LOS SAUCOS, Mexico Dense clouds of migrating monarch butterflies used to snap branches and cast shadows across the forests of central Mexico, but severe weather is posing a new threat to the annual phenomenon.
The yearly 2,000-mile journey, which takes four generations of butterflies to complete, starts in Canada and ends in the Mexican state of Michoacan, which normally enjoys mild weather from November to March.
Millions of the insects swarm to these arid hills each year, their orange-and-black wings creating a flickering fog of color that mesmerizes locals and tourists.
"When I first saw the monarchs in their sanctuary, I thought it was more of a plague than something beautiful," said David Bernal, a guide at the Piedra Herrada resting place two hours drive west of the capital, of a childhood visit.
"I was afraid. There were so many, they clouded our path."
A loss of forests and food sources has for years thinned the number of monarchs coming to Mexico. But scientists fear that a new pattern of punishing winter storms may mark the start of an irreversible decline of the transcontinental migration.
In early February, normally one of Mexico's driest months, 15 inches of precipitation fell on hilly central regions, battering monarch reserves with snow, sleet and freezing rain.
Fewer butterflies arrived this year than ever before, and as many as half of them are thought to have perished in February. The snowstorms that recently buried U.S. cities like Philadelphia and Washington began as unseasonable Mexican rains when warm winter air became loaded with ocean moisture.
"Populations are only so resilient," said Chip Taylor, a University of Kansas entomologist who has studied the migrations for two decades.
"Will butterflies come back? Yes, but the numbers will be so vanishingly small that it may mean the end of this spectacular phenomenon," Taylor added.
The monarchs' transcontinental to and fro is woven through local myth since past generations saw the butterflies as returning ancestral souls. Today, the monarch is a proud local emblem that inspires taxi companies and soccer teams.
In three of the past 10 winters, at least half the monarch butterflies arriving in Mexico died due to the topsy turvy weather that many scientists link to climate change. Mexico will host a global climate change summit in November that aims to set binding international goals for reducing carbon pollution.
Even before strange weather became commonplace, the monarch was imperiled due to a loss of food and habitat.
As they sail across the Great Plains, monarchs survive on milkweed that is being crowded out by large-scale farming.
Meanwhile, illegal loggers clear protected land of oyamel fir trees whose slender needles are a favorite roosting place.
President Felipe Calderon, a Michoacan native, once vowed to use the army to halt logging, but Mexican forest set aside for monarchs is still being picked apart by "tree theft and mafia-style logging," said U.S. researcher Lincoln Brower of Sweet Briar College in Virginia.
Brower, 78, has studied monarch butterflies since the 1950s. He was one of the first people to see the Mexican overwinter sites after they were identified by scientists in 1975, a sight he said caused him to "practically fall on the ground."
"Now I may outlive the monarchs," he said.
(Editing by Catherine Bremer; Editing by Will Dunham)