As summer fades to fall, the great Monarch migration to Mexico begins and so does June Bernard's most delicate work.

In one hand, she gently holds the gauzy, still-damp wing of a Monarch recently emerged from a chrysalis that looks like a thumb-sized Chinese lantern. With the other, Ms. Bernard uses a toothpick to pick up a sticky, white tracking tag slightly bigger than a ladybug and places it on the underside of an orange-and-yellow, black-veined wing that's as bright as stained glass.

She softly presses it in place before releasing the butterfly unharmed. It walks along her hand, slowly unfurling the wings it will use to sail south on the wind with 100 million or so other Monarchs that roost for the winter in select groves of oyamel trees in Mexico's Transvolcanic Mountains.

The tag, encoded with numbers and letters identifying the location and name of the tagger, is supplied by Monarch Watch, an 18-year-old University of Kansas program that documents the still mysterious long-distance Monarch migration and promotes conservation of butterfly habitats that are threatened at both ends of the trip.

"Monarchs are not endangered yet, but they are a species of concern because of development and pesticide use here and logging of their sanctuaries in Mexico," said Ms. Bernard, a docent and educator at the Pittsburgh Zoo who has been tagging up to 100 Monarchs a year for 12 years.

Around her home in Hampton, she grows milkweed, which Monarch larvae feed on exclusively. She's been tagging butterflies for eight years at the zoo, where she recently held a lecture and class that attracted 77 people who tagged and released 85 butterflies.

The zoo's horticulture department also helps by including milkweed in plantings around the grounds and even its parking lots.

Each year an estimated 10,000 Monarch Watch volunteers tag about 80,000 butterflies in the eastern half of the United States and send them on their journey of almost 2,000 miles.

Unlike most other insects in temperate climates, Monarchs cannot survive cold winters and migrate to milder winter roosts. The Monarchs living in southwestern Pennsylvania are leaving now. Those west of the Rocky Mountains fly to groves of trees along the southern California coast, while those in the East fly to Mexico.

They are the only butterflies to make such a long, two-way migration every year, flying and gliding in massive groups -- often to the same grove of trees or even to the same tree.

Such a long-distance migration -- the kind usually associated with birds or whales -- is even more amazing because Monarchs only make the round-trip once. Yet their descendents, several generations removed, will find their way to the same destination on their trips.

Scientists have yet to figure out how that pinpoint homing system works, but the tagging program is providing valuable data about flight triggers and pathways, said Monarch Watch Director Chip Taylor, a professor of ecology and evolutionary biology at the University of Kansas.

"We're just getting geared up to analyze the data, but preliminary indications are very exciting," Mr. Taylor said. "It appears that the timing and pacing of the migration is very consistent from year to year and is not dependent on the weather as had been thought. The Monarchs are very predictable."

What's also increasingly evident, according to Monarch Watch, is that butterfly populations are threatened by development of rural habitats and pesticide use in the Northeast, which not only eradicates milkweed but also can kill the butterflies. In some more urban areas, smog also damages the plants on which the larvae feed.

Threats to habitats in Mexico also are severe. Monarchs are known to congregate in 11 to 14 wooded sites, each containing several acres. The oyamel trees on which the butterflies cluster are valuable sources of lumber and often are cut down, even in government-designated butterfly sanctuary areas where cutting is illegal.

"There's a strong demand for lumber in the area where the butterflies roost," Mr. Taylor said. "The people who live there make only $10 a day, so it's easy to see how they'd be tempted to cut down the trees worth about $300 each."

Mexican organized crime operations also are known to carry out nighttime raids on the forest, employing chain saws and trucks to clear trees from several acres in a single night. Logging not only removes the roost trees but opens up the forest canopy and makes the Monarchs more vulnerable to freezing weather.

In December 1995, an estimated 5 million to 7 million Monarchs died after a freak snowstorm hit their overwintering sites. Snowstorms in 1992 and 2004 killed similar numbers.

Mr. Taylor said this year is the worst for Monarch populations since 2004, when the winter kill was followed by a cold summer, which depressed breeding.

"It's been the same this year. We had a cold April, May and June when the butterflies were coming north and then drought conditions in Minnesota," he said. "The result is falling populations."

In the 18 years since Monarch Watch began, more than 1 million monarchs have been tagged. About 12,000 of those tags have been recovered.

That's around a 1 percent return rate, almost exclusively from the wings of Monarchs that have died in Mexico and been found by local residents who are paid 50 pesos -- about $3.65 -- for each tag they turn in.

One tag, "CHG024," was found on March 4, 2004, in El Rosario, Mexico, 1,872 miles from where Ms. Bernard had pressed it onto the underside of a wing. Although she's tagged close to 1,000 butterflies, it's the only one of her tags that's been recovered.

"It's like playing and hitting the lottery. I was thrilled when I got the letter telling me the tag had been found," said Ms. Bernard, who plans to visit the Mexican sanctuaries in February 2011.

"We send the butterflies off and you just don't know what will happen. Finding the tag justifies the effort and lets us know we're doing good work."

For more information, go to www.monarchwatch.org.