Birds of tropical regions migrate according to the rhythmic succession of wet and dry seasons—a profoundly influential factor on the annual cycle of animals and plants alike.

The migratory behaviour of birds has a unique regularity in Africa, where life zones are arranged symmetrically by latitudes away from the Equator. Some migrants never cross the Equator. Thestandard-wing nightjar (Macrodipteryx longipennis), which nests in a belt extending from Senegal in the west to Kenya in the east along the equatorial forest,migrates northward to avoid the wet season. Theplain nightjar (Caprimulgus inornatus), on the other hand, nests in a dry belt from Mali in the west to theRed Sea andKenya in the east during the rains and then migrates southward toCameroon and the northernCongo region during the dry season.

Other birds migrate across the Equator to their alternate seasonal grounds. Abdim’s stork (Sphenorhynchus abdimii) nests in a belt extending from Senegal to the Red Sea; after the wet season, it winters from Tanzania through most of southern Africa. The pennant-wing nightjar (Cosmetornis vexillarius), in contrast, nests in the Southern Hemisphere south of the Congo forests during the austral, or Southern Hemisphere, summer, then starts north with theonset of the rainy season. It spends its winters in savannas from Nigeria to Uganda.

In coastal and pelagic regions

Among the migrating seabirds, a distinction must be made between the coastal and the pelagic, or open-sea, species. Birds such as guillemots, auks, cormorants, gannets, and gulls—all common to the seashore—stay in the zone of thecontinental shelf. Except during the breeding season, they are dispersed over a vast area, often preferring specific directions of travel.Gannets (Sula bassana) nesting around theBritish Isles spread in winter along the Atlantic coast ofEurope and Africa toSenegal, the young travelling farther than the adults. Pelagic birds, most of which belong to the orderProcellariiformes (petrels andalbatrosses), cover much greater distances and, from a few small nesting areas, roam over a large part of the oceans.

Wilson’s petrels (Oceanites oceanicus), which nest in the western sector of the Antarctic (South Georgia Island, Shetland Islands, and South Orkney Islands), spread rapidly northward in April along the coasts of North andSouth America and stay in the North Atlantic during the summer. In September they leave the western Atlantic, travelling east, then southeast, along the coasts of Europe and Africa toward South America and their Antarctic breeding grounds, arriving there in November. These petrels thus travel in a great loop through the wholeAtlantic Ocean, in a flight patterncorrelated with the direction of prevailing winds. The same pattern is used by other seabirds normally carried by the winds. Albatrosses, such as thewandering albatross (Diomedea exulans) that nests on small Antarctic islands, circle the globe during their migrations. One such bird, banded as a chick at Kerguelen Island in the southernIndian Ocean and recovered at Patache, Chile, travelled in less than 10 months at least 13,000 kilometres (8,100 miles)—perhaps as much as 18,000 kilometres (11,200 miles)—by drifting with the prevailing winds.

In the Pacific,short-tailed shearwaters (Puffinus tenuirostris) nest in enormous colonies along the coasts of southernAustralia and inTasmania, thenmigrate across the western Pacific to Japan, remaining in the North Pacific and theArctic Ocean from June to August. On the return migration they go east and southeast along the Pacific coast of North America, then fly diagonally across the Pacific to Australia.

Arctic terns (Sterna paradisaea), whose breeding range includes the northernmost coast of Europe,Asia, andNorth America, spend the winter in the extreme southern Pacific and Atlantic, chiefly along Antarcticpack ice 17,600 kilometres (11,000 miles) from their breeding range. American populations of the Arctic tern first cross the Atlantic from west to east, then follow the coast of western Europe.Arctic terns thus travel further than any other bird species.

Modes of migration

The migration flights of birds follow specific routes, sometimes quite well defined over long distances. The majority of bird migrants, however, travel along broad airways. A singlepopulation of migrants may be scattered over avast territory so as to form a broad front hundreds of miles in width. Such routes are determined not only by geographical factors—e.g., river systems, valleys, coasts—and ecological conditions but are also dependent upon meteorological conditions; i.e., birds change their direction of flight in accordance with the direction and force of the wind. Some routes cross oceans. Smallpasserine (perching) birds migrate across 1,000 kilometres (620 miles) or more of sea in areas such as theGulf of Mexico, theMediterranean Sea, and theNorth Sea.American golden plover, wintering in the Pacific, fly directly from theAleutian Islands (southwest ofAlaska) toHawaii, the 3,300-kilometre (2,050-mile) flight requiring 35 hours and more than 250,000 wing beats.

The speed of migratory flights depends largely on the species and the type ofterrain covered. Birds in migration go faster than otherwise.Rooks (Corvus frugilegus) have been observed migrating at speeds of 51 to 72 kilometres (32 to 45 miles) per hour; starlings (Sturnus vulgaris) at 69 to 78 kilometres (43 to 49 miles) per hour;skylarks (Alauda arvensis) at 35 to 45 kilometres (22 to 28 miles) per hour; and pintails (Anas acuta) at 50 to 82 kilometres (31 to 51 miles) per hour. Although the speeds would permit steadily flying migrants to reach their wintering grounds in a relatively short time, the journeys are interrupted by long stops, during which the birds rest and hunt for food. The redbackedshrike (Lanius collurio) covers an average of 1,000 kilometres (620 miles) in five days as follows: two nights for migration, three nights for rest, five days for feeding.

Most migrations occur at relatively low altitudes. Small passerine birds often fly at less than 60 metres (200 feet). Some birds, however, fly much higher. Migrating passerines, for example, have been observed at altitudes as great as 4,000 metres (14,000 feet). The highest altitude recorded thus far for migrating birds is 9,000 metres (29,500 feet) forgeese near Dehra Dun in northwest India.

Pelicans,storks,birds of prey,swifts,swallows, andfinches are diurnal (daytime) migrants. Waterbirds,cuckoos,flycatchers,thrushes,warblers,orioles, andbuntings are mostly nocturnal (nighttime) migrants. Studies of nocturnal migrants using radar on telescopes focussed on theMoon show that most migratory flights occur between 10pm and 1am, diminishing rapidly to a minimum at 4am.

Most birds aregregarious during migration, even those that display a fierceindividualism at all other times, such as many birds of prey and insectivorous passerines. Birds with similar habits sometimes travel together, a phenomenon observed among various species of shorebirds. Flocks sometimes show a remarkable cohesion. The most characteristic migratory formation of geese, ducks, pelicans, and cranes is a “V” with the point turned in the direction of flight. In flocks of northern baldibis (Geronticus eremita), each member of the group takes a turn leading the formation in flight, before dropping back to fly in the wake of another to save energy.