Urbanization profoundly impacts avian communities, altering their diversity and composition. This study investigates the effects of urbanization intensity on avian diversity and community structure across five cities in Bangladesh, covering 503 systematically sampled 1 km² grids in three levels of urbanization gradients (high, moderate, and low). Across these grids, a total of 31,601 individual birds, belonging to 238 species from 60 families and 19 orders, were recorded, representing approximately one-third of the country's avian fauna. Species richness and sampling completeness varied along the gradient, with highly urbanized areas exhibiting significantly lower species richness (136 species) compared to moderately (185 species) and low urbanized areas (183 species). Taxonomic (χ² = 19.93, p < 0.001) and phylogenetic (χ² = 21.34, p < 0.001) diversity declined with increasing urbanization intensity, whereas functional diversity showed no significant variation. Community structure analyses revealed significant differences among all three urbanization levels, reflecting distinct avian assemblages in each (R = 0.02, p < 0.001). Generalized additive models (GAMs) identified air pollution levels, distance from the city edge, and cropland percentage as significant predictors, negatively affecting species richness, taxonomic diversity, and phylogenetic diversity. Functional diversity, in contrast, was positively influenced by waterbody coverage but declined with increasing cropland areas. Indicator species analysis identified 19 species significantly associated with specific urbanization levels, reflecting habitat preferences and urban adaptability among these species. The findings underscore the detrimental effects of urban intensification on avian diversity and highlight the importance of mitigating air pollution and conserving waterbodies to sustain biodiversity in urbanized habitats.