Spring fills the skies of North America with birds that only months ago were hundreds and even thousands of miles of the south, where winter temperatures are more hospitable and food more plentiful.
Like clockwork, the mass migration is carried out with regularity and precision to the wonder of earthbound observers.
The cause and mechanics of bird migration -- now at its peak in the United states -- remain partly a mystery. But research by ornithologists at the US Fish and Wildlife Service and various academic institutions is providing some answers that satisfy scientific curiosity and shed more light on man's relationship with his feathered friends.
"The more we find out, the more we realize how little we know" about migration, says bird habitat specialist Chandler Robbins of the US Fish and Wildlife Service at the Patuxent Wildlife Research Center in Maryland.
Still, "People have started to uncover the secrets of migration and utilize this knowledge for the betterment of our society," concludes a book summarizing the latest research on the subject. the book, entitled "Migration of birds" and published by the Fish and Wildlife Service, is a revision of a work that first appeared in 1935.
It suggests, for example, that development of wilderness land in some instances may do much more than scare off birds. It may also cause global changes in migration patterns that will exert great pressures on native bird populations.
Why do birds migrate? Although it happens in coordination with the changes in season, migration has more to do with the availability of food than discomfort with weather, recent research shows.
Cold weather causes the disappearance or hibernation of insects, a major source of food for birds. also, snow obscures food that is close to the ground, and the shorter days of winter reduce feeding time.
STill, weather may act as the stimulus for migration, giving birds a signal to relocate well before food supplies actually run short. But how weather, length of days, and other factors weigh in giving birds a signal to migrate is not fully understood.
Most birds do migrate, although exactly how they move from their spring breeding grounds to their winter destinations with almost pinpoint accuracy -- and back again -- is not clear.
Experts agree that birds probably use a number of navigation tools to guide their travels, which for some species includes long-distance flying at night.
The sun, stars, and the Earth's magnetic field all may provide compass bearings for birds. Their acute senses also may play an important role.
Prof. William Keeton at Cornell University has found that birds can hear very low-frequency sounds that go undetected by the human ear. Earth's upper-level jetstream emits low-frequency sounds, as do ocean breakers, weather fronts, and winds whipping over mountains. Professor Keeton has no evidence that birds use these sounds to guide their migration, but he says it is a possibility.
Birds have keen vision, and natural landmarks also help guide migration for some species. Even birds that travel the oceans may use island reefs and currents as guideposts.
Still, most of these navigational aids only help birds get where they want to go, much as a compass helps a hiker find the desired direction. "The real mystery is how they know where they want to go," says Professor Keeton.
Birds are the "perfect flying machine," with their hollow, air-filled bones and light, but tough, covering of feathers, points out the Fish and Wildlife Service publication. It offers the example that a golden plover travels more than 2,400 miles on its migration from Nova Scotia to South America in about 48 hours of continous flight. The feat is achieved with the consumption of two ounces of body fat -- comparable in energy terms to a 1,000- pound airplane flying 20 miles on a pint of fuel, instead of the gallon that is required.
The arctic tern is the champion marathon flier, migrating from its breeding range as far north as land extends in North America to its southern destination in the Antarctic region 11,000 miles away. The journey takes several months.
But this is nothing compared to a theory put forth in the early 1700s by a scientist who concluded that migratory birds left the Earth altogether in the cold season and spent the winter months on the moon.
Other, more plausible, theories also have been disproved with modern research , such as the notions that birds migrate at very high altitudes and at great speeds.
Although the Fish and Wildlife Service cites an instance where a Himalayan mountain climber at 16,000 feet lookep up to see a flock of geese flying by about two miles higher, it points out that the bulk of migration occurs below 3, 000 feet. The speed of flight varies from species to species, but generally ranges from 10 to 50 m. p. h.