American naturalists have long recognized that monarch butterflies migrate southward in the fall but they had no idea whether they wintered somewhere or simply laid their eggs and died out along their journey. Then, in 1975, huge congregations of monarchs were discovered in the Sierra Madre Mountains, west of Mexico City. Twelve wintering sites were discovered, all in pine forests near an elevation of 9000 feet; within these colonies, naturalists documented monarch concentrations of up to 4 million butterflies per acre. We now know that virtually all monarchs east of the Rocky Mountains winter in this area, leaving the species vulnerable to cold weather spells, regional pollution and local logging activity.
Monarchs that survive the winter breed in March and the migrants that we observe in spring represent a mix of last autumn's adults and younger generations. Why such small, fragile creatures make such a long journey to congregate in such a limited area remains a mystery.
