Migration (biology)

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ I hold that the origin of migration is not to be found merely in conditions peculiar to a remote past, but that the conditions inhere in the organic complex of the bird, and are thus handed down from generation to generation. Starting with this assumption I examined the behaviour which normally accompanies the seasonal life-history of the individual, and found, in that behaviour, manifestations of cyclical change leading to definite biological consequences. ”

“ 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. ”

“ 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. ”

“ Both of these theories assume that migration is an ingrained habit, but both have been criticized on biological and geological grounds, so neither should be accepted without qualification as definitely accounting for the origin of bird migration. ”

“ Migration seems to be a general instinct in the animal world, developed when a species becomes enormously abundant. At such times this instinct apparently overcomes all others and the creatures move on regardless of obstacles and conditions that may mean certain death to the vast majority. Such migrations among mammals have often been recorded, one of the most notable examples being that of the little lemmings which migrate at periodical intervals in a way which has often been described. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ As we have seen from the foregoing discussion, migration exists in many forms throughout the world and probably arose to satisfy many different needs in different orders of birds at the same time. New pattens, traditions, and routes are arising today as well as disappearing. Currently, the migration patterns we see are a composite result of historic influences mixed with present day influences. Even though the migration of several different species may be very similar, the patterns exhibited today can be the result of quite different evolutionary processes. ”

“ Migration owes its origin to the potentiality of flight, enabling birds to advantage themselves by extended dispersals, which through heredity become instinctive, regular and periodical. Geological changes, especially the passing away of the glacial epoch, only influenced by opening up new lands for summer colonisation, but climatic conditions prevented these lands from becoming permanent abodes and fostered the habit of periodical migration. ”

“ The direction of migration is determinate, except in the case of those distributive agents which act constantly in the same direction. The general tendency is, of course, forward, the lines of movement radiating in all directions from the parent area. This is well illustrated by the operation of winds which blow from any quarter. In the case of the constant winds, migration takes a more or less definite direction, the latter being determined to a large degree by the fruiting period of any particular species. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Migration introduces them to an entirely new state of existence, temperature, food, and antagonists. The migrating animals are replaced by others, which likewise experience new surroundings; and thus the extirpation of a single plant may determine a long series of changes. Instances of this kind are not uncommon. What must these changes have been throughout the remote ages which have turned sea-shore mud into uplifted mountain chains, and sunk long-stretching, sunny hills into the ocean depths! ”

“ 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.” ”

“ 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. ”

“ By selection of staggered peak migration dates, evolution has distributed the members of this family more or less evenly throughout the entire season. Likewise, in the eastern Mediterranean area, we find a similar situation in spring migration for three closely related buntings ”

“ There is no invariable law governing the distance of migration, although in general it is found that where a species has an extensive range, the subspecies that breed farthest north go farthest south to spend the winter. ”

“ 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. ”

“ 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. ”

“ Migration is the act of changing an abode or resting place, the wandering or movement from one place to another, but technically the word is applied to the passage or movement of birds, fishes, insects and a few mammals between the localities inhabited at different periods of the year. The wandering of a nomadic tribe of men is migration; the mollusc, Wandering from feeding ground to feeding ground in the bed of the ocean, migrates ”

“ Migration has long since become a definite hereditary habit that recurs in annual cycles, probably because of physiological stimuli associated with the reproductive period. In seeking its origin it is necessary to study the history of the birds' occupation of their present ranges, and from the information available to consider what appear to be reasonable theories. ”

“ 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. ”

“ 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. ”

“ While it is beyond question that certain general directions of flight are consistently followed by migratory birds, it is well to remember the term "migration route" is to some extent a theoretical concept referring to the lines of general advance or retreat of a species, rather than the exact course followed by individual birds or a path followed by a species with specific geographic or ecological boundaries. ”

“ 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 ”

“ In order to grasp the problems of migration it is necessary to get rid of the puerile and insular aspect of the subject, namely that migrants are merely those birds which come to us, like the swallow and cuckoo in the spring, and those, like the fieldfare and brambling, which visit us in winter but are not with us in summer. ”

“ 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. ”