Despite the accumulation of structural descriptions of bird nests and considerable diversity in these structures across species, we know little about why birds build the nests that they do. Here we used phylogenetic comparative analyses to test one suggested explanation, specifically for Old World babblers (Timaliidae): that building a domed nest coevolved with building a nest on the ground. We show that babblers that build domed nests build them at a lower height than do babblers that build cup-shaped nests, and that in this radiation the evolution of domed nests depended on the transition to building a nest on the ground. Our results are consistent with the hypothesis that babblers add a roof to the nest in order to confer protection against increased predation risk on the ground. We believe that this is the first formal identification of evolutionary pathways that have led to the diversity in nest structure and location that we see today.