A tiny transmitter is now riding on the backs of monarch butterflies as they begin one of the animal kingdom's longest journeys — a flight of up to 2,800 miles from Canada and the United States to the mountain forests of central Mexico. Researchers can follow those flights in near real time using a free smartphone app called Project Monarch, and some of the butterflies carrying the new trackers were tagged at a sanctuary in Madison, Wisconsin.
The tagging effort in Wisconsin was carried out by the Southern Wisconsin Bird Alliance, according to WRAL, which reported on the app as part of a broader look at monarchs' fall migration south to central Mexico. The group operates the Goose Pond Sanctuary near Madison, and per the Southern Wisconsin Bird Alliance, it tagged 1,499 monarchs during the fall 2025 migration alone — part of more than 10,000 butterflies tagged on its protected lands since 2012.
How a Butterfly-Sized Transmitter Works
The trackers behind Project Monarch help researchers follow monarchs, according to the National Audubon Society. The system uses smartphones to support that monitoring, according to developer Cellular Tracking Technologies.
The system already has a proof of concept. In November 2024, researchers logged the first continuous high-resolution migration track of a tagged monarch nicknamed Lionel, who was tagged in Cape May, New Jersey, and whose route extended 726 miles to St. Augustine, Florida, per Cellular Tracking Technologies. Ultra-light transmitters have been deployed to trace individual fall migration paths.
A Fragile Rebound, Still Far Below Historic Levels
The tracking push comes as monarch numbers show a mixed picture. Survey results released by the World Wildlife Fund in March found that eastern monarch colonies occupied 7.24 acres of forest during the 2025–2026 overwintering season in central Mexico, a 64% jump from 4.42 acres the previous winter. The organization also reported on the 2023–2024 overwintering season.
Despite that gain, eastern monarch populations remain down roughly 80% since the 1990s, when overwintering colonies routinely covered up to 45 acres of Mexican forest, according to the same WWF report. The organization identifies illegal logging, fires, drought, extreme weather, habitat loss, herbicides, and insecticides as threats. Monarchs' fall migration is described by the U.S. Fish and Wildlife Service.
Federal Protection Still Pending
The Fish and Wildlife Service proposed listing the monarch as a threatened species under the Endangered Species Act in December 2024, alongside a Section 4(d) rule meant to allow continued scientific tagging and citizen-science programs even under federal protection. As of mid-2026, a final decision remained under extended administrative review, the agency says, leaving formal protections in limbo even as tracking data accumulates.
That uncertainty adds weight to the tracking work of Project Monarch and similar tagging programs. Community-science monarch-tagging programs are also part of the broader effort. Researchers are also using tracking technology to gather more real-time detail.
Part of a Wider Wave of Community Tagging
Part of a Wider Wave of Community Tagging Wisconsin's tagging effort is part of broader efforts to study monarch migration. Those efforts now sit alongside Project Monarch's automated network as researchers work to learn more about the insects' migration routes.
