As autumn arrives in the Northern Hemisphere, an extraordinary movement of life begins. Billions of birds - across more than 4,000 species - leave their summer breeding grounds and head south. Some fly to South America. Some go to Africa. Some even make their way from the Arctic to Antarctica, traveling pole to pole each year.
Migrating birds use multiple navigation tools to find their way. They detect Earth’s magnetic field through tiny iron particles in their beaks and a light-sensitive protein in their eyes. They navigate by the position of the Sun during the day. They use the rotation of the stars at night. Some inherit “maps” of major rivers, coastlines, and mountain ranges. Together, these tools let them travel thousands of miles to specific locations and back, year after year.
Migration is triggered mostly by changes in day length. As days shorten in autumn, hormonal changes signal birds to fatten up, then take off. Some smaller birds - like the ruby-throated hummingbird - fly nonstop across the Gulf of Mexico, a single 18-hour journey over open water. Larger birds like geese often fly in V-formations to save energy. The whole annual migration is one of the largest organized movements of life on the planet.
