Approximately two weeks after a bird has felt the urge to leave—that “homesick,” upward pull—it is overcome by a stronger lift. This feeling seems to affect the members of an entire flock synchronously. It flies off in one sweeping, perfectly choreographed movement.

This unrehearsed synchronicity—the ability of thousands of birds in a single flock to suddenly take off, flying in such dense swarms without colliding—continues to baffle ethologists. It can only be assumed that this behavior is driven by evolutionarily developed instincts.

Zugunruhe describes the angst birds seem to feel when seasons change and the migration period begins. Even a caged bird “knows” it should be traveling too. They hop about, flutter their wings, and flit from perch to perch.