Every spring and fall, a spectacle unfolds in the night sky as millions of birds attempt long, perilous journeys between their summer breeding and wintering grounds.

Most of the thousands of bird species that engage in this annual migration travel at night, when wind currents are smoother and the moon and stars guide their way.

The birds typically follow established flyways, generally north-south routes that offer the best opportunities for rest and refueling along the way.  Multiple bird species share these flight paths as they contend with rough weather, dehydration, starvation, and the threat of predation. (Read more about the legendary treks of migratory birds.)

Because migration is such an integral part of the avian life cycle,