Migration
Migration
Migration is the act of moving from one place to another, and is often associated with seasonal movements of animals between their breeding territory and a wintering range. This activity is most readily observed in birds, but has been documented in many other animals as well, including insects, fish, whales, and other mammals. Migration is a complex behavior that involves timing, navigation and other survival skills.
Types of migration
The migration behaviors of different species of animals can be categorized as either complete, partial, differential, or irruptive. In complete migration, all of the members of a population will travel away from their breeding habitat at the end of that season, often to a wintering site hundreds or even thousands of kilometers away. The arctic tern (Sterna paradisaea) is an example of a complete migrant. Individuals of this species travel from the arctic to the antarctic and back again during the course of a year, a round-trip migration of more than 18,600 mi (30,000 km).
In other species, some individuals will remain at the breeding ground year round, while other members of the same species migrate away. These species are called partial migrants. American robins (Turdus migratorius), considered indicators of spring in some areas, are year-round residents in others.
Differential migration occurs when all the members of a population migrate, but not necessarily at the same time or for the same distance. The differences are often based on age or sex. Herring gulls (Larus argentatus), for example, migrate a shorter and shorter distance as they grow older. Male American kestrels (Falco sparverius) spend more time at their breeding grounds than do females, and when they do migrate, they do not travel as far.
Irruptive migration occurs in species which do not migrate at all during some years, but may do so during other years, when the winter is particularly cold or food particularly scarce. For example, some populations of blue jays (Cyanocitta cristata) are believed to migrate only when their winter food of acorns is scarce.
Directions of migration
Migration is usually thought of as a southward movement in autumn and a return northward in the spring. These migrations, termed latitudinal migrations, are the most common type, but there are several other directions in which migratory animals may travel. A mirror image of latitudinal migration, often called austral migration, occurs in the Southern Hemisphere. These migrations tend to be shorter than those in the Northern Hemisphere, mainly because of the scarcity of land in the cold regions of the Southern Hemisphere.
Some birds, such as prairiefalcons (Falco mexicanus), travel east to west and back. These movements, called longitudinal migrations, are probably related to seasonal changes in the choice of prey and its location.
Elevational migration occurs in many animals that live on mountains. A short migration to lower elevations in winter will accomplish the same as a much longer southerly migration, since valleys have warmer climates than do mountaintops. A form of elevational migration also occurs in zooplankton, tiny animals that drift with the currents in the open ocean. In the summer, when tiny plants are abundant, the zooplankton live near the ocean surface, feeding on the plants. In the winter, they migrate 1,090 yd (1,000 m) deeper and do not feed at all.
Migration pathways
Migratory animals travel along the same general routes each year. Several common “flyways” are used by North American birds on their southward journey. The most commonly used path includes a 496-682 mi (800-1,100 m) flight across the Gulf of Mexico. In order to survive this arduous journey, birds must store extra energy in the form of fat. All along the migration route, but particularly before crossing a large expanse of water, birds will rest and eat, sometimes for days at a time. The recommencement of the journey will begin only when a certain amount of body fat is reached.
The requirement of stopover sites in addition to breeding and wintering sites makes migratory animals particularly susceptible to habitat loss, and thus many migrants are among our most endangered species.
Although most migrants travel at night, a few birds prefer daytime migrations. The pathways used by these birds tend to be less direct and slower than those of night migrants, primarily because of differences in feeding strategies. Night migrants can spend the day in one area, foraging for food and building up energy reserves for the night’s non-stop flight. Daytime migrants must combine travel with foraging, and thus tend to keep to the shorelines, which are rich in insect life, capturing food during a slow but ever-southward journey.
Advantages of migration
Despite the dangers of long-distance travel, many different animals have developed migration strategies, presumably as a defense against the greater dangers of environmental uncertainty and competition. The breeding range of migratory animals is likely to cover a region whose environment becomes increasingly inhospitable after the breeding season is over (i.e. there is less food available in the winter), so it is advantageous to travel elsewhere. The wintering ranges will have food available year-round, but also many more animals competing for that food, as well as increased competition for shelter in which to raise their young. Hence the advantage of returning to the breeding grounds, where competition is less severe.
Navigation
Perhaps the most remarkable aspect of migration is the navigational skills employed by the animals. Birds such as the albatross (Diomedea sp.) and lesser golden plover (Pluvialis dominica) travel hundreds of miles over the featureless open ocean, yet unerringly
KEY TERMS
Habitat— The place that an animal lives, which provides everything necessary for the animal’s survival.
Navigation— The process of finding one’s way to a known destination across unfamiliar terrain.
home in on the same breeding grounds year after year. Salmon (Oncorynchus sp.) migrate upstream from the sea to the very same freshwater shallows in which they were hatched. Monarch butterflies (Danas plexippus), which began life in the United States or Canada, travel to the same wintering grounds in southern California or Mexico that had been used by ancestors many generations removed.
How are these incredible feats of navigation accomplished? Different animals have been shown to use a diverse range of navigational aids, involving senses often much more acute than our own. Sight, for example, may be important for some animals’ navigational skills, although it may often be secondary to other senses. Salmon can smell the water of their home rivers, and follow this scent all the way from the sea. Pigeons also sense wind-borne odors and may be able to organize the memories of the sources of these smells in a kind of internal map. It has been shown that many animals have the ability to sense the magnetic forces associated with the north and south poles, and thus have their own built-in compass. This magnetic sense, along with the sense of smell, are believed to be the most important factors involved in animal migration, but researchers are continually discovering new and unusual navigational systems throughout the animal kingdom.
Resources
BOOKS
Baker, Robin, ed. The Mystery of Migration. New York: The Viking Press, 1981.
Halliday, Tim, ed. Animal Behavior. Norman, OK: Univ. of Oklahoma Press, 1994.
PERIODICALS
Long, Michael E. “Secrets of Animal Navigation.” National Geographic 179, no. 6. (June 1991).
OTHER
National Center for Ecological Analysis and Synthesis. “Migration Patterns” <http://www.nceas.ucsb.edu/nceas-web/kids/mmp/migration.htm> (accessed December 2, 2006).
David E. Fontes