Avian functional space occupancy and trait–environment associations along a composite urbanisation gradient in urban green spaces

Abstract Urbanisation profoundly alters avian communities by selecting for different functional traits. As cities become increasingly important for biodiversity conservation, urban green‐space management requires evidence identifying both the trait combinations that become underrepresented with increasing urban intensification and the habitat features associated with particular functional traits. We surveyed bird communities across 24 urban green‐space patches in Harbin, Northeast China. We integrated patch‐level land‐cover composition, within‐patch vegetation structure and direct anthropogenic pressure into a composite urbanisation gradient (CUG) and assessed changes in taxonomic diversity, functional diversity, functional space occupancy and trait–environment associations. Species richness declined consistently along the CUG. Functional richness also declined, but this result was sensitive to how residual spatial structure was modelled and should be interpreted with caution. The area occupied within the two‐dimensional functional space was progressively smaller across increasing CUG levels, particularly in the region aligned with larger body mass, higher hand‐wing index and aquatic‐associated traits. In high‐CUG green spaces, the core region of this two‐dimensional functional space was characterised by greater representation of mid‐high foraging strata, canopy foraging strata and woodland habitat use. RLQ and fourth‐corner analyses showed that different habitat features were associated with different avian functional traits. Water‐body cover was associated with larger body mass, water foraging and wetland habitat use, whereas vegetation cover and foliage height diversity were associated with forest habitat use. Synthesis and applications . Our findings provide trait‐based insights for blue–green urban planning. These associations suggest that urban planning should not treat green spaces as homogeneous habitat patches but should consider both blue‐space allocation and nature‐based vegetation management. Embedding small wetlands or water bodies in dense urban areas, maintaining blue–green connectivity and promoting vertically complex woody vegetation may help retain complementary avian functional groups and limit the risk of functional homogenisation in rapidly urbanising landscapes.