of monarchs in a mountain in the state of Michoacan. The next year the Urquharts traveled to Mexico to see the spectacle themselves, and the August 1976 issue of National Geographic announced the news to the world.
The most remarkable aspect of monarch migration is that each butterfly makes the trip only once. Monarchs that leave the wintering grounds in late February or early March lay eggs on their way north, then die.
Consequently, several generations of monarchs separate those that return in the spring from those that head south in the fall. Yet somehow each fall, inexperienced monarchs return to their ancestors’ traditional wintering areas.
On the winter grounds monarchs are sluggish and inactive. They congregate on trunks of fir trees by the tens of millions. When February arrives, they still have plenty of stored energy for the trip north.
Mating occurs before migration begins, and females lay eggs on milkweed plants as they move northward. This insures a new generation of monarchs in northern Mexico and south Texas.
Subsequent generations lay eggs as they work their way north until monarchs return as far north as Canada.
The monarch migration follows the appearance of milkweed from south to north.
Though these burnt-orange and black insects are among the most beautiful butterflies, both the caterpillars and adults taste terrible to anything that eats them. In laboratory experiments blue jays vomit within minutes of eating a monarch because monarchs are what they eat.
Milkweed leaves are highly toxic to most animals. Though monarchs are unaffected by the toxin, they incorporate the poison into their own body tissues.
The poison is retained through metamorphosis so even adults have high concentrations of the foul tasting chemicals. Curiously, the toxin is more highly concentrated in the wings and exoskeleton than in the body. Thus, a predator that nips even a piece of wing learns that monarchs are distasteful.
As added protection, a monarch’s abdomen is tough and leathery, difficult for a predator to bite. These adaptations enable predators to learn that monarchs taste badly without necessarily killing the butterfly.
This also explains why we often see monarchs with badly battered wings — battle scars from the ongoing struggle between predator and prey.