Urbanization and pollution pressures drive functional homogenization in bird feeding guilds

Urbanization and pollution are rapidly transforming ecosystems across the Global South, yet their impacts on functional biodiversity remain underexplored. Birds, as ecologically diverse and responsive taxa, are valuable indicators of environmental change and functional simplification. We examined how urbanization and pollution jointly influence the functional structure of avian communities across an urban-rural gradient in Sialkot District, a rapidly industrializing region of northeastern Pakistan. Using standardized point counts across 18 sites stratified by urban intensity and pollution levels, we quantified bird community composition and feeding guilds. We applied multidimensional indices of functional diversity (FRic, FEve, FDis, RaoQ) and ordination methods to assess trait-based filtering. Functional richness and divergence contracted markedly with increasing pollution and urbanization. Functionally specialized guilds particularly insectivores, piscivores, and carnivores were absent from the most polluted urban sites, which were dominated by ecological generalists such as omnivores and scavengers. Trait dissimilarity and redundancy declined sharply with pollution, and ordination revealed clear guild turnover along the pollution gradient, indicating homogenization of trophic roles. Functional evenness and dispersion showed weaker responses, suggesting selective persistence of a few tolerant guilds in urban cores. Feeding guild richness also declined with pollution, reflecting a strong loss of trophic diversity under anthropogenic stress. These findings provide rare empirical evidence from South Asia linking environmental contamination to multidimensional losses of avian functional diversity. By integrating trait-based metrics with ecological gradients, this study highlights the need to incorporate functional indicators into conservation planning, particularly in rapidly urbanizing regions where ecological data remain limited.