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Urbanization is a global phenomenon that is encroaching on natural habitats and decreasing biodiversity, although it is creating new habitats for some species. The Eurasian kestrel (Falco tinnunculus) is frequently associated with urbanized landscapes, but it is unclear what lies behind the high densities of kestrels in the urban environment. Occupied nest sites in the city of Vienna, Austria, were investigated along a gradient of urbanization (percentage of land covered by buildings or used by traffic). Field surveys determined the abundance of potential prey (birds and rodents), and the results were compared to the birds' diets. A number of breeding parameters were recorded over the course of three years. High breeding densities in urban habitats do not necessarily correlate with high habitat quality. The high density of kestrel nests in the city center is probably due to the ready availability of breeding cavities. Highly urbanized areas in Vienna are associated with unexpected costs for the city-dwelling raptor, in terms both of prey availability and of reproductive success. The kestrel appears to be exploiting the urban environment but, given the poor reproductive performance of urban kestrels, it is likely that the species is falling into an ecological trap. [ABSTRACT FROM AUTHOR]