Monarch Butterflies Now Wear Tiny Tags — for Scientists to Track Them in Real Time

Thanks to tiny new ultralight radio tags small enough for a fragile butterfly to carry, scientists are getting a rare, real-time look at how western monarchs move through their winter habitat.

It’s part of a breakthrough tracking effort reshaping what we know about one of North America’s most iconic migrations.

In November, the Xerces Society for Invertebrate Conservation joined with scientific collaborators to deploy roughly 100 ultra-light Bluetooth tags along the Central Coast, offering an unprecedented look at how Western monarchs move through their fragmented winter habitat.

Eyelash glue was delicately applied to each monarch’s thorax and used to attach the tiny tag.

The solar-powered tag, developed by Cellular Tracking Technologies, weighs about 60 milligrams, which works out to only about a tenth of the weight of a monarch. By broadcasting its signals to billions of Bluetooth-enabled devices, the system allows researchers to follow individual butterflies through an app called Project Monarch Science.

The public can follow along on the monarch’s journey, too. If you want to participate in tracking monarch populations, you can download the Project Monarch App, where you can log any sightings, and the Bluetooth technology in your phone will also help triangulate other butterflies that might be in the area.

In late 2024, a tagged monarch named Lionel migrated from New Jersey to Florida, logging thousands of detections along the way. “For the first time, we got a full southbound track of a monarch for over a thousand miles along that track,” said Dr. David La Puma, director of global market development at CTT.

More than 400 monarchs like Lionel have since been tagged, revealing detailed flight paths, unexpected detours and long-distance movements never documented before.

The monarch picture in California

In California, the project tagged over 40 monarchs in Santa Cruz County, covering Natural Bridges, Lighthouse Field, and Moran Lake. Additional researchers tagged monarchs in San Luis Obispo and Santa Barbara, bringing the West Coast total to about 100 individual butterflies. The effort required more than a year of preparation and buy-in from the California Department of Fish and Wildlife.

Early detections already hint at butterfly behavior that scientists had previously only theorized about. “We knew they moved between overwintering sites, but the frequency we’re seeing is just very quick,” said Ashley Fisher, conservation biologist at the Xerces Society.

According to the Bluetooth tracking, one monarch traveled more than 100 miles in just two days, passing several known winter sites and surprising researchers with long-distance movement during the overwintering season — when butterflies are hibernating, conserving energy for the upcoming spring migration.

The Western monarch population has plummeted by more than 99% since the 1980s due to habitat loss, pesticide use and human-caused climate change affecting migration and nectar sources.

Protecting the coastal groves where they overwinter is a cornerstone of recovery — but scientists have long lacked a clear view of how individual butterflies travel during the season.

This California deployment is key for understanding how Western monarchs use coastal habitat, especially as populations remain low, with a preliminary estimate of about 8,000 butterflies counted so far this winter.

By revealing which sites monarchs rely on, how often they leave to forage and where they disperse in early spring, the data will help California agencies and conservation groups identify critical habitat, guide restoration and reduce threats like pesticide exposure.

“The detailed day-to-day movements are blowing the lid off our understanding” of these creatures, Fisher said.

The tagged butterflies will also help researchers understand when and where monarchs move among overwintering sites and the surrounding areas, said Emma Pelton, senior conservation biologist with the Xerces Society.

“These details will help us identify where to prioritize conservation of western monarch habitat, as well as provide the most accurate picture of their home range to date,” she said.

Where can I see monarchs in and around the Bay Area?

For the best Western monarch butterfly viewing, head south along the coast to groves in Monterey, Santa Cruz and San Luis Obispo. Some exceptional viewing sites include:

• Pacific Grove’s butterfly grove near Monterey
• Natural Bridges State Beach in Santa Cruz
• Lighthouse Field State Beach in Santa Cruz
• Pismo State Beach in San Luis Obispo

There have also been reports of monarchs in the East Bay, at spots like:

• The Ardenwood Historic Farm in Fremont
• Berkeley Aquatic Park in Berkeley
• Oyster Bay Regional Shoreline and Point Pinole Regional Shoreline
• Albany Hill Park in Albany

Xerces has a map of all the monarch butterfly overwintering sites in California, but please note that some of these locations might not be open to the public.

