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Animals migrate to reproduce, eat, or seek warmer climates. Those that migrate travel long distances in groups from one part of the world to another, past oceans, over plains, or through the sky on a decided route. Some species must travel thousands of miles every year, while others make the trip just once in their lifetime.
Warmer weather often means a larger or better food source. Animals, like geese, who spend spring and summer in the cool northern hemisphere, move south for the winter on a seasonal schedule. They follow the same path from year to year, often stopping at the same landmarks. Herbivorous mammals, like African antelope, follow green grass depending on precipitation and drought. Their yearly travel patterns might not be along the same path, depending on the weather and their feeding. Those wanderers, called "nomadic migrators," include the behavior of massive plagues like locusts. Generally, though, animals travel closer to the warmer equatorial zone and away from the poles during winter.
Another cycle that triggers migration is mating, gestation, birth, and raising young. Some animals, like Pacific trout, make one long migration over the course of their lives. From small streams, they travel to open waters to mature, then return to their birthplace when they are ready to spawn. The ideal location for an animal's reproduction might have a special food source, less predators, or a geographical feature necessary for new life. Tortoises must have soft, sandy beaches to bury their leathery eggs. Frogs need low, leafy branches to overhang ponds not in danger of drying up too soon.
Migration takes an enormous amount of energy. Leading up to the long trip, animals build up fat for stored energy, or pare down weight for easier movement. Hormonal chemicals trigger these changes. In animals equipped to navigate long distances, the position of the sun and stars in the sky, water currents, temperature, wind conditions, and their "biological clock" help them reach the appropriate destination.