Large yearly fluctuations in congregation sizes of migratory Danainae butterflies in the Western Ghats, India

We documented the overwintering congregations of migratory Danainae butterflies (Nymphalidae) over six migratory seasons in the threatened tropical forests of the Western Ghats, India. Congregation sizes fluctuated drastically (>90%), indicating vulnerability to chance events. Continued monitoring is needed to explain these fluctuations and identify declining trends for timely conservation action.