Bird brains fit the bill: morphological diversification and the evolution of avian brain size

Brain size varies greatly across and even within lineages. Attempts to explain this variation have mostly focused on the role of specific cognitive demands in the social or ecological domain. However, their predictive power is modest, whereas the effects of additional functions, especially sensory information processing and motor control, on brain size remain underexplored. Here, using phylogenetic comparative models, we show that the socio-cognitive and eco-cognitive demands do not have direct links to relative brain size (that is the residual from a regression against body mass) once morphological features are taken into account. Thus, specific cognitive abilities linked to social life or ecology play a much smaller role in brain size evolution than generally assumed. Instead, parental provisioning, generation length, and especially eye size and beak and leg morphology have a strong direct link to relative brain size. Phylogenetic lability analyses suggest that morphological diversification preceded changes in the rate of brain size evolution and greater visual input, and thus that morphological diversification opened up specialized niches where efficient foraging could produce energy surpluses. Increases in brain size provided general behavioural flexibility, which improved survival by reducing interspecific competition and predation, and was made possible by intense parental provisioning. Indeed, comparative analyses in a subset of species show that thicker beaks are associated with larger size of brain regions involved in behavioural flexibility (telencephalon, pallium). Thus, morphological evolution had a key role in niche diversification, which subsequently may have facilitated the evolution of general cognitive flexibility.