A new time tree of birds reveals the interplay between dispersal, geographic range size, and diversification.

The spatial and temporal dynamics of biodiversity are shaped by complex interactions among species characteristics and geographic processes. A key example is the effect of dispersal on geographical range expansion and gene flow, both of which may determine speciation rates. In this study, we constructed a time-calibrated phylogeny of over 9,000 bird species and leveraged extensive data on avian traits and spatial occurrence to explore the connections between dispersal, biogeography, and speciation. Phylogenetic path analyses and trait-dependent diversification models reveal that geographic range size is strongly associated with the hand-wing index, a proxy for wing aspect ratio related to flight efficiency and dispersal ability. By contrast, we found mixed evidence for the effect of dispersal on diversification rates: dispersive lineages show either slightly higher speciation rates or higher extinction rates. Our results therefore suggest that high dispersal ability increases range expansion and turnover, perhaps because dispersive lineages expand into islands or other geographically restricted environments and have lower population sizes. Our results highlight the nuanced and interconnected roles of dispersal and range size in shaping global patterns of avian diversification and biogeography and provide a richly sampled phylogenetic template for exploring a wide array of research questions in macroecology and macroevolution.