El Rosario is the largest and best-known sanctuary, reached from Angangueo. A steep hour's walk at around 3,000 metres brings you to the colony, where butterflies hang in such density that fir branches bend under the weight and the ground is carpeted with them.

Sierra Chincua is the other main Michoacán sanctuary, generally quieter than El Rosario with a gentler approach and the option of riding part of the way on horseback.

Piedra Herrada and El Capulín, closer to Valle de Bravo in the state of México, are the practical choice from Mexico City and see fewer international visitors.

The colonies at midday are the moment to be there: when the sun warms the trees above about 12 degrees, the butterflies release and the air fills with them. Cold mornings keep them clustered and static — impressive, but the flight is the thing.

The season runs from around November to March, with January and February the most reliable. Arrive around late morning and be prepared for altitude on the climb.

Practicalities

Entry fees are modest and a local guide is included — guiding jobs are one of the main reasons surrounding communities protect this forest rather than log it.

Silence is required at the colonies, and it is enforced: noise disturbs clustered butterflies and wastes energy they cannot spare. Stay on the marked path, and never touch or pick up a butterfly, including one on the ground. The migration's population fluctuates enormously between years with weather and habitat loss, so check recent counts if you are travelling specifically for it.