Tiny Trackers Are Revealing the Secret Journeys of Monarch Butterflies

Every fall, monarch butterflies travel some 3,000 miles from as far as southern Canada to winter in the oyamel fir forests of central Mexico. For decades, scientists could only guess where they rested and refuelled along the way. Now tiny solar-powered tags are filling in the map.

As reported by The Associated Press on November 4, radio tags were fitted to nearly 1,750 monarchs this year. About 175 tagged butterflies had been detected in Mexico since this season's migration began.

Surprises in the data

Tracking started last year through the Project Monarch Collaboration, and it has already turned up firsts. Scientists recorded the first evidence of a butterfly from Utah, west of the Rocky Mountains, joining eastern monarchs in central Mexico. Others flew down the East Coast to Florida, crossed the Gulf of Mexico and carried on through Mexico.

This season, three tagged monarchs turned up 800 miles away across the Gulf after a cold front brought strong winds, the first such crossing recorded by Project Monarch.

"There's just all these unique stories of their paths, connections between the population, that we didn't know existed or we hypothesized but couldn't prove," said Emma Pelton of the Xerces Society for Invertebrate Conservation.

A careful, gentle process

The $200 tags, made by Cellular Tracking Technologies, weigh about as much as a drop of water. Most tagged butterflies weigh more than 400 milligrams so they are not overburdened. Volunteers and scientists cool the insects to calm them, protect their legs and antennae, and attach the transmitter with a dab of adhesive before release.

Rebeca Quiñonez-Piñón, senior scientist at the National Wildlife Federation, is leading work in the "Texas funnel," a narrow corridor many eastern monarchs pass through each fall and spring. She said a properly trained tagger keeps the butterfly "completely safe."

Independent experts note limits. Louie Yang, an insect ecologist at the University of California, Davis, said the tags show which receiver detected a butterfly rather than its exact spot, and more study is needed on any effect on behavior. Still, he called the technology "pretty remarkable."

Why it matters

Eastern monarch numbers have fallen more than 80 percent since the mid-1990s, and western numbers by about 95 percent. Knowing where the butterflies stop could help protect those places before they disappear.

"Pollinators are crucial to protect and ensure that our ecosystems remain healthy," Quiñonez-Piñón said.

Curious readers can follow individual butterflies on the free Project Monarch Science app.