TEXT6+ The term migration is derived from the Latin word ╥migrare,╙ ╥to go from one place to another.╙ This complex set of behaviors, exceptionally well developed in birds, involves the seasonal movement of animals back and forth between breeding locations and non-breeding locations.In North America, migrating birds generally move from north to south for winter, and from south to north for summer, although one group of seabirds (many shearwaters and storm-petrels) nest in the southern hemisphere during the northern winter and spend the non-breeding period ╥wintering╙ in the northern hemisphere during our summer. Some species first migrate latitudinally or even laterally for considerable distances over regular routes before arriving at their destinations. Migration reflects the fact that, to survive, an animal must be adapted to the demands of its environment. Environments having favorable factors in one season may lose them in another. One adaptive response to the pressures of an unfavorable area is to leave it and move to another environment with better conditions. When environmental conditions change regularly with the seasons, it may be adaptive for an animal to take on the substantial risks and energy requirements of migrating. While birds are well equipped for both short-range and long-range movements, and are well-known for seasonal migrations, they are not alone as migrants. Other seasonal, long-distance migrators include whales, butterflies, eels, salmon, trout, tuna, sharks, caribou, bison, and bats.Why Birds MigrateNumerous factors have been suggested as influencing migratory behavior, a complex mix of instinct and learning. Northern breeding territories may offer a greater supply of food and longer days in which to gather it for nestlings. Although many birds (Snowy Owls and grouse, for example) can withstand severe winters, others move to more temperate conditions. Changes in day length associated with the onset of seasons may be one of the most important cues triggering migration. Physiology of MigrationThe metabolism of a migrating bird undergoes profound changes after its breeding season, in late summer, usually just after molting. The action of the hormones prolactin and corticosterone, released at different times of the day by the pituitary and adrenal glands, stimulates the bird to accumulate large amounts of subcutaneous fat.Such fat deposits provide energy for long migration flights. Experiments suggest that a male Ruby-throated Hummingbird weighing about 4.5 grams (0.16 oz.), 40 percent of it fat, could fly nonstop for 26 hours. At an average speed of 40 km per hour, it could travel across the Gulf of Mexico in that period.When spring migration begins, increased action of the pituitary gland helps stimulate the gonads of migratory birds for the breeding cycle ahead. The pituitary gland controls both sexual and migratory cycles simultaneously, but the control of the migratory urge depends in addition on the action of several endocrine glands.Seasonal variations in the muscle metabolism of migratory birds seem to support different levels of locomotive activity and, consequently, energy output. In non-migratory birds, by comparison, seasonal changes seem to adapt the species only to trends in temperature and food. Biologists have found significant differences, for example, in the muscle fibers of birds with different migratory strategies. Long-distance migrants have smaller fibers, higher capillary densities, and shorter diffusion distances. That is, there are more capillaries, and thus more effective oxygen delivery to the muscles during extended periods of exertion, such as long migratory flights. This is an example of the close relationship between the ecology and physiology of an organism. Patterns of MigrationContemporary migration patterns undoubtedly have their origins in much earlier times--times when topography and climate differed from those of the present. Some biologists theorize that at the time of the last ice age the ancestors of today╒s migratory birds were residents in their present non-breeding range, south of their present nesting ranges. It is speculated that they gradually left the southern non-breeding grounds in spring and moved northward as the weather moderated in the north, where they began to breed and raise their families. With onset of severe weather in fall and winter, and an associated reduction in food, these birds retreated southward from their newfound nesting territories to their original homes to winter, thus beginning the migratory pattern.Many species of birds that summer in the United States leave the country to winter in the Caribbean region and in Mexico and Central and South America. The main non-breeding area for many North American species is in Mexico and Central America, south to Panama. Nowhere in the Americas, nor in any other part of the world, is there such an extraordinary concentration of winter residents. In Central America, during the North American winter, every biological zone literally swarms with North American migrants.Some Long-Distance MigrantsThe Arctic tern makes one of the longest and most spectacular of all migration journeys. In North America, Arctic terns nest from Greenland and islands in the Arctic, Alaska, and Canada to Massachusetts, and they migrate from the Arctic to the Antarctic. Those that nest in eastern Canada, for example, start their autumn journey by crossing the Atlantic to Europe; they next fly