Correction Appended
Donald G. McNeil Jr. answered select reader questions about this article. Read his answers.
Orley R. Taylor, top, who runs the Monarch Watch project at the University of Kansas, monitors the butterflies and is leading efforts to create way stations, above, for them to use during migration.
LAWRENCE, Kan. — Pinching a bright orange butterfly in one hand and an adhesive tag the size of a baby’s thumbnail in the other, the entomologist bent down so his audience could watch the big moment.
“You want to lay it right on this cell here, the one shaped like a mitten,” the scientist, Orley R. Taylor, told the group, a dozen small-game hunters, average age about 7 and each armed with a net. “If you pinch it for about three seconds, the tag will stay on for the life of the butterfly, which could be as long as nine months.”
Dr. Taylor, who runs the Monarch Watch project at the University of Kansas, is using the tags to follow one of the great wonders of the natural world: the annual migration of monarch butterflies between Mexico and the United States and Canada.
The northward migration this spring was the biggest in many years, raising hopes of butterfly enthusiasts throughout North America. But a drought in the Dakotas and Minnesota meant that not nearly as many butterflies started the return trip. And without the late-summer hurricanes that normally soak the Texas prairies and sprout the nectar-heavy wildflowers where the monarchs refuel, many are presumably finding that leg of the journey a death march. Dr. Taylor has already halved his prediction for the size of the winter roosts in central Mexico, to 14 acres from 30.
Nevertheless, the 4,000-mile round trip made by millions of monarchs holds a central mystery that Dr. Taylor and a network of entomologists are trying to solve.
The butterfly that goes from Canada to Mexico and partway back lives six to nine months, but when it mates and lays eggs, it may have gotten only as far as Texas, and breeding butterflies live only about six weeks. So a daughter born on a Texas prairie goes on to lay an egg on a South Dakota highway divider that becomes a granddaughter. That leads to a great-granddaughter born in a Winnipeg backyard. Come autumn, how does she find her way back to the same grove in Mexico that sheltered her great-grandmother?
Wildebeest, in their famous migration across the Serengeti, learn by following their mothers — or aunts, if crocodiles get Mom. But the golden horde moving south through North America each fall is a throng of leaderless orphans.
Birds orient themselves by stars, landmarks or the earth’s magnetism, and they, at least, have bird brains. What butterflies accomplish with the rudimentary ganglia filling their noggins is staggering.
They are one of the few creatures on earth that can orient themselves both in latitude and longitude — a feat that, Dr. Taylor notes, seafaring humans did not manage until the 1700’s, when the clock set to Greenwich time was added to the sextant and compass.
All monarchs start migrating when the sun at their latitude drops to about 57 degrees above the southern horizon.
But those lifting off anywhere from Montana to Maine must aim themselves carefully to avoid drowning in the Gulf of Mexico or hitting a dead end in Florida. The majority manage to thread a geographical needle, hitting a 50-mile-wide gap of cool river valleys between Eagle Pass and Del Rio, Tex.
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To test their ability to reorient themselves, Dr. Taylor has moved butterflies from Kansas to Washington, D.C. If he releases them right away, he said, they take off due south, as they would have where they were. But if he keeps them for a few days in mesh cages so they can see the sun rise and set, “they reset their compass heading,” he said. “The question is: How?”
The skill is crucial because of storms. For example, 1999 was a banner year for monarchs on the East Coast; they were blown there by Hurricane Floyd.
Dr. Barrie Frost, a professor of neuroscience at Queens University of Canada, is fairly certain that they don’t use the earth’s magnetic field or the sky’s polarized light. He builds butterfly flight simulators, big barrels open to filtered sunlight with an airflow that a butterfly must navigate with a tiny wire glued to it. Computers sort out the random flitting to say which direction they were aiming for. Repolarizing the light or flipping the magnetic field with a coil does not redirect them, he said.
Dr. Frost believes that sun reckoning launches the monarchs generally only to the south, while mountain chains and the Gulf of Mexico funnel them toward southern Texas.
But once in Mexico’s mountains, they gain elevation and make several sharp turns. Dr. Frost suspects that they are guided by the smell of the previous year’s corpses.
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Correction: Oct. 7, 2006
A related video about the annual migration of monarch butterflies inaccurately described the butterflies’ life cycle. Monarch butterflies lay eggs on milkweed plants. Monarch caterpillars do not lay eggs.
