The Monarch butterfly (Danaus plexippus) is a very familiar species due to its size and striking pattern of orange, black, and white. It also is unusual because, unlike most butterflies, some Monarchs migrate for the winter. Life begins when eggs are laid on a host plant that will be used by the caterpillars.
Milkweed (genus Asclepias) is the preferred food plant for the caterpillars of the Monarch butterfly, but the milkweed is a toxic plant. The toxins created by the plant, called glycosides, are its defense--intended to keep animals from being able to eat the plant. Because only the caterpillars of the Monarch have adapted to be unaffected by the defense, they have no competition for the food source. Monarch caterpillars are able to eat leaves of the milkweed and store the glycosides in their own bodies, which makes the caterpillar toxic. Adult monarchs retain the toxins, but the obvious coloration of the Monarch butterfly makes it an easy target for a predator such as a bird. If a bird eats a Monarch butterfly, the toxic plant glycosides stored in the butterfly make the bird sick. The bird may vomit but does not die. Remembering the color pattern of the butterfly, the bird learns from the experience and no longer is interested in eating Monarchs. Within that bird’s territory, other Monarchs can fly about unmolested.
Another butterfly, smaller than the Monarch but mimicking its color pattern, can be found in areas inhabited by the Monarch. Compare the left photo of the Monarch to the right photo of its mimic. This mimic, called a Viceroy (Limenitis archippus), does not feed on milkweed and is not toxic. It would be acceptable food for a bird, but if the bird has already learned not to eat a Monarch, the bird will leave the Viceroy alone as well. If a bird ate a Viceroy first, it might not learn that the color pattern is meant to be a warning. That would make the mimicry less effective, so there are fewer Viceroys than Monarchs. Lower numbers of Viceroys increase the chances that the first contact a bird has would be with a Monarch. As fall approaches, some individual Monarchs begin a migration. The most famous migration takes the butterflies to Mexico, but eastern populations also overwinter in Florida and along the coast of Texas. The migration period extends from about August through October.