Monarch butterflies migrate to a surprisingly small number of destinations. The eastern population, which breeds across the central and eastern United States and southern Canada, flies to mountain forests in central Mexico. The western population, found west of the Rocky Mountains, heads to coastal California. Some smaller populations skip the migration entirely and live year-round in warmer regions.
The Eastern Population: Central Mexico
The vast majority of North American monarchs belong to the eastern population, and they all funnel into a tiny area of central Mexico for the winter. Their destination is the oyamel fir forests that grow on mountaintops roughly 10,000 feet above sea level in the states of Michoacán and México. This habitat is so specific and so limited that scientists measure the population each winter by counting how many hectares of forest the butterfly clusters occupy.
These forests exist within the Monarch Butterfly Biosphere Reserve, a protected area that has been monitored since the early 1990s. In December 2024, monarchs occupied 1.79 hectares (about 4.4 acres), nearly double the 0.9 hectares measured in 2023. That sounds encouraging, but it remains well below the decade average of 2.81 hectares, which itself is far smaller than the colonies researchers documented when monitoring began.
Why Oyamel Fir Forests Matter
Monarchs don’t just happen to end up in these forests. The oyamel firs create a microclimate that keeps the butterflies alive through winter. At night, the tree trunks average about 1.4°C warmer than the surrounding air, acting like a slow-release heat source that researchers have called the “hot water bottle effect.” That small temperature buffer protects the butterflies from freezing, especially when they’re wet from rain or fog. During the day, the same trunks stay about 1.2°C cooler than the ambient forest temperature. This prevents the monarchs from warming up too much and burning through their fat reserves, which they need to survive the full 154-day wintering season.
Butterflies that cluster on the thick trunks survive winter storms at substantially higher rates than those clinging to outer branches, where wind strips away heat. The forest canopy also blocks direct sun and rain, creating a stable pocket of cool, humid air that keeps the monarchs in a low-energy resting state for months.
The Western Population: Coastal California
Monarchs living west of the Rockies take a shorter trip. They migrate to overwintering sites scattered along the California coast, stretching from Mendocino County in the north down to northern Baja California, Mexico. Instead of fir forests, these butterflies cluster in groves of eucalyptus, Monterey pine, and Monterey cypress trees near the ocean, where the mild coastal climate keeps temperatures stable.
The western population is far smaller than its eastern counterpart and has experienced steep declines in recent decades. Annual volunteer counts at California overwintering sites have repeatedly recorded historically low numbers, though occasional upticks have offered some hope.
Populations That Don’t Migrate at All
Not every monarch makes the journey. Non-migratory populations live year-round in southern Florida, parts of Arizona, southern California, Mexico’s Yucatán Peninsula, Central America, and Caribbean islands including Puerto Rico and the Dominican Republic. These butterflies breed continuously wherever milkweed is available and never enter the suspended state that defines the migratory monarchs’ winter. In Costa Rica, monarchs make short regional movements but remain isolated from the main migratory population.
How They Get There
Eastern monarchs travel between 1,500 and 3,000 miles to reach Mexico, covering 50 to 100 miles per day over a journey that can take up to two months. The generation that makes this southward trip is biologically different from the summer butterflies that came before it. Often called the “super generation,” these fall monarchs live six to eight months instead of the typical two to five weeks, and they delay reproduction until the following spring. They are the only generation that completes the entire southward migration in a single lifetime.
The return trip works differently. No single butterfly flies all the way back north. Instead, the overwintering monarchs head north in spring, lay eggs, and die. Their offspring continue northward, breeding and dying across successive generations until monarchs repopulate their full summer range by midsummer.
How Monarchs Navigate
Monarchs find their way using a sun compass corrected for the time of day. They track the sun’s position and the patterns of polarized light in the sky through specialized photoreceptors in a small region at the top of each compound eye. Their brains integrate this directional information to maintain a consistent southwest heading.
What makes this compass work over the course of an entire day, as the sun moves across the sky, is a built-in clock. Remarkably, the circadian clocks that provide this time correction are located in the antennae, not the brain. Monarchs with painted or removed antennae lose their ability to orient correctly, even though they can still fly. There is also evidence that monarchs possess a magnetic compass based on light-sensitive proteins in their eyes, though the sun compass appears to be the primary navigation tool.
Threats to Overwintering Habitat
The three main pressures on monarch populations are the widespread use of herbicides (which destroys milkweed along breeding routes), climate change, and habitat loss from land use changes. At the Mexican overwintering sites, illegal logging has historically thinned the oyamel fir canopy, reducing the thermal protection the trees provide. Climate change threatens to push the suitable temperature zone higher up the mountains, and since the firs already grow near the summits, there may be nowhere left for the forest to shift.
In California, coastal development and the removal of overwintering tree groves have reduced available habitat. Because both the eastern and western populations concentrate in such small areas during winter, even localized damage to these sites can affect millions of butterflies at once.