- Key People:
- George Gaylord Simpson
- Jack Miner
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Migration, like reproduction and other phases (as molting in birds), is part of the life cycle and depends on a complex internal rhythm that affects the whole organism, particularly the endocrine glands (glands of internal secretion) and the gonads. Migration must thus be viewed in relation to the entire annual cycle.
Each year birds return to particular areas to breed, and remain there until the members of the brood can care for themselves. There is no relation between the reproductive and migratory stimuli, yet the two phenomena, although independent, are nevertheless stimulated by the same factor.
A physiological study of certain migrants has revealed that metabolic patterns usually change prior to migration, and fats accumulate in the body tissues. The whitethroat (Sylvia communis) weighs an average of 12 to 13 grams (about 0.4 ounce) during the breeding season, 16 to 19 grams (about 0.6 ounce) in the autumn, and 20 to 24 grams (about 0.8 ounce) in the winter. Food consumption increases with the autumn molt, reaching a peak at the beginning of the migration season. These fundamental physiological changes, chiefly under the control of the thyroid gland, are correlated with migratory activity. Such fluctuations are not observed in nonmigratory species.
Variations in metabolism and related phenomena are controlled by an endocrine gland, namely the pituitary gland, which is located in the lower part of the brain and acts as a command post, sending out instructions in the form of secretions called hormones. That the pituitary has a cycle independent of environmental factors is demonstrated by the regularity with which phases such as reproduction occur from year to year in the lives of some birds, and by the diverse response of various species and populations to the same environmental factors. That the pituitary is, however, influenced by environmental factors, such as variations in day length and the intensity of the Sun, has been demonstrated experimentally.
Gonadal development and the deposition of fat, for example, are influenced by the pituitary, which responds to increasing day length in springtime by accelerating the rate of gonadal development. The pituitary thus governs the development of gonads and, in addition, affects all metabolic processes, including development of the thyroid gland, so as to prepare the animal physiologically for migration. If only the pituitary and variations in day length were involved, migration would be triggered at definite times, because the pituitary cycle is fixed, and photoperiodism is a highly predictable phenomenon; such a lack of flexibility, however, would inevitably cause migrant populations to suffer catastrophes because ecological conditions are irregular—meteorological events, such as the arrival of spring, and biological phenomena, such as flowering, foliation, hatching of insects, and availability of food, are highly variable from year to year. The pituitary thus serves only to prepare the bird for flight; the proper ecological conditions, on the other hand, are necessary to initiate it. The availability of food is an important factor. Temperature and weather conditions also have an influence—a sudden period of cold weather during autumn may induce the immediate departure of many migrants.
Sensitivity to changes in the weather and other environmental conditions varies markedly among species. Some, such as the woodcock, snipe, lapwing, starling, and lark, rely on surrounding conditions to initiate their spring and autumn migrations, and the patterns of their flight depend on temperature and barometric pressure. Others, such as the swift, cliff swallow, Baltimore oriole, and short-tailed petrel, are less weather dependent, and, since the dates of their arrival and departure are not regulated by the weather, they occur with remarkable regularity each year.
The factors that stimulate migration in animals other than birds are not yet well understood. Ecological conditions play a great part in the migratory activity of mammals, who react to general food shortage by moving to another region. Whale, for example, leave the Antarctic region as winter modifies the oceanographic conditions. Seals disperse when the food supply in the area of their breeding colonies is depleted. Environmental factors are of primary importance in the migration of fishes and marine invertebrates. Annual movements of water masses change physical conditions such as temperature and salinity; biotic conditions are influenced accordingly.
Origin and evolution of migration
The origins of migration remain in the realm of pure conjecture; neither observation nor experiment has resolved the matter. The explanation, however, must be related to geographical and climatological factors that have prevailed since the Neogene Period, which ended some 2,600,000 years ago. The great Quaternary ice ages, which came later, were very important in altering the distribution of animals over a large part of the world, but migrations occurred long before.
Migration, as it is now known among modern birds and mammals, probably appeared gradually by stages. Some animals changed their habitat only slightly, never leaving the same general region. The movements of other animals were more erratic, their dispersal being oriented toward the most favourable places. Such movements are the first stages of true migration—a phenomenon characterized by elaborate mechanisms—which gradually acquired stability through natural selection. At first, many populations must have perished rather than attempt to flee from unfavourable conditions. Only a fraction of such populations probably sought more favourable conditions elsewhere, but natural selection favoured the “migrants,” and migratory tendencies were retained.
In some cases, original habitats were in present-day wintering areas, and animals developed a tendency to leave in spring in order to breed in other territories. Seasonal changes of weather and food supply in these newly settled regions forced the animals to migrate in fall, and they thus retreated to their former range. Among birds nesting in the Northern Hemisphere, hummingbirds, tyrant flycatchers, tanagers, orioles, bee-eaters, and swifts have distinct tropical affinities; in recent geological times these birds gradually spread northward as glacial ice receded and the continent became warmer. Other birds, such as plover, ducks, and geese, originally lived in what is now their breeding area. Gradual climatic changes forced them to spend their winters in regions far to the south. Migrations thus appear to be the consequence of invasions or emigrations, during which animals settle in new areas during a segment of the annual cycle.
Migratory birds use the routes by which their ancestors first invaded new regions after the glacial recession. The yellow wagtail (Motacilla flava) and the wheatear (Oenanthe oenanthe) settled in Alaska; they migrate annually into other parts of the Western Hemisphere but spend their winters in the warm regions of southeastern Asia and even Africa, probably following the migratory route of their ancestors. A typically North American species, the gray-cheeked thrush (Hylocichla minima), which has extended its breeding area to northeastern Siberia, returns to spend the winter in the central regions of South America.
