Abstract/Summary

Evolutionary history has been punctuated with extraordinary transitions leading to great radiations of diversification. None have captivated researchers more than the transition from terrestrial lifestyles to flight in birds. Over the last few decades, methods in biomechanical modelling and comparative phylogenetic methods have seen significant advances and innovations. In more recent years, the synthesis of these two approaches has been used to broaden our understanding of the evolutionary processes involved during a transition. The evolution of flight provides an adequate case for studying a transition, given its ample fossil record and decades of foundational research. Despite this, questions remain as to how the first birds took to the sky. The work presented in this thesis applies modern phylogenetic methods and biomechanical models to questions surrounding the transition to flight in birds and showcases how this synthesis of methods can be implemented within the context of a transition. Chapter 1 illustrates how we can use modern phylogenetic methods to study trends in forelimb evolution independently of underlying allometric relationships while simultaneously estimating and assessing rates of evolution. The study uncovers a trend in the increase of Avialae forelimb length over time, simultaneous but independent of body size. In Chapter 3, biomechanical models are used to infer the feeding behaviours of species in the extinct genus Pelagornis, offering unique insight into the ecology of species of the past. We test a specific hypothesis that Pelagornis could have fed by skimming, finding that Pelagornis would not have been able to meet the energetic requirements to overcome the induced drag of skimming during flight. A synthesis of these methods is used in Chapters 2 and 4. Chapter 2 assesses whether flight performance is increasing through the early stages of the transitions in flight, using aerodynamic models to estimate flight indices in Mesozoic birds. The study reveals an increase in flight performance through the Mesozoic, from the Early Cretaceous to the Late Cretaceous. Chapter 4 applies the same aerodynamic model to extant taxa and illustrates how these methods can be used to understand the evolution of flight in the modern radiation of birds, finding evidence of divergent evolution in modern birds' flight efÏciency, sinking rate and turning radius. Collectively, these works highlight the potential insights gained by synthesising biomechanical and phylogenetic methods in studying transitions, specifically, in this case, the transition to flight in birds.