Passage
Each autumn, millions of monarch butterflies leave the cooling regions where they spent the summer and travel enormous distances to warmer wintering grounds. What makes the feat remarkable is that no single butterfly makes the round trip. [A] The insects that fly south have never seen the destination. [B] Their great-grandparents left it the previous spring. [C] Somehow, the route is passed down without being taught. [D] No parent survives long enough to guide the next flight.
How the butterflies find their way is a puzzle scientists are still unraveling. Evidence suggests they use the position of the sun as a kind of compass, adjusting for the time of day so that they keep heading in a steady direction.
The wintering sites are strikingly specific. Generation after generation, the butterflies cluster in the same small patches of forest, where the temperature and shelter suit their needs. Crowded together on the trees, they wait out the cold months in a resting state.
Because the journey depends on these few sites and on food plants along the route, the migration is fragile. Loss of habitat at either end, or in between, can threaten a spectacle that has unfolded for countless generations.