Avian cranial evolution is influenced by shape interactions between hard and soft tissue traits

Changes in the structure and relative size of the brain are thought to be key transformations in the evolution of birds, reflecting innovations and diversity of neurosensory and cognitive capabilities. These changes do not occur in isolation, being accompanied by many other derived morphological characteristics. In the avian head alone, these include the evolution of a toothless beak, an increase in relative eye size, and a reduction and restructuring of jaw muscles. Several developmental trade-offs have been proposed to explain the interrelationships among these traits, but how these developmental patterns translate into evolutionary correlations among cranial traits is poorly understood. Here, we use two-block partial least squares analyses and Ornstein-Uhlenbeck models of adaptive trait evolution to explore the phenotypic evolution of hard and soft cranial tissues and test hypotheses of correlated trait evolution. In pairwise analyses, we found that all traits are significantly correlated, and found support for a form of adaptive trait evolution across the whole head in which traits interact reciprocally via the neurocranium. Together, these results highlight the integrated nature of the avian head and reveal that the evolution of diverse phenotypes is a result of complex multiple interactions among hard and soft tissue traits.