“This incredible little butterfly that weighs the same as a paperclip [is] migrating hundreds to thousands of miles each direction and surviving the winter,” said Isis Howard, a conservation biologist working on the endangered species team at the Xerces Society for Invertebrate Conservation.
Santa Cruz’s Natural Bridges State Park is ranked among the top 10 western monarch overwintering sites in California — a sheltered grove where monarchs cluster to conserve energy and keep warm in the winter. Monarch butterflies, scientifically known as Danaus plexippus, soar through the sky, dancing in circles around each other and hanging like leaves from the branches of the coastal eucalyptus trees in the grove. 
A visitor observes the monarch butterfly clusters from a tree in Natural Bridges State Park.
“They come here mid-to-late October, and they’re coming here to stay [for] the winter. Here, [when] they first arrive, they fly around and get as much nectar as possible, because they need to survive the whole winter,” said Lily Aravanis, a field interpreter at Natural Bridges.
“Later in the season … we’ll have hundreds, hopefully thousands of butterflies that cluster in the trees. They all hang on to the same branch in this monarch cluster,” she continued.
The configuration of the eucalyptus trees makes Natural Bridges an easy choice for traveling monarchs. Three rings of trees scattered around the park create a still environment that blocks out wind and bad weather, and the open centers within the rings provide access to sunlight so they can warm up their wings and refuel on nectar. Although eucalyptus is an invasive species, its long leaves are perfect for tiny butterfly legs to cling to.
“Eucalyptus has the benefit of blooming in the winter. Because they’re from Australia, they have a funky timing on their blooming, which is exactly when the monarchs need the nectar the most,” Isis Howard said.
The Western Monarch Count has reported the five lowest monarch counts in the past 10 years. Overall, western monarchs have declined by 99 percent since the 1980s, as of 2026. Due to dwindling monarch populations in recent years, Santa Cruz sites have taken the lead in the top counts of western monarchs.
Where do the Monarchs go?
When the monarchs arrive at Natural Bridges, they have just completed their full migration cycle, leaving them tired and ready to nectar as much as they can before the winter. A pause in reproduction is triggered when the butterflies experience temperatures below 40 degrees for four nights in a row. This is called reproductive diapause. Up until this point, all their energy has been spent to migrate and breed.
“We call that the super generation, and it really is super. An overwintering monarch can live anywhere from six to nine months. Breeding monarchs, on the other hand, live anywhere from three to five weeks,” Howard said. “[Monarchs are] the only butterfly we know of that does a two-way migration as an adult.”
A monarch butterfly migration infographic presented at the Natural Bridges Visitor Center describes each migration generation.
Though their migration is driven by a need for warmer temperatures, nectar plants — such as milkweed — are also necessary for them to complete their journey. Without them, caterpillars that hatch from female monarch eggs cannot survive. Monarchs traveling throughout the San Francisco Bay Area search for this plant environment to protect their offspring, as milkweed doesn’t naturally grow in Santa Cruz. After the Bay, they travel across the Central Valley, the Sierra Nevadas, and even up to the border of Oregon.
“The ones who were born here are going to continue the migration up toward the Sierra Nevadas,” she continued, “where they’re going to find milkweed, lay their eggs, and then those caterpillars are going to take up the next leg of the migration.” 
A male monarch butterfly and female monarch butterfly displayed at the Natural Bridges Visitor Center.
How Does Climate Change Impact These Butterflies?
This year, a super El Niño is predicted to arrive — the largest in living memory. Warmer ocean temperatures will shift trade winds, leading to increased storm activity along the California coast. Lily Aravanis suggested that bigger storms can be detrimental to the safety of these butterflies, mentioning that severe storms can knock butterflies out of trees, leaving them vulnerable to predators and outside of their optimal range of temperatures: between 55 and 90 degrees Fahrenheit.
“Butterflies don’t heal, so any sort of damage that’s done to their wings, that’s permanent. The loss of scale due to rubbing or weather, or if a bird takes a chunk out of their wing, none of that’s going to regenerate,” said Howard.
Dena Spatz, the statewide wildlife biologist for California State Parks in the Natural Resources Division, noted that forest fires are another pressing issue, as eucalyptus is already highly flammable. Combined with worsening drought seasons in California and development projects, climate change impacts have continued to harm monarch habitats and food sources.
“We know lots of monarch groves have been cut down. Aging trees are becoming less suitable for monarchs. Milkweed habitats across the west have been lost to development,” Spatz said.
Howard also emphasizes the need for Californians to move away from pesticide usage.
“[In] 2024, there was a mass mortality event at the Pacific Grove Monarch Sanctuary, and we got our volunteers to collect some of those butterflies. We sent them off to a lab, and hundreds of butterflies died in one event due to recently applied pesticides,” Howard said.
Aside from reducing pesticide and herbicide usage, Aravanis advises people to plant nectar plants or milkweed if they are somewhere it may naturally grow (such as the Bay Area and the Central Valley) as well as visit the park that works so hard to support these creatures.
“The main butterfly season that we have here [at Natural Bridges State Park] is October through about February. The park is open seven days a week,” Aravanis said. “Folks are absolutely encouraged to come and say hello to the butterflies.”
