

The monarch’s wingspan averages 90 to 100 mm (about 4 inches). The coloration of the orange wings, marked by black veins and a black border with two rows of spots, warns predators of the insect’s bad taste. The viceroy butterfly (see brush-footed butterfly) and the monarch share similar coloration. Indeed, like the monarch, the viceroy is unpalatable to some of its predators. Hence, it is believed that the two noxious organisms resemble one another as a form of defense against predators and that the relationship between them serves as an example of Müllerian mimicry.

In North America thousands of monarchs gather in autumn and migrate southward, sometimes traveling almost 3,000 km (about 1,800 miles) to overwinter on the California coast or in the mountains of the oyamel fir forest in Mexico. The monarchs begin to return north in the spring, feeding on nectar along the way. Eggs are laid only on milkweed plants, and a new generation hatches, matures, and continues the northward trip.



The overwintering forests of the monarch are protected within the Monarch Butterfly Biosphere Reserve, a UNESCO World Heritage site. Although logging is banned there, illegal logging and degradation of the forest have threatened the survival of monarch populations. Likewise, ongoing land development in areas along the monarch’s different migratory routes is considered a serious threat to the species’s ability to successfully perform its annual migratory feat. Some researchers also suspect that a loss of milkweed plants associated with the expansion in the early 21st century of the use of genetically modified herbicide-resistant crops in the United States has further placed the monarch’s long-term survival in jeopardy. But while the threats are many, evidence indicating that monarch populations are in decline is sparse. Scientists’ ability to determine the monarch’s population status has been challenged by difficulties in monitoring populations accurately and by a limited knowledge of naturally occurring annual and geographical population fluctuations.