We examined nest predation rates at artificial nests in two contrasting habitats (open woodland and shrubland) inside the Reserve of Ñacuñán, west-central Argentina and in open woodlands within the reserve and a nearby grazed area, to assess whether between-habitat differences in vegetative structure were associated with changes in nest predation levels. We also quantified diurnal and nocturnal predation rates to further characterize the predator assemblage of the reserve. Nest predation rates and the estimated daily mortality rates were quite high in both open woodland and shrubland and did not differ significantly between them. Nest predation rate and daily mortality rate of nests in open woodlands of the reserve were high but similar to the rates for nests in adjacent grazed open woodlands. Nearly all predation events occurred during daylight hours. This result was not an artifact of the use of quail eggs because the proportion of depredated nests during the night was similar when we used smaller eggs of captive passerines. Our results support previous findings on the prevalence of birds as nest predators in this area. Avian predators seemed to be efficient at locating our artificial nests in all habitats, leading to the lack of differences between treatments. Conversely, our results suggest that extensive cattle grazing does not greatly modify nest predation risk for above-ground nests, although this conclusion is provisional given the conditions under which our experiments were carried out.