Monarch Butterfly Overwintering in California

Caitlin Dempsey


Monarch butterflies (Danaus plexippus) are known for their mass migration, traveling thousands of miles across North America. The population east of the Rocky Mountains migrates to Mexico, while the western monarch butterfly population travels to the coastal areas of California.

Unlike many migratory species, it is not a single monarch butterfly that completes the round trip. Instead, the monarch butterfly migration cycle spans multiple generations of monarchs, driven by instinct and environmental cues.

Why is coastal California important for monarch butterfly migration?

Monarch butterflies undergo four stages: egg, larva (caterpillar), pupa (chrysalis), and adult. At no point in these stages can a monarch butterfly freeze and survive. So the monarchs migrate with the changing weather to always be in locations where the temperatures stay above freezing. Monarch butterflies west of the Rockies travel up to 100 miles a day, for a total trip of about 2,000 miles to reach coastal California eucalyptus, Monterey pine, and Monterey cypress groves.

Coastal California’s diverse geography, with its long coastline, varied topography, and distinct microclimates, creates ideal conditions for monarchs during winter. The state’s coastal areas, particularly along the Central Coast, provide a perfect combination of moderate temperatures, humidity, and forested areas that provide a wind break, that are essential for the butterflies’ survival during the colder months.

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A natural shaded relief map showing areas of conservation in California for the monarch butterfly.
The monarch butterfly “Terrestrial Significant Habitats” dataset is a part of California’s Areas of Conservation Emphasis (ACE) program, version 3.0. This dataset helps identify important land habitats across California that might be crucial areas for the survival and reproduction of the monarch butterfly. Monarch ACE data: California Department of Fish and Wildlife. Map: Caitlin Dempsey.

The microclimate in key coastal forests in California are conducive to the monarchs’ survival during winter. The temperature and humidity levels are within the range that allows monarchs to remain in diapause comfortably. These forests’ coastal locations also protects the butterflies from extreme cold and provide a stable environment.

Regions like Santa Cruz, Pismo Beach, and Pacific Grove offer a mild climate, with temperatures that rarely dip below freezing. The presence of eucalyptus, Monterey pine, and Monterey cypress trees in these areas offers the perfect roosting spots for the butterflies. These trees provide shelter from the elements and maintain a microclimate suitable for the monarchs’ survival during the colder months.

The temperature profile of California coastal forests allows monarch butterflies to enter diapause

Shorter days and cooler temperatures trigger changes in the monarchs, leading them into a state of diapause, a form of semi-hibernation. This means that during the cooler parts of the night and day, the butterflies gather in near motionless clusters and only become active once the air temperature reaches above 55°F. Diapause is a crucial physiological state in the life cycle of monarch butterflies, allowing them to conserve energy and survive on stored fat reserves through the winter months with moderate amounts of supplemental nectar.

A female monarch butterfly on an orange flower in the sun.
A female monarch butterfly rests on a flower in the demonstration garden at Natural Bridge State Beach. Photo: Caitlin Dempsey.

The coastal forests also produce native and non-native flowering plants that ensure that monarchs have access to nectar throughout their stay. The Australian eucalyptus trees was introduced into California during the mid-1850s. These tall, straight trees flower during the wintertime, providing a source of nectar that themonarchs have adapted to.

A group of orange and black butterflies drinks nectar from a white flowering tree.
The eucalyptus tree flowers during the winter months, providing the monarch butterfly with an essential source of nectar. Photo: Caitlin Dempsey.

Visiting an overwintering site in California

Natural Bridges State Park in California is one of the key overwintering sites for the western population of monarch butterflies. While there are numerous overwintering monarch butterfly groves in the coastal areas of Central and Southern California, this park is the only State Monarch Preserve in California, according to the California Department of Parks and Recreation. The presence of a eucalyptus tree grove situated in a shallow canyon at Natural Bridges State Beach provides ideal habitat for monarchs to survive the winter months.

Depending on the weather, November and December are typically the best times to see large populations of overwintering monarch butterflies at Natural Bridges State Park. I visited the monarch preserve at the very end of November shortly after a census counted 7,000 butterflies according to the docent at the grove.

The monarch preserve within Natural Bridges State Park is located at the bottom of a shallow canyon. To enter the preserve, I walked along a winding and gently sloping boardwalk to descend into the canyon. The length of the boardwalk is relatively short and is wheelchair and stroller accessible.

A view of Eucalyptus with clusters of monarch butterflies.
Can you see the monarch butterflies? In the early morning, the pale orange undersides of their wings help the monarch butterflies to blend in with the leaves of the eucalyptus trees in the grove. Photo: Caitlin Dempsey.

Entering the grove, dominated by eucalyptus trees and English ivy, I would never have guessed that 7,000 monarch butterflies were finding winter refuge in this preserve. Until the sun’s rays pierce through the upper branches of the tall trees, the monarch butterflies are clumped in large groups that blend in with the tree leaves. From the ground, these clusters of monarch butterflies look like leaves as the pale orange underside of the wings camouflages their presence when their wings are folded.

