Meeting Abstract
The asymmetrical flight feathers of extant birds are an important adaptation for avian flight. Barb geometry (barb angle and barb length) controls feather asymmetry and stability, both of which are critical for aerodynamic performance. We hypothesize that different barb geometries can produce feathers with the same width, but with varying degrees of aerodynamic stability due to redundancy in theoretical morphospace. However, the relationship between barb geometry and vane asymmetry across the evolutionary history of asymmetrical flight feathers is unknown. Here we demonstrate that barb geometries significantly differ among vanes with different functions within the wing of extant birds. In particular, leading vanes that function as the cutting edge of an airfoil during flight exhibit a distinct range of barb geometries characterized by small barb angles, which have been shown to increase a vane’s resistance to aerodynamic forces. We also observed small leading vane barb angles in the highly asymmetrical forewing feathers of volant Mesozoic stem birds