The impact of introduced herbivores on the composition and structure of plant communities has been widely studied. However, little is known about how they affect wildlife. We studied the impact of feral horses under different grazing regimes on the communities of birds in a nature reserve in the Pampas grasslands in Argentina. The areas that had predominantly tall grass (enclosures and areas of moderate grazing intensity) showed the greatest species richness and total abundance of birds. Some species, e.g. the southern lapwing (Vanellus chilensis), were associated with the presence of horses, while others, e.g. pipits (Anthus spp), were more common in lesser grazed areas. The presence of feral horses was associated with an increase in the rate of predation of eggs which varied from 12.5% within the enclosures to 70% in grazed areas. It is suggested that the increase in predation rate was due to the increased visibility of the nests and an increase in the density of opportunist carnivores. Small areas of grassland in a good state of conservation could serve as sources that would maintain communities of birds in the more transformed sections.