From the article:
The number of monarchs overwintering in central Mexico increased from last year’s low, according to the annual census released today by World Wildlife Fund-Mexico and partners. The butterflies were found covering an area of 4.42 acres, which means the eastern monarch population that migrates between Canada and Mexico each year shows an improvement from last year, which was the second worst year ever recorded at 2.22 acres. The annual survey measures the area of forest in which monarch butterflies hibernate each winter, providing a reliable indicator of the eastern monarch’s population status.