A long-standing issue in biology is whether the intelligence of animals can be predicted by absolute or relative brain size. However, progress has been hampered by an insufficient understanding of how neuron numbers shape internal brain organization and cognitive performance. Based on estimations of neuron numbers for 111 bird species, we show here that the number of neurons in the pallial telencephalon is positively associated with a major expression of intelligence: innovation propensity. The number of pallial neurons, in turn, is greater in brains that are larger in both absolute and relative terms, and positively co-varies with longer post-hatching development periods. Thus, our analyses show that neuron numbers link cognitive performance to both absolute and relative brain size through developmental adjustments. These findings help unify neuro-anatomical measures at multiple levels, reconciling contradictory views over the biological significance of brain expansion. The results also highlight the value of a life history perspective to advance our understanding of the evolutionary bases of the connections between brain and cognition.

This research was funded by MINECO (PID2020-119514GB-I00, to D.S.), the Czech Science Foundation (18-15020S, to P.N.), the Grant Agency of Charles University (1438217 to M.K.) and the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie postdoctoral fellowship (grant agreement 838998, to F.S).