Read Ebook: The Migration of North American Birds (1935) by Lincoln Frederick Charles

Font size: 10 px 15 px 20 px 25 px 30 px 35 px

Background color: BlackWhiteGrayLight GrayDark GrayDim Gray

Text color: BlackWhiteGrayLight GrayDark GrayDim Gray

Apply/Save settings

Ebook has 266 lines and 33502 words, and 6 pages

Page

Introduction 1

Mystery of migration 3 Historical accounts 3 Advantages of migration 4 Theories of causes of migration 5

When birds migrate 7 Movements of species and groups 8 Nocturnal and diurnal migration 11

How birds migrate 13 Speed of flight andspeed of migration 13 Altitudes at which birds travel 22 Orientation 23 Segregation during migration 25

Where birds migrate 27 Distances of migration vary 27 Short and undetermined migration 27 Variable migrations wi thin species 28 Fall flights not far south of breeding ranges 29 Long-distance migrations 30

Routes of migration 33 Wide and narrow migration lanes 34 Atlantic oceanic route 39 Atlantic coast route and tributaries 42 Mackenzie Valley-Great Lakes-Mississippi Valley route and tributaries 45 Pacific coast route 47 Pacific oceanic route 49 Arctic routes 52

Evolution of migration routes 52

Vertical migration 55

Vagrant migration 55

Perils of migration 56 Storms 56 Aerial obstructions 57 Exhaustion 59

Influence of the weather on migration 59

Problems of migration 61 Banding studies 61 Movements of residents 62 Migration of the white-throated sparrow 63 Migration of the yellow-billed loon 63

Conclusions 65

Bibliography 66

Index 69

Where do the birds go each fall that have nested in our dooryards and frequented the neighboring woods, hills, and marshes? Will the same ones return again to their former haunts next spring? 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. Lack of information on the subject may mean the loss of an important resource by unconsciously letting it slip from us. Ignorance of the facts may be responsible for inadequate legal protection for such species as may urgently need it. More general knowledge on the subject will aid in the perpetuation of the various migrants, the seasonal habitats of some of which are in grave danger from man's utilization, sometimes unwisely, of the marsh, water, and other areas they formerly frequented.

The migrations of birds were probably among the first natural phenomena to attract the attention and intrigue the imagination of man. Recorded observations on the subject date back nearly 3,000 years, to the times of Hesiod, Homer, Herodotus, Aristotle, and others. In the Bible are several references to the periodic movements of birds, as in the book of Job , where the inquiry is made: "Doth the hawk fly by Thy wisdom and stretch her wings toward the south?" Jeremiah , wrote: "The stork in the heavens knoweth her appointed time; and the turtle , and the crane, and the swallow, observe the time of their coming." And the flight of quail that saved the Israelites from starvation in their wanderings in the wilderness of Sinai is now recognized as a vast movement of migratory quail between their breeding grounds and their winter home in Africa.

Throughout the ages the return flights of migratory birds have been important as a source of food after a lean winter, and as the harbinger of a change in season. The arrival of certain species has been heralded with appropriate ceremonies in many lands, and among Eskimo and other tribes the phenomenon to this day is the accepted sign of the imminence of spring and of warmer weather. The pioneer fur traders in Alaska and Canada offered rewards to the Indian or Eskimo who saw the first goose of the spring, and all joined in jubilant welcome to the newcomer.

Always hunted for food, the large flocks of ducks and geese became objects of the enthusiastic attention of an increasing army of sportsmen as the North American Continent became ever more thickly settled. Most of the nongame species were found to be valuable also as allies of the farmer in his never-ending warfare against weed and insect pests. The need for laws protecting the valuable game and nongame birds and for regulated hunting of the diminishing game species followed as a natural course. In the management of this wildlife resource it has become obvious that continuous studies must be made of the food habits of the various species, their environmental needs, and their travels. Hence bird investigations are made by the Biological Survey, the Bureau charged by Congress under the Migratory Bird Treaty Act with the duty of protecting those species that in their yearly journeys pass back and forth between the United States and Canada.

For half a century the Biological Survey has been collecting data on the interesting and important phenomenon of the migration of North American birds. The field men of the Bureau have gathered information concerning the distribution and seasonal movements of the different species in many extended areas, from the Arctic coast south to the pampas of Argentina. Supplementing these investigations is the work of hundreds of volunteer ornithologists and bird students throughout the United States and Canada, who each year, spring and fall, forward to the Bureau reports on migrations observed in their respective localities. Added, to the mass of data thus assembled is a rapidly growing file of records of birds that have been banded and of the subsequent recovery of the marked individuals.

