Birds do not spread evenly across the map when they travel. Geography — coastlines, mountain ranges, river valleys and the food and shelter along them — funnels migration into broad corridors called flyways. Following a flyway lets birds chain together reliable stopover sites, and it concentrates millions of birds into predictable routes, which is exactly what makes large-scale migration possible to watch and study.
In the Americas, migration is traditionally described as four north–south flyways: the Atlantic, along the eastern seaboard; the Mississippi, following the great river system through the continent’s heart; the Central, over the Great Plains; and the Pacific, down the western coast and mountains. They are not rigid lanes — many birds drift between them — but they capture where the bulk of the traffic flows.
Elsewhere the corridors are just as dramatic. The East Atlantic Flyway carries waterbirds between the Arctic, Europe and West Africa; the East Asian–Australasian Flyway links Siberia and Alaska to Southeast Asia, Australia and New Zealand and supports some of the longest migrations on Earth. Narrow crossings — Gibraltar, the Bosphorus, Panama — pinch raptors and soaring birds into rivers of birds you can stand and count.
Not all movement fits the seasonal map. ‘Irruptions’ are irregular, hard-to-predict invasions: when a seed crop fails in the far north, finches, crossbills, redpolls and owls push far south of their usual range in search of food, sometimes in huge numbers, then largely vanish again the following year.