Mexico’s Environment Department said Thursday that the orange-and-black butterflies had been spotted by park rangers entering Coahuila state, on the border with Texas.
The butterflies head for the fir- and pine forest of central Mexico, where they spend the winter. No butterfly lives to make the full round trip, and it is unclear how they find the route each year.
Last year, butterflies that reached the wintering grounds covered 2.79 acres (1.13 hectares). That was a 69 percent rebound from the previous February’s 1.65 acres (0.67 hectare), which was the lowest since record-keeping began in 1993.
The migratory route can be 3,400 miles (5,470 kilometers) long.