Speciation history shapes patterns of assemblage species richness in birds

Speciation is the ultimate source of biodiversity but how it impacts patterns of species richness remains unclear. While increasing total richness, co-occurrence need not if speciation generates spatially non-overlapping lineages. Here we apply a dynamic model of speciation and community assembly to phylogenetic and geographic data for 40 passerine bird families to estimate the time required for species to assemble locally and test to which extent this explains variation in assemblage richness. While variable across clades, we estimate an average lag-time from speciation to colonisation of ∼8Myr, similar to time between speciation events. Concomitantly, the maximum proportion of co-occurrence is highest in old, species-poor clades, having had more time to accumulate locally. Phylogenetic patterns, previously interpreted as ecological limits to local richness, are consistent with slow, constant colonisation. Our results highlight how time-for-colonisation limits the formation of species assemblages and the strong legacy of speciation history on current biodiversity patterns.