Passage
Each autumn, across eastern North America, millions of orange-and-black monarch butterflies begin a journey of remarkable length. Leaving gardens and roadside fields, they travel south, sometimes more than two thousand miles, to a cluster of mountain forests where they will wait out the winter. What makes the trip astonishing is not only its distance but the fact that no monarch makes it twice, and the butterflies that set out have never seen the place they are headed.
The monarch’s year unfolds over several generations. Through the spring and summer, a series of short-lived butterflies breeds its way northward, each generation living only a few weeks. Then, as the days grow shorter and the nights cooler, something changes. The butterflies that emerge in late summer belong to a special generation. They do not breed right away; instead they live for months, storing energy for the long flight south. Scientists sometimes call this the migratory generation.
How do these travelers, generations removed from the last ones to make the trip, find their way? Part of the answer is a built-in compass. Monarchs appear to steer by the sun, holding a steady southward heading as the sun moves across the sky. To do this they must correct for the time of day, and researchers think an internal clock, located partly in the antennae, allows them to make that correction. A butterfly whose sense of time is disturbed will drift off course.
The sun compass is not the whole story. On cloudy days, when the sun is hidden, the monarchs keep heading south, which suggests they have another cue to fall back on. Many scientists think the butterflies can also sense the Earth’s magnetic field, using it as a backup direction-finder. The two systems together may explain how the insects hold their course across changing weather and terrain.
Cues from the environment tell the monarchs when to leave and, roughly, where to go. Shortening day length is the main signal; cooler temperatures reinforce it. These changes prepare the migratory generation and set it in motion. But the cues explain the timing and the general direction better than they explain the precise destination — how the butterflies converge, year after year, on the same small patches of forest remains only partly understood.
When they arrive, the monarchs gather in enormous numbers, clustering on the branches of high-mountain trees whose cool, moist air keeps them in a kind of slowed-down state through the coldest months. There they rest until the lengthening days of late winter stir them awake. Then the survivors turn north, breed along the way, and hand the return journey to the generations that follow.
For all that researchers have learned, the monarch migration keeps some of its secrets. We can describe the compass, name the cues, and trace the route, yet the deepest question — how a butterfly inherits both the urge and the ability to reach a forest it has never known — remains a subject of study. The monarchs cross a continent on instructions we are still learning to read.
