Monarch butterfly numbers lowest since 1995

One of the world's largest annual migrations is at risk of disappearing. Milions of butterflies usually move from Northern America to Mexico in the winter. But experts say their numbers are declining. Mexico monitors their numbers by measuring the areas they cover in the forests where they cluster together in their thousands. In 1995 it was almost 180,000 square kilometres. But last month they were down to an area of less than 6,000 square kilometres.