Why do birds come and go with the seasons? In this BrainPOP movie, Tim explains the natural phenomenon of migration, the instinctive movement of large groups of animals. You'll learn about the three different reasons animals have for moving, and why these movements tend to take place at the same time every year. You'll also find out about other kinds of migrations, like the nomadic movement of elephants, and why locusts stay in one place till the food runs out. Find out how animals know where they're headed, when to take off, and what kind of environmental clues animals use to calibrate their internal clocks. Plus, find out why migration's not just for the birds, either! Look, the butterflies are migrating!