Purple Crow Butterfly Migration

Every November, up to 10 million Purple Crow butterflies (Euploea spp.) descend on southern Taiwan's Maolin, Meinong, and Siaolin river valleys — a migration ranked second only to North America's Monarch overwintering event among all recorded large-scale butterfly aggregations worldwide. Peak monitoring by the Endemic Species Research Institute has logged over 10,000 butterflies per hour crossing a single highway transect during the November migration front. Daytime roosting sites concentrate hundreds of thousands of individuals per hectare in camphor and Mikania vine thickets, producing canopies of iridescent blue-black wings visible at a distance. Taiwan pioneered a butterfly corridor system on major highways: timed traffic slowdowns during peak migration have cut vehicle-strike mortality by 85% since 2007. The valleys are within 90 minutes of Kaohsiung by public transport, and all key roosting sites carry no entry fee. The overwintering period runs November through March, with January offering the densest concentrations. Aggregating up to 10 million individuals, this migration is 30% larger than the typical eastern Taiwan whale-watching season's estimated 3,000 sperm whales, offering a density comparison that underscores its sheer scale.