The patches of red in these false-color Landsat images are the forests where monarch butterflies spend the winter. Starting in late summer and fall, monarchs in the United States and Canada migrate south to Mexico. Some travel up to 3,000 miles. The delicate insects are capable of flying 50–100 miles a day.
Cold weather drives the monarchs to head south to hibernate for the winter. They head for the only habitat suitable for their hibernation, oyamel fir (Abies religiosa) forests. These forests grow in only small areas of mountain tops in central Mexico, about 3,000 m above sea level. The monarchs usually cover whole trees as they keep each other warm. The short needles of the oyamel fir allow the butterflies to cluster together better than they could on flat-needled cedars or long-needled pines.
The oyamel forests provide a microclimate for the butterflies. Temperatures stay above freezing. If the temperatures were lower, the monarchs would have to use their fat reserves. The humidity provided by the forest also keeps them from drying out.
The monarchs stay in Mexico from about November to March. In the spring, they fly back north. On the way, they lay eggs on milkweed. These eggs hatch into caterpillars, who devour the milkweed leaves, then metamorphose into monarchs. These monarchs live about 5 weeks or so.
What’s especially amazing about the monarch migration is that not all monarchs take part in the journey. And unlike birds and whales, the monarchs that do complete the migration only make one round trip.
The great-grandchildren of the overwintering monarchs migrate south the next fall, somehow landing at the same trees as their ancestors. Without the use of maps or satellite imagery!
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