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, increase in relative eye size, and 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.