Ali Khan Faizee, Vijay Ramesh, V.V. Robin. Western Ghat Birds Exhibit a Unique Pattern of Seasonal Elevational Shifts and a Combination of Thermal Regime Dependent Shift Strategies.

Abstract

Elevational migration is globally exhibited by montane bird species through elevational range shifts to track seasonal changes in weather patterns and resources. Studies investigating the patterns and mechanisms of such elevational shifts have been limited in spatial scale owing to the difficulty of sampling data across montane gradients. Here, we integrated data from the worlds largest participatory science project (eBird) with systematic surveys to examine patterns and mechanisms that may explain bird elevational range shifts in the Nilgiri Hills of the Western Ghats Biodiversity Hotspot. We calculate the extent of shifts in elevational ranges for birds present year-round in the Nilgiris between the hottest and coldest months and quantify the associations between these shifts and species traits using phylogenetic generalized least squares regressions. We found that 77% (54/70) of the species investigated showed seasonal shifts in their elevational ranges. Birds in the Nilgiris showed nearly equal proportions of downslope (37%) and upslope (33%) winter shifts. All species had significant range overlap between seasons but employed different shifting strategies. Species with narrow thermal regimes predominantly shifted downslope in winter (niche tracking), while those with the widest thermal regimes shifted upslope (niche switching), revealing that species adapt to climatic variability through contrasting thermal strategies. Our results suggest that climatic constraints, specifically thermal regime, could be a major driver of seasonal shifts in elevational ranges of birds in the Nilgiris. Understanding these elevational shift strategies is critical for conservation planning in tropical montane systems facing accelerating climate change.