Many mammals, birds, fishes, insects, and other animals regularly move from one place to another at certain times of the year. This movement is called migration. Some animals make long journeys back and forth across land and ocean. Other migrations take a vertical direction. Certain water animals, for example, move from deep water to the surface according to the season.

Birds

The kind of migration most easily seen is that of birds. Bird migration has been known for a very long time. More than 2,500 years ago the prophet Jeremiah wrote about the regular arrival of the turtledove, crane, swallow, and stork. Aristotle, a philosopher and scientist of ancient Greece, also wrote about bird migration.

Many kinds of birds migrate each year between their breeding grounds and warmer regions near the equator where they spend the winter. Birds of the Northern Hemisphere (the lands north of the equator), for example, fly south for the winter. In tropical regions birds migrate according to the cycle of wet and dry seasons. They may travel to another part of the tropics to avoid heavy rains.

The greatest migrator among birds is the Arctic tern. Each year it spends the summer in the Arctic and then flies half way around the world to Antarctica for the winter. This journey covers about 11,000 miles (18,000 kilometers).

The routes that birds follow during their migrations are determined partly by geographical features such as river systems and mountains. They also depend on weather conditions—for example, birds change their direction of flight according to the direction and force of the wind. During migration even birds that spend most of their time alone become social. Some birds show remarkable grouping. The best-known migratory formation is the V shape of a flock of geese, ducks, pelicans, or cranes.

Mammals

Few land mammals migrate because walking is slow and requires much energy and time. Nevertheless, many hoofed animals that live in regions where the climate and plant growth vary widely must seek fresh grazing lands in different seasons. For example, reindeer, or caribou, travel long distances between their summer and winter homes. Large African mammals migrate with the wet and dry seasons, which can greatly change the places they live. Zebras, wildebeests, and many kinds of antelopes make seasonal movements over a large range. During the rains herds spread out. Then during the dry season they gather around watering holes.

Among the land mammals that migrate, one of the strangest is the lemming, a small rodent that lives in northern North America, Europe, and Asia. Every few years lemmings move in enormous numbers outward from the centers of population. They swim rivers, cross mountains, and sometimes jump from cliffs into the sea.

Bats, the only true flying mammals, migrate more commonly than land mammals do. A few kinds of bats native to Europe and Asia make short flights to winter quarters. Other kinds of bats with stronger powers of flight make longer trips. Red bats, large hoary bats, and silver-haired bats make long flights from the northern part of their range in Canada to the southern United States.

Some mammals that live in the sea also migrate. Various kinds of whales travel from cold polar regions to warmer waters near the equator to breed and find food in winter. Some types of seals, such as northern fur seals and harp seals, also swim to certain places to breed and then return to their native waters. Dolphins and porpoises migrate as well, but little is known about their travels.

Fishes

Eels and salmon are among the best-known migratory fishes. Certain kinds of eels spend most of their lives in freshwater and then migrate to the sea to breed. European eels travel thousands of miles to their breeding place in the Sargasso Sea in the North Atlantic. Salmon hatch in freshwater rivers and then swim out to sea. They find plenty of food and grow quickly. They spend one to three years in the sea before returning to lay their own eggs in the river where they hatched. Salmon are thought to find their way back to their home river largely by recognizing the smell in the water from their trip to the sea.

Besides eels and salmon, such important sea fish as herring and cod also have regular movements according to the season. They are closely watched by fishing fleets.

Insects

The locust is the most important and most dangerous of migratory insects. In various parts of the world great swarms of locusts spread across the land, doing much damage by devouring crops.

Much more welcome insect migrants are several kinds of butterfly. The best known of these is the monarch of North America. It has an extensive breeding range and has been known to migrate more than 1,800 miles (2,900 kilometers). Butterflies may travel as far as 80 miles (130 kilometers) in a day.

Reptiles and amphibians

Most reptiles and amphibians are unable to travel great distances and thus do not migrate. Instead they lapse into a state of inactivity. This state makes it possible for them to stay in one place for an entire year. They may hibernate in the winter and estivate in the summer (see hibernation). Their only migratory movements are made during the reproductive period. Many types of frogs and toads travel each year to breeding ponds and lakes. Sea turtles cover long distances to sandy beaches where they lay their eggs.

Why and how animals migrate

Migration is part of the life cycle of an animal. Scientists know more about what causes birds to migrate than they do about other animals. The pituitary gland, in the lower part of the brain, controls the process. This gland produces substances called hormones that prepare the bird's body for both flight and reproduction. The activity of the pituitary speeds up as the days grow longer in springtime. A change in the environment during this time—for example, a drop in the food supply or a sudden change in temperature—will cause the birds to begin their migration. Food shortages are also a common cause of migrations in mammals and other animals.

Scientists do not fully understand how migrating animals find their way over long and complex routes. Many birds are known to have a compass sense, which allows them to fly in a particular direction. They use the position of the sun and stars to get their bearings. Birds and other animals use landmarks such as mountains, rivers, and coasts to navigate as well. Scientists think that even fishes use geographical clues to recognize their underwater range.