Sealed surfaces limit bird diversity, whereas tall structures may support abundance of some species, while both filter traits in urban landscapes

• Species richness decreased with increased sealed area. • Bird abundance increased with greater building height. • Insectivores decline with sealing; high-level foragers prefer tall buildings. • Larger birds and high-level nesters dominate areas with high buildings. • Vertical density with low sealings can support diverse bird communities. The use of vertical space by avian species remains is a relatively unexplored area in urban ecology. Although the impacts of horizontal urban structures, such as sealed surfaces, are well understood, the influence of building height on bird communities is less clear, especially in Central Eastern Europe. We explored how three-dimensional urbanisation influences species richness, abundance, and key functional traits. We selected five cities in western Hungary, with 20 sites per city, using a 2 × 2 factorial design that combined two structural variables: building height (low vs. high) and sealed surface cover (low vs. high). We conducted point counts at 100 sites and assigned functional traits to each bird species, including adult and nestling diet, foraging technique, nesting site, migration status, and body mass. Species richness decreased with higher sealed surface, but was not affected by building height, while bird abundance increased with building height, but remained unaffected by sealed surface. Community-weighted means (CWMs) of nestling diet indicated a decline in insectivores with increased sealing. Foraging strategy shifted towards high-level foragers in areas with taller buildings. Nesting site CWM increased vertically with both building height and sealing. CWM body mass also increased with building height, with larger-bodied birds dominating tall structures. Our findings highlight that urban verticality, alongside surface sealing, filters bird communities both taxonomically and functionally. Therefore, incorporating three-dimensional habitat structure is essential for understanding urban biodiversity loss. These results suggest that vertical densification may support bird communities only if combined with low surface sealing and sufficient habitat heterogeneity including a diversity of native trees and bushes.