Western Tamil Nadu witnessed a massive butterfly migration once the Northeast monsoon set in, with swarms reported in Salem, Namakkal, Erode, Tiruppur and Coimbatore districts. The species on the move were the Blue Tiger, the Dark Blue Tiger, the Common Crow and the Double-branded Crow, all heading west and southwest towards the Western Ghats foothills. At Rasipuram, Blue Tigers streamed past at 72 per minute, and in Coimbatore the flow peaked at 135 butterflies per minute. The Nature and Butterfly Society (TNBS) has been recording this phenomenon since 2013. The butterflies generally make the opposite journey, from the Western Ghats to the Eastern Ghats and the plains, in April–May.