The phrase "longest migration" means the journey that takes the most time and distance for a particular animal or group of animals to move from one place to another. Full definition
One of the world's longest migrations of zebras occurs in the African nation of Botswana, but predicting when and where zebras will move has not been possible until now. (sciencedaily.com)
MIGRATION Gray whales make an extraordinarily long migration from the Arctic Ocean (northwest of Alaska in the Chukchi Sea) to the Baja peninsula off Mexico, and back each year. (enchantedlearning.com)
Owing to its extraordinarily long migration on a coastal route that passes close to large human populations, it is probably the most viewed of any large whale. (nps.gov)