These data, together with other carded records gleaned by the Biological Survey from a vast literature, constitute a series of files that now contain well over 2,500,000 entries, easily the greatest existing accumulation of information pertaining to the distribution and movements of North American birds. Not only do the facts thus assembled form the basis of regulatory action for the protection of the birds, but they also make it possible to publish scientific accounts of the ranges and migrations of the different species. They furnish the basis of this publication.

The several important bird-protective measures adopted by State and Federal Governments, particularly those having as their objective the conservation of the migratory song, insectivorous, and game species, can be effective only if they have intelligent public support. 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.

HISTORICAL ACCOUNTS

Of observers whose writings are extant, Aristotle, naturalist and philosopher of ancient Greece, was one of the first to discuss the subject of bird migration. He noted that cranes traveled from the steppes of Scythia to the marshes at the headwaters of the Nile, and that pelicans, geese, swans, rails, doves, and many other birds likewise passed to warmer regions to spend the winter. In the earliest years of the Christian era, the elder Pliny, Roman naturalist, in his Historia Naturalis, repeated much of what Aristotle had said on migration, and added comments of his own concerning the movements of the European blackbird, the starling, and the thrushes.

In spite of the keen perception shown in some of his statements, Aristotle also sponsored some superstitions on bird migration that persisted for several centuries. One of these, that of hibernation, became so firmly rooted that in 1878, the American ornithologist Coues listed the titles of no less than 182 papers dealing with the hibernation of swallows . The hibernation theory accounted for the autumnal disappearance of certain species of birds as their passing the cold season in a torpid state, hidden in hollow trees, caves, or the mud of marshes. Aristotle ascribed hibernation not only to swallows but also to storks, kites, doves, and others. Some early naturalists wrote fantastic accounts of flocks of swallows seen congregating in the marshes until their accumulated weight bent into the water the reeds on which they clung and thus submerged the birds. It was even recorded that when fishermen in northern waters drew up their nets they sometimes had a mixed "catch" purported to consist of fish and hibernating swallows. Clarke quotes Olaus Magnus, Archbishop of Upsala, who in 1555 published a work entitled "Historia de Gentibus Septentrionalibus et Natura", wherein he observed that if swallows so caught were taken into a warm room they soon begin to fly about but would live only a short time.

The hibernation theory survived for more than 2,000 years and is still occasionally repeated by credulous persons to account for failure to locate definitely the winter home of the chimney swifts , which each autumn gather in immense flocks in southern Georgia and northern Florida and then suddenly disappear. Thereare, however, records of occurrence during migration for a few points in the West Indies, Mexico, and Central America, and it is probable that these birds spend the winter season in the great rain-forest area of the Amazon Valley in Brazil, passing most of the daytime high in the air with other swifts that are local residents.

Aristotle was also the originator of the theory of transmutation, basing it upon the fact that frequently one species will arrive from the north just as another species departs for more southerly latitudes. From this he reasoned that although it was commonly believed that such birds were of two different species, there really was only one, and that this one assumed the different plumages to correspond with the summer and winter seasons.

Some who easily accepted the disappearance of the larger birds as migratory travelers were unable to understand how the smaller species, some of them notoriously poor fliers, could make similar journeys. They contended that the larger species, as the storks and cranes, carried their smaller companions as living freight. In some of the Mediterranean countries it is still believed that these broad-pinioned birds serve as aerial transports for the hosts of small birds that congregate on the shores of the Mediterranean Sea, awaiting opportunity for this kind of passage to their winter homes in Africa. Similar beliefs are found among some tribes of North American Indians.

ADVANTAGES OF MIGRATION

Nevertheless it cannot be said that the winter or summer areas are entirely unsuited to the requirements of every migrating species at other seasons, for some individuals pass the winter in areas that are frequented only in summer by other individuals of their species. The extensive breeding ranges of such species present wide climatic variations, so that some individuals may actually be resident in a region where others of their kind are present only in winter.

The tendency in many species to move southward at the approach of winter is not always due to the seasonal low temperatures, since experiments have demonstrated that many summer insect feeders, when confined in outdoor aviaries, comfortably withstand temperatures far below zero. The main consideration is the depletion of the food supply, caused either by disappearance or hibernation of insects, or by the mantle of snow or ice that prevents access to the seeds and other forms of food found on or close to the ground or submerged in water. Possibly also the shortened hours of daylight materially restrict the ability of the birds to obtain sufficient food at a time when the cold requires an increased supply to maintain body heat. It is noteworthy that chickadees and some other of our smaller birds have no fear of Arctic weather, as their food supplies are mainly arboreal and so are always available. Also, when there is a good supply of food in the form of pine seeds in Canadian woods, nuthatches and crossbills will remain through the winter. When these birds appear abundantly in winter at points in southern latitudes, it may be concluded that there is a shortage of their food in the North, or that they have been lured farther south by the greater abundance of this food there.

THEORIES OF THE CAUSES OF MIGRATION

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. The two now most commonly accepted are diametrically opposed to each other.

NORTHERN ANCESTRAL HOME THEORY

According to one of these, nonmigratory birds swarmed over the entire Northern Hemisphere in earlier ages, the conditions of food and habitat being such as to permit them to remain in their haunts throughout the year. The entire northern area then afforded the two important avian requirements--suitable breeding conditions and a year-long food supply. This is the condition today in the Tropics, and it is noteworthy that many tropical birds are nonmigratory. Gradually, however, in the Northern Hemisphere the glacial ice fields advanced southward, forcing the birds before them, until finally all bird life was concentrated in southern latitudes. As the ages passed and the ice cap gradually retreated, each spring the birds endeavored to return to their ancestral homes in the North, only to be again driven south at the approach of winter. As the size of the ice-covered area diminished, the journeys made became ever longer, until eventually the habits of migration were fixed to accord with the climatic conditions of the present age.

Thus, this theory supposes that today migratory birds follow the path of a great racial movement that took place in a distant past, associated with advances and recessions of the ice. The actions of the birds themselves lend some support to this theory, as every bird student has noted the feverish impatience with which certain species push northward in spring, sometimes advancing so rapidly upon the heels of winter as to perish in great numbers when overtaken by late storms. It is probable, however, that at that season the reproductive impulse urges the birds on to their northern breeding grounds.

SOUTHERN ANCESTRAL HOME THEORY

The opposing theory is simpler in some respects and supposes that the ancestral home of all birds was in the Tropics and that as all bird life tends to overpopulation there was a constant effort to seek breeding grounds where competition would be less keen. Species that strove for more northern latitudes would be kept in check by the ice and forced to return southward with the recurrence of winter conditions. As the ice retreated, vast areas of virgin country became successively suitable for summer occupancy, but the winter habitat remained the home to which the birds returned after the nesting season. It is a fact that some species spend very little time on their breeding grounds; the orchard oriole , for example, spends only 2 1/2 months in its summer home, arriving in southern Pennsylvania about the first week in May and leaving by the middle of July.

Both theories assume that migration is an ingrained habit, but neither is supported by positive biological data. Both have been criticized also on geological grounds, and neither can be accepted without qualification. It is apparent, however, that whether the ancestral home of any species was at the northern or at the southern limits of its present range, or even in some intermediate region, the search for conditions favorable for breeding in summer and for feeding in winter has been a principal factor underlying the origin of migration.

THEORY OF PHOTOPERIODISM

A modern view, based on studies of living behavior, favors the theory of photoperiodism, propounded by recent investigators as the cause of the annually induced movements of the birds. This theory holds as its major premise that quantity of light and length of day are the stimulating causes of migration. Its proponents urge that migration is a phenomenon far too regular to be created anew each season merely under stress of circumstances, such as need for food; and that it begins before the necessity for a change in latitude becomes at all pressing. Swallows, nighthawks , shore birds, and others may start their southward movement while the summer food supply in the North is at peak abundance; while robins , bluebirds , and others may leave an abundant food in the South in spring and press toward northern points when the food supplies there are almost entirely lacking and when severe cold and storms are likely to play havoc with the advance migrants. The regularity of arrival and departure is one of the most impressive features of migration, and since birds travel in almost strict accordance with the calendar, the proponents of the theory ask: What phenomenon to which we may attribute the stimulating impulse occurs with such precise regularity as the constantly increasing light in spring?

Experimental work has abundantly demonstrated the effect of increased light upon the growth, flowering, and fruiting of plants. Similarly, experiments with the common junco, or snowbird , reported by Rowan , resulted in increased development of the sexual organs by the end of December, although the birds were confined in outdoor aviaries in Canada and had been exposed to temperatures as low as -44? F. From the first of November until early in January, the juncos were subjected to ever-increasing light, supplied in the aviaries by electric bulbs. As regards illumination, they were thus artificially provided with conditions approximating those of spring. At the close of this period, it was found that the sexual organs of the birds had attained the maximum development normally associated with spring. With gradual reduction of the lighting over a period of little more than 1 month, the organs returned to their normal winter condition.

After a consideration of all evidence, including the fact that no ultraviolet rays were used, it was concluded that the explanation lay in the increased exercise taken during the periods of increased light. A simple test whereby certain birds were forced by mechanical means to take more exercise, the light being so reduced that there was merely sufficient glow for them to see the advancing mechanism that forced them into movement, showed that the rate of development of the sexual organs exactly paralleled that in the birds that were exposed to extended periods of illumination in the outdoor aviaries. Other features in this experiment--such as the behavior of the birds themselves--also indicated that more activity due to increased light is the governing cause of the spring development of the sexual organs. If this development be accepted as a controlling cause of migration, then this experiment must be recognized as of great importance.

Upon closer analysis, however, it is found that this theory, like those before discussed, is open to serious objections. First, some of our summer residents that migrate south for the winter do not stop in equatorial regions, where they might find the periods of day and night about equally divided, but push on beyond, some penetrating as far south as Patagonia. Also it might be asked: If the lengthening day is the stimulating factor, why should our summer birds wintering in the Tropics ever start northward, as in their winter quarters the variation in the length of day from winter to summer is imperceptible. Like all the other theories advanced, this also, as at present understood, is subject to unanswered criticism.

It is known that at any given point many species leave in fall and return in spring. Since bird banding has had such wide application as a method of study, it is known also that in some species one of the parent birds frequently returns and nests in the same tree, bush, or box that held its nest in the previous season . One ordinarily thinks of the world of birds as quiescent during two seasons each year, at nesting time and in winter. For individual species this is obviously the case, but when the entire avifauna of the continent is considered it is found that there are at almost all periods some latitudinal movements.

MOVEMENTS OF SPECIES AND GROUPS

Some species begin their fall migrations early in July, and in some parts of the country distinct southward movements can be detected from then until the beginning or middle of winter. For example, many shore birds start south in the early part of July, while the goshawks , snowy owls , redpolls , Bohemian waxwings , and many others do not leave the North until forced to do so by the advent of severe winter weather or by lack of the customary food. Thus, an observer in the northern part of the United States may record an almost unbroken southward procession of birds from midsummer to winter, and note some of the returning migrants as early as the middle of February. Purple martins have been known to arrive in Florida late in January on their way north, and the northern movement may continue among late arrivals into the first week of June. 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.

A study of these facts indicates that sometimes there exists a very definite relationship between what we may term northern and southern groups of individuals of the same species. A supposition, on which additional banding work is expected later to give definite facts, is that in the case of some species that have an extensive latitudinal breeding range and a normal migration, the individuals that nest farthest south migrate first in fall and proceed to the southern part of the winter range; those that occupy the central parts of the breeding range migrate next, and travel to regions in the winter range north of those occupied by the first group; and finally the individuals breeding farthest north are the last to begin their fall migration and these remain farthest north during the winter. In other words, this theory supposes that the southward movement of the species is normally such that the different groups maintain their relative latitudinal positions, both spring and fall.

The black-and-white warbler furnishes an example. The breeding range of this bird extends west and northwest from South Carolina and New Brunswick as far as Great Bear Lake in northwestern Canada . The bird spends the winter in southern Florida, the West Indies, central Mexico, Central America, and northwestern South America. In the southern part of its breeding range it is nesting in April, but the summer residents of New Brunswick do not reach their breeding grounds before the middle of May. Therefore, about 50 days are required for these northbound birds to cross the breeding range, and if 60 days be allowed for nest building, egg laying, incubation, care of the young, and molt, they would not be ready to start southward before the middle of July . Then another 50-day trip south, and the earliest migrants from the northern areas would reach the Gulf coast in September. But both adults and young have been observed at Key West, Fla., by the middle of July, and on the northern coast of South America by August 21. Since the birds at Key West were fully 500 miles south of the breeding-range, it is evident that they must have come from the southern part of the nesting area.

Many similar cases might be mentioned, such as the black-throated blue warblers , which are still observed in the mountains of Haiti in the middle of May when others of the species are en route through North Carolina to breeding territory in New England or have even reached that region. Redstarts and yellow warblers , evidently the more southern breeders in each case, are seen returning southward on the northern coast of South America just about the time that the earliest of those breeding in the North reach Florida on their way to winter quarters.

Share on: WhatsApp @Hash Facebook Tweet