Millions of orange and black monarch butterflies migrate to Mexico on a 2,000-mile (3,220-km) journey each year from Canada across North America.
But the butterflies occupied only 2.1 hectares in December 2020, compared with 2.8 hectares the previous winter season, the conservationist group WWF said in a statement.
Climate change had a "considerable impact" on the butterflies' migration pattern, as well as the reduction in their milkweed breeding habitant in the United States, the WWF said.
Mexico's western state of Michoacan is home to the country's largest monarch butterfly reserve, typically a major tourist attraction. Visitors to the reserve have plummeted during the pandemic, sanctuary officials said.