The Whole Migration Fits in Three Hectares
Monarchs breeding across a continent, from Manitoba to Maine, fly up to 3,000 miles south each autumn. Almost every eastern one ends up in a few patches of fir forest on mountaintops west of Mexico City. Tradition says they arrive around Día de Muertos.
Forest area covered by every overwintering eastern monarch colony in winter 2025–26, roughly four football pitches. — WWF-Mexico & CONANP, 2026
the same colonies at their 1996–97 peak, 6× larger than this winter
rise from 1.79 ha the year before, still below the ~6 ha target
Thirty-three winters, one shrinking footprint
Each December, surveyors measure how many hectares of forest the clustered butterflies cover. That area stands in for the population.
Where this winter sits on the spectrum
On a log scale, 2.93 ha is a clear recovery from the record low and still less than half the stability target.
A reserve the size of a city, a colony the size of a park
The Monarch Butterfly Biosphere Reserve protects 56,259 ha. The butterflies used about 0.005% of that area.
Monarchs pack onto the trunks and branches of oyamel firs at about 2,900–3,300 m. The dense canopy works like a blanket, keeping them cool enough to rest without freezing. With so many butterflies on so little forest, one storm or one illegally logged slope can hit a large share of the population. In 2025–26, five of the nine colonies (2.27 ha) were inside the reserve and four (0.66 ha) outside it.
The western population is down to thousands
Monarchs west of the Rockies mostly winter in coastal California groves. Volunteers count them every Thanksgiving season.
The 2025 count found 12,260 butterflies at 249 sites, the third-lowest total since counts began in 1997. The two lower totals were 2020 (1,901) and 2024 (9,119). In 2022 volunteers counted 335,479. Estimates for the 1980s run to about 4.5 million, a published estimate rather than a count.
One generation that lives eight times longer
Summer monarchs live a few weeks. The autumn “super generation” holds off breeding and flies the whole route.
flown south by the “super generation”
USFWScovered per day on the way
USFWSmax lifespan of migrants, vs weeks for summer broods
USFWScolonies in 2025–26: 3 Michoacán, 6 State of Mexico
WWF-Mexico/CONANPof the 2.93 ha sat inside the reserve (5 colonies)
WWF-Mexico/CONANPmonarchs implied at 21.1 million per ha (modeled)
Thogmartin et al. 2017Sources & notes
- WWF-Mexico & CONANP (Mar 2026), Area of Forest Occupied by the Colonies of Monarch Butterflies in Mexico, Overwintering Period 2025–2026: 2.93 ha, nine colonies (3 Michoacán, 6 State of Mexico); five inside the reserve (2.271 ha), four outside (0.661 ha).
- WWF-Mexico, Monarch Butterfly Survey Report 2024–2025 (Mar 2025): 1.79 ha; up from 0.90 ha in 2023–24.
- Long-term overwintering series 1993–94 to 2025–26 (WWF-Mexico/CONANP annual surveys, as compiled by Monarch Watch): record high 18.19 ha (1996–97), record low 0.67 ha (2013–14).
- Thogmartin, W.E. et al. (2017), “Density estimates of monarch butterflies overwintering in central Mexico,” PeerJ 5:e3221: median 21.1 million monarchs per hectare (95% range of the source estimates is very wide). Population figures here (≈62 million for 2025–26; ≈384 million for 1996–97) are MODELED as area × 21.1 million, not counts.
- U.S. Pollinator Health Task Force (2015), National Strategy to Promote the Health of Honey Bees and Other Pollinators; Semmens, B.X. et al. (2016), Scientific Reports 6:23265: ~6 ha (≈225 million monarchs) as the recovery/stability target.
- UNESCO World Heritage Centre, Monarch Butterfly Biosphere Reserve (inscribed 2008): 56,259 ha, ~100 km northwest of Mexico City, in Michoacán and State of Mexico.
- U.S. Fish & Wildlife Service, “The monarch super generation and their phenomenal migration”: 3,000+ miles, roughly 50 miles a day, migrants live up to eight months.
- Yale Environment 360, “To Protect Monarch Butterfly, A Plan to Save the Sacred Firs”: colonies cluster on oyamel fir (Abies religiosa) at about 9,500–10,800 ft (≈2,900–3,300 m).
- Xerces Society, Western Monarch Count: 1,901 (2020), 335,479 (2022), 233,394 (2023), 9,119 (2024), 12,260 (2025, 249 sites). The 1980s figure (~4.5 million) is a published estimate (Schultz et al. 2017, Biological Conservation), not a count.
- Notes: the map is a hand-drawn schematic; flyway lines are generalized routes, not tracking data. “≈4.1 football pitches” uses a 105×68 m pitch (0.714 ha). The Día de Muertos arrival link is a cultural tradition, not a measured date.