southward along the western coasts of Europe and Africa to reach their non-breeding range in Antarctica, an annual round-trip roughly equal to the circumference of the Earth.Many shorebirds are also long-distance migrants. Some go to the West Indies and Central America to spend the northern winter. Others fly deep into South America, to lake shores of the high Andean plateaus and along both coasts of South America.Of all migrating North American shorebirds, the American Golden-Plover is perhaps the most well-known for its travels across an enormous loop over the New World. After nesting in the tundra of Alaska and northern Canada, these birds assemble in Labrador, then fly over 2,400 miles of ocean to the Brazilian coast, then onward through to Brazil and Uruguay to spend the northern winter on the Argentine pampas. On their northward return in spring, they travel a different course, over northwest South America and Central America to the Gulf of Mexico, then up the Mississippi Valley to their tundra breeding grounds, thousands of miles north of their non-breeding quarters. The entire route is a giant ellipse with a major north-south axis of 8,000 miles.By contrast, comparatively few North American land bird migrants pass beyond the South American tropics. However, nighthawks, Barn and Cliff swallows, and some of the thrushes and vireos winter south of the Equator in Brazil. Bobolinks travel about 5,000 miles from their summer homes in southern Canada and the northern United States to southern Brazil and northern Argentina. Some individual nighthawks and Barn Swallows travel still farther; of all North American land birds, they probably have the longest migration routes in traveling from their summer homes in Alaska south to Argentina--7,000 miles away.Migration By Day or NightLarger birds that live secretively by day (rails and woodcock, for example) and most small insect-eating birds migrate by night. Examples include cuckoos, tyrant flycatchers, nuthatches, creepers, wrens, most thrushes, vireos, wood warblers, tanagers, orioles, and the majority of sparrows. How nocturnal migrants find their way is discussed below.Small birds of strong flight, such as swifts, swallows and hummingbirds, which can feed on the wing, usually migrate by day, although swifts and hummingbirds migrate also at night. Hawks, eagles, cranes, storks, pelicans, and vultures migrate by day; herons by day and night; and gulls probably both by day and by night.Migrating AltitudesCommon passerines migrate mainly below 5,000 feet but tend to fly higher in spring than in autumn, and higher at night than by day. Radar studies have detected migrating flocks, perhaps plovers or sandpipers, at altitudes between 8,000 and 15,000 feet.Rate of TravelThe daily rate of migration by birds varies considerably. For example, the American Robin, a slow but steady migrant, moves northward in spring at an average of about 38 miles a day during its 3,000-mile trip from Iowa to Alaska. Waterfowl and shorebirds fly much farther--usually from 500 to 1,000 miles in a sustained flight.Some birds travel long distances in a remarkably short time. A Lesser Yellowlegs, banded in August on Cape Cod, was identified six days later on the island of Martinique in the West Indies--1,900 miles distant. It had traveled an average daily distance exceeding 300 miles. How Do Birds Find Their Way?The mechanisms of orientation (taking up a direction) and navigation (maintaining a direction independently of landmarks) by migrating birds have been among the great ornithological mysteries since humans began to observe birds.Orientation by the SunEarly investigators suggested that migrating birds might use the sun as a guide. Waterfowl (several species of ducks) use the sun compass for a guide in orientation, and even European songbirds that are chiefly nocturnal migrants, as well as the North American White-throated Sparrow, have been shown through experimentation to possess a sun compass.Orientation by the StarsNocturnal orientation by the stars, with the direction appropriate to the season, has been confirmed through experiments for the White-crowned Sparrows, Bobolink, Yellow-billed and Black-billed cuckoos, Rose-breasted Grosbeak, and Indigo Bunting. This is possible, of course, only on clear nights.Other CuesAdditional cues suspected of aiding birds in migration are surface features of the earth, which are learned in early migrations. When stars are obscured, or landscape features are not obvious, birdsmay use the structure of wind turbulence over geographic features for orientation.The Earth╒s Magnetic FieldOther studies have shown that the earth╒s magnetism influences the orientation of migratory birds. Ring-billed Gulls and European Robins can use the earth╒s magnetic field to determine their migratory direction.Crystals of magnetite have been found contained within the brains of migrating animals, including pigeons and tuna. These crystals apparently aid in detecting the earth╒s magnetic field and using it for navigation.ConclusionMigrating birds have many potential sources of cues that may guide migratory flight, including topography of the land; meteorological conditions; the position of the sun, stars, and moon; the earth╒s magnetic field; and perhaps others. Bird navigation is very complex. It appears that many of these cues are used simultaneously by migrating birds, weighted differently in varying circumstances, to ensure arrival at their ultimate destination.
