Barb geometry of asymmetrical feathers reveals a transitional morphology in the evolution of avian flight

Abstract

The geometry of feather barbs (barb length and barb angle) determines feather vane asymmetry and vane rigidity, which are both critical to a feather's aerodynamic performance. Here, we describe the relationship between barb geometry and aerodynamic function across the evolutionary history of asymmetrical flight feathers, from Mesozoic taxa outside of modern avian diversity (Microraptor, Archaeopteryx, Sapeornis, Confuciusornis and the enantiornithine Eopengornis) to an extensive sample of modern birds. Contrary to previous assumptions, we find that barb angle is not related to vane-width asymmetry; instead barb angle varies with vane function, whereas barb length variation determines vane asymmetry. We demonstrate that barb geometry significantly differs among functionally distinct portions of flight feather vanes, and that cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles. This cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds, revealing a fundamental aerodynamic adaptation that has persisted from the Late Jurassic. However, in Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds. In both modern birds and enantiornithines, trailing vanes have larger barb angles than in comparatively stemward taxa like Archaeopteryx, which exhibit small trailing vane barb angles. This discovery reveals a previously unrecognized evolutionary transition in flight feather morphology, which has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor. Our findings suggest that the fully modern avian flight feather, and possibly a modern capacity for powered flight, evolved crownward of Confuciusornis, long after the origin of asymmetrical flight feathers, and much later than previously recognized.

Specialized primary feathers produce tonal sounds during flight in rock pigeons (Columba livia)

“…This morphology was previously noted by Bachmann et al (2007), but the authors did not speculate at its function. This small, 5-cm-long region of P10 (between 50 and 70% of IV length) has barb characteristics that decrease its stiffness (Ennos et al, 1995;Feo et al, 2015) and promote aeroelastic flutter, which is necessary for the production of tonal sounds during flight. P10 feathers stiffened with a temporary fixative (hairspray) were effectively silenced (i.e.…”

“…First, we quantified variation in gross feather shape between male and female rock pigeons to determine whether a link between sexually selected displays and sound production could exist, as is common in other species (Clark and Prum, 2015;Prum, 1998). We then examined several characteristics of feather barbs, which are known to influence feather shape and stiffness (Ennos et al, 1995;Feo et al, 2015), two characteristics associated with aeroelastic flutter (Clark et al, 2013a,b). Finally, we experimentally tested the necessity of particular feather morphologies for natural, in vivo sound production, and the sufficiency of individual feathers to reproduce in vivo sounds in laboratory simulations of flapping wings.…”

Inspiration for wing design: how forelimb specialization enables active flight in modern vertebrates

“…Feathers are tailored for their function in flight; vanes are more resistant to upward aerodynamic forces, which feathers experience more typically compared with downward ones [62]. Modern primary feathers have evolved specialized asymmetric vanes, with shorter barbs leading and longer barbs trailing the rachis [67]. Asymmetry in feathers has been thought to improve aerodynamic function; moving the rachis towards the leading edge gives each feather a more aerofoil-like cross-section [68,69].…”

“…Male hummingbirds dive at high speeds in a mating display, spreading their tail feathers at the correct moment to produce a chirp-like sound due to tail feather flutter [72]. Feather mechanical properties depend on the vane, formed from barbs with independently variable lengths and attachment angles [67]. The middle and tip of each flight feather can resist larger out-of-plane forces compared with the base, which is protected by smaller covert feathers [62].…”

Paleophysiology: From Fossils to the Future

“…Recent work indicates that a high diversity of melanosomes, associated with melanin-based coloration, is a good proxy for high metabolic rate in vertebrates, and that pterosaurs and dinosaurs excluding maniraptorans (the group including birds) had a low diversity of melanosomes, implying slower metabolism [51]. The barb structure of feathers, together with changes in the functional relation between the tail region and the anterior appendages (wings) during the transition from theropod ancestors to true birds, indicates that endothermy in birds evolved during the Cretaceous, well after the Late Jurassic origin of gliding flight in the bird clade [52,53].…”