The monarch butterfly is a migrating marvel: It's the only insect that every year travels for two months and up to 5,000 kilometers (3,100 miles), from Canada and the United States to Mexico, where it spends the winter. There, the monarch butterfly roosts in the pine and fir forests in the Sierra Madre mountains west of Mexico City, which protect the insects, and help keep them from freezing.

Not just the distance is amazing — also the butterflies' sense of direction fascinates scientists and experts. The monarch butterfly divides its migration over three generations, meaning no single butterfly lives long enough to make the roundtrip back to the US and Canada. And still, year in and year out, they seem to know exactly where they are heading.

The butterfly synchronizes its lifecycle with milkweeds by laying its eggs on the plant in the US. In fall, when the milkweeds start to die, a young generation of monarch butterflies leave for Mexico. As soon as the first milkweeds appear in spring, the monarch butterfly returns to begin the cycle anew. There's evidence that they possess a genetically coded instinct for which direction to fly.

Milkweed is considered a pest in farming — industrial agricultural practices including monculture cropping and intensive pesticide use has decreased the plant that monarchs rely upon. It's one of the factors linked to monarch butterflies' decline.

The baby butterflies that set off on the migration have never been to Mexico and therefore can't know the route. But still, the monarchs always find their way back to the Mexican reserve, where they wait out the winter. Studies in 2014 and 2016 suggest the sun offers orientation. In addition, the insects possess an inner magnetic compass corresponding to the Earth's magnetic field.

Despite monarch butterflies being perfectly equipped to take on the vast distance each year, their numbers are decreasing nonetheless. It is hard to count individual butterflies, though, so experts came up with a specific method of assessing how many butterflies have migrated back to Mexico: They look at the area of forest the monarch butterfly covers.

And this area has declined dramatically. Since the winter of 1996-1997 it has shrunk, from about 44 acres (18 hectares) to 6 acres (2.5 hectares) in the winter of 2017. The reasons for this are diverse, among them more intense storms and busy hurricane seasons. Such extreme weather disturbs the insects' migration route and uproots trees, which the butterflies rely on in order to survive.

Other reasons for the butterflies' decline include illegal logging and land use changes. Monarch butterflies migrate to pine and fir forests that thrive at about the same altitude as prime avocado-growing land. In February 2018, Mexican police shut down an illegal avocado plantation that had been set up in the Monarch butterfly's overwintering grounds.

On the positive side, Mexican officials claim to have nearly eliminated illegal logging in butterfly refuge zones. Fighting such deforestation is key, because the spectacular migration of thousands of Monarch butterflies can only continue if there are enough trees for the monarch butterfly to find shelter in.