Milkweed Butterflies
After missing out couple of years in trot ( 2015 & 2016 ), the Milkweed butterflies are back to what they have been doing for years unknown- migrate to Western Ghats during September and October . From end of September to November beginning , every year , from time immemorial , Nymphalidae butterflies like Dark Blue Tiger , Blue Tiger , Common Crow and Double-branded Crow along with a few other species will undertake their migration journey from plains and Eastern Ghats of Tamil Nadu to the Western Ghats . ( The migration here is discussed only from Tamil Nadu perspective ). Butterflies undertake migration mainly to escape from harsh climate for their living and lack of sufficient food and environment for their survival . The Monarch Butterfly migration in America is a well-known and closely followed event .
In India , especially in South India , the Danainae butterflies mentioned above , undertake their journey every year , fairly regularly , and in large numbers of tens of thousands . The data available with Tamil Nadu Butterfly Society ( which has been tracking these events since 2012 ) indicates that this migration to Western Ghats just before the commencement of North East Monsoon happened between September last week and October middle . Their return to the plains and Eastern Ghats happens during next April- May ( not the ones that migrated but their off-springs ), before the commencement of South West Monsoon in the hills . Tamil Nadu Butterfly Society ( TNBS ) has been observing and documenting the migration since 2012 and the past data showed that 2015 and 2016 are the worst years with respect to butterfly migration . It was almost nil and this was attributed to the failed monsoons . The rains in the current South West Monsoon was more than the normal across the state by 29 % between June and September , especially for the month of August and the good rains continued until the first two weeks of September and thereafter too . Coimbatore Dt has recorded 169 % excess rain during this period for example . We also saw some good rains during the months of April and May this year . This has resulted in an unprecedented generation of Emigrants butterfly during May-June throughout the Tamil Nadu Ghats and Plains and their migration . ( See Pattampoochi – Wings 1 Veins 3 for more details on the same ).
Migration , for the running year ( 2017 ) was observed continuously between the period 5th September and 15th September ( with 13th September being the peak day ) and subsequently up to 30th September in patches and in random manner . Movements of species like Dark Blue Tigers , Double-branded Crows , Common Crows and Common Emigrants are in good numbers . From observations made at Coimbatore , they were steadily moving towards hills to the direction of Palakkad gap and Siruvani Hills from Coimbatore . Observations were also made at Anaikatti Hills of Coimbatore Dt . They were seen moving between 930 AM and 3 PM at a height of 2-15m , making steady movement in one direction . Sample observations ( multiple observations of smaller time windows and mathematical projections ) from a specific location in Coimbatore resulted in an estimated 38000 specimens moving in a single day for a width of 1.5 Km . That means an estimated lakhs and lakhs of butterflies could have reached the hills as part of the migration .
Past data with us show that they usually migrate during last week of September and this is an early migration due to favourable conditions advanced .
Observations made at Erode , Thiruppur and the foot hills of Coimbatore ( like Marudhamalai ) are supporting the theory that the origin of migration may be from the Eastern Ghats Hills of Yercaud , Kolli and the surroundings apart from the plain population . They might have travelled a distance of about 150 Km .
Butterfly Migration 2017 – An update