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Bring binoculars or a telephoto lens to see the butterflies up close

A close up of a cluster of monarch butterflies in the trees.
Bring binoculars or a camera with a telephoto lens to see the butterflies up close. Photo: Caitlin Dempsey.

When visiting the monarch preserve, I recommend that you bring binoculars or a camera with a telephoto lens in order to see the butterflies up close as they cluster high up in the trees. I estimated that even the lowest clusters of monarchs was hanging about 30-40 feet off the ground. The camera lens on your phone will probably not be adequate enough to see the full detail of these butterfly clusters.

A cropped closeup view of the monarch butterflies.
Monarch butterflies cluster together on eucalyptus leaves for warmth in the grove at Natural Bridges State Beach. Photo: Caitlin Dempsey.

For my trip, I brought a 70–350 mm lens for my Sony mirrorless camera to be able to view and photograph the monarch butterflies up close. If you don’t have binoculars with you, there are often volunteers down in the grove with a monitor setup where you can see a closeup view of a butterfly cluster.

A monitor shows a close up look at monarch butterflies in the trees.
A monitor set up by volunteers shows a close up view of a monarch butterfly cluster. Photo: Caitlin Dempsey.

What time of day and weather conditions are the best to see the monarch butterflies at Natural Bridges State Beach?

For monarch butterflies to fly, their muscles need to reach a certain temperature, typically around 55°F (13°C). Monarch butterflies are unable to move at all when it’s below 45°F. In cooler temperatures, especially in the early morning or during the cooler months of overwintering, their muscles are too cold for flight. Basking in the sun warms these muscles, making flight possible.

The best time to come to the grove is around 11am on a sunny day in November or December. The butterflies remain inactive until the sun starts to break through to the upper branches of the trees. I got to the grove around 10am when the butterflies were in their inactive stage, the branches of the eucalyptus trees weighed down by the number of butterflies on each of them.

A cluster of monarch butterflies with the sun dappling through.
Sun dapples through the Eucalyptus trees, warming up clusters of monarch butterflies which start to disperse. Photo: Caitlin Dempsey.

Around 11am, the sun’s rays start to reach the very tops of the eucalyptus trees. As the sun rises higher into the sky, the rays reach further and further down into the branches. Gradually, more and more butterflies start to spread their wings and fly out from the clusters. The air gradually becomes more cluttered with monarchs.

As the butterflies warm up in sunlight they start emerge into the air of the forest. The collective movement of thousands of wings beginning to beat sends clicking sounds throughout the grove.

A view up in to the sky with monarch butterflies flying and tree branches off to the side.
As the sunlight begins to warm up the grove, monarch butterflies emerge from their clusters and being flying around. Photo: Caitlin Dempsey.

Monarch butterflies spread their wings in the sun primarily for thermoregulation, a process crucial for their survival and daily activities. Being ectothermic (cold-blooded) creatures, monarchs rely on external heat sources, like the sun, to regulate their body temperature.

The monarch butterflies seek out spots on the shrubs nearby to spread their wings to absorb the heat from the sun. By spreading their wings and orienting their bodies towards the sun, monarchs absorb solar radiation more effectively. The dark scales on their wings help in absorbing heat, which raises their body temperature to a functional level.

A monarch butterfly with its wings full spread out in the sun.
A monarch butterfly will fully spread out their wings to absorb the warmth of the sun during the winter months. Photo: Caitlin Dempsey.

Warming up using the sun’s heat is more energy-efficient than generating heat through metabolic means. Since food sources can be scarce, especially during migration or overwintering, conserving energy is crucial for the monarchs’ survival. The overwintering western population of monarchs is the longest-lived population of the migration cycle. Thanks to diapause, this generation of monarchs lives between 6-8 months compared to only 2-6 weeks for the summer populations.

Monarch butterflies with wings spread in the sun on bushes.
Monarch butterflies spread their wings in the sun to warm up. Photo: Caitlin Dempsey.

While most of the activity was happening above my head, I learned it was important to also pay attention to the boardwalk around me. The occasional monarch butterfly would pause to hang out on the boardwalk.

A monarch butterfly sitting on a boardwalk.
A monarch butterfly rests on the boardwalk in the grove. Photo: Caitlin Dempsey.

The monarchs at Natural Bridges State Beach will stay in the grove until about late January, early February. The exact duration of the monarchs’ stay can vary slightly each year, depending on factors such as weather conditions and climatic changes. Warmer temperatures in early spring signal the end of diapause and prompt the butterflies to begin their migration back to their breeding grounds in the valleys west of the Rockies.

The generation flying northeast back to the valleys just west of the Rockies will leave around February and March. There are three generations of monarchs that lay eggs on milkweed plants at stops along the away before the fourth and final general of the northeasterly migration cycle reaches the spring/summer grounds. Once fall arrives, that fourth generation makes the return migration all the back to the coastal forests of California.

A Monarch butterfly on a milkweed plant.
A Monarch butterfly on a milkweed plant. Photo: Caitlin Dempsey.
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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.