Urbanization induces bird color homogenization

Abstract Urbanization acts as an environmental filter that allows bird species with certain ecological traits, such as a generalist diet and nesting on buildings, to occur in urban centers. However, its effect on other bird traits, such as plumage color, has still not been studied. The aim of this study was to analyze changes in plumage diversity and composition along urbanization gradients of three cities in central Argentina: Mar del Plata, Balcarce and Miramar. Bird surveys were made in urban, suburban and rural areas using transects during two breeding seasons. Color description and bird size were obtained from the literature. Color diversity was calculated using functional diversity indices: FD and FDis. Null models that control for species richness were used to estimate standardized effect sizes. Color composition was analyzed using Non-metric multidimensional scaling. FD and FDis decreased with percentage of impervious cover and increased with habitat diversity, whereas FDis also showed an interaction between city type and impervious cover. After both indices were controlled for species richness, they showed decreases with impervious cover and effects of city type. Highly urbanized areas were dominated by grey color, plumage dimorphism, polymorphism and small and medium sizes. Sites with high habitat diversity were inhabited by species of yellow and green coloration, whereas rural areas were occupied by large species with combinations of black, brown and white. More urbanized areas show not only fewer species, but also species that look more similar than expected by chance. These results suggest that urbanization acts as an environmental filter for bird colors, allowing the presence of birds with similar colors.