India, Dec. 3 -- Once a year, Big Butterfly Month - marked in September every year - turns Delhi into an open-air observatory, its gardens, forest edges, and city parks alive with the flutter of wings. This year, with 70 recorded species, Delhi saw two more than last year. But the bigger finding was the number of rarities observed. Absentees returned. And, for the second year in a row, new Himalayan species made their way into Delhi's checklist.

Ecologists said the discovery of new species in Delhi is more than a mere statistical bump. It signals the surprising ways in which butterflies are arriving, and in some instances settling here. This year, Delhi has seen an unusual run of favourable conditions - an ample monsoon, expanding green ...