Abstract
Aim Species richness is unevenly distributed among clades, reflecting differences in the extent to which species co‐occur, their rates of geographical turnover and total area occupied. Here, we study richness variation among 10 large clades of New World passerine birds, assessing the dynamics of range expansion and diversification that led to the build‐up of co‐occurring species. Location New World. Methods We defined co‐occurrence based on the presence in 1°×1° grid cells, an area substantially smaller than the geographical scale of speciation in birds, implying that range expansion is the main mechanism by which species accumulate within grid cells. For each species, we estimated its ‘diversification rate’, a measure of the extent to which it is a member of an actively diversifying lineage. Within each group, we ranked all species by this measure, before dividing them into quartiles. Results Neither clade age nor geographical area correlates with overall clade richness, but the maximum number of co‐occurring species does. Within each group, locations supporting large numbers of species in actively diversifying lineages generally differ from those containing many species belonging to more ancient groups. The tropical Andes and Amazon are notable for continuously generating and maintaining species, such that the most species‐rich clades have predominantly radiated in these areas. Based on the timing of their speciation events, we infer that a lack of reproductive isolation may limit the co‐occurrence of species that are members of actively diversifying lineages, but not those that are generally long diverged. Main conclusions Spatiotemporal differences in the rates of range expansion and diversification are likely set by the capacity of regions to generate and maintain species. This, rather than the rate at which reproductive isolation accrues, area occupied, or time for diversification, appears to be the major factor underlying the disparity in clade richness between New World passerine groups.