Avian elevational migration patterns in the Western Ghats, as revealed by participatory science and systematic surveys.

Elevational migration is short-distance migration through which species track seasonal changes in weather and resources along elevational gradients. Key identified drivers of elevational migration include climatic variability, resource availability, and predation. Although the spatiotemporal coverage of participatory science platforms like eBird can be used to quantify elevational migration patterns, such documentation is missing for most of the worlds ecozones. We investigate the extent of seasonal elevational migration for birds present year-round in the tallest mountains of the Western Ghats biodiversity hotspot - the Nilgiris. We combined rigorously curated participatory science data with systematic field surveys. We estimated the elevational shifts of 70 species of birds between the hottest and coldest quarters in the eastern Nilgiris. We ran regression models to quantify the associations between these shifts and their suspected drivers. Around 70% of the species in the region shifted in their elevational ranges, and the frequency of shifts was higher at lower and mid elevations. Downslope shifts were positively associated with a narrow thermal range and species lower temperature limit, while the opposite associations were seen for upslope shifts. We suspect species might be tracking their thermal regimes, and the temperature limits of species can be a major driver of elevational migration in the Nilgiris. The predominance of partial elevational migration in the region provides opportunities for testing theories on biotic drivers of these movements in the future. We highlight the combined usage of field and participatory science in examining ecological patterns in understudied parts of the world.