Migration (animal)

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

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

“ Why should a bird subject itself to the rigors of a long migratory journey twice a year if it can find all the requirements suitable for existence in one locality? It seems well to consider briefly the ends that are served by this annual round trip between breeding grounds and winter quarters. Obviously, the migratory habit enables a species to enjoy the summer of northern latitudes and to avoid the severity of winter. In other words, migration makes it possible for some species to inhabit two different areas during the seasons when each presents favorable conditions. ”

“ 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. ”

“ Much has been learned, and present knowledge is being placed on record, but it must be left to future study to clear away many of the uncertainties that continue to make bird migration one of the most fascinating subjects in the science of ornithology.
Each kind of bird seems to have its own reaction to its environment, so that the character of movement differs widely in the various species, and seldom do any two present the same picture. ”

“ The relation of the world's land masses to each other and the distribution and association of biotypes within these land masses influence the direction birds migrate. Topography may aid, hinder, or prevent the progress of a migrant depending on the bird's particular requirements. Old World migrants must contend with east-west tending mountain ranges and deserts, whereas New World travelers can proceed north and south across a landscape with its major mountain ranges and river systems oriented in the same direction as the birds migrate. ”

“ Probably northern migrants that winter in equatorial regions and beyond have their migratory urges controlled by similar rhythms or biological clocks. Also, no evidence suggests that the southward migration of birds is controlled by changing periods of light even among species such as white-crowned sparrows, for which this is a controlling factor in the spring. ”

“ 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 ”

“ Typical migration consists of definite movements that are repeated regularly year after year, and it is to these that the term is generally restricted. It is desirable, however, if only for purposes of comparison, that some account be taken of the movements of some other birds, which, while not typical, do possess some of the characteristics of true migration. Data on this subject are being collected through bird banding. ”

“ They never leave the holes until they take their departure, and then they go tumbling and rolling to the sea. There is no trace of this first stage of migration to be observed next morning, except sometimes in their outward course an odd one tumbles into our yards, where they are found. ”

“ There probably is no single aspect of the entire subject of bird migration that increases our admiration so much as the unerring certainty with which birds cover thousands of miles of land and water to come to rest in exactly the same spot where they spent the previous summer or winter. Records from birds marked with numbered bands offer abundant proof that the same individuals of many species will return again and again to identical nesting or winter feeding sites. ”

“ 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. ”

“ In fact, the choice of migration highways is so wide that it seems as if the routes of no two species coincide. Differences in distance traveled, in time of starting, in speed of flight, in geographical position, in latitudes of breeding and of wintering grounds, and in other factors, all contribute to this great variation of migration routes. Nevertheless, there are certain factors that serve to guide the avian travelers along more or less definite lines, and it is possible to define general lines of migration for the majority of species. ”

“ The migration season is full of peril for birds. Untold thousands of smaller migrants are destroyed each year by storms and attacks by predatory animals. These mortality factors, and others, help keep bird populations in check. Perils of migration are among these causes. ”

“ It affects different birds in different ways. In North America, migratory movements are continent wide, and no evidence has indicated any particular part of the landscape influences all birds in the same manner. Certain bird populations may use general areas in migration, but they are usually not rigidly restricted to them because of topography. ”

“ Like the bird's other habits its migratory behavior is just as characteristic as the color of its plumage and, like it, evolved through natural selection because it was advantageous for the survival of the population. Its origin has been thought by some to be a mystery locked in past ages, but by study of the history of how birds came to occupy their present ranges, information becomes available which suggests theories that may be developed and explored. ”

“ We are sometimes told that we must seek the origin of migration in the physical changes that have occurred in the ancient history of the earth—in glacial conditions which gradually forced birds to the south, or in the "stability of the water and mobility of the land" which brought about a gradual separation of the feeding area from the breeding area—and which continued for a sufficient length of time to lead to the formation of an instinct, and that the instinct persists because it is serviceable in promoting the welfare of the race. ”

“ In conclusion then, we can say that the weather may be the impetus for migration for many species, but it cannot stimulate a bird to migrate unless it is physiologically prepared. Arrivals on the ground are not necessarily indicative of the number of birds passing overhead. During the fall, peak migrations usually follow the passage of a cold front when the temperature is falling, the barometer is rising, winds are from the west or northwest, and the sky is clearing. ”

“ The migration of birds as it is known today had its beginning in times so remote that its origins have been entirely obscured, and it can be interpreted now only in terms of present conditions. The causes underlying migration are exceedingly complex. The mystery that formerly cloaked the periodic travels of birds, however, has been largely dispelled through the fairly complete information that is now available concerning the extent and times of the seasonal journeys of most of the species. ”

“ 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.” ”

“ It may be questioned, what object could the birds have in risking an oversea voyage, away from chance of food and hope of rest, when the land-bridge remained open for them? Each individual or group of individuals which arrived at any particular place a little in advance of the migrating multitudes of its own species, or others which fed upon the same kind of food, would certainly gain advantage, and would be the most likely to develop strong flight and the power of endurance in its descendants ”

“ The causes of the annual migrations of birds have received much study and been the subject of much speculation. The general consensus of opinion in this connection seems to be that the birds are controlled largely by what we in our ignorance term instinct. The true beginning of the migration seems to be in the fall, when the birds are driven from their homes by cold or, perhaps more accurately in most cases, by scarcity of food. This, however, is not the whole story, since many species start southward before cold weather approaches and while food is yet abundant. ”

“ The migration of birds is as a rule between the breeding area or home and the winter quarters, but there are many migrants which never reach breeding quarters in spring, and many others which leave the regular breeding quarters or the place of residence in winter to perform a very real migration under peculiar stress of circumstances. ”

“ 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. ”

“ 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 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. ”

“ It should be remembered, however, that while homing may involve flight from a point that the bird has never before visited, the flight is always to a known point—that is, the bird's nest—while, on the other hand, the first migratory flight is always from the region of the bird's birth to a region it has never before visited. ”

“ It is easy to see how the migration that had once taken place over continuous land would be kept up, first over lagoons and marshes, then over a narrow channel, and subsequently over a considerable sea, no one generation of birds ever perceiving any difference in the route. ”

“ They may play at and actually perform a migration which is so closely bound up with the life of the species. That this impulse has not always sufficient strength to force them to perform the Whole journey is apparent from the fact that many non-breeders, young or sexually mature, on their northward journey through our islands or along our coasts, never reach the breeding area; the food supply on the way attracts them more than the memory of home; they linger with us until the breeding season is over and the return journey has begun. ”