Few insect migrations are as thoroughly documented as that of the monarch butterfly, Danaus plexippus. Every autumn, monarchs hatched in the north-eastern United States and southern Canada fly south and west to a small set of forests in the mountains of central Mexico, where they overwinter in dense, cool aggregations that can cover whole tree trunks. In the following spring, the survivors fly north again, breeding along the way, and their descendants - not the original migrants - return to the northern breeding grounds. That a four-generation cycle of butterflies, weighing less than a gram each, should navigate such a complex pattern has made the monarch one of the most intensively studied of migratory species.

Until the mid-twentieth century, the Mexican overwintering sites were not known to scientists, even though local people had observed the wintering butterflies for generations. The discovery is attributed to the Canadian zoologist Fred Urquhart, who spent decades tagging monarchs in North America and encouraging volunteers to report recaptures. Eventually, in 1975, his tags were traced to the Oyamel fir forests in the states of Michoacan and the State of Mexico, where monarchs were found in such numbers that individual trees bent under their weight. The rediscovery reshaped the conservation agenda for the species, which had previously been thought of as widely distributed and unthreatened.

Monarch migration is triggered by a combination of cues. As days shorten and nights cool in the late northern summer, hatched adults enter a non-reproductive phase called diapause, in which flight and feeding continue but egg-laying does not. This extends their lifespans from a typical four or five weeks to several months, long enough to reach Mexico and overwinter. Navigation uses the sun as a primary cue, corrected by an internal clock that compensates for the sun's movement across the sky through the day. Laboratory experiments in which butterflies are kept under shifted light cycles show that such manipulation can turn their flight direction by a predictable amount, confirming the role of the internal clock.

The monarchs use additional cues as they travel. Geomagnetic information allows some orientation under cloudy skies, and topography provides landmarks for the final approach to the mountains. Entomologist Dr. Rosa Martinez, who has worked on the monarch sites for more than twenty years, has shown that the butterflies concentrate in forests whose microclimate - cool, but rarely freezing, and sheltered from wind - allows them to survive the winter without exhausting the fat stores they have accumulated en route. The exact microclimate requires a forest of specific density and composition, which is harder to replace than a simple count of trees might suggest.

Several pressures threaten the migration. In Mexico, illegal logging reduces and fragments the overwintering forests. In the United States, the widespread adoption of herbicide-tolerant crops has coincided with a near-elimination of milkweed, the only plants on which monarch caterpillars can feed, from agricultural fields. Herbicide spraying kills milkweed that once grew between crop rows. And climate changes, including more frequent freezing events in Mexico and more erratic spring conditions on the breeding grounds, further strain the population. Estimates of wintering populations, counted by the area occupied in the Mexican forests, have fallen from nearly twenty hectares in the mid-1990s to well under five in most recent years.

Response measures have been mixed. Mexico has upgraded some of the overwintering forests to protected status, and enforcement against illegal logging has improved, though it remains incomplete. In the United States and Canada, citizen-led milkweed planting programmes have restored hundreds of thousands of acres of habitat, and farmers in several regions have adopted practices that leave verges and roadsides unsprayed. Dr. Martinez cautions that these efforts, though valuable, have yet to reverse the trend in the wintering population, and that a single catastrophic event - a hard freeze during an overwintering period, for example - could precipitate a collapse.

The monarch is a symbol as well as a biological subject. Its migration has entered school curricula across North America, and its arrival at the Mexican overwintering sites coincides, by long-standing coincidence, with the Mexican celebration of the Day of the Dead. Whether all of this public attention will translate into the scale of action the species now requires is not yet clear. What is clear is that the monarch's combination of small size, predictable pattern and measurable decline makes it an unusually legible case study of what migratory species face across the world.