Animal migration is a phenomenon whereby animals travel long distances from one environment to another. The reasons for it often relate to the availability of food, to climatic changes or to mating cycles. Usually, the animals travel in large groups. For example, in the Serengeti National Park in Tanzania, some 1.7 million wildebeest make a mass migration to new grazing pastures every year, together with hundreds of thousands of other large mammals such as zebras and gazelles.
Although we often think of migration as only affecting larger animals, it occurs in species at all levels of the animal kingdom, including mammals, fish, birds, reptiles, amphibians, crustaceans, and insects. Ordinarily, migration takes place seasonally or annually, such as when northern-hemisphere birds fly south for the winter, or else relates to a major change in an animal's life cycle, such as the 'salmon run' when salmon swim from the ocean into rivers to lay their eggs. Some butterflies, including the Monarch butterfly, engage in multi-generational migrations over thousands of miles—a distance that takes so long to cover that no individual butterfly will complete the whole journey. While most migrations are a normal response to cyclical environmental changes, irregular migrations can also take place in response to dramatic, unforeseen changes, such as famine, overpopulation, or unexpected weather patterns.