When animals regularly move from one place to another, they are said to migrate. Generally, migration is dependent on the variations in climate and the availability of food.
Insect-eating birds that breed in summer, in temperate regions move to equatorial parts in autumn, as the insects are not found in the temperate zones in winter. In spring, the birds fly back again. The reason for the migration of Northern birds is very interesting. When the ice retreated northwards, areas of new land became exposed. Insects colonized this land and birds followed them. But in autumn, they returned to the winter feeding areas they had always used.
Zebras and gazelles migrate around the plains of East Africa following the rains that make the grass grow. In summer, blue whales move from tropical regions to polar areas where the water is rich in plankton.
Migration is also related to breeding habits. Animals often migrate to a particular region to breed. The Spiny lobsters that live in the shallow waters migrate to the deep waters to breed. Else from the rivers of Europe and North America move to the Sargasso Sea. Turtles migrate thousands of miles to lay their eggs. Of course from the sea, up rivers to the hatched, to breed is only too well known!