Some birds achieve primate-like levels of cognition, even though their brains tend to be much smaller in absolute size. This poses a fundamental problem in comparative and computational neuroscience, because small brains are expected to have a lower information-processing capacity. Using the isotropic fractionator to determine numbers of neurons in specific brain regions, here we show that the brains of parrots and songbirds contain on average twice as many neurons as primate brains of the same mass, indicating that avian brains have higher neuron packing densities than mammalian brains. Additionally, corvids and parrots have much higher proportions of brain neurons located in the pallial telencephalon compared with primates or other mammals and birds. Thus, large-brained parrots and corvids have forebrain neuron counts equal to or greater than primates with much larger brains. We suggest that the large numbers of neurons concentrated in high densities in the telencephalon substantially contribute to the neural basis of avian intelligence.

Department of Cognitive Biology University of Vienna 1090 Vienna Austria

Department of Zoology Faculty of Science Charles University Prague CZ 12844 Prague Czech Republic

Instituto de Ciências Biomédicas Universidade Federal do Rio de Janeiro CEP 21941 902 Rio de Janeiro Brazil

Instituto Nacional de Neurociência Translacional Ministério da Ciência e Tecnologia Conselho Nacional de Pesquisas CEP 04023 900 São Paulo Brazil

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