West zone districts witness large-scale movement of butterflies; roadkill ends migration of hundreds midway

Hundreds or more butterflies were not simply appearing randomly; they were moving steadily west and southwest toward the Western Ghats foothills. The movement followed the Northeast monsoon and became a major field observation across Tamil Nadu’s western districts.

Members of The Nature and Butterfly Society, or TNBS, monitored the movement across Salem, Namakkal, Erode, Tiruppur and Coimbatore. In one striking count, Blue Tigers passed Rasipuram in Salem at 72 individuals per minute. Coimbatore recorded movement from Mettupalayam to Singanallur.

The activity peaked in Coimbatore on October 17, from 8.30 a.m. to 4 p.m., with rates ranging from five to 135 butterflies per minute. Movement continued across Erode and Tiruppur on October 18. Coimbatore Nature Society members also helped monitor the migration, alongside TNBS observers.

Butterfly migration is the seasonal movement of butterflies from one region to another. It can involve large numbers traveling along a repeated route. In this event, the movement mattered because it revealed a broad, coordinated shift toward the Western Ghats foothills after the Northeast monsoon.

The butterflies belonged to the Danainae subfamily of milkweed butterflies. They were Blue Tiger, or Tirumala limniace; Dark Blue Tiger, or Tirumala septentrionis; Common Crow, or Euploea core; and Double-branded Crow, or Euploea sylvester. TNBS observers recorded their steady westward and southwestward movement.

TNBS has monitored this regional phenomenon since 2013. Its observations show that the current event resembles movement recorded in mid-September after several spells of rain. The article also describes a later seasonal movement, from the Western Ghats toward the Eastern Ghats and plains, during April and May.

The observations show a substantial movement, but the article does not give one total number for all butterflies. Instead, observers measured the flow at particular places and times. These rates make the scale visible, especially because some counts reached more than one hundred butterflies every minute.

At Rasipuram in Salem, Blue Tigers passed at 72 individuals per minute. In Coimbatore, movement between Mettupalayam and Singanallur peaked on October 17, from 8.30 a.m. to 4 p.m. Rates there ranged from five to 135 butterflies per minute. These are field counts, not an estimate of the entire migration.

Movement continued across Erode and Tiruppur on October 18, with Tigers and Crows still being seen. The butterflies normally migrate at heights of five to 25 meters and above, but many flew much lower during this event. The article does not state a total distance or total population.

The Northeast monsoon coincided with the butterflies’ movement because the migration began after the monsoon’s onset and reasonable rain fell across Tamil Nadu. Rain was especially noted in the districts forming the observed migration corridor. The article presents this timing as a repeated pattern, not as a fully explained cause.

TNBS has monitored the phenomenon since 2013. Its records show westward and southwestward movement after the Northeast monsoon, matching an earlier movement observed in mid-September after a few spells of rain. The current event therefore fit an established local pattern.

Rain may mark the seasonal conditions associated with movement, but the article does not identify the precise biological trigger. It reports the timing and the related field observations. After reaching the foothills, butterflies actively swarmed available nectar plants, showing that the monsoon period also coincided with usable resources along their route.

The butterflies normally migrate at heights between five and 25 meters and above. During this movement, however, naturalists widely observed them flying much closer to ground level. That unusual flight path mattered because roads cut across the areas where the butterflies were moving.

With butterflies flying low, vehicles could strike them more easily. The article reports roadkill in hundreds across the region. It directly links the low-level flight path with the deaths, but it does not identify why the butterflies descended. The cause remains unstated in the source.

This observation adds a warning to an otherwise impressive migration record. Monitoring groups documented both the movement and its danger. The event also shows why route conditions matter: a migration corridor can connect foothills and plains while carrying traffic. The article gives no specific mitigation measure, but continued field monitoring records where such risks occur.

The October migration fits a two-part seasonal pattern described by TNBS. After the Northeast monsoon, butterflies move west and southwest toward the Western Ghats foothills. This direction has been observed repeatedly since 2013, including movement after rain in mid-September.

The contrasting movement normally occurs during April and May of the following year. Butterflies then travel from the Western Ghats hills toward the Eastern Ghats and the plains. The article connects that return movement with the arrival of the Southwest monsoon. Together, these observations describe movement in opposite directions during different seasons.

The pattern matters because it shows that the October event was not isolated. It resembled earlier TNBS observations and continued across several districts. The article does not predict exact future dates or numbers. It does indicate that monitoring can compare present movements with the seasonal cycle recorded across the region.

Rainfall appears in the article as a seasonal signal linked with butterfly movement. Reasonable rain was recorded across Tamil Nadu, especially along the districts involved in the migration corridor. TNBS also observed a similar movement after rain in mid-September, reinforcing the connection between rainfall and timing.

The Western Ghats foothills provided nectar plants when the butterflies arrived. Blue Tigers, Dark Blue Tigers, Common Crows and Double-branded Crows actively swarmed these plants. The monitored route stretched across districts including Salem, Namakkal, Erode, Tiruppur and Coimbatore, linking observations across the landscape.

These features support migration by combining a seasonal cue, food resources and a connected route toward the hills. The article does not detail how the corridor is formed or which plants were used. It shows, however, that foothills and available nectar were important destinations, while rainfall coincided with the movement’s timing.