Biological migration

“ We know that insects and plants are so transported. Birds and ocean-currents carry seeds from land to land. Insects on a blade of grass or a fallen bough are carried down a river by the current to found colonies of their race far from their place of origin. And such circumstances give rise to races and varieties among species modifiable by the peculiarities of their new localities. The accidental migrations of man may be considered as belonging to the epoch of culture-migration, since they must more usually have occurred with a race advanced in the art of navigation. ”

“ Only to a certain extent can it be safely contended that the present route of a species is an indication of its earlier journeys, or that the direction of original dispersal is recapitulated in the present line of migration. Heredity, experience, and imitation would certainly tend to preserve and confirm the general direction; the shortest and easiest passage from food-base to food-base would become an hereditary route, unless circumstances arose which caused a change. ”

“ Band recoveries, netting records, and personal observations help us to critically examine migration routes and probe deeper into the origin and evolution of these pathways. We are beginning to realize certain deviations occur from the "normal" north and south movements expected in most species. ”

“ When we can feel assured that all the individuals of the same species, and all the closely allied species of most genera, have, within a not very remote period descended from one parent, and have migrated from some one birth-place; and when we better know the many means of migration, then, by the light which geology now throws, and will continue to throw, on former changes of climate and of the level of the land, we shall surely be enabled to trace in an admirable manner the former migrations of the inhabitants of the whole world. ”

“ I am far from supposing that all the difficulties in regard to the distribution and affinities of the identical and allied species, which now live so widely separated in the north and south, and sometimes on the intermediate mountain-ranges, are removed on the views above given. The exact lines of migration can not be indicated. We can not say why certain species and not others have migrated; why certain species have been modified and have given rise to new forms, while others have remained unaltered. ”

“ There is a similar liability to error in interpreting migrations on the basis of a single or a few species, for a single species or a few species may be transported by unusual or accidental means, so to speak, when there is no normal pathway for general migration, and when no systematic migration takes place. ”

“ Some degree of mobility is present in 73 per cent of the species common to these regions, while of the total number of species in which the mode of migration is evident, viz., 95, 66 per cent are wind-distributed, 20 per cent animal-distributed, and 14 per cent are water-distributed. ”

“ In some species the migration is so prolonged that the first arrivals in the southern part of the breeding range will have performed their parental duties while others of that species are still on their way north. ”

“ In some years it is extremely interesting to watch the steady influx of migrants of various species, often indicative, to my mind, of a long and severe winter ensuing. At other times migration goes on scantily or spasmodically. ”

“ 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 ”

“ All this takes place by a very gradual process, requiring thousands of years, thus giving ample time for heredity to infuse the migratory habit into the nature of the birds. Every new generation would learn the route and other needful details from their predecessors, and thus the process would go on in an unending circuit year by year.
After the foregoing was written, my attention was called to the following quotation from Dr. J. A. 80 Allen’s valuable paper on the “Origin of the Instinct of Migration in Birds.” ”

“ But, again, many other organisms may make the journey from one continent to another together with the birds and insects. Of course all parasites, the number of which is legion, fleas, lice, mites, moulds, etc., migrate with the organisms upon which they live. In the earth which often remains sticking to the claws of birds there are also small animals and plants or their germs. Thus the voluntary or involuntary migration of a single larger organism may carry a whole small flora and fauna from one part of the earth to another. ”

“ This suggests that if we are correct in assuming that birds first appeared in a tropical climate, and from such climate migration started, it is probable that by gradually prolonging their northern visits and accustoming themselves to northern insect and vegetable life, these regions became populated by their resident species. It is evident that the present migratory species are simply compelled to return, and three compelling causes are demonstrable. ”

“ With our present knowledge of bird migration it is difficult at best to recognize distinct broad belts of migration down the North American continent encompassing groups of distinct populations or species. ”

“ In fact, with the entire continent of North America crossed by migratory birds, the different groups or species frequently follow lines that may repeatedly intersect those taken by others of their own kind or by other species. The arterial or trunk routes, therefore, must be considered merely as indicating paths of migration on which the tendency to concentrate is particularly noticeable. ”

“ Now, in studying the composition of a fauna, and especially its origin, it is of the utmost importance to be able to determine approximately the percentage of accidental arrivals and of the ordinary migrants—that is to say, of those which have reached the country owing to accidental distribution, and of animals which have adopted the usual course of migration. ”

“ With marine animals of all kinds, we may safely infer a large amount of migration during climatal and other changes; and when we see a species first appearing in any formation, the probability is that it only then first immigrated into that area. It is well known, for instance, that several species appeared somewhat earlier in the palæozoic beds of North America than in those of Europe ”

“ Where, as on mountain ranges, we find, contrary to the general rule, the same species in different localities, the migration from the door of the ark loses all semblance of probability, unless we are pleased to imagine that creatures, now without the instinct of migration, for a long time possessed it, and roamed about the world through many a sultry plain to pick out a hill-side here and there with a temperature suited to their constitutions. ”

