Alteration of habitat has been highlighted as the most important factor causing declines in populations of grassland birds. In this study we characterized bird assemblages in cultivated and natural grasslands under four different agricultural schemes in Uruguay. We surveyed birds along transects to determine species richness, composition, and population density along this agricultural gradient. Although species richness was lower in the most natural grassland type (24 species), community composition and abundance patterns highlighted the importance of natural grasslands for the conservation of habitat specialists. An analysis of similarity showed that bird communities in the four grassland types were distinct (ANOSIM global R = 0.68). Several species using grassland facultatively were characteristic of cultivated habitats, whereas the opposite was true for grassland obligates and natural grasslands. An indicator-species analysis further supported the association of grassland-facultative and grassland-obligate birds with cultivated and natural grassland, respectively. Population-density patterns varied by species; some were more abundant in cultivated habitats, others in natural grasslands, but threatened species attained relatively high densities in natural grasslands only. Some threatened species, such as the Ochre-breasted Pipit (Anthus nattereri) and Pampas Meadowlark (Sturnella defilippii), were largely restricted to natural grasslands, so their long-term survival will depend on the conservation of suitable patches of these habitat types.