ALL AFLUTTER OVER A NATURAL WONDER

end of August (when temperatures are falling and days are shorter), they slip into a state of semi-hibernation, slowing down bodily functions and storing energy for a longer life.

The science of their internal compass is an even greater enigma, because no single butterfly makes this astounding migration twice.

Silvestre has been working at El Rosario for 17 years and he doesn’t know the answer, but he’s still staggered by the phenomenon year after year.

Other local residents, however, didn’t always feel that way.

“Some people thought they were a plague,” he tells me as we perch on fallen logs in a sun-splintered clearing. “They used to fry the butterflies, remove any poisonous parts and eat the rest in tacos.”

A greater death threat came in 2010, when North Americans destroyed fields of milkweed – a favourite food for monarch caterpillars – resulting in the population’s drastic decline.

In 2014, the presidents of Canada, USA and Mexico agreed the conservation of butterflies was a priority, and Silvestre assures me numbers have risen since then.

Noise upsets the butterflies, so we sit in

offers a five-day Mexico’s Colonial Cities tour, which includes a visit to see the monarch butterflies in season (November to March).

Prices are from £995 per person. International flights extra. See for more information. In shady areas, clusters keep warm by clinging to tree trunks, transforming the forest into a mass of reptilian scales. Above us, wings bluster like autumn leaves across the blue sky, their shadows dancing at my feet.

Some will return home, others will stay, spiralling to the ground now their time is up.

And so the cycle of life continues, an epic migration and a journey with no beginning or end.