The world’s greatest animal migrations - SilverKris

"I watched descriptions of tiny monarchs—about 10 cm across and weighing less than a gram—arrive by the thousands to huddle on oyamel fir branches and trunks in towering orange-and-black clusters, where they spend winter together until spring and then begin multi-generational journeys back north; the phenomenon is centred on Mexico’s UNESCO World Heritage-listed Monarch Butterfly Biosphere Reserve, with popular sites including El Rosario, Sierra Chincua, Cerro Pelón and Piedra Herrada, and the season runs from November through March." - SilverKris