In the spring of 1949, in a coastal town in Lower Saxony, ornithologist Gustav Kramer trapped a few migratory songbirds in cages and put them outside. Every night, he noticed, the birds oriented themselves in the same direction, wings partially spread and quivering, their beaks tilted skyward. They hopped on their perches, agitated and excited, fluttering to the side of the cage facing their migration route. And because songbirds use the North Star to navigate, they did it only after sunset, when they could see the night sky. Kramer called the condition zugunruhe, a German compound of hug (to move) and unruhe (unrest). Migratory restlessness.

It happens in people, too. There’s a time to leave, when you feel a stirring. For most, it’s in the fall, when, as Joni Mitchell sang in “Urge for Going,” “The sun turns traitor cold / And all trees are shivering in a naked row.” Others feel it in the spring; they are pulled outdoors when the land is unlocked from winter, when the ice and cold air roll away to leave a landscape filling with summer—a time to wander, to follow a path through the forest, to find a swimming hole and sun-warmed stones, to walk up a mountainside and fall asleep in the grass.

In songbirds, the urge to leave is signaled by a cocktail of hormones and environmental cues, mainly a steep decline of melatonin at night. In the spring, they fly to the northern hemisphere to raise their young on the bloom of insects wafting through the air. When winter starts to strip the land of food and the chicks have grown, everyone returns south, to the berries and seeds and bugs of South America, or Africa, or southern Asia. They’ve been doing this for millennia, inheriting the urge and ability to cover thousands of miles, over mountains and oceans, with a precision that still baffles scientists; an arctic tern will migrate the distance of the moon and back three times in her lifetime. It’s an instinct so sharp and ancestral that some birds still end their migrations at the edge of where the ice caps once were, landing on the far side of a wall melted 15,000 years ago.

Zugunruhe in people doesn’t always precede migration. You might just feel restless in the mind, a wandering imagination shaped by the changing seasons. “I ask to be melted,” Henry David Thoreau journaled on an early April morning in 1852. He went on to explain his zugunruhe like this: “A few weeks ago, before the birds had come, there came to my mind in the night the twittering sound of birds in the early dawn of a spring morning, a semi-prophecy of it, and last night I attended mentally as if I heard the spray-like dreaming sound of the midsummer frog and realized how glorious and full of revelations it was. Expectation may amount to prophecy.”

Summer was coming; he felt it in his dreams. Maybe Gustav Kramer’s caged songbirds felt that too: their bodies not only shaking with anxiety to go blindly north, to follow the stars, but with the waking dreams of expectation, of the prophecy of early summer mornings beyond the horizon, of food and partnership, sex and parenthood, of perfect timing. When we want to take flight, perhaps it is just to escape the cold or our winter-bound homes—or maybe, as with Thoreau and the songbirds, it’s a prophecy: We sense that something better is beyond the horizon, if only we take flight.