Experts in Mexico said Tuesday there is a tentative sign of hope for the mass migration of monarch butterflies, whose numbers dropped to their lowest level ever last year.

The head of Mexico’s nature reserves, Luis Fueyo, said the first butterflies have been seen entering Mexico earlier than usual this year.

Fueyo said it is too early to say whether butterfly numbers will rebound this year from a series of sharp drops, but noted “this premature presence could be the prelude to an increase in the migration.”

He said the first butterflies have been sighted in the northern border state of Coahuila. Most normally arrive in October from the United States and Canada, where they spend the summer.

“This year, we are seeing them present in Mexican territory earlier than usual,” Fueyo said.

By November they settle in mountaintop forests where they spend the winter. Fueyo said authorities will wait to make a definitive count after the butterflies have settled in completely, something that usually occurs by December.

In February, Mexico, the United States and Canada agreed to form working groups on the conservation of Monarch butterflies, after steep and steady declines in the previous three years. Last year, the black-and-orange butterflies covered only 1.65 acres (0.67 hectares) in the pine and fir forests west of Mexico City, down from more than 44.5 acres (18 hectares) at their recorded peak in 1996.

Because the butterflies clump together by the thousands in trees, counting individuals in near impossible; instead they are counted by the area they cover.

The head of international affairs for Mexico’s Environment Department, Enrique Lendo Fuentes, said the three nations “will probably have a joint plan of action before the end of November.”

The plan, experts said, would be to create a corridor of milkweed-friendly areas along the entire three-nation migratory route. That is important, because the butterfly has to reproduce along the way; the same generation doesn’t make the entire trip.

For its part, Mexico has already taken a number of steps, like setting up a network of observers to track monarchs from the time they enter the country, to try to identify milkweed patches that the butterflies might use along their way to their mountain wintering grounds. Activists have launched a web site where Mexicans can report and share sightings.

Monarchs lay eggs on milkweed leaves that hatch into caterpillars, morph into chrysalises and transform into butterflies. The butterfly’s dramatic decline is being driven by widespread planting of genetically engineered, herbicide-resistant crops that tend to drastically decrease the amount of milkweed available.

Mexican writer and environmentalist Homero Aridjis, said the butterflies face a number of challenges, including climate change and illegal logging in Mexico, in addition to the loss of habitat in the United States.

“We are alarmed, because we don’t yet what is going to happen” with butterflies, whose migration — but not the existence of the species — is consider at risk of disappearing.

“We don’t know what size population is going to come” to Mexico, “so we are a little alarmed.”