One of the most striking patterns in evolutionary biology is that clades may differ greatly in the number of species they contain. Numerous hypotheses have been put forward to explain this phenomenon, and several have been tested using phylogenetic methods. Remarkably, however, all such tests performed to date have been characterized by modest explanatory power, which has generated an interest in explanations stressing the importance of random processes. Here we make use of phylogenetic methods to test whether ecological variables, typically ignored in previous models, may explain phylogenetic tree imbalance in birds. We show that diversification rate possesses an intermediate phylogenetic signal across families. Using phylogenetic comparative methods, we then build a multipredictor model that explains more than 50% of the variation in diversification rate among clades. High annual dispersal is identified as the strongest predictor of high rates of diversification. In addition, high diversification rate is strongly associated with feeding generalization. In all but one instance, these key findings remain qualitatively unchanged when we use an alternative phylogeny and methodology and when small clades, containing five species or less, are excluded. Taken together, these results suggest that large-scale patterns in avian diversification can be explained by variation in intrinsic biology.