Migration is a pattern of behavior in which animals travel from one habitat to another in search of food, better conditions, or reproductive needs. There are two important factors that make migration different from other types of animal movement: First, migration happens seasonally, and second, migration involves a return journey. This makes it different from emigration, when animals travel to find a new, permanent place to live. Many animal species migrate, including species of fish, crustaceans, amphibians, reptiles, insects, and mammals. These animals might journey by land, sea, or air to reach their destination, often crossing vast distances and in large numbers.

One of the main reasons animals migrate is to find food. In Tanzania, wildebeests (Connochaetes taurinus), zebras (Equus quagga), and gazelles (Eudorcas thomsonii) migrate in huge herds. They roam the Serengeti looking for fresh grass and water, which are hard to find during the dry season. Humpback whales (Megaptera novaeangliae) migrate for food as well. In the summer, they travel to feeding grounds near the polar ice, where the water is full of krill and small fish. In the winter, they migrate back to warmer waters to raise their calves.

Other animals migrate because of the climate or seasons. For example, monarch butterflies (Danaus plexippus) migrate to avoid cold temperatures in the winter. These butterflies cannot survive freezing temperatures, so they fly from Canada all the way to Mexico, where they gather to keep warm over winter. They make the return journey over many generations, stopping to lay eggs on milkweed plants along the way. The caterpillars eat the milkweed and then finish the journey as butterflies.

Finally, some animals migrate for reproductive reasons: either to find a mate, raise their young, or to spawn. For example, salmon start life in rivers and migrate to the sea to feed and grow. After spending up to seven years in the ocean, they migrate back to the rivers they were born in so that they can spawn. Christmas Island red crabs (Gecarcoidea natalis) migrate for similar reasons. They spend most of their life in the forest but migrate to the ocean to reproduce.

Recent improvements in technology have helped us understand migration better, but there is still a lot we do not know. Scientists are not yet entirely sure how animals know where to go and when to leave, especially when they have never made the journey before. Some researchers suggest that these animals use a mix of stimuli, such as sunlight, the Earth’s magnetic field, and chemical cues, to find their way.