For birds that spend their summers in the northern hemisphere.

For birds who spend their summers in the northern hemisphere, this journey can span from 700 to over 17,000 kilometers, with some flights lasting as long as four months. Birds who migrate across oceans may soar without stopping for over 100 hours. Sleeping and eating on the fly, they navigate the endless ocean by the stars, wind currents, and Earth’s magnetic field. Tracking the specifics of these epic expeditions is notoriously difficult. And while birds often take the most direct route possible, storms and human development can alter their paths, further complicating our attempts to chart migration.