Migration, Bird

the annual more or less distant movement of all or some members of a bird population from the breeding ground to the wintering ground and back again. Bird migration is an adaptation to seasonal climatic changes that make a region suitable for bird life in one season (in the northern hemisphere, summer; in the tropics, the wet season) and not suitable in others. It is a genetically fixed phenomenon that results from the settlement of a species in a new region or from a change of environmental conditions in a species’ native land. For this reason, some populations of a given species may be nonmigratory and others migratory.

The length of the migration period often depends on the feeding habits of different birds. Most grain-eating birds migrate earlier than insectivorous birds. The last insectivores to migrate are those that catch insects in the air (swifts, swallows). Some species, for example, snipes, arrive at the breeding grounds early and leave early. However, as a rule, the earlier a bird arrives at its breeding grounds, the later it leaves. In most bird species, the males return to the breeding ground earlier than the females, with adults returning earlier than the young. In autumn this pattern is reversed.

The distance of the migration depends on the ecological needs of the birds: grain-eaters winter close to the nesting ground, insectivorous birds travel farther south, and aquatic birds travel south to regions with unfrozen bodies of water. The distances traveled are also determined by the competition among bird species, both local and migratory, for a particular wintering place and by the history of the migratory species. The most extensive migration is made by the arctic tern, which nests in the arctic and winters in the antarctic. Crows, rooks, snow buntings, and many thrushes winter close to their breeding ground.

Sometimes the breeding and wintering regions overlap. In such cases, birds that have nested farther north displace the birds in the overlapped area for the winter, thus creating a false impression of a sedentary mode of existence. This type of migration characterizes crows.

Southerly bird populations generally are nonmigratory or make short migrations, whereas more northerly ones travel farther south. The females of some species winter farther south than the males. In many species that reproduce at an age of one year or older, those individuals under the breeding age spend the summer outside the nesting ground.

Some species make intermediate migrations: after their first nesting siskins and redpolls migrate farther north, where they nest again; mallards travel to molting places (males, after the females begin incubating the eggs; females, after reproduction); and starlings travel in the direction of wintering places after reproduction. Young garganeys migrate thousands of kilometers after nesting. Similar migrations by other species are shorter, often only several kilometers. Thus, autumn bird migrations may initiate with short flights, may follow an intermediate migration, or may originate from nesting and hatching places. Most bird species migrate after molting. Some birds stop molting during migration, and others molt upon reaching the wintering ground. Changes in food supply and weather conditions in a given season may stimulate or arrest migration of some bird species. However, sometimes unfavorable conditions do not result in migration but in destruction of the birds.

The capacity for migration is associated with the deposition of fat in the birds, the advent of special day-length biological rhythms, the yearning to move in a definite direction, and the formation of flocks. In the spring, the lengthening of the day serves as a signal for many bird species to prepare for migration. The periods of preparation for autumn migration are established in the spring and are subsequently controlled by an endogenous “circumannual” biological rhythm. In birds that migrate long distances, migration begins immediately after preparation. In birds that do not make long migrations, departure may be hastened or delayed by external conditions. The direction of migration is conditioned by the location of favorable wintering places and their accessibility. Many species fly from the European USSR southwesterly to Western Europe and to Africa. Starlings from the Baltic region fly west to the British Isles, and grosbeaks fly from Europe southeast to southern Asia. In choosing migration routes, birds make maximal use of terrain that is favorable to their species. However, when necessary, birds are able to overcome such obstacles as the Mediterranean Sea, the Sahara, or the Gulf of Mexico.

The ability to find the true direction of migration is congenital. Some birds orient themselves according to the sun, the stars, and, possibly, the geomagnetic field. In young birds the ability to find their nesting grounds in the spring and the previous year’s wintering place in the fall is developed before departure. As a result, as the banding of birds shows, the birds return year after year to their nesting and wintering places. In siskins and crossbills this ability is manifested weakly; some species wander all winter.

Most birds migrate by day and night; a few species fly only by day. Diurnal migration begins after sunrise and lasts two to five hours; sometimes the most distance is covered toward the end of the day. Nocturnal migration, which begins 40 minutes to an hour after sunset and usually lasts all night, occurs at altitudes from a few hundred meters to several kilometers. Birds making diurnal migrations may also fly at high altitudes, but sometimes, when there are head winds or when the flights are short, the birds fly at low altitudes of a few to several meters.

Most birds fly in flocks; only a few species make solitary migrations. Flock formation is advantageous for orientation, reduces losses from predators, synchronizes the behavior of the birds, and possibly facilitates the transmission of experience. There are several flock formations: rank, file, wedge, and more complex multilayer ones. Such formations facilitate the use of aerodynamic conditions and broaden the field of vision. The average velocity is from 30–50 km/day (titmice—which make short migrations) to 200–300 km/day (warblers, wagtails, and flycatchers—which make long migrations).

Migration usually lasts one to two months, with its duration depending very little on distance. Speed increases toward the end of the migration; thus, the autumn migrations in middle latitudes proceed more slowly than the spring flights. The birds do not fly every day; they alternate one or two days (or nights) of flight with five to ten days of stopover. Expenditures of energy in flight are 12 to 14 times greater than at rest, and the birds’ fat reserves are rapidly exhausted. During stopover, the birds accumulate new reserves of fat. The greater the amount of energy expended for migration to the wintering place and back, the less energy needed for thermoregulation during the winter (as a result of wintering in warmer conditions).