Is cooperative breeding associated with bigger brains? A comparative test in the Corvida (Passeriformes)

The cognitive demands of a social existence favour the evolution of relatively large brains and neocortices in primates. Comparable tests of sociality and brain size/structure in birds have not been performed, despite marked similarities in the social systems of birds and mammals. Here, we test whether one aspect of avian sociality, cooperative breeding, is associated with an increase in brain size across 155 species of the passeriform parvorder Corvida. Using conventional and phylogeny-corrected statistics, we examined the correlated evolution of relative brain size and: the presence/absence of cooperative breeding, percentage of nests that are cooperative and cooperatively breeding group size. Most of the comparisons yielded non-significant results, which suggests that cooperative breeding is not related to relative brain size in this parvorder. There are a number of potential explanations for our findings. First, changes in brain region size may be correlated with cooperative breeding without affecting overall brain size. Secondly, cooperatively breeding birds might not possess more complex social behaviour than non-cooperatively breeding birds. Thirdly, relatively large brains might be ancestral in this parvorder. This may predispose them to evolve the range of complex behaviours found in this group, including extreme sociality. Finally, ecological and/or developmental factors might play a more significant role than social behaviour in the diversification of avian brain size. Assessing these alternatives requires more information on the neural and cognitive differences between bird species.