For many Americans and Canadians, a telltale sign of the changing seasons is a V-shaped flock of honking Canada Geese flying overhead during their migration.
These birds get attention for being large, common and noisy. But billions of other American birds migrate each fall and spring with hardly any fanfare, making their journey at night.
The cover of darkness protects migrating songbirds — like warblers, sparrows and orioles — from hungry predators like hawks. But their journey has different challenges: tall buildings, bright lights, and other dangers from humans.
These birds travel in huge pulses when weather conditions are favorable, sometimes with hundreds of millions traveling in a single night. But before now, it was hard to predict exactly where and when these pulses would happen.