Aim: Diet is key to understanding species’ resource use, relationships with their environment and biotic interactions. We aimed to identify the major strategies that shape birds’ diet space, and to investigate their spatial distributions in association with biogeographic, bioclimatic and anthropogenic drivers. Location: Global Time period: Current Major taxa studied: Birds Methods: We analysed score-based assessments of eight diet categories for 8937 out of 10964 extant bird species. We constructed a multivariate diet space by ordinating these data in a principal coordinates analysis, and assessed its dimensionality as a balance between the representation of original diet scores and parsimony. We averaged species’ positions along each dimension for 12705 species assemblages and used quantile regressions to infer the relative contributions of species richness, climate, primary productivity, topography and human footprint to the spatial distribution of the diet space at a global scale. Results: Birds’ diet space was structured by four dimensions ordinating species along continuums ranging from insectivory to plant-based strategies, granivory to frugivory, common to rare diets and nectarivory to carnivory and piscivory. Although orthogonal at the species level, these dimensions were correlated among species assemblages, with regional variation consistent with past climatic and tectonic events. Human footprint packed bird assemblages in the diet space, while warm climate, high productivity and high topography were associated with high variability in the prevalence of dietary strategies among assemblages. Main conclusions: The tremendous variability in bird diets can be explained by a few basic ecological continuums sustained by morphological and ecophysiological differences among species. Strong biogeographic legacies on top of bioclimatic drivers distribute this diet space in species assemblages through environmental filtering and niche packing. However, these patterns are altered at macroecological scales by human-mediated functional homogenization, which may in turn affect the global distribution of bird functions and services.