A few weeks back, I was lucky enough to be able to visit Piedra Herrada, one of the sites of the monarch butterfly migration here in Mexico. It was a long walk up a steep path that started at around 10,000 ft in elevation. As we made our way up higher, the butterflies started circling around and becoming more active, with thousands of them flying in all directions. And then when we stopped, we could see tens of thousands more resting in the trees, like in this picture.
Monarch butterflies fly south every year from Canada and the US down to Mexico and it's not the same butterflies that start the trip as end it as they only live a month or two at most. What I found particularly interesting though is that even though it's never the same butterflies returning, they always migrate back to the same sites.