“ Darwin, I believed that all changes of structure in organisms result from changes of function, I held that the cause of such changes of function is migration. Assuming as the antecedent of migration a great geologic change, such as upheaval of the East Indian Archipelago step by step into a continent, it was argued, in an essay I then wrote, that, subjected primarily to new influences in its original habitat, each kind of plant and animal would secondarily be subjected to the altered conditions consequent on spreading over the upheaved regions. ”

“ Many have insisted that wherever migrants cross the sea they do so along submerged coast-lines or over submerged land-bridges, arguing that the gradual evolution which has made the advantageous adoption of a habit of migration possible was unable to eliminate the hereditary tendency to follow the exact route by which their ancestors passed from place to place. ”

“ Traditions that have lived for countless generations may die overnight if experienced individuals are lost or no longer active (Hochbaum 1955) ; migration patterns remain constant only as long as the factors influencing these - 97 - patterns remain constant. But the landscape and the interacting ecological stresses are forever changing, and we would expect the adaptive behavior of birds to respond with them. One of these responses to an expanding habitat is colonization of new territory and expansion of a species' range with accompanying development of a migratory habit. ”

“ In the case of lands newly raised above the sea and connecting formerly separated areas, it is necessary that they should first be taken possession of by vegetation, before they can become passable by animals, for the migration of mammals from continent to continent is an entirely distinct phenomenon from the annual migration of birds. ”

“ We have also learned that, time being given, animals and plants manifest wonderful powers of migration, that they can vary within considerable limits without ceasing to be practically the same species, and that under certain conditions they can endure far longer in some places than in others. We also see evidence that it is not on limited islands, but on the continents, that land animals and plants have originated, and that swarms of new and vigorous species have issued from the more northern regions in successive periods of favourable Arctic climate. ”

“ Little imagination is required to see how enlightening it might be could we analyze thoroughly the patterns of all migrating species. When the detailed facts are available, it seems likely that general trends will emerge which may be of great significance to the study of migration in general. A final point which must eventually be clarified is determination of the extent of variability in the pattern of each species from year to year and locality to locality. ”

“ The old pathways have apparently become implanted as part of the migratory instinct in all members of particular populations even after extending their ranges considerable distances from the original. ”

“ The general anatomical and physiological attributes of birds enable them to develop more diverse and spectacular migratory behavior than any other group of animals. Their potential for long sustained flights is of primary importance in pre-adapting birds to successful migrations. Migration has long since become a definite hereditary habit of many species of birds that recurs in annual cycles, evidently because of physiological changes which prompt a search for an environment suitable for reproduction and survival. ”

“ He who rejects it, rejects the vera causa of ordinary generation with subsequent migration, and calls in the agency of a miracle. It is universally admitted, that in most cases the area inhabited by a species is continuous; and when a plant or animal inhabits two points so distant from each other, or with an interval of such a nature, that the space could not be easily passed over by migration, the fact is given as something remarkable and exceptional. The capacity of migrating across the sea is more distinctly limited in terrestrial mammals, than perhaps in any other organic beings ”

“ 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. ”

“ There is also infinite variety in the routes covered during migration by different species. In fact, the choice of migration highways is so wide that is seems as if the routes of no two species coincide. ”

“ In the numerous cases in which the same group now occupies countries separated by oceans or seas, by lofty mountain-chains, by wide deserts, or by inhospitable climates, we have to consider how the migration which must certainly have taken place has been effected. It is possible that during some portion of the time which has elapsed since the origin of the group the interposing barriers have not been in existence; or, on the other hand, the particular organisms we are dealing with may have the power of overpassing the barriers, and thus reaching their present remote dwelling-places. ”

“ In those cases the birthplace may have been the original home, before the migratory habit was acquired, and scarcity of food the reason why it was acquired; and 'the influence of scarcity in causing migration is well shown by the occasional or irregular migrations of certain prolific animals which do not ordinarily leave their birthplaces, although, when these become overstocked, migrations take place, just as human colonists go out from thickly settled countries to find new room for growth in foreign lands. ”

“ Even where migration does not bring it face to face with new competitors or new enemies or new prey, it inevitably experiences new proportions among these. Further, an expanding species is almost certain to invade more than one adjacent region. Spreading both north and south, or east and west, it will come among the plants and animals, here of a level district and there of a hilly one—here of an inland tract and there of a tract bordered by the sea. ”

“ The effective conversion of migration into invasion is greatest when the movement is continuous, and least when it is discontinuous, since, in the latter, species are usually carried not only out of their particular habitat but even far beyond their geographical area, and the migration, instead of being an annual one with the possibility of gradual adjustment, may not recur for several years, or may, indeed, never take place again. ”