Detailed shape characterization reveals complementary ecomorphological patterns in brain shape and size evolution across the avian radiation
Like mammals, birds have exceptionally large brains for their body sizes, which is thought to have facilitated their diversification through environmental change, and has been associated with many ecological adaptations. Although size metrics are informative and well-studied, they are limited in their ability to characterize localized neuroanatomical variation. Size and shape together can holistically capture the diversification of avian brain form, and how these changes relate to function and ecology. We employ high-density geometric morphometric (GM) shape data across 300 avian species to capture shape variation in whole brain endocasts, and compare how evolutionary patterns differ between shape and size. While ecomorphological signal is mixed across brain regions, we find that the majority of shape variation falls along an elongated-globular spectrum, where terrestrial and aquatic birds generally have anteroposteriorly elongated brains, while aerial bird brains are typically more globular. The supermajority (>90%) of endocranial shape variation is independent of evolutionary allometry, demonstrating that shape characterizes key aspects of avian neuroanatomy that are missing from volumetric data. Our results suggest that neuroanatomical diversity in birds is not driven by the dominance of any particular factors, but rather through clade-specific divergence in localized brain structures from the avian-wide allometric and ecomorphological patterns.