I.

Introduction

Animal Migration, seasonal or periodic movement of animals in response to changes in climate or food availability, or to ensure reproduction. Migration most commonly involves movement from one area to another and then back again. This round-trip, or return migration, may be of a seasonal nature, as in the spring and autumn migrations of many birds. Or it may require a lifetime to complete, as in various species of Pacific salmon that are born in freshwater streams, travel to ocean waters, and then return to the stream where they were born to breed before dying.

Migration occurs in a wide range of animals, from microorganisms in freshwater lakes, which shift seasonally from deep to shallow water as a result of temperature changes; to whales, which move in autumn from subpolar to subtropical seas to have their young and then return in late spring to the colder, food-rich waters. Humans migrate as well: The Kung bushmen of the Kalahari Desert, for example, follow migrating game animals that they rely upon for food. They also leave drought-ridden areas to find other water sources.

In addition to round-trip migration, some migrations are nomadic in nature. Nomadic migrations involve irregular movement patterns that are dependent on temporary local conditions. For example, many of the large grazing animals that live in herds on the plains of eastern Africa move in response to varying local conditions of food and climate. In these migrations, the animals follow no regular route and do not return to any one place. Another type of migration—removal migration, or one-way migration to new sites—is exhibited by migratory locusts of Africa and Asia. These locusts are well known for their enormous mass movements when their populations peak and food becomes scarce. They move to new areas, almost blackening the sky as they pass overhead. Rarely do they return to their place of origin.

Irruption is a specific migratory cycle occurring in extreme climates. The best-known example of irruption is seen in lemmings of the arctic tundra. These small animals reach a peak in their population every three or four years, at which point they migrate overland in great numbers. Most of them die in the process; only a few survive to start the migratory cycle again. Another migration pattern is remigration, in which the round trip is divided between generations: the first generation of animals migrates to an area and reproduces, and the return trip is made by their offspring.