Frederick Charles Lincoln

“ 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. ”

“ Much migration takes place at night and great stretches of the open sea are crossed to reach destinations. Nights are rarely so dark that all terrestrial objects are totally obscured, and features such as coastlines and rivers are just those that are most likely to be seen in the faintest light, particularly by the acute vision of birds from their aerial points of observation. Even if terrestrial objects are completely obscured on a very dark night, the migrants are still able to assess their surroundings during the day before starting out again. ”

“ 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. ”

“ Migration is a phenomenon far too regular to be created anew each season merely under stress of circumstances, such as need for food; and it begins before the necessity for a change in latitude becomes at all pressing. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ 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 search for favorable conditions under which to breed in summer and to feed in winter, as influenced by competition for space, has been the principal factor underlying the extension of ranges, usually by young, nonconditioned individuals. This is exemplified by the northward extension in historic times of a number of species. Many of these range extensions have closely followed man's settlement of the area and the subsequent changes in habitat that man has made. ”

“ 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. ”

“ Differences in distance traveled, time of starting, speed of flight, geographical position, latitudes of breeding and wintering grounds, and other factors contribute to this great variation of migration routes. Nevertheless, there are certain factors that serve to guide individuals or groups of individuals along more or less definite lines, and it is possible to define such lines of migration for many species.
Except in a few species, individuals probably do not follow precisely the same route twice. This is especially true in the group of soaring birds that utilize thermals. ”

“ People have started to uncover the secrets of migration and utilize this knowledge for the betterment of our society.
Each kind of bird seems to have its own reaction to the environment, so that the character of movement differs widely in the various species, and seldom do any two present the same picture. ”

“ While some migrants are still traveling south, some early spring migrants can be observed returning north through the same locality. For example, many shorebirds start south in the early part of July, while the goshawks, snowy owls, redpolls, and Bohemian waxwings do not leave the North until forced to do so by the advent of severe winter weather or a lack of 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. ”

“ 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. ”

“ The concept of bird populations being confined to four fairly definite and distinct migration "flyways" is probably most applicable to those birds that migrate in family groups, namely geese, swans, and cranes, but does not appear to be very helpful in understanding the movements of the more widely dispersing ducks. The "pioneering spirit" in Canada geese, for example, is limited by their social structure the young travel to and from specific breeding and wintering areas with their parents. These young later in life usually breed in the same areas as did their parents. ”

“ In other words, in the North spring comes with a rush, and, during the height of migration season in Saskatchewan, the temperature in the southern part of the Mackenzie Valley just about equals that in the Lake Superior area, 700 miles farther south. Such conditions, coupled with the diagonal course of the birds across this region of fast-moving spring, exert a great influence on migration and are probably factors in the acceleration of travel speed. However, it should be remembered that the birds are getting closer to the breeding season and may be stimulated to travel faster for this reason. ”

“ This explanation of the stimulus for migration may apply very broadly to birds that winter in temperate parts of the world and nest in the same hemisphere but fails in those birds wintering in the tropics, where little change in length of day occurs and even decreases during the spring in regions south of the Equator. It might be asked: "If the lengthening day is the stimulating factor, why should our summer birds, wintering in the tropics, ever start north?" ”

“ 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. ”

“ Traditions that have lived for countless generations may die overnight if experienced individuals are lost or no longer active (Hochbaum 1955) ; migration patterns remain constant only as long as the factors influencing these - 97 - patterns remain constant. But the landscape and the interacting ecological stresses are forever changing, and we would expect the adaptive behavior of birds to respond with them. One of these responses to an expanding habitat is colonization of new territory and expansion of a species' range with accompanying development of a migratory habit. ”

“ 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. ”