An extraordinary natural spectacle unfolds every year above the 10,000-foot-high forests in central Mexico. Millions of orange-and-black butterflies arrive and nest atop fir, juniper and pine trees, after flying in from thousands of miles away in Canada. But the epic migration of the monarch butterflies is facing increasing threats, including US farmers ripping up milkweed plants — which monarch caterpillars eat — the construction of a border barrier in Texas and climate change. The World’s Jorge Valencia has our story.