Flight, the most power-demanding mode of locomotion, is seen as one of the greatest transformations in the history of vertebrate evolution. The way bone and muscle formations come together to help a bird take flight, glide through the air, dive onto prey, land on water and power over our tallest mountains has always fascinated researchers. Yet, the origin of flight is still not entirely understood.
The current perspective on how flight has evolved uses the adult bird skeletal and muscular structures as the hallmarks for defining whether ancestral birds had the ability to fly. Adult flying birds are equipped with large wings, strong pectoral muscles and tight, specialized joints and bones that allow them to fly. If a fossil presents these adaptations, then it must have been capable of flight. However, if a fossil has small wings and less constrained joints and bones, then it is concluded that its skeletal structure would have supported underdeveloped muscles that would have been too weak for flight. While this approach appears to be well established in the avian literature, young developing flight birds challenge this notion. They do not have the flight structures of an adult bird, much like the early winged dinosaurs, yet they are still capable of aerodynamically active flapping behaviours. This means they have the moves (the flap), but not the capacity (skeletal and muscular ability) to take flight.
Ashley Heers, a postdoctoral fellow at the American Museum of Natural History, and her colleagues set out to examine how a developing bird goes from flapping its wings to flying. Using X-Ray Reconstruction of Moving Morphology (XROMM), they explored how flight movements correlate with the development of the skeletal, joint and muscular structures necessary for flight. The experiment examined wing and leg movements in a pre-flight behaviour test in the precocial ground bird Alectoris chukar, where adult and immature birds were required to flap-run over obstacles to mimic pre-flight scenarios. By analysing flight structures in adults and young birds, the authors tested three hypotheses: (1) immature birds have different flapping movements from adults; (2) differences in pre-flight movements between adults and youngsters are due to different wing size; and (3) immature birds move differently because they struggle more to go over obstacles.
The team was able to establish that immature birds flap-running on a 60–65 deg incline displayed significant differences in pre-flight behaviours when compared with adults, but that those differences were not due to the ratio of wing length to substrate width, nor to the different levels of effort. The authors determined that, initially, juvenile birds use their legs and wings to perform the same movements as the adults during locomotion, in spite of lacking the flight adaptations of the older birds. However, as the youngsters’ skeletal structures develop and they begin to rely more on their skeleton for powered flight, their movements become more aerodynamic. Therefore, although immature and adult birds have different skeletal structures, the youngsters are nevertheless capable of using adult-like flapping movements to mimic pre-flight behaviour.
These findings offer insight into how birds develop the ability to fly, but also provide additional information on how flight was first acquired in ancient vertebrates. The authors propose that the smaller ancestral flapping forelimbs, similar to those of immature flight birds, first evolved to improve leg movements (i.e. running from predators), which led to increased pectoral muscle strength, then to the aerodynamically active, larger forelimbs that we now call wings. Therefore, it seems that a bird always flapped before it flew.