Bird migration

“ In fact, bird migration has been described as a phase of geographic distribution wherein there is a more or less regular seasonal shifting of the avian population caused by the same factors that determine the ranges of the sedentary species. If this view is correct, then it must be recognized that the far-reaching works of man in altering the natural condition of the earth's surface can so change the environment necessary for the well being of the birds as to bring about changes in their yearly travels. ”

“ 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. ”

“ As bird populations become more and more migratory, we might expect their flight capabilities to be enhanced accordingly. These changes in morphology are readily seen in wing shape. Several groups of birds have closely related species or populations some of which are migratory and others sedentary. The sedentary species or populations have more rounded wings because of the relative length of the wing quills. On the other hand, populations that migrate great distances, such as albatrosses, falcons, swifts, various shorebirds, and terns, have more pointed wings. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ What dangers do they face on their round-trip flight and in their winter homes? These and other questions on the migratory habits of birds puzzle the minds of many who are interested in the feathered species, whether it be the farmer who profits by their tireless warfare against the weed and insect pests of his crops, the bird student who enjoys an abundance and variety of feathered inhabitants about him, or the hunter who wants a continuation from year to year of the sport of wild-fowling. ”

“ It is an observed fact that some birds in a migratory movement alter their course to travel along a leading line, but whether this feature in the landscape caused the migrants to change their course is only theory (Thomson 1960) . Besides topography, many other factors can influence this type of flight behavior including weather, wind speed and direction, time of day, species, age, and experience of the bird (Murray 1964) .
Large bodies of water constitute real barriers to soaring birds dependent on thermals and air currents. ”

“ I doubt not that, throughout the ages, geological changes have been an important factor in directing or limiting the scope of migration, and moreover are so still; just as climatic changes and the relative abundance or scarcity of enemies have influenced the course of its evolution. These are all contributory factors operating in the external environment. But there are, besides, internal factors which form part of the inherited constitution of the bird, and, being passed on from generation to generation, afford the conditions under which migration is constantly being renewed. ”

“ Today, such conditions are found only in more southern regions where migrations are much shorter or nonexistent. Gradually, however, in the Northern Hemisphere the glacial ice fields advanced southward, causing a southward movement of conditions favorable to northern birds, until finally all bird life was confined to southern latitudes. As the ages passed, the ice cap gradually retreated, and each spring the birds whose ancestral home had been in the North moved in again to fill newly opened breeding - 96 - habitat only to be driven south again at the approach of winter. ”

“ 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 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. ”

“ 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 few or many birds we meet with, either on the coast or inland, resting on passage, may represent a lost or wandering party of stragglers or weaklings from a vast army which has passed over; they may or may not be on the route or course normally followed by the majority. The cartography of bird migration is a study in itself. Mr Abel Chapman, describing his experiences in the Mediterranean, says—"For forty hours we were passing across (or beneath) the lines of an army of migrants—say 500 miles in width ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ Birds are adapted in their body structure, as no other creatures, to life in the air. Their wings, tails, hollow bones, and internal air sacs all contribute to this great faculty. These adaptations make it possible for birds to seek out environments most favorable to their needs at different times of the year. This results in the marvelous phenomenon we know as migration—the regular, seasonal movement of entire populations of birds from one geographic location to another. ”

“ Today we realize that birds do not migrate by "hitching" rides with other birds and that the scope of the migration phenomenon is worldwide, not simply limited to the United States, the Northern Hemisphere, or the world's land masses. The migration heritage is developed just as extensively in Old World warblers migrating to and - 6 - from Europe and Africa as in our wood warblers traveling from Canada and the United States to South America and back. ”

“ The truth lies in the opposite direction, as even small land birds are so little averse to ocean voyages that they not only cross the Gulf of Mexico at its widest point, but may even pass without pause over the low, swampy coastal plain to the higher regions beyond. Under favorable conditions birds can fly when, where, and how they please. Consequently the distance 4 covered in a single flight is governed chiefly by the food supply. Exhaustion, except as the result of unusual factors, cannot be said to be an important peril of migration. ”

“ For more than half a century the Fish and Wildlife Service and its predecessor, the Biological Survey, have been collecting data on the interesting and important phenomenon of the migration of North American birds. The field men of the Service have gathered information concerning the distribution and seasonal movements of the -3- different species in many extended areas, from the Arctic coast south to the pampas of Argentina. ”

“ 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?" ”