Monarch butterflies have made a big comeback in their wintering grounds in Mexico, after suffering serious declines, investigators said Friday.

The area covered by the orange-and-black insects in the mountains west of Mexico City this season was more than three and a half times greater than last winter. The butterflies clump so densely in the pine and fir forests they are counted by the area they cover rather than by individuals.

The number of monarchs making the 3,400-mile (5,500-kilometer) migration from the United States and Canada declined steadily in recent years before recovering in 2014. This winter was even better.

This December, the butterflies covered a total of 10 acres (about 4 hectares), compared to 2.8 acres (1.13 hectares) in 2014 and a record low of 1.66 acres (0.67 hectares) in 2013.

While the news was good, the monarchs still face problems: The butterflies covered as much as 44 acres (18 hectares) 20 years ago.

The United States is working to reintroduce milkweed, a plant key to the butterflies' migration, on about 1,160 square miles (3 million hectares) within five years, both by planting and by designating pesticide-free areas.

Milkweed is the plant the butterflies feed and lay their eggs on, but it has been attacked by herbicide use and loss of open land in the United States.

In Mexico, meanwhile, illegal logging more than tripled in the monarch butterflies' wintering grounds in 2014, reversing several years of steady improvements. Illegal logging had fallen to almost zero in 2012.

Authorities said the reserve's buffer area lost more than 20 acres (9 hectares) in 2015 due to illegal logging in one area, but said the tree cutting was detected and a number of arrests were made.

"Now more than ever, Mexico, the United States, and Canada should increase their conservation efforts to protect and restore the habitat of this butterfly along its migratory route," said Omar Vidal, director of the World Wildlife Fund in Mexico.

The forest canopy acts as a sort of blanket against the cold for butterflies that form huge clumps on tree branches during their winter stay in Mexico.

Monarch expert Lincoln Brower wrote in a research paper that the 2015 forest loss was actually 25 acres (10 hectares) in the reserve area, and said the illegal logging "questions the effectiveness of current strategies to protect the already precarious overwintering habitat of the monarch butterfly."

The logging took place in a particularly sensitive area of the reserve, and if butterflies can't find shelter there, "they may be forced into forested areas with less microclimatic protection," exposing them to potential cold and rain that can prove fatal, Brower wrote.

The migration is an inherited trait: No butterfly lives to make the full round trip, and it is unclear how they find their way back to the same patch of pine forest each year.

Some scientists suggest the butterflies may release chemicals marking the migratory path and fear that if their numbers fall too low the chemical traces will not be strong enough for others to follow.

The long-term trend is troubling. After their peak in 1996, when the monarchs covered more than 44 acres (17.8 hectares), each time the monarchs have rebounded, they have done so at lower levels. The species is found in many countries and is not in danger of extinction, but experts fear the migration could be disrupted if very few butterflies make the long trip.

Largely indigenous farm communities in the mountain reserve have received government development funds in return for preserving the 139,000-acre (56,259 hectare) reserve that UNESCO has declared a World Heritage site.

Some of the communities earn income from tourist operations or reforestation nurseries to grow and plant saplings.