Tracking the mysterious monarch

Each autumn, strong transatlantic winds carry vagrant birds to our southern shores: waders and warblers blown from their seasonal migrations down the coast of north America. With them, from time to time (notably in 1995 and 1999) come a fluttering line of butterflies glowing like stained-glass at sunset. The orange-and-black monarch butterfly, Danaus plexippus, has been snatched from the spectacular migration that remains both a world wonder and a nourishing scientific mystery.

Sue Halpern's book is remarkably well timed. Thanks to the vulnerability of its caterpillar to pollen drifting from a GM brand of maize, the monarch has lately become something of a global symbol. But as it happens, her research has been building for years, and this particular hazard warrants only passing mention.

She writes as a journalist of the new, reflective, kind who does science, and sometimes literature, a service by travelling into the field with its practitioners and making both them and their work come alive; that Barry Lopez and David Quammen come to mind puts Halpern in distinguished company. Studying the monarch requires a lot of travel, since the core of its extraordinary lifestyle is a journey of up to 3,000 miles between Canada and the mountains of southern Mexico. Unlike the migration of birds, no single butterfly ever makes the round trip; as the monarchs return north, they pass through three or four generations, hatching from eggs laid on milkweed, the plants whose toxins make them poisonous to birds.

The following autumn, without memory to guide them, tens of millions funnel in again from the vastness of eastern north America to scattered patches of Mexican pine forest as small as 50 acres. At the remote Rosario reserve, branches are bowed to the ground by the sheer weight of insects, and visitors shuffle through black-and-orange wings piled like fallen leaves.

While the mass movements of monarchs have obsessed biologists for a century, it was only in the mid-1970s that their precise wintering grounds on the mountain sides of Michoacβn became known. Even since then, the positive phenomenon of an active, north-south migration has been far from agreed.

As a habitat for hibernation, the oyamel pine forests at the 10,000-foot contour have a microclimate warm enough not to freeze the monarch and cool enough not to use up the energy of the fats stored in its abdomen. But can this justify a journey of up to 3,000 miles?

One sceptic, Adrian Wenner of the University of California, sees the migration as little more than a passive southward drift when winds go north in autumn, carrying the butterflies into the physical bottleneck of southern Mexico. But most biologists now seem to accept that monarchs do navigate their way south, gliding on thermals to save energy and perhaps using the sun for a compass.

One established route is an Atlantic flyway down the east coast of the US. At Cape May, south of Atlantic City, there seems to be a regular peak in the southward flow of monarchs in the third week of September, which matches well with the arrival of wind-blown monarchs on Ireland's south coast (big storms carry the butterflies across in as little as four days).

Sue Halpern's fluent, questing, and action-packed narrative presents a memorable gallery of biologists, but also attends to the army of amateurs whose observations of monarchs across a continent (notably those butterflies with numbered, confetti-size tags on their hind wings) are helping to fill in the map of evidence.

Their enthusiasm may be judged by a visit to the monarch's own website at the University of Kansas: www.monarchwatch.org. An index would have helped, too.

Michael Viney is the author of Another Life, the Weekend's nature and environment column