Brain size dependent speciation and extinction rates in birds and the cognitive buffer hypothesis

Both brain and body size vary greatly across birds, with both traits hypothesized to play a key role in diversification patterns across the group. Larger brains facilitate the evolution of greater behavioural plasticity, which could facilitate diversification in two ways, either by driving increased speciation (behavioural drive) or by making lineages more resilient to extinction (cognitive buffer). Here, we model brain size dependent diversification dynamics from published endocranial volume data for a sample of 2291 extant and fossil bird taxa, jointly modelling speciation rate, extinction rate and fossilisation rate separately for perching birds and other groups. We find that increasing relative brain size primarily increases diversification rate through decreasing extinction rate, supporting the cognitive buffer hypothesis and suggesting that larger brains in birds promote diversification by making species more resilient to extinction. Although speciation and extinction are dependent on relative brain size in both perching birds and other groups, this dependence is non-linear. In perching birds, the highest diversification rates are found over a narrow range of relative brain sizes, in contrast to other bird groups which show lower diversification rates over a broader range of relative brain sizes.