At El Rosario reserve, one of five butterfly sanctuaries in Mexico, officials expect the insects to occupy far more forest this year than the 2.2 hectares (5.4 acres) they took up last year, which saw the smallest migration in 14 years.

"There is good news for the monarch butterfly this year," said Eduardo Rendon, of the Monarch Program that brings together government officials and environmental groups. "The omens are there will be many more, after last year's adverse weather meant there were so few."

Experts should know by December how much territory will be occupied by the monarchs this year, the best indication of their numbers. The largest migration on record was in 1996-97, when the insects took up 18 hectares (44 acres).

The migration has long been a focus of study and source of mystery for scientists and wildlife watchers. Not one butterfly makes the round-trip journey, and the descendants of those who start it head instinctively for a place they have never been.

After leaving Mexico, it takes three or four generations of monarch butterflies to reach their summer grounds in Canada and the northern United States.

The last generation, which has a longer life span, then makes the journey south to Mexico for the winter.

The fragile insects are not considered in danger of extinction, but their numbers are threatened by the use of pesticides in the United States and Canada, and by logging that erodes their wintering grounds in Mexico.

Wildlife officials are working to provide tourism jobs to poor residents around the sanctuaries, which draw some 120,000 visitors a year.

"They know the butterflies are not rivals, because about 2,000 people are benefiting from tourism jobs," Rendon said.

Still, residents say they are not provided sufficient incentives to protect the reserves rather than take advantage of lucrative timber resources.

Earlier this month an ultralight aircraft that had accompanied the butterflies from Canada landed in another Mexican reserve with the first arrivals.