The Origins of the Bird Brain: Multiple Pulses of Cerebral Expansion in Evolution

Birds demonstrate extraordinary cognitive and emotional capabilities. The majority of these performances are most likely supported by their developed cerebrum. Birds, as well as mammals, have a much larger cerebrum compared to reptiles, given a similar body size. Since the common ancestral reptiles of birds and mammals had a relatively small brain according to paleobiological evidence, birds and mammals must have evolved to expand their brains independently after they diverged into different lineages. In the lineage leading to modern birds, brain expansion occurred multiple times, possibly in response to different selective pressures. This chapter includes focused discussions on three major pulses regarding brain evolution of the bird lineage. In each discussion, possible important selection factors to trigger the brain expansion are proposed. First, a discussion is on the emergence of amniotes (the common ancestor of reptiles, birds, and mammals) in the Paleozoic Era. Adaptation to terrestrial habitats and increased parental investment might play essential roles in brain expansion. Second, a discussion focuses on how theropod dinosaurs in the bird stem lineage evolved their brains in the Mesozoic Era. In the bird stem lineage, predatory behavior and body miniaturization were probably associated with the development of the brain. Finally, we discuss the evolutionary process of cerebrum expansion in modern birds during the Cenozoic Era. Acquisition of powered flight and endothermic metabolism are proposed as the main contributing factors of cerebral expansion in modern birds.