Monarch

Eastern North American monarchs travel up to 3,000 miles from summer breeding grounds in southern Canada and the United States to overwintering sites in central Mexico. This journey is unique because no single butterfly completes the round trip—it takes multiple generations, with the long-lived generation born in late summer flying south and a new generation returning north in spring.

Eastern monarchs congregate by the millions in oyamel fir forests in the mountains of Michoacán, Mexico, where they cluster densely on trees to conserve warmth. Western monarch populations, by contrast, winter at hundreds of coastal sites in California, from Mendocino County to San Diego County.

Monarch caterpillars feed exclusively on milkweed plants (Asclepias species), which contain toxic compounds called cardenolides or cardiac glycosides. These toxins are sequestered in the caterpillar's body and persist into adulthood, making monarchs unpalatable and even poisonous to many predators like birds.

The eastern monarch population has declined by more than 80% since the 1990s due to habitat loss, pesticide use, and climate change. In 2022, the IUCN listed the migratory monarch butterfly as Endangered on its Red List of Threatened Species, highlighting the urgency of conservation efforts.