Migration (bird)

“ 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. ”

“ 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. ”

“ There probably is no single aspect of the entire subject of bird migration that challenges 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. The records from birds marked with numbered bands offer abundant proof that the same individuals of many species will return again and again to their identical nesting sites. ”

“ 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. ”

“ 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.
It has been frequently observed that migrating birds have a tendency to follow major topographic lines on the earth's surface when their trend is in the general direction of the birds' journey. Bird migration is generally thought of as a north-and-south movement, with the lanes of heavier concentration following the coasts, mountain ranges, and principal river valleys. ”

“ In addition, if daylength influences when birds are stimulated to migrate, why should they not all leave the same locality at the same time? Or, if weather controls the departure of birds from a given area, should not all the migrants leave when conditions are optimal and refrain from departing when conditions are not so? Actually, the conditions that place a bird in a disposition to migrate are probably the result of a combination of factors affecting different species - 16 - differently. ”

“ 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. ”

“ 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. ”

“ 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.” ”

“ 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. ”

“ To increase such support, information must be more generally available on that little understood but universally fascinating subject of bird migration. A brief presentation of facts on the migratory habits of the birds, scientifically gathered by the Bureau of Biological Survey over many years, will be helpful to bird-study classes, to conservation organizations, and to farmers and others individually interested in the welfare of the birds. ”

“ The actual status of these individual birds is uncertain. In this group too we have the Greenland wheatear, so closely allied to our familiar early migrant that, unless the bird can be measured, its identification is uncertain.
5. Irregular Migrants: birds which may be classed in other groups. Some of these are really winter residents, but their visits are so irregular that they may for convenience be classed with spasmodic or occasional invaders, such as Pallas's sand-grouse, which arrive at uncertain intervals in large numbers. ”

“ In the first place, it may be well to define briefly certain phases of bird movement that are often overlooked or confounded with the generally accepted understanding of what migration covers. In the popular mind, and, it may be added, this is the correct view, a migratory species is one that regularly resorts to a given locality for the purpose of rearing its young, after which both old and young retire to some other, often widely different locality, where they pass the time before the next breeding season. ”

“ Every year professional biologists and voluntary cooperators, working under permit, place bands on thousands of birds, game and nongame, large and small, migratory and nonmigratory, with each band carrying a serial number and the legend, NOTIFY FISH AND WILDLIFE SERVICE, WASHINGTON, D.C., or on the smaller sizes, an abbreviation. When a banded bird is reported from a second locality, a definite fact relative to its movements becomes known, and a study of many such cases develops more and more complete knowledge of the details of 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ American robins and bluebirds may leave abundant food in the South and press northward when food supplies there are almost entirely lacking and severe cold and storms are likely to cause their wholesale destruction. Regularity of arrival and departure is one of the most impressive features of migration, and since birds travel in a rather strict accordance with the calendar, we might ask: "What phenomena, other than the regular changes in length of day, occur with sufficient precision to act as a stimulus for migration?" ”

“ Flying along the insect-teeming shores of the Gulf of Mexico, the 2,000 extra miles that are added to the migration route are but a fraction of the distance that these birds cover in pursuit of their food.
Although most of our smaller birds make their longest flights at night, close observation shows that travel is continued to some extent by day. This is particularly the case during the latter half of a migratory season, when the birds manifest a desire to hasten to their breeding grounds. ”

“ The popular application of a term does not do away with the need of definition, especially as there are many complicated phases of migration. 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. ”

“ Many song sparrows, meadowlarks, blue jays, and other species make such short migrations that the movement is difficult to detect because individuals, possibly not the same ones, may be found in one area throughout the year while other individuals that move south may be replaced by individuals from the north. Information on different movements of this type, within a species, can be gained by observing birds marked with numbered bands, colored materials, or identification of racially distinct museum specimens. ”

“ Of the way in which Migration is performed there is still much to learn–but one thing is certain, all Birds do not migrate in the same manner. Some gather in flocks, great or small, others seem to accomplish their northward journey in pairs, or at any-rate arrive at their breeding-quarters already paired. Some undoubtedly voyage by night, others may be seen to travel by day. Of the Birds which in spring arrive unpaired, it is now incontestable that the males outstrip or precede the females. ”

“ If we plot numbers observed against time, the rising and receding curve takes the form of a bell. In the northern part of the United States there are two general migration waves. The first one in early spring consists of "hardy" birds including many of our common seed eaters like the finches, sparrows, and others. ”

“ When birds start their southward migration, the movement necessarily involves the full width of the breeding range. Later, there is a convergence of the lines of flight taken by individual birds, owing to the conformation of the land mass, and as the species proceeds southward the width of the occupied region becomes less and less. ”

“ If this be true, it explains why in southern latitudes so many of the earliest fall arrivals are females that may have deserted the breeding grounds after the eggs were laid.
Migratory flights are frequently accomplished in close flock formation, as with the shore birds, blackbirds, and waxwings, and especially -34- some of the sparrows—the snow buntings, longspurs, juncos, and tree sparrows. Other species, however, though they travel in flocks, maintain a very loose formation; examples are the turkey vultures, the hawks, swifts, blue jays, swallows, warblers, and bluebirds. ”

“ The period of the migratory flight in the autumn of any particular genus or species is most probably referable to two causes: the first one of temperature, affecting the time of nesting; the second is the period at which the young arrive at maturity, or rather that period when they throw off paternal control or are thrown off themselves. When able to act independently and procure food on their own account, they flock together and migrate in a body. We know that, with rare exceptions, the young of the year migrate some weeks in advance of the parent birds ”