A huge number of danaine butterflies undertake seasonal migration between the eastern plains and the Western Ghats in Southern India. It is a spectacular, yet poorly studied ecological phenomenon and our knowledge about the patterns, drivers, and threats is fragmented and largely anecdotal. This study investigated three aspects of the phenomenon: the pattern of danaine butterfly migration and their breeding in Southern India; species composition, numbers, and sex ratio of butterflies in congregation sites; and the plants from which they gather pyrrolizidine alkaloids. Of the six migrating species reported during the study, Tirumala septentrionis, and Euploea sylvester outnumbered all other species. The butterflies were found aggregating on seven plant species for the uptake of secondary chemicals, with Crotalaria being the most important genus used in the Western Ghats. The wet forests of the Western Ghats act as vital habitats for the migrating adult danaines and ensure nectar resources and secondary chemicals which are essential for building up fuel stores for their migration, survival in aggregations, and breeding. This work will contribute to effectively conserve butterfly habitats inside and outside Protected Areas, especially the highly vulnerable summer congregation sites in the Western Ghats, and help preserve their ecological functions.