What attracts monarch butterflies to a certain spot?

Monarch butterflies gravitate toward coastal areas, said Natalie Johnston, education manager at Pacific Grove Museum of Natural History, and the largest population in the state this year was recorded in Santa Cruz.

Martha Nitzberg, an interpreter at Natural Bridges State Beach in Santa Cruz, said their location is an apt example of how a particular place can offer up a welcoming, attractive environment for migrating butterflies.

First, like many other top viewing spots in the state, it’s by the ocean, where winter temperatures are moderate and frost can’t permeate the ground and vegetation.

At Natural Bridges, monarchs find extra protection from the elements, too. Located in a canyon, a series of concentric rings of trees helps block wind and protect the monarchs.

“They find a microclimate, as they don’t like wind,” Nitzberg said.

Nitzberg said they even have owls living nearby that scare off other birds that could be potential predators for these butterflies.

When is peak monarch butterfly viewing season?

Nitzberg said the butterflies start to become more numerous in the middle of October, and their visit tends to peak in Santa Cruz right around Thanksgiving. This year’s peak count at Natural Bridges, Nitzberg said, was 1,700 on Nov. 28.

At Pacific Grove, by contrast, Johnston said they recorded fewer than 200 butterflies at their peak this year.

Fall is also the best time to visit if you’re hoping to see a lot of action, as they’re actively hunting for flowers and nectar to build up stores for winter.

But keep in mind: Monarch butterflies can’t actually fly if it’s under 56 degrees Fahrenheit. So in the colder months or times of day, it’s unlikely you’ll see any fluttering around at all.

What should you know about viewing monarchs?

Nitzberg said it’s best to bring binoculars, although many state parks like Natural Bridges also set up spotting scopes and hand out some binoculars to visitors.

Without binoculars, monarchs can admittedly sometimes be hard to spot, she said. Visitors might be more likely to mistake their dark “clumps” high up in the eucalyptus trees for pinecones, until they get a closer look.

If you’re hoping to see them flying about, Johnston recommends visiting in the afternoon, when the sun has had time to warm their tiny bodies up a touch.

Why do monarch butterflies migrate?

“The sun is the key to their migration,” Nitzberg said. “The butterflies are always looking for spring.”

On their migration path, the butterflies are looking for milkweed and flowers. And as the seasons turn and plants dry up, the monarchs continue northward, inland, and, oftentimes, to higher elevations, in search of places where they’re most likely to find springtime conditions into summer.

But because butterflies live only a handful of weeks, the journey north and east can see up to four generations of monarchs passing the baton to each other as the season progresses.

Their short lifespan makes them “like little flitting beauties,” Johnston said. ”You get a glimpse of them, and then they’re gone.”

The reverse is also true: In the fall, the monarchs head south, following the sun toward warmer temperatures.

These butterflies, Nitzberg explained, experience a hormone shift on their travels south that allows them to accumulate fat and live longer. This can extend a monarch’s lifespan to as many as 9 months: a “super generation,” Johnston said.

“Instead of just going dormant like most insects do when it gets cold, they all migrate down and become social, and hang out in these beautiful clusters,” she said.

That’s also what makes them prime for tagging, Johnston explained – not only because they live long enough to get useful tracking information, but also because one butterfly’s location can reveal that of a cluster of monarchs, which can then be counted and interpreted.

“By tracking the monarch butterfly population, we can essentially use that to extrapolate how other important pollinators like other species of butterflies or even bumblebees, which require the same similar conditions, how they’re doing,” she said. “We can study them and learn from them in a way that we can’t with other insects.”

What can you do to aid monarchs’ migration?

In your own garden, if you have milkweed plants native to your area, planting them can help the butterflies find food. However, Nitzberg warned against planting non-native milkweed, as it remains active in winter and can therefore aid in the spread of disease and confuse monarchs’ migrations.

And don’t forget that protecting the caterpillar is the first step to a healthy monarch — so if you have a lot of yellow jackets or wasps, removing those can create a haven for caterpillar populations.

But it’s not just monarchs at risk, stressed Nitzberg, so creating a garden without pesticides can benefit all species of butterflies.

“Poisons are really hard on all butterflies, especially these migrating monarchs,” she said.