Search

The great diversity of avian species relates to their prodigious capacity for locomotor variation. The derived structure and function of the avian tail represents one evolutionary innovation that contributes to this variation. The tail supplements the role of the wings in flight, increasing lift and maneuverability while decreasing drag. These functions are made possible through specialized caudal anatomy, consisting of an articulated fan of feathers (rectrices) supported by a truncated series of caudal vertebrae and a unique terminal element, the pygostyle. The morphology of the caudal skeleton is variable across birds, but despite its evolutionary significance, little is known about the causes of this variation. This dissertation explores how ecomorphological patterns and trait interactions shape caudal skeletal variation in a phylogenetic comparative context. First, the relationship between flight behavior and caudal skeletal morphology is examined. Pygostyle shape is found to be a good predictor of foraging style, suggesting that the tail skeleton evolves in response to functional demands. Next, the association between the rectrices and the underlying skeleton is tested. Indeed, a strong correlation between these two tissues is detected, supporting the hypothesis that the rectrices and pygostyle coevolve. In addition to its functional/locomotor role, the tail fan is used for intraspecific display in many species, with sexual selection acting to generate sexually dimorphic ornamental rectrices. The evolution of these ornaments is predicted to necessitate changes in the caudal skeleton to support and maneuver such structures. However, no sexual dimorphism in caudal skeletal morphology is detected in species with dimorphic tail fans. In conjunction with the previous finding that caudal bones and feathers coevolve, these results illustrate the complexity of trait interactions within the tail and suggest that such covariation patterns are malleable. Finally, integration between the subregions of the tail skeleton is evaluated. The pygostyle and free caudal vertebrae exhibit significant intracolumnar covariation, suggesting the tail skeleton evolves as a coordinated whole. Together, these results offer insight into how form-function relationships and trait interactions have influenced the evolution of the unique avian caudal skeleton and provide a framework for exploring macroevolutionary patterns and ontogeny in this system.