Evolution of Avian Flight

Abstract

Abstract The origin of birds and of bird flight has drawn scientific interest since the inception of evolutionary thinking. Though early investigations were hampered by a paucity of fossils, new discoveries have filled in many gaps and provided unprecedented detail into morphological changes that attended the evolutionary appearance of birds and bird flight. Birds are now widely regarded as the descendents of theropod dinosaurs. In contrast, form–function relationships and behaviours that might have facilitated the evolutionary acquisition of flight remain widely debated. Given the versatility of extant birds, we should not expect to find only one solution to this problem. Nevertheless, much debate seems to stem not from looking for multiple plausible functions and behaviours, but rather from traditional ‘ground up’ versus ‘trees down’ assumptions and a general lack of experimental and ecological support for inferred form–function relationships. Many researchers have therefore called for more rigorous hypothesis testing, and a plethora of new techniques and perspectives are up to the challenge. Key Concepts: Birds are the descendents of bipedal theropod dinosaurs. Evolutionary assembly of the avian body plan occurred gradually, with bird‐like wings evolving before bird‐like skeletons, in conjunction with a reduction in body size and then a cranial shift in centre of mass. These changes took place via a series of transitional anatomical stages that were presumably associated with transitional functions and behaviours before becoming co‐opted for flapping flight. Many origin‐of‐flight hypotheses have been proposed, and it is challenging to discriminate among them. Much debate seems to stem from traditional ‘ground up’ versus ‘trees down’ assumptions and a general lack of experimental and ecological support for inferred form–function relationships. A growing number of researchers have therefore suggested that hypotheses must be consistent not only with the fossil record, but also with experimental support for form–function relationships and behaviours inferred by origin‐of‐flight scenarios. New techniques and perspectives are meeting this challenge and providing more rigorous insight into the origin of flight in birds. For example, recent work on developing birds with dinosaur‐like anatomies clearly demonstrates that behaviours involving the cooperative use of wings and legs (e.g. wing‐assisted incline running and steaming) act as a developmental (or evolutionary) bridge between leg‐based terrestrial locomotion and wing‐based aerial locomotion, by allowing developing birds (or evolving dinosaurs) to supplement their wings with their legs until the wings can fully support body weight during flight.