To read the full article in the New Yorker, go to Monarch butterflies migrate.

Walking around my family’s property in upstate New York last summer, I noticed something I hadn’t seen in years: scores of monarch butterflies flying around the milkweed that rings the perimeter of the yard. Six months later, at the end of January, biologists attending the Trinational Monarch Science Meeting, in Mexico City, confirmed what I and many others had been seeing throughout the summer and fall: the eastern monarch population was a hundred and forty-four per cent larger than it had been a year earlier. The announcement offered a modicum of hope amid dire warnings of mass extinctions and ecological catastrophe. Just two weeks earlier, the Xerces Society for Invertebrate Conservation had issued a finding that the monarch population west of the Rockies dropped by around ninety per cent in the past year and is on the verge of collapse.

Though they weigh less than a gram, they travel thousands of miles each fall to overwintering sites that provide the right microclimate to enable them to survive for months with little food or water. To track these migration patterns, citizen scientists have been gluing small tags on monarchs’ wings since the nineteen-fifties. The data from recovered tags is continually overlaid on maps that show where the butterflies are going and where they are coming from. That’s how we now know that monarchs east of the Rockies typically start the journey in Canada and the upper Midwest, aiming for the Oyamel-fir forests in Mexico’s Transverse Neovolcanic Mountains. In the spring, the butterflies lay eggs in Texas before dying; successive generations move northward in a kind of relay race that follows the proliferation of milkweed, their host plant. Monarchs that begin their journey west of the Rockies do something similar: after wintering on the coast of California, shielded by stands of eucalyptus or Monterey pine, they move inland to the Central Valley, but also north to Washington State and southern British Columbia, and to Nevada, Arizona, Idaho, and possibly Montana….