Avian elevational migrants in India's Western Ghats challenge climatic constraint theory through broader environmental tolerances.

Abstract

O_LIAvian elevational migration is a common but often overlooked form of short-distance migration. Numerous hypotheses have been proposed to explain this behavior, yet few studies have tested those comprehensively. We examine the climatic constraint hypothesis - a key hypothesis explaining elevational migration - to evaluate its predictions for avian elevational migration in the monsoon-dominated Western Ghats of India. C_LIO_LIWe used citizen science data from eBird to quantify elevational migration patterns of resident birds of the region. Using robust data curation protocols and new bias correction methods, we modeled migration patterns of 164 species across summer, monsoon and winter seasons. To formally test the climatic constraint hypothesis, we modeled the likelihood and distance of migration against body size and environmental niche breadths, using phylogenetically informed logistic regression and Bayesian generalized linear mixed models. C_LIO_LIOur results show that 43% of resident birds exhibited elevational migration in at least one season. Most species migrated between winter and summer (57), followed by migrations between summer and monsoon (39) and then monsoon and winter (38). Species predominantly moved upslope in summer (82%) and downslope in monsoon (87%), with no discernible pattern of direction in winter. Species with broader temperature tolerances were more likely to migrate across all seasons. Additionally, species with broader precipitation niches and larger body sizes moved further downslope during the monsoon and winter, respectively. C_LIO_LIWe found that species with broader tolerances to environmental conditions were more likely to be migratory and moved further along the elevational gradient, contradictory to the expectations of the climatic constraint hypothesis. Our analyses suggest that elevational migrants of the Western Ghats are not climatically limited but likely possess flexibility to track ecological resources that vary with season and elevation. C_LI