Frederick Charles Lincoln

“ 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. ”

“ 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 follows only the general trend of the land mass. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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." ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ The nature and extent of the changes wrought by man on the North American Continent are easily apparent. Forests have been extensively cut away and their places have been taken by second growth or cultivated land, and wide stretches of prairie and plain have been broken up, irrigated, and devoted to agriculture. These great changes are exerting a profound effect upon the native bird populations, and the various species may be either benefited or adversely affected thereby. ”

“ 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 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. ”

“ The theory is sometimes advanced that the older and more experienced birds lead the way, showing the route to their younger companions. This explanation may be acceptable for some species, but not for those in which adults and the young migrate at different times. The young cowbird that is reared by foster parents flocks with others of its kind when grown and in many cases can hardly be said to have adult guidance in migration. An inherited migratory instinct with a definite sense of the goal to be reached and the route to be followed must be attributed to these birds. ”

“ Well timbered and watered, the entire region affords ideal conditions for the support of its great hosts of migrating birds. This route is followed by such vast numbers of ducks, geese, shore birds, blackbirds, sparrows, warblers, and thrushes, that observers stationed at favorable points in the Mississippi Valley during the height of migration can see a greater number of species and individuals than can be noted anywhere else in the world. ”

“ Night travel is probably best for the majority of birds, chiefly from the standpoint of feeding. Digestion is rapid in birds, and yet the stomach of a bird killed during the day almost always contains food. To supply the energy required for long flight, it is essential that food be obtained at comparatively short intervals, the longest of which in most species is during the hours of darkness. ”

“ HOW BIRDS MIGRATE SPEED OF FLIGHT AND SPEED OF MIGRATION
There is wide-spread misconception concerning the speed at which birds normally fly, and even regarding the speed they can attain when occasion demands, as when closely pursued by an enemy. It is not unusual to hear accounts of birds flying "a mile a minute." While undoubtedly some birds can and do attain a speed even greater than this, such cases are exceptional, and it is safe to say that even when pressed, few can develop an air speed of 60 miles an hour. ”

“ Among land birds that have a definite migration, the Ipswich sparrow (Passerculus princeps) has what is probably the most restricted migration range of any species. It is known to breed only on Sable Island, Nova Scotia, and it winters along the Atlantic coast south to Georgia. Living constantly within sound of the surf, it is rarely more than a quarter of a mile from the outer beach, and is entirely at home among the sand dunes and their sparse covering of coarse grass. ”

“ 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. ”

“ In a few species, the males and females apparently arrive at the breeding grounds together and proceed at once to nest building. In fact among the shore birds, ducks, and geese, courtship and mating may take place, in whole or in part, while the birds are in the South or on their way north, so that when they arrive at the northern nesting grounds they are paired and ready to proceed at once with the raising of their families. Mallards and black ducks may be observed in pairs as early as January, the female leading and the male following when they take flight. ”

“ Both the soaring and the sailing of birds show them to be proficient in the use of factors employed in aerial transportation that only recently have become understood and imitated by aeronautical engineers. The use of ascending currents of air, employed by all soaring birds, and easily demonstrated by observing the gulls that glide hour after hour along the windward side of a ship, are now utilized by man in his operation of gliders. Moreover, the whole structure of a bird renders it the most perfect machine for extensive flight that the world has ever known. ”

“ 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. ”