Many mammals, birds, fish, insects and other animals travel regularly from one place to another during a certain time of the year. This journey is called migration. Animals change their living areas because they may get more food, better places to build nests or more sunlight somewhere else. Some animals travel thousands of kilometers across land and water. Others travel only short distances. Water animals, for example, often move vertically from deeper water to the surface area. Birds are animals that travel the greatest distances. Many types of birds migrate each year between their breeding grounds in the northern part of the world and warmer regions near the equator where they spend their summer months. In tropical regions birds often travel from wet to dry areas. The Arctic tern spends the summer in the Arctic regions and then flies half way around the world to the Antarctic for the winter. When flying, most birds follow geographical features, like mountains, rivers or islands. A few types of butterflies are also long-distance travelers. The monarch butterfly, for instance, spends most of the winter in Mexico and then travels back to the United States in the spring. Mammals do not migrate so much because walking is slow and takes up too much time. There are, however, hoofed animals that do travel longer distances in order to search for better grazing places. Reindeer or caribou can move long distances between their winter and summer homes. They migrate in herds of up to 100,000 animals. Caribou spend the summer in the Canadian Arctic and then travel up to one thousand kilometers to the forests along the American border. Zebras and other wild animals that live in the Serengeti area migrate every year in search of new pastures and food. During annual rainfall they spread out and during the dry season they gather together around waterholes. Some sea mammals also migrate. Whales travel from colder Polar Regions to warmer water where they mate and bring up their babies. Then they travel back to the colder waters, where there is more food. Seals often do the same. Although most reptiles do not migrate so much there are some that follow a yearly cycles. Sea turtles spend most of their life at sea but each year they come to the coasts and lay their eggs in nests on the beach. The most famous breeding place is Ascension Island in the middle of the Atlantic Ocean. Turtles come from the coast of Brazil over a distance of 2000 km and make their nests there. The best known migratory fish is the salmon. It is born in freshwater rivers and then swims to sea. There it finds a lot of food an grows very quickly Salmon can spend one to three years in ocean waters and then swim back to the rivers where they hatched to lay their own eggs. Most of them find their way back home because of the smell of the water in which they were born.