The last monarchs of the summer are hatching right now. Almost every one will fly to a mountainside in Mexico it has never seen. Here is that journey, stage by stage.
Eggs laid in mid-August become the final generation of the year. Ordinary summer monarchs live two to six weeks. This brood is built for something far longer.
Shortening days switch off its reproduction before it even emerges. Freed from laying eggs, this generation can live up to eight months rather than a few weeks.
Monarch caterpillars eat milkweed and nothing else. As nights cool, the milkweed toughens and dies back, and there is suddenly nothing left in the north to stay for.
They switch to nectar, favouring late bloomers like goldenrod, and bank it as body fat. That fat has to cover the whole flight south and the entire winter after it.
Monarchs steer by the sun, correcting for its movement across the sky using a body clock housed in their antennae. Under heavy cloud they fall back on Earth's magnetic field.
Rather than flapping the whole way, they ride rising columns of warm air and glide between them. The record for a single tagged monarch is 265 miles in one day.
At dusk they drop into sheltered trees and pile onto the branches in their thousands. The same roosting sites are used year after year, by butterflies that never overlap.
Volunteers stick numbered tags on monarch wings each autumn. Butterflies tagged in Canada turning up in Mexico is how the route was finally pinned down, in 1975.
Survivors reach oyamel fir forests in central Mexico, 2,400 to 3,600 metres above sea level. The cool, damp air holds them just above freezing without drying them out.
They cluster until spring, fly north and lay eggs in Texas. No single butterfly makes the round trip: it takes three or four short-lived generations to reach Canada again.