Migration refers to periodic or seasonal movements of animals over a relatively long distance, from one habitat or climate to another. Migrations may be made by particular individuals, or by an entire population of a species. Migration is an extremely "expensive" undertaking for animals to make, in terms of the energetic costs of the extensive movements from one place to another. Migration can also be a risky venture, during which animals are exposed to predators, the possibility of becoming irretrievably lost, and other natural hazards. However, there are also substantial benefits to be gained by migrating. These are usually associated with having access to a seasonal abundance of food in certain kinds of habitats. It is because of the opportunity to exploit such abundant resources that many species of animals have adopted the extraordinary behavioral attribute of migration.
Individuals of some species undertake regular short-distance migrations, usually to take advantage of seasonally rich habitats at higher altitude in the mountains. These consistent, seasonal movements are referred to as "vertical" migrations. For example, in the Rocky Mountains, individual grizzly bears (Ursus arctos) commonly migrate to high-altitude tundra meadows in the early summer, where they feed on a lush growth of sedges and other plants. Later in the summer they migrate downward to feed in lower-altitude valleys and meadows, where they also spend the winter sleeping deeply in a den. Some birds of the mountains also undertake migrations of this sort, as is the case of Clark's nutcracker (Nucifraga columbiana).
A much larger number of animal species undertake longer-distance seasonal migrations. For example, caribou (Rangifer tarandus) living in the subarctic regions of Alaska and northern Canada spend the long winter in the conif erous boreal forest, where habitats are relatively protected from the cold and wind. They then migrate northward in the summer to graze in the open tundra, where there is a lush productivity of herbaceous plants to feed upon, a relatively windy landscape where the density of biting flies is somewhat less, and fewer wolves (Canis lupus) and other predators of the vulnerable caribou calves.
Some large mammals of temperate prairie and tropical savanna also undertake long-distance, seasonal migrations. The once-abundant buffalo (Bison bison) of North America once migrated in enormous herds from their summer range in the northern prairie, to their winter habitats further south. Wildebeest (Connochaetes taurinus) still have similarly impressive migrations in the savanna and grassland of eastern Africa.
Some species of fish are also long-distance migrants. Atlantic salmon (Salmo salar) breed in large rivers, but the juveniles migrate to the ocean, where they feed for several years until they become reproductively mature, after which they migrate back up their birth streams to breed. This migratory habit is known as anadromous, and it is shared by the various species of Pacific salmon (Oncorhynchus spp.), such as the coho salmon (Oncorhynchus kisutch). In contrast, American eels (Anguilla rostrata) are known as catadromous, because they breed in the ocean (in the Sargasso Sea), and the young elvers migrate up rivers and lakes to grow into adults.
Most of the species of birds that breed in the temperate, boreal, and tundra zones undertake long-distance migrations, from their northern breeding habitats to their wintering habitats in the subtropics or tropics of the southern parts of their range. (Of course, in the Southern Hemisphere, these directions are reversed.) In North America, the many species of migratory birds include almost all of the flycatchers, hummingbirds, tanagers, thrushes, vireos, warblers, and most of the other songbirds that breed in the forests, tundra, and wetlands of North America. These birds are essentially tropical species that somehow discovered that more northerly ecosystems supported a great abundance of insects, spiders, and other critical foods during the growing season of spring, summer, and autumn. They learned to migrate to those seasonally profitable habitats, where they spend their breeding season, which is the time that birds need access to the greatest abundance of insect foods. This is because of the enormous demands of their ravenous young, which in most cases do not feed themselves and must be nourished by the parents, even for some time after they have learned to fly. By undertaking long-distance migrations, these species of birds are able to successfully raise their large families, while avoiding the relatively intense competition for food that occurs in most tropical habitats.
During the past several decades, ecologists and birdwatchers have been reporting alarming declines in the populations of many species of migratory birds in North America. Most of the declining species breed in mature temperate and boreal forests. The reasons for the songbird declines are not totally understood, but the most important factors are thought to be the following: (1) extensive deforestation in the tropical wintering range; (2) disturbance of mature-forest habitat in the northern breeding range; (3) fragmentation of breeding habitats into "islands" too small to sustain populations over the longer term, and that are easily penetrated by forest-edge predators and nest parasites; (4) loss of critical habitats used for staging and migration; and (5) effects of pesticides and other toxic chemicals. One of these migratory songbirds, the Bachman's warbler (Vermivora bachmanii), used to breed in mature hardwood forests in the southeastern United States. Unfortunately, it became extinct several decades ago. This tragic loss was probably caused by the loss of the critical wintering habitats of Bachman's warbler on the island of Cuba in the Caribbean. These tropical forests were destroyed and converted into sugar-cane plantations.
Not all of the avian migrants are songbirds ( many species of sandpipers, plovers, raptors, and seabirds also undertake long-distance migrations. Some of them travel extraordinarily long distances. One example is the whimbrel (Numenius phaeopus), which breeds on the northern tundra, and then migrates over land and the ocean to spend the winter on the pampas of Argentina. However, the longest migrations of any animal are made by the arctic tern (Sterna paradisaea, some of whom breed in the High Arctic, at the limits of land on the northern tips of Ellesmere Island, Greenland, and Spitzbergen. Once their broods are raised, these terns migrate south to spend their winter foraging in Antarctic waters. Therefore, twice each year, they travel virtually the entire length of the globe!
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