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Ancient human wonder at the annual disappearance and reappearance of neotropical birds gradually evolved into recorded observation and scientific study. Descriptive accounts were written three thousand years ago by Homer, Hesiod, and Herodutus. Aristotle wrote of cranes migrating from Eurasia to Africa. By the late 1700's, careful studies in the United States were beginning to accumulate extensive information on populations and behaviors of birds in this hemisphere. Increasingly during the nineteenth and twentieth centuries, structured field observation, banding, radio tracking, and radar were used to investigate migration. Much has been learned about bird migration, although there continue to be many unanswered questions.

  • Purpose of migration: Migration most likely evolved to augment successful breeding and reproduction. Birds that winter in the southern hemisphere travel to northern regions seeking food supplies that are widely available during longer summer daylight hours. Hormonal signals triggered by changes in weather, length of day, and food supplies stimulate return to wintering sites.
  • Species that migrate include:
    • birds that need open water for feeding, such as ducks, geese, herons, sandpipers, plovers;
    • birds that eat insects, such as warblers, swallows, and thrushes;
    • birds that hunt animals that hibernate or migrate, such as hawks, kites and vultures.
  • Navigational cues for migrating: Migratory birds use a variety of navigational cues for finding their way between their breeding and wintering sites. Birds that travel by day, including hawks and vultures, swallows, swifts, and nighthawks use the angle of the setting sun (and the pattern of polarized light created), land features such as mountains and rivers, and wind direction. Birds that travel by night or over vast ocean distances, such as warblers, swallows, and thrushes, use a combination of star patterns, the earth's magnetic field, and odors. At least some birds can detect ultraviolet radiations and very deep sound vibrations such as distant ocean waves. When one set of cues is obscured, as the sun and stars may be by cloud cover, more reliance is placed on alternate cues. Short-distance migrants learn routes as young birds from experienced family members; long-distance migrants add learned experience to their genetic programming for flying in particular directions during particular periods.
  • Physiology of migrating birds: Birds' streamlined shape and lightweight, partially hollow skeletons minimize air resistance in flight. The wings of migrating birds generally are longer and more pointed than wings of non-migratory species. The blood of migratory birds is enhanced by a high concentration of red blood cells and, in some birds, contains two different forms of hemoglobin that allow for quick adaptation to variable oxygen availability at changing altitudes.
  • Time of day for migration: Most songbirds and shorebirds and some waterfowl migrate at night, using the more favorable nighttime flying conditions of stable air masses, fewer predators, and reduced threat of dehydration or overheating. Hawks and vultures, which soar and glide on thermal air currents, migrate by day as do swallows, swifts, and nighthawks, which feed on daytime flying insects.
  • Preparation for migration includes increased foraging for food to produce rapid weight gain and greater fat storage, changing activity rhythms during darkness, and flocking and orienting in the migratory direction.
  • Length of migration: Migration distances vary among species, from a few hundred miles to the 22,000 mile round-trip journey of the Arctic Tern between the Arctic and Antarctica. Some species may travel only a short distance from high-elevation breeding sites in mountain regions to lower elevations within the same mountain ranges. One-way migrations range from several weeks to four months, accomplished in spurts of flights from several hours to several days.
  • Flyway routes: Bird migration in North America generally takes place along four broad geographical routes. The Mississippi Flyway is the longest, stretching 3000 miles from the arctic coast of Alaska south along the Mackenzie and Mississippi rivers to the Gulf of Mexico.
  • Conservation of bird habitats: A variety of organizations has emerged in recent years that seek to alter trends of deforestation, fragmentation, and pollution of breeding and wintering grounds. Individual actions contribute greatly to bird conservation, in the forms of backyard propagation of plant cover, limits in use of garden pesticides, and restraint of destruction of undisturbed habitat by urban sprawl.