Spatial structure in metacommunities and their relationship to environmental gradients have been linked to opposing theories of community assembly. In particular, while species sorting predicts strong environmental influences, the neutral theory, the mass effect and the patch dynamics frameworks all predict differing degrees of spatial structure due to dispersal and competition limitations. Here we study the relative influence of environmental gradients and spatial structure in bird assemblages of the Chilean temperate forest. We carried out bird and vegetation surveys in South American Temperate forests at 147 points located in nine different protected areas in central Chile, and collected climatic and productivity data for these localities. Species turnover rates between sites were calculated, as well as three indices of bird diversity. A stepwise multiple regression was used to select a small number of environmental factors that influenced bird species diversity. Although diversity indices were spatially autocorrelated, environmental factors were sufficient to account for this autocorrelation. Moreover, species turnover rates between sites were not significantly related to distance. We then tested a complex hypothesis about climate, vegetation and avian diversity interactions using structural equation modeling (SEM). The SEM shows that there are both direct and indirect effects of climate and area on avian diversity, as well as important influences of vegetation structure. Taken together, the weak spatial structure of species turnover and diversity, as well as the strong environmental effects on bird diversity support the idea that species sorting has a predominant role structuring avian assemblages in the region.