Butterfly

A migration done in shifts

A single monarch never makes the entire round trip from Canada to Mexico and back. Up to four or five generations are required: short-lived summer butterflies move north, while a special “super generation” emerging in late summer postpones reproduction, lives 8 months, flies south to overwinter in the oyamel fir forests of central Mexico, and starts the trip back north in spring.

The puzzle is that this generation has never visited Mexico — it is genetically programmed to find a forest none of its ancestors for several generations have seen.

Toxic by diet

Monarch caterpillars eat only milkweed leaves, sequestering cardiac glycosides from the plant in their tissues. The adults retain the toxin, making them unpalatable to most bird predators. The viceroy butterfly mimics the monarch’s coloration to share in the protection — a classic example of Müllerian mimicry.

How they navigate

Monarchs use a time-compensated sun compass to navigate, combining the position of the sun with an internal circadian clock so they can keep heading southwest even as the sun moves across the sky. They likely also detect the Earth’s magnetic field as a backup on cloudy days.

Decline

The eastern monarch population has declined by roughly 80% since the 1990s. Causes include herbicide-driven loss of milkweed in Midwestern farmland, illegal logging of overwintering forests in Mexico, and climate-driven storms at the wintering sites. The species was listed as Endangered by the IUCN in 2022.