North America’s monarch butterfly population has more than tripled in Mexico this winter, despite reaching dangerously low numbers in the past.

The butterflies covered 4.01 hectares in their overwintering habitat of pine and fir forest in a reserve in Central Mexico this season. In 2014, the insects covered 1.13 hectares, while in 2013 they covered an all-time low of 0.67 hectares.

Each year, monarchs migrate more than 5,000 kilometres from Canada to Mexico. They arrive in late October and usually return in March.

This season, the monarchs covered the area so densely they had to be counted by the area they covered instead of individually. An estimated 150 million insects made the trip this year, compared to only 35 million in 2013.

This increase is great news to North American conservationists, but the numbers are still far off from the almost 18 hectares the butterflies covered two decades ago, when there were an estimated one billion monarchs made the tropical trip.

The dramatic drop in numbers is due to a lack of milkweed, the plant on which monarchs lay their eggs and feed, and illegal logging in their overwintering habitat. Conservationists in the United States have since started to reintroduce about 100,000 hectares of milkweed, previously lost to herbicide use, but illegal logging remains a serious issue for monarchs. In 2015, nine hectares of overwintering habitat was lost to the activity.

In 2014, Canada, the United States and Mexico set a goal of having 220 million butterflies in the reserve by 2020, a task U.S. authorities believe is still within reach.

“The good news coming from Mexico makes me enormously enthusiastic,” Dan Ashe, the director of the U.S. Fish and Wildlife Service, said in a press conference this week. “It indicated that we have the capacity to save the Monarch butterfly of North America.”