VERNIGHT TEMPERATURES IN the fir forests of central Mexico hover just above freezing in January. It’s the ideal temperature for the enormous colonies of monarch butterflies that, in late August, begin their 3,000-mile migration south to this small mountainous region. They overwinter until March, when they begin the journey back to the north. This sliver of fir forest, first discovered by biologists in 1975, has a microclimate that draws the monarchs that spend their summer months in the vast swath of Canada and the United States east of the Rocky Mountains. Scientists estimate that by October, when all the butterflies have arrived, each acre occupied by a colony can hold up to 25 million monarchs. It’s thought that, even though monarch butterfly populations have dropped 84 percent in just the last two decades, more than a billion of the colorful insects still find their way home to a protected reserve
