For years, people have worried about North America’s monarch butterflies. The black-and-orange insects have been on the decline since the 1990s. But their numbers are finally on the rise.

Every year, monarchs head from the U.S. to Mexico to spend the winter. There, they gather on the branches of fir trees to hibernate. Scientists estimate the monarchs’ population by measuring the area they cover. Last winter, the monarchs occupied 10 acres of forest—an area 3.5 times greater than the year before. “We are very excited and encouraged by this good news,” says Joanna Gilkeson of the U.S. Fish and Wildlife Service.