New Perspectives on the Ontogeny and Evolution of Avian Locomotion

Abstract

Close correspondence between form and function is a central tenet of natural selection. One of the most striking, textbook cases for form-function congruence is the evolution of flight and the body plan of birds: compared with other tetrapods, extant adult birds have highly modified integuments and skeletons, and it has traditionally been assumed that many of these modifications are adaptations or exaptations for flight. However, developing birds that lack many of the morphological signatures of flight capacity nevertheless use their developing wings for a variety of flapping behaviors, such as wing-assisted incline running and even brief flight. Immature birds thereby demonstrate that rudimentary "flight" apparatuses are more functional than traditional assumptions about form-function relationships would predict. Here, I review the ontogeny of avian locomotion, highlighting how the developmental acquisition of flight in extant birds can improve our understanding of form-function relationships in the avian body plan, and provide insight into the evolutionary origin of flight among extinct non-avian theropod dinosaurs.

Innovation: an emerging focus from cells to societies

“…Determining whether a phenotypic trait is an innovation based on the presence/absence or quantitative criteria alone is difficult, because the trait itself may not be specific to the function being innovated and vice versa [17]. Thus, as alluded to above, birds may use their wings for one or more of several functions in addition to flight, including flap-running, paddle-swimming, thermoregulation, brooding and displays [67].…”

Dynamic body acceleration increases by 20% during flight ontogeny of greylag geese Anser anser

“…The study of the development of flight in birds has been notably concerned with the biophysical and behavioural changes experienced by juveniles in an attempt to draw parallels with the evolution of flight (Dial et al , Heers et al , , Heers and Dial , Heers ). Studies on ground‐dwelling birds have found relationships between changes in wing and flight feather morphology and flapping behaviour during flight ontogeny – younger birds exploit the drag produced by their flexible, symmetrical and open feather structure to contribute to weight support whereas older birds change their flapping behaviour to generate greater lift with their stiffer wings and more asymmetrical flight feathers (Heers et al ).…”

Feathers for escape: the transition from juvenile to adult in red-legged partridges (Alectoris rufa)

“…With maturation, wing-tip shape changes from a more pointed form in juveniles (for increased flight velocity) to a more blunt form in adults (for better flight manoeuvrability) (Heers & Dial, 2015). There is a clear relationship between feather morphology and aerodynamic performance, with older birds having stiffer distal primaries, a higher number of barbicels and a higher degree of overlap between barbules (Heers, 2016). These characteristics generate greater lift/drag ratios than in juvenile wings, which have flexible rachis and less cohesive barbules (Schmitz et al, 2015).…”