Rates of diversification vary greatly among taxa. Understanding how species-specific traits influence speciation rates will help elucidate mechanisms driving the production and maintenance of biodiversity over broad spatiotemporal scales. Ecological specialization and range size are two characteristics thought to predict differences in speciation rates among clades, yet each mechanism predicts both increases and decreases in speciation. We estimate a continuous index of specialization using avian bill morphology. We determine the relative effect of specialization and...