Mexico City ? Scientists are marveling at the impressive comeback of Monarch butterflies, which once again are carpeting the fir trees of central Mexico with orange and black wings — despite a deadly freeze last year that killed hundreds of millions.

Hard rains and biting cold in the central states of Michoacan and Mexico in January 2002 killed 75 percent to 80 percent of the Monarch butterflies that make a 2,000-mile journey from the eastern United States and Canada.

The unprecedented numbers of deaths — some estimated as many as 500 million butterflies perished — followed by drought conditions last summer and decreased levels of butterfly sightings in the United States prompted concern that fewer numbers of the insects would arrive south of the border this year.

But the butterflies came — en masse. Scientists estimate anywhere from 200 million to more than 500 million monarchs are now hanging in enormous clusters in a 20-acre area of forest.

“That’s at least twice what we expected,” said Orley “Chip” Taylor, an entomologist at Kansas University and director of Monarch Watch, a network of Monarch butterfly researchers based in Lawrence, Kan.

“It’s a little bit of a mystery. … There’s obviously something we really have to learn about where these butterflies come from and how successful they are. … But it’s quite clear that they have recovered.”

The key to the butterflies’ comeback this year was the size of the colonies that arrived in Mexico before the killer rains of January 2002, said Lincoln Brower, a professor emeritus of zoology at the University of Florida, who has been studying Monarchs for 47 years.

A Monarch butterfly lands on a leaf in Mexico's Monarch Butterfly Biosphere Reserve in Valle de Bravo in this Nov. 28, 2002, file photo. Scientists are marveling at the impressive comeback of Monarch butterflies after a freeze last year that killed hundreds of millions.