Europe also has several good examples of changes in migration routes through range extension. One of the best examples is the serin. During the past century, this European finch has spread its breeding range from around the Mediterranean Sea to include the entire continent. While the Mediterranean populations remain sedentary, the more northern breeding birds are migratory. Most likely, those birds that did not migrate from the North were eliminated by severe weather. Similarly, the wheatear, yellow wagtail, and Arctic warbler have extended their breeding ranges eastward across the Bering Sea into Alaska, but the wheatear, for instance, migrates all the way back across Asia to Africa where it winters with other wheatears coming from Europe, Iceland, and Greenland.
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. Kipp(1942, 1958) demonstrated this using orioles. The sedentary black-headed oriole of India has a well-rounded wing whereas the closely related black-naped oriole is migratory between India and Siberia and has primaries that are much more pointed and well developed.
Thus it seems the origin and evolution of migration have roots in the present that are deep in the past. The important thing to consider in the evolution of a migratory trait is whether a population can adapt to new conditions by genetic modification of its physiology and habits. The migratory habit has evolved in those populations in which, on the average, more individuals survive by moving to a different area part of the year than if they remained in the same area all year.
WHERE WE STAND
The migration of birds had its beginning in times so remote its origins have been largely obscured and can be interpreted now only in terms of present conditions. The causes underlying migration are exceedingly complex. 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. More and more sophisticated approaches including radar, radio telemetry, computer processing of banding data, and physiological and behavior studies are being developed.
With the widespread use of these new techniques, we are beginning to realize the benefits, aside from aesthetic reasons, for studying migration. Radar alone has aided tremendously in documenting flock size, heights, and speeds of migration as well as the descriptions and locations of patterns and routes of specific migrants in relation to aircraft flight lanes. Recent studies have indicated local, nonmigratory populations of various blackbirds cause nearly all of the rice damage in southern States and the "hordes from the North" contribute very little to the losses. In addition, the transport of arborviruses from one continent to another via these long distance migrants is being investigated. 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. 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 nature and extent of the changes wrought by man on the North American Continent are readily apparent. Extensive forests have been burned or cut away, rolling prairies turned over with the plow, and wetlands drained or filled. Their places have been taken by a variety of human activities. These great changes are exerting pressure on native bird populations, and various species may be either benefited or adversely affected.
The Federal Government has recognized its responsibility to migratory birds under these changing conditions. Enabling acts allow for carrying out migratory bird treaty obligations in cooperation with other countries, and now most species have legal protection under regulations administered by the U.S. Fish and Wildlife Service. The effectiveness of conservation laws, however, is increased in the same measure that the people of the country become acquainted with the migratory bird resource and interest themselves personally in the well-being of the various species. Long before European man came to America, the birds had established their seasonal patterns of migration throughout the Western Hemisphere. The economic, scientific, and esthetic values of these migratory species dictate they be permitted to continue their long-accustomed and to some extent still-mysterious habits of migration.
BIBLIOGRAPHY
Able, K. P.