As shown above, the monarch butterfly population is so dense in their mountainous, winter residence of central Mexico that they cover the trees like leaves.
Some branches noticeably sway from the weight of the myriad monarchs even though they each weigh less than a gram. Many of the butterflies on this tree have journeyed 3,000 mi (5,000 km) or further to get to this several-hundred-acre colony, near the village of Michoacan. They overwinter until March before winging their way north to the U.S. and Canada, where they’ll feed, most often on milkweed, until late summer.
They begin their southern migration in early fall, and by the first few days of November, virtually all of them have arrived in Michoacan. The butterflies that make this arduous circuit are the great-great-grand-butterflies of the ones that left the subtropics the previous spring! Genetic programming related to the monarchs’ internal circadian clock guides them, likely using the Sun as a navigation aid, to their winter destination. — Steve Spiegel / Jim Foster