"Their numbers always fluctuate, but if you look at a chart of the past 10 years, it appears the trend is going lower," said Eduardo Rendon, a local World Wildlife Fund coordinator.

In a mysterious migration that fascinates biologists and delights tourists, tens of millions of bright orange butterflies make the grueling annual trip south to sit out the winter months in central Mexico's temperate fir forests.

Tracking them is hard as those that arrive in November are the great-grandchildren of those that left the previous March.

But Mexican and US biologists are studying different points along the migration route to work out what is hurting most -- US pesticides, bad weather, deforestation in Mexico, predatory birds or even climate change.

"There are many factors that could explain why there are fewer butterflies, but we don't know which ones are most important," said Eduardo Ramirez, director of the Monarch Butterfly Biosphere Reserve in Mexico's Michoacan state.

"We want to find out what is to blame and in what proportion so we can try to reverse this trend."

It is impossible to count the butterflies, which form blizzard-like clouds in several protected reserves, so the biologists guess by measuring how much space they take up.

Rendon's team is working to plot numbers of dead butterflies against tree-density and weather data.

"You can see dead butterflies that are still whole, as if nothing happened to them. We think they are dying because of climatic conditions," he said.

LOGGING RAMPANT

At El Rosario, which drew 133,000 tourists last year, the butterflies are clustered high above where they used to gather -- making them more vulnerable to cold weather -- because logging has thinned out the lower slopes.

In 2002, some 65 million died when snow fell on El Rosario.

The government says a crackdown on illegal logging, which has left bald patches across the fir-covered hills, has cut the crime in half, but environmentalists say it remains rampant.

"Deforestation is threatening the butterflies' survival, yet you still see logs piled up behind local houses," said local-born environmental campaigner Homero Aridjis.

"The loggers are armed and well-organized and they work at night. The government inspectors are scared of them so all they do is stop the lumber trucks, by which time it's too late."

For locals, many of whom are dirt-poor and unemployed, selling logs is a temptation that's hard to resist.

"Since they banned logging we all care for the woods. But there is no benefit for us. We get nothing from it," said Rosa Gonzalez, 52, a widow who struggles to feed three unemployed sons on the $6 a day she earns as a guide at El Rosario.

Many butterflies also die from harsh weather en route, and others are gobbled up by birds in Mexican forests, evidenced by hundreds of torn-off wings scattered on the forest floor.