Detour migration to circumvent the Himalayas in the Montagu's Harrier Circus pygargus.

Background: Migrating birds do not always travel along the shortest possible routes between breeding and wintering sites. Rather, detours are a common phenomenon in response to availability of foraging habitats, generic wind patterns, predation risk, and ecological barriers. The Himalayas are a formidable ecological barrier within the Central Asian Flyway (CAF), but hitherto research has focused on high-altitude flights of species that cross the Himalayas, and thus information on species that circumvent this mountain range is lacking in this understudied migration system.

Methods: We tracked Montagu's Harriers Circus pygargus for 5 years from their wintering range in India, and found that these long-distance migrants travelled by a grand westward detour around the Himalayas to their breeding areas in Kazakhstan. We calculate the energetic optimality of the detour on the basis of a well-known theoretical model and explain the general migration patterns of Montagu's Harriers in the CAF. Additionally, we compare ecological factors such as ground elevation, habitat greenness (NDVI), land cover and wind patterns along the actual migration route with the hypothetical shortest route to explain why Montagu's Harriers follow a detour migration pattern in the CAF.

Results: The observed (detour) route was on average 1245 ± 94.4 km (27%) longer than the hypothetical shortest direct route. The detour did seem to be optimal for Montagu's Harriers as per the model that considers a distance of up to 1288 km to be optimal. With the detour, harriers circumvented the high altitudes of the Himalayan plateau, effectively avoiding high ground elevations over 4000 m above mean sea level (AMSL). Harriers followed the same detour during spring and autumn migrations, encountering both supporting and opposing winds, and thus the detour cannot be explained by generic wind patterns. The detour was facilitated by the availability of open natural ecosystems (ONEs) and stopover sites with higher productivity west of the mountain range along the floodplains of the Amu Darya river and in the Thar Desert during spring and autumn respectively.

Conclusion: We argue that circumventing the mountain range, as illustrated by our pioneer study on the Montagu's Harrier, could be a common behaviour among migrating landbirds in the CAF. We also emphasize the importance of the protecting ONEs along the western detour for the long term conservation of migratory birds in the CAF.