By Ines Keane
Start with the sound. UNESCO’s description of the reserve says that when the butterflies take flight they fill the sky, and that the beating of their wings is “like light rain.” That is the World Heritage listing talking, not a travel brochure, and it sets the tone for a place where the main event is something very small, repeated a very large number of times.
Where it is
UNESCO places the reserve in rugged forested mountains about 100 km northwest of Mexico City. It covers 56,259 hectares: 13,551.552 hectares of property and 42,707.498 hectares of buffer zone. Wikipedia puts the reserve in two states, Michoacán and the State of Mexico, and rounds the area to 56,000 hectares in some lines and 56,259 in others. We take the rounding to be just that.
Who comes, and when
According to UNESCO, up to a billion monarch butterflies return each autumn from breeding areas as far away as Canada. They cluster in the oyamel fir forests, and their combined weight bends the branches. Wikipedia says the butterflies occupy only a fraction of the reserve from October to March, and that open sanctuaries receive visitors from November to March. It gives the oyamel fir as growing between 2,400 and 3,600 meters above sea level.
The trip does not end there. UNESCO says that in spring the butterflies begin an eight-month migration to eastern Canada and back, during which four successive generations are born and die. How they find their way back to the overwintering site, UNESCO says, “remains a mystery.” Wikipedia lists some of the cues for the southward leg: the angle of sunlight, the decline of milkweed, shorter days and falling temperatures.
Fourteen colonies, eight protected
UNESCO counts 14 overwintering colonies and says the property protects 8 of them, an estimated 70 percent of the eastern population’s overwintering butterflies. It also says the property holds more than half of the eastern population’s overwintering colonies. Those two statements sit close together on the same page, and we report them as written.
How the count works
The annual figure is not a head count. Xerces Society, a conservation group, says the survey led by WWF-Mexico measures the area of forest where monarchs hibernate each winter, and calls that “a reliable indicator” of the eastern population. The Society gives 0.67 hectares for 2013-14 as the lowest year on record and 0.9 hectares for 2023-24. WWF’s own page gives 2.93 hectares for 2025-26, against 1.79 the winter before, a 64 percent increase. Wikipedia gives 2.10 hectares in 2020 and notes that 6 to 60 million butterflies can pack into a hectare of colony. That range is wide, and we do not convert the hectares into a number of butterflies.
A short history of the protection
Wikipedia says researchers found the overwintering areas in 1975, that a first decree followed in 1980, and that a 1986 decree covered 16,110 hectares across five sites. In September 2000 the reserve was enlarged to 56,259 hectares, and in 2008 UNESCO inscribed it as a natural World Heritage site under criterion (vii). UNESCO names illegal logging as a known threat and says forest cover, and the microclimate it creates, is the key management requirement.
Dates in order
- 1975: overwintering areas found, according to Wikipedia.
- 1980: first protection decreed, according to Wikipedia.
- 1986: decree for 16,110 hectares, according to Wikipedia.
- September 2000: enlarged to 56,259 hectares, according to Wikipedia.
- 2008: World Heritage inscription, according to UNESCO and Wikipedia.
- 2013-14: lowest area on record, 0.67 hectares, according to the Xerces Society.
- 2025-26: 2.93 hectares, according to WWF.
Side by side
Wikipedia lists five sanctuaries open to visitors in one section and four in a later one, the difference being Piedra Herrada. It also gives the butterflies’ presence as October to March but visitor access as November to March. UNESCO does not discuss visiting at all. Our reading: the season for seeing them is shorter than the season for their being there, and anyone planning should ask the local sanctuary, not a summary.
What kind of sources these are
UNESCO’s World Heritage listing, which is the official description; Wikipedia, which anyone can edit; and the web pages of two conservation groups, WWF and the Xerces Society, both of which campaign for the species. We read no Mexican government page and no sanctuary’s own site.
What we left out
Entry fees, guide arrangements, road and weather conditions, safety advice and anything about who patrols the forest. We left out the communities’ names and the reserve’s land-ownership figures. We also left out the Wikipedia mention of a separate sanctuary southeast of the capital, since its details do not belong to this reserve.
Quick answers
When can you see monarch butterflies in Mexico? According to Wikipedia, the butterflies are in the reserve from October to March, and open sanctuaries receive visitors from November to March.
How many monarchs spend the winter there? UNESCO says up to a billion return each autumn to 14 overwintering colonies, and the property protects 8 of them.
How is the monarch population measured? The Xerces Society says the survey measures the hectares of forest the colonies occupy. WWF reports 2.93 hectares for 2025-26.
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