Have you ever heard loud honking calls and looked up to see geese flying in a V-formation? The geese are getting ready for a long trip, or they may be on their way already. Geese are among the animals that travel south for the winter. In spring, they return to their summer homes in the north. Their journeys are known as migrations. Animal migration is the relatively long-distance movement of individuals, usually on a seasonal basis. It is found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans.
Why do animals migrate? In most cases, they migrate to have the best possible living conditions all year-round. Geese, like other migrating birds, travel south to have a ready food supply. The grass they feed on becomes scarce during the northern winter. But in summer it’s plentiful again, and they return north to mate, lay eggs, and hatch baby geese.
Geese aren’t the only birds that migrate. Arctic terns make one of the longest journeys of all. These seabirds spend the summer in the Arctic, where there are plenty of small fish to eat. The birds lay eggs and rear their young here. As winter approaches, the birds fly south. They fly all the way to the Antarctic, where it’s summer and they can find plenty of fish. Their round-trip journey takes them about six months, three months each way! It’s about 20,000 miles (35,000 kilometers) long. Many kinds of ducks, hawks, songbirds, and other birds also migrate.
BATS AND BUTTERFLIES
Other flying creatures besides birds migrate, too. Some kinds of bats migrate. Monarch butterflies make amazing journeys.
Each autumn, millions of monarch butterflies in North America migrate. Monarchs from west of the Rocky Mountains fly to California. Monarchs east of the Rockies fly to Mexico. The butterflies’ journeys may take a month. During and after the long flight, the butterflies gather together. Thousands of monarch butterflies cover the trees in these gathering places.
Most adult monarchs migrate south only once. The females lay their eggs and then die. The young butterflies make the return trip back north in the spring.
CARIBOU AND WILDEBEEST
Some large mammals migrate, too. Caribou, a kind of deer, live in northern Canada in summer. They feed on grass on Arctic plains. In the fall, the caribou gather in large herds and migrate to forests farther south in Canada. The herds move slowly, stopping along the way. But they may travel long distances, sometimes more than 1,900 miles (5,000 kilometers) in a year.
Large African antelopes called wildebeest migrate long distances during dry seasons, when rainfall stops. The wildebeest migrate in search of water to drink and fresh grass to feed on. A herd of migrating wildebeest may have as many as 1 million animals.
Long migrations are also common in the sea. Humpback whales spend the summer in the icy waters of the Arctic and Antarctic. There, they find plenty of shrimp-like krill. Krill are the main food of these whales. In winter, the whales travel toward the equator to give birth in warmer waters. By the time the whales return to their summer home, their young are big enough to make the journey.
Gray whales summer in icy waters off the coast of Alaska. In fall they migrate to warmer waters near Mexico, where they breed.
Salmon are born in streams and rivers, where they spend their first two or three years. Then they migrate to the sea. When they are ready to breed, the salmon journey back upriver. They find the stream they were born in. Here, they lay eggs and die.
Green turtles also migrate to lay their eggs. Some kinds migrate long distances. They swim across the Atlantic Ocean from South America to lay their eggs on Ascension Island. It’s a distance of about 1,300 miles (2,000 kilometers). After the eggs are laid, the adult turtles return to Brazil’s coastal waters. They leave their young to hatch on the beach and find their way to the sea. Two years later, the young turtles swim back to the beach of their birth to lay their own eggs.
FINDING THEIR WAY
No one understands exactly how migrating animals know when to migrate or how they find their way. Some experts think that the shortening days in fall or the lengthening days in spring may act as a signal to birds and other animals. Such signals indicate that it is time to start their long journey.
Once on their way, birds are thought to navigate by the Sun and stars. Research shows that birds also may be sensitive to Earth’s magnetic field—the space around Earth where a magnetic force is felt. Fish and turtles may find their way back to their breeding grounds by remembering the smell of the water.