Tiny trackers reveal hidden paths of migrating monarchs
New solar-powered tags weighing less than a water drop are helping scientists map the exact routes of butterflies crossing the Texas funnel.
By Scott Marshall ·
Nearly 1,750 monarch butterflies carry tiny solar-powered radio tags this year, a number that finally allows researchers to trace their movements in real time. The devices, which cost $200 each and weigh about as much as a drop of water, are part of a tri-national effort to solve a decades-old mystery: where these insects stop for food and rest during their 3,000-mile journey.
The migration has long baffled scientists. For years, they could not build a clear map of the routes, daily flight distances, or overnight shelters used by the swarms moving from southern Canada to the oyamel fir forests of central Mexico. Habitat loss, pesticides, and climate change have eroded these stopovers, making it difficult to track the population without direct data.
Rebeca Quiñonez-Piñón, a senior scientist at the National Wildlife Federation, leads the study. Her team mixes live tracking with community science to monitor butterflies in the "Texas funnel," a narrow corridor through which eastern monarchs travel each fall and spring. The specific path through Texas shifts annually depending on where water, nectar, and trees are available.
The data has already produced surprises. Since tracking began last year, scientists confirmed the first butterfly in Utah, west of the Rockies, joining eastern populations in Mexico. Others were detected migrating down the Eastern Seaboard to Florida, crossing the Gulf of Mexico, and continuing through Mexico, a connection previously unseen in such volume.
Emma Pelton, a senior conservation biologist with the Xerces Society, noted that the new findings prove connections between populations that were only hypothesized before. She described the team as thrilled after one year of data, particularly because older tracking devices were too heavy or large for the insects.
The tags come with limitations. Louie Yang, an insect ecologist at the University of California, Davis, said the technology identifies where a receiver detected the butterfly, not its exact location. A detection does not guarantee the butterfly is still alive, and further studies are needed to see if the tags alter natural behavior. Only monarchs weighing more than 400 milligrams are selected for tagging to prevent overburdening them.
Eastern monarch populations have dropped more than 80% since the mid-1990s, while western populations have fallen by about 95%. Tierra Curry, a senior scientist with the Center for Biological Diversity, explained that climate change causes northern areas to stay too hot, delaying migration cues. If temperatures freeze before the butterflies reach Mexico, they rarely survive the trip. The Trump administration delayed a decision on federal protections for monarchs late last year, and a court has ordered a final decision by 2030.
