming's " migrations " are instructive. They are quite sporadic. When, owing to combination of some favourable circumstances they suddenly increase, enormous numbers forsake the highlands for the lowlands of Norway; not in a methodical way, but quite lawlessly; that means to say they radiate from their centres of dispersal. At any given spot, however, they seem to keep to the same direction, and no obstacles seem to divert their course. Those which arrive at the much indented coast are known even to rush into the sea, where of course they get drowned. There is no sense in this. The overcrowded condition of their home impels them to leave, and this impulse continues blindly. They do not attempt to settle anywhere between their home and the sea. A year or two after the irruption not a lemming is there to be found, and where during their stampede they come across suitable districts, they find these already occupied by resident lemmings.

Such and similar irruptions have no doubt taken place often during the world's history; and yet such sporadic stampedes into a foreign country hardly ever lead to its regular settlement, especially when such a country possesses already a kindred fauna of its own.

Fishes

Many fishes make periodic migrations for breeding purposes, which by their numbers and the distances travelled much resemble those of birds, but very little is known about these fishes. Take the incredible masses of herrings and their kindred; the collecting of the cod and its allies on their breeding-ground. According to D. S. Jordan (A Guide to the Study of Fishes, New York, 1905) some kinds are known mainly in the waters they make their breeding-homes, as in Cuba, southern California, Hawaii or Japan, the individuals being scattered at other times through the wide seas. The tunny, which has a world-wide distribution, arrives off the south coast of Portugal in the month of May; enormous numbers pass through the Straits of Gibraltar and support great fishing industries in the Mediterranean. In the month of August they return to the ocean! ( Apesca do Alum no Algarve em 1898, por D. Carlos de Braganza, Lisboa, 1899; with many maps). Many fresh-water fishes, as trout and suckers (quoting Jordan) forsake the large streams in the spring, ascending the small brooks where their young can be reared in greater safety. Still others, known as anadromous fishes, feed and mature in the sea, but ascend the rivers as the impulse of reproduction grows strong. Among such fishes are the salmon, shad, alewife, sturgeon and striped bass in American waters; Clupea alosa, the Allis shad, and C. finta, the Twait shad, Alepocephalus rostratus, the " maifisch " of the Rhine, in Europe. " The most remarkable case of the anadromous instinct is found in the king-salmon or quinnat ( Onchorhynchus tschawytscha ), of the Pacific coast. This great fish spawns in November, at the age of four years and an average weight of twentytwo pounds. In the Columbia river it begins running with the spring freshets in March and April. It spends the whole summer, without feeding, in the ascent of the river. By autumn the individuals have reached the mountain streams of Idaho, greatly changed in appearance, discoloured, worn and distorted. On reaching the spawning-beds, which may be moo m. from the sea in the Columbia, over 2000 m. in the Yukon, the female deposits her eggs in the gravel of some shallow brook. The male covers them and scrapes the gravel over them. Then both male and female drift, tail foremost, helplessly down the stream; none, so far as certainly is known, ever survive the reproduction act. The same habits are found in the five other species of salmon in the Pacific. The salmon of the Atlantic has a similar habit, but the distance travelled is everywhere much less, and most of the hook-jawed males drop down to the sea and recover, to repeat the act of reproduction." Few fishes are katadromous, i.e. their usual habitat is in rivers and lakes, but they descend into the sea for breeding purposes. The common eel is the classical example.

Insects

D. Sharp makes the following remarks ( Cambridge Nat. Hist. vi.): " Odonata are among the few kinds of insects that are known to form swarms and migrate. Swarms of this kind have been frequently observed in Europe and in North America; they usually consist of a species of the genus Libellula, but species of various other genera also swarm, and sometimes a swarm may consist of more than one species.

" Locust swarms do not visit the districts that are subject to their invasions every year, but as a rule only after intervals of a considerable number of years.. The irregularity seems to depend upon three facts, viz. that the increase of locusts is kept in check by parasitic insects; that the eggs may remain more than one year in the ground and yet hatch out when a favourable season occurs, and that the migratory instinct is only effective when great numbers of superfluous individuals are produced.. .. It is well established that locusts of the migratory species exist in countries without giving rise to swarms or causing any serious injuries.. .. When migration of locusts does occur it is attended by remarkable manifestations of instinct. Although several generations may elapse without a migration, it is believed that the locusts when they migrate do so in the direction taken by predecessors. They are said to take trial flights to ascertain the direction of a favourable wind, and that they alight and wait for a change. The most obscure point is their disappearance from a spot they have invaded. A swarm will alight on a locality, deposit there a number of eggs, and then move on. But after a lapse of a season or two there will be few or none of the species present in the spot invaded. In other cases they again migrate after growth to the land of their ancestors. It has been ascertained by the United States Entomological Commission that such return swarms do occur." See J. A. Palmeri, Om Foglarnes flyttningsvagar (Helsingfors, 1874). The same in German: Ober die Zugstrassen der Vogel (Leipzig, 1896). In this and the work of von Middendorff, already cited, reference is made to almost every important publication on the subject of migration, which renders a notice of its very extensive literature needless here, and a pretty full bibliographical list is given in Giebel's Thesaurus ornithologiae (i. 146-155). Yet mention may be made of Schlegel's Over het trekken der Vogels (Harlem, 1828); Hodgson's " On the Migration of the Natatores and Grallatores as observed at Kathmandu " in Asiatic Researches (xviii. 122-128), and Marcel de Serres's Des Causes des migrations des animaux et particulierement des oiseaux et des poissons (Harlem, 1842). This last, though one of the largest publications on the subject, is one of the least satisfactory. S. F. Baird's excellent treatise " On the Distribution and Migrations of 'North American Birds," Am. Journ. Sc. and Arts (2nd ser. 1866), pp. 78-90, 187-192, 337-347; reprinted Ibis 186 7, pp. 2 572 93. N. A. Severzoff, " Etudes sur le passage des oiseaux dans l'Asie centrale," Bull. Soc. Nat. (Moscow, 1880), pp. 234-287; Menzbier, " Die Zugstrassen der Vogel im europa,ischen Russland," op. cit. (1886), pp. 291-369; Palmeri, Referat fiber den Stand der Kenntniss des Vogelzuges, Intern. Ornith. Congr., Budapest, 1891; W. W. Cooke and C. H. Merriam, Report on Bird Migration in the Mississippi Valley, U.S. Dep. Agric.-Economic Ornithol., publ. 2 (Washington, 1888); Gaetke, Die Vogelwarte Helgoland (Braunschweig, 1891). In English: Heligoland as an Ornithological Observatory (Edinburgh, 1895); A. Newton, article " Migration," Dict. Birds (1893). (H. F. G.)