The number of monarch butterflies wintering in Mexico is up for the second year in a row but their famous migration remains imperiled, researchers said in a press conference in Mexico City today. Every year, millions of butterflies journey from their breeding grounds in the U.S. Midwest and Canada to their winter habitat in the oyamel fir and pine forests of central Mexico. Beginning in 1993, researchers have ventured into those forests and measured the total area covered by the butterflies, the established proxy for their numbers. According to the most recent survey, conducted in December 2015 and released today, monarchs covered 4 hectares of forest this winter. That’s nearly four times more area than the 1.13 hectares the butterflies occupied last year, indicating that the population continues to rebound from the all-time recorded low of 0.67 hectares in the winter of 2013–14. But monarch numbers remain low compared with the late 1990s and early 2000s, when the butterflies often covered 8 hectares or more. The main threat to the butterflies’ 4000-kilometer journey continues to be the loss of milkweed in the United States, says Omar Vidal, the director general of the World Wildlife Fund Mexico, which administers the annual count. Monarchs lay their eggs exclusively on milkweed, which used to thrive in between rows of crops in farmers’ fields. But since farmers have adopted herbicide-resistant corn and soybeans, they have been able to spray their fields with powerful chemicals that have all but wiped out milkweed where the monarchs need it most.