Elevation-driven filtering constrains railway effects on multidimensional avian diversity during the late breeding season on the Qinghai–Tibet Plateau

High-elevation ecosystems are shaped by abiotic filtering, yet how transportation infrastructure influences biodiversity under such constraints remains unclear. The Qinghai–Tibet Railway, a major transportation corridor on the Qinghai–Tibet Plateau, provides an opportunity to study the effects of elevation and railway on avian diversity. We surveyed avian assemblages along 68 transects spanning elevational gradients and distances from the railway. We quantified independent associations of elevation and railway with taxonomic, functional, and phylogenetic diversity (with NDVI as a covariate) and used standardized effect sizes (SES) to evaluate community structure. Elevation was the dominant driver; species richness, Shannon diversity, local contribution to beta diversity (LCBD), and functional and phylogenetic beta diversity all decreased with increasing elevation. Functional and phylogenetic alpha diversity differed between raw and standardized metrics: raw phylogenetic diversity (PD) and mean phylogenetic distance (MPD) decreased with elevation, while standardized metrics did not; raw functional diversity showed no trends, but SES.FRic increased with elevation. Trophic guilds varied in responses: herbivores declined in all taxonomic metrics, omnivores and carnivores declined in richness and Shannon, and railway effects raised LCBD only in omnivores with increasing distance. Most communities had negative SES values and were classified as random or clustered, indicating functional and phylogenetic clustering relative to null expectations. These results confirm elevation as the primary driver of multidimensional avian diversity, with railway-associated variation playing minor roles. We suggest that infrastructure effects in alpine systems mainly involve local taxonomic reshuffling, while stochastic processes or environmental filtering may constrain functional and phylogenetic responses to anthropogenic disturbance.