Evidence of niche packing among bird species from analyses of functional trait patterns along an urbanization gradient in South China

Increasing urbanization has altered the functional traits of birds, with potential repercussions for niche use. However, how urbanization affects a species' occupied ecological niche—whether through ‘packing’ where species compete for limited resources, or ‘expansion’ where new resources allow species to spread—is still unclear. Here, we investigated the diversity and niche occupancy of bird species across an urbanization gradient in Guangzhou, China. Bird occurrence records from online public databases, including Bird Report, eBird, and the GBIF for 1998–2022 were used to model the potential distributions of 288 bird species in Guangzhou with MaxEnt at a 1 km × 1 km resolution, evaluated by cross-validation and AUC. Niche occupancy dynamics were assessed using the standardized effect size of functional richness (SES.FRic), and generalized linear mixed models employed relationships among urbanization, net primary productivity (NPP), and diversity metrics. Migratory and resident bird communities showed similar distribution patterns, and all bird species richness was significantly negatively correlated with the urbanization index. With increasing urbanization, SES.FRic of bird communities decreases, indicating a typical pattern of niche packing, where birds optimize adaptation by increasing population density, enhancing resource use efficiency, and adjusting life history strategies within a limited niche space. Furthermore, NPP possibly promotes bird species richness by enhancing foraging and nesting resources. However, this effect is insufficient to offset the structural changes and resource heterogeneity induced by urbanization, which continue to constrain niche expansion. Our findings underscore that conservation strategies in Guangzhou should focus on enhancing green spaces quality, increasing resource availability, and promoting targeted habitat management.