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 range size and shape on speciation rates across 559 species within the Emberizoidea superfamily, a morphologically diverse clade distributed across the Americas and associated islands. We find a significant positive correlation between specialization and speciation rate, and a negative correlation with range size. Only the effect of specialization persisted after removing island endemics, suggesting that ecological specialization is an important driver of diversity across large macroevolutionary scales and the relative importance of specific drivers may differ on islands and continents.