More monarch butterflies appear to have made the long flight from Canada and the U.S. to their winter nesting ground in western Mexico, raising hopes after their number dropped to a record low last year.

But experts still fear that unusually cold temperatures will threaten the orange and black insects.

While an official census won't be ready until mid-January, observers are seeing healthy populations of butterflies bunched together on fir and pine trees in protected sanctuaries, said Gloria Talavera, director of the official monarch butterfly reserve.

"We're encouraged, because we've seen more," Talavera said Monday.

But cold forecast for this winter, she said, "could put at risk the whole migratory phenomenon.... We will be saying a prayer each day until mid-February."

As well, the butterflies may warrant U.S. Endangered Species Act protection because of farm-related habitat loss blamed for sharp declines in cross-country migrations of the orange and black insects, the U.S. Fish and Wildlife Service said on Monday.

Monarch populations are estimated to have fallen by as much as 90 per cent during the past two decades because of the destruction of milkweed plants they depend on to lay their eggs and nourish hatching larvae, according to the Xerces Society for Invertebrate Conservation.

The loss of the plant is tied to factors such as increased cultivation of crops genetically engineered to withstand herbicides that kill native vegetation, including milkweed, the conservation group says.

Threatened by pesticides, logging

Monarchs, unique among butterflies for the regularity and breadth of their annual migration, are also threatened by widespread pesticide use and logging of mountain forests in central Mexico and coastal California where some of them winter, said biologist Karen Oberhauser from the University of Minnesota.

The Fish and Wildlife Service said on Monday a petition requesting U.S. government protections for monarchs — filed by the Xerces Society and others — "presents substantial information indicating that listing may be warranted."

The agency's initial review will take about a year to complete.

The butterflies, revered for their delicate beauty after emerging from a jade green chrysalis ornamented by gold stitching, are roughly divided into two populations in the U.S. according to their fall migration patterns.

Monarchs from east of the Continental Divide wing across 4,800 kilometres to Mexico, while those from west of the Divide in Rocky Mountain states like Idaho make a relatively shorter journey to California.

Big drop in migration since 1990s

An estimated one billion monarchs migrated to Mexico in 1996, compared with just 35 million last year, according to Marcus Kronforst, a University of Chicago ecologist who has studied monarchs.

The monarchs' navigation remains mysterious. While they are known to orient themselves by the sun's position, and by the Earth's magnetic field on cloudy days, it is unclear how new generations find their way to wintering sites they have never seen, Oberhauser said.

With files from Reuters