This is a phenomenon that continues to amaze me, and makes the monarch seem that much more incredible. Some eastern monarchs travel nearly 2,000 miles to reach their overwintering grounds in central Mexico. Some eastern monarchs overwinter in Florida and remain reproductive throughout the winter. The overwintering populations in central Mexico numbers in the tens of millions. The number fluctuates each year.
Monarchs enter a reproductive diapause while overwintering.
They don't begin laying eggs again until spring. Monarchs are the only species of butterfly that participate in a true migration. They migrate south, then when spring arrives, return northward to reproduce. The location of the overwintering grounds in central Mexico was only discovered about 25 years ago. They do not become mixed up and unable to migrate as some extreme sources would propose.