Frederick Charles Lincoln

“ The arguments against the theory of vision and memory are chiefly that much migration takes place at night and that great stretches of the open sea are crossed without hesitation. Nevertheless, the nights are rarely so dark that all terrestrial objects are totally obscured, and such features as coast lines and rivers are just those that are most likely to be seen in the faintest light, particularly by the acute vision of a bird and from its aerial points of observation. But some birds fly unerringly through the densest fog. ”

“ 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 the Eskimos 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. ”

“ 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?" ”

“ 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 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. ”

“ 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. ”

“ Much is being done by legislation for the welfare of the birds. The effectiveness of these conservation laws, however, is increased in the same measure that the people of the country become acquainted with the facts in the life histories of the migrants and interest themselves personally in the well-being of the various species. Long before the white man came to America the birds had established their seasonal lanes of migration throughout the Western Hemisphere. ”

“ Contrary to what many observers believe, the arrival of birds in an area, whether they stop or continue on, is more often controlled by the weather at the point of departure than at the point of arrival. During the peak of migration, suitable weather may occur at an observation site, but strong migratory movements may be arrested before the birds arrive there because the weather was not suitable at the point of departure or somewhere in between. ”

“ It is to 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. The spring migration might, of course, be more nearly considered as true "homing." ”

“ While birds that have recently arrived from a protracted flight over land or sea sometimes show evidences of being tired—as, for example, pintail ducks that have flown from the North American mainland to the Hawaiian Islands—their condition is far from being a state of exhaustion. With a few hours' rest and a crop well filled with proper food, most birds exhibit eagerness to resume their journey. The popular notion that birds find the long ocean flights excessively wearisome and that they sink exhausted when terra firma is reached, generally does not agree with the facts. ”

“ Every pair of birds requires a certain amount of territory for the performance of its reproductive duties, the extent of which varies greatly between different species. This territory must be large enough to provide adequate food, not only for the parent birds but also for the lusty appetites of their young. In the Arctic summer, 24 hours of daylight allow the young to feed or be fed almost continuously and rapid growth is apparent. The short breeding season in northern latitudes exposes the vulnerable young to predation for a brief period and prevents a build up of predator populations. ”

“ 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. ”

“ Sportsmen sitting in their "blinds" frequently observe the passage of flocks of ducks and geese, but great numbers of these birds also pass through at night, the clarion call of the Canada goose, or the conversational gabbling of a flock of ducks being common night sounds in spring and fall in many parts of the country. The sibilant, nocturnal calls of the upland plover or Bartramian sandpiper and of other shore birds during their spring and fall flights form vivid memories in the minds of many students of migration. ”

“ 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. ”

“ 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. ”

“ The speed of migration, however, is quite different from that attained in forced flights for short distances. A sustained flight of 10 hours a day would carry herons, hawks, crows, and smaller birds from 100 to 250 miles, while ducks and geese might travel as much as 400 to 500 miles in the same period. Measured as air-line distances, these journeys are impressive and indicate that birds could cover the ordinary migration route from the northern United States or even from northern Canada to winter quarters in the West Indies or in Central America or South America in a relatively short time. ”

“ It cannot be said that the winter or summer area of every species is entirely unsuited to the requirements of all of its members at other seasons, because some individuals pass the winter season in areas that are frequented only in summer by other individuals of their species. Such species may have extensive breeding ranges with wide climatic variations so that some individuals may actually be permanently resident in a region where others of their kind are present only in winter. ”

“ In other words, migration is so ingrained that each species moves north in spring when the average weather that will be encountered is not unendurable. The hardy birds travel early, fearless of the blasts of retreating winter. The more delicate species come later, when there is less danger of encountering prolonged periods of inclement weather. Some of these hardy birds pause and allow the spring season to advance, and then by rapid travel again overtake it, or in some cases actually outstrip it. ”

“ Nocturnal and diurnal migration
When one recalls that most birds appear to be more or less helpless in the dark, it seems remarkable that many should select the night hours for extended travel. Among those that do, however, are the great hosts of shore birds, rails, flycatchers, orioles, most of the great family of sparrows, the warblers, vireos, and thrushes, and in fact, the majority of small birds. ”

“ 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. ”

“ For migratory birds the winter season is a period of rest, a time when they have no cares other than those associated with the daily search for food or escape from their natural enemies. Their migrations, however, are a vital part of their life cycles, which have become so well adjusted that the seasons of travel correspond in general with the major seasonal changes on their breeding grounds. ”

“ A migration route, therefore, may be anything from a narrow path that adheres closely to some definite geographical feature, such as a river valley or a coast line, to a broad boulevard that leads in the desired direction and which follows only the general trend of the land mass. ”

“ The mystery that formerly cloaked the periodic travels of birds, however, has been largely dispelled through the fairly complete information now available concerning the extent and times of seasonal journeys of most species. Many gaps still remain in our knowledge of the subject, but present knowledge is being placed on record, and the answers to many uncertainties that continue to make bird migration one of the most fascinating subjects in the science of ornithology must be left for future studies. In some areas we are on the threshold of discovery. ”

“ As will be seen from the map, a seemingly natural and convenient highway extends through the Bahamas, Cuba, Hispaniola, Puerto Rico, and the Lesser Antilles to the South American coast. Resting places are afforded at convenient intervals, and at no time need the aerial travelers be out of sight of land. It is not, however, the favored highway, and only about 25 species of birds go beyond Cuba to Puerto Rico along this route to their winter quarters, while only 6 species are known to reach South America by way of the Lesser Antilles. The obvious drawback is lack of adequate food. ”

“ 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?" ”

“ 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. ”

“ The great majority of North American birds seeking winter homes in the Tropics thus elect the short cut across the Gulf of Mexico in preference to the longer although safer land or island journey by way of Texas or Florida. During the height of the migration some of the islands off the coasts of Louisiana and Texas are wonderful observation places for the student of birds, as the feathered travelers literally swarm over them. ”

“ The period of migration is a season full of peril for birds. Untold thousands of the smaller migrants are destroyed each year by storms, in unfamiliar habitats, and through attacks of predatory birds, mammals, and reptiles. If each pair of adult birds should succeed in raising two fledglings to maturity, the population of migratory birds would have a potential annual increase of 100 percent and the world would soon be heavily overpopulated with them. ”

“ The swallow is a day migrant while the warbler travels at night. The migration of the warbler is made up of a series of long nocturnal flights alternated with days of rest and feeding in favorable localities. The swallow, on the other hand, starts its migration several weeks earlier and catches each day's ration of flying insects during its aerial evolutions, while slowly migrating. The 2,000 extra miles flown along the insect-teeming shores of the Gulf of Mexico are exceeded by the great distances covered by these birds in normal pursuit of food. ”

“ In cases where the migration is a long one, however, the notion must be abandoned that the birds' flight is restricted to particular narrow routes that follow river valleys and the like, as many species seem to disregard utterly such apparently good natural flyways as river valleys. For example, the Arkansas River has a general east and west course for a great part of its length, and while it does constitute a highway for - 34 - many perching birds en route from the Mississippi Valley to the Rocky Mountain region, some of the hawks and many ducks and shore birds pay the valley scant attention. ”