Migratory describes animals that travel from one place to another at certain times of the year, usually following the same routes and schedules their species has used for thousands of years. Monarch butterflies are migratory: they fly from Canada and the northern United States all the way to Mexico each fall, then their descendants make the return journey north in spring. Arctic terns are perhaps the most impressive migratory birds, traveling from the Arctic to Antarctica and back each year, covering more than 40,000 miles.

Animals migrate for survival, usually to find food, better weather, or safe places to raise their young. In winter, many birds fly south where insects and plants are still abundant, then return north in spring when food becomes plentiful again. Caribou migrate across the Arctic tundra following the growth of plants they eat. Even some whales are migratory, swimming between cold feeding waters and warm breeding areas.

You might see migratory used in phrases like migratory patterns (the routes animals regularly follow) or migratory species (animals that migrate). Scientists who study these journeys are amazed by how young animals, making the trip for the first time without parents to guide them, somehow know exactly where to go.