MEXICO CITY (AP) — The stunning and little-understood annual migration of millions of Monarch butterflies to spend the winter in Mexico is in danger of disappearing, experts said Wednesday, after numbers dropped to their lowest level since record-keeping began in 1993.

Their report blamed the displacement of the milkweed the species feeds on by genetically modified crops and urban sprawl in the United States, extreme weather trends and the dramatic reduction of the butterflies’ habitat in Mexico due to illegal logging of the trees they depend on for shelter.