migration: The movement of an animal from one region, location, or habitat to another in order to breed, grow, or find food; an instance of this; esp. (of a bird, mammal, or fish) the periodic travel to and from a region at a particular season and along a well-established route.——Oxford English Dictionary

migration: when birds or animals travel regularly from one part of the world to another cliff etc.——Longman Dictionary of Contemporary English  @en