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 prairie falcons (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.