Building a bird: ontogenetic and evolutionary construction of the avian body plan (13.2)

In the process of invading aerial media, birds and their dinosaur ancestors have undertaken some of the most dramatic transformations in vertebrate history. Flight is the most challenging form of locomotion, and extant flight‐capable adult birds have many anatomical traits that are presumably adaptations or exaptations for meeting such demands. Immature birds lack these hallmarks of flight capacity, yet in many species juveniles must navigate the same hazardous environment as adults. How is this accomplished? Here, I explore ontogenetic relationships between anatomy and performance, and show that flight‐incapable juveniles transition to flight‐capable adults via stages that incrementally improve locomotor performance and culminate in flight capacity. Developing birds reach refugia by flap‐running up slopes or over water and supplementing their incipient wings with their legs, until the wings can fully support body weight during flight. Such cooperative use of wings and legs acts as a developmental bridge that allows juveniles to transition from leg‐based terrestrial locomotion, to wing‐and‐leg‐based locomotion, to wing‐based aerial locomotion. Many of the anatomical changes occurring during this transition are also observed in theropod dinosaurs during the evolution of flight. Thus developing birds offer important insight into both ontogenetic and evolutionary construction of the avian bauplan.