Temperature and Precipitation Niche Dynamics Shape Avian Elevational Shift Strategies in the Hengduan Mountains

ABSTRACT Aim Intensifying climatic seasonality drives spatial reorganisation in mountains, but the niche processes underlying seasonal elevational shifts remain insufficiently understood. We test how seasonal changes in climate niche position (optima) and niche breadth (the range of climatic conditions that a species experiences over space and time) along temperature and precipitation gradients predict the direction and distance of bird elevational shifts. Location Hengduan Mountains, southwest China. Taxon Breeding birds. Methods We analysed 42,458 records of 206 species from field surveys and citizen‐science data. We quantified elevational shift direction and distance using the signed and absolute differences between seasonal median elevations. Using fine‐resolution climatic data, we estimated key climatic niche metrics—seasonal optima, annual breadth, seasonal breadth change, and overlap—and evaluated their power to predict shift direction and distance alongside functional traits using phylogenetic Bayesian models. Results We identified three post‐breeding elevational strategies: upslope, downslope, and resident. Although moving in opposite directions, upslope and downslope shifters exhibited congruent seasonal directions of climatic niche change. Bayesian analyses revealed that shift direction in downslope species was associated with seasonal changes in both climatic optima and niche breadth, whereas for upslope species, it was linked primarily to seasonal niche breadth change. Furthermore, shift distance across both groups corresponded to the magnitude of seasonal niche breadth change, while diet and dispersal capacity showed distinct associations with these spatial responses. Conclusions We show that opposing spatial trajectories arise from distinct niche mechanisms. Downslope shifts combine the tracking of climatic optima with seasonal changes in niche breadth, whereas upslope shifts are linked predominantly to the reorganisation of niche breadth. This divergence reflects different sensitivities to mean climatic conditions versus seasonal variability along both temperature and precipitation axes, highlighting that integrating niche position and breadth dynamics is essential for predicting how species redistribute along elevational gradients as seasonal climatic contrasts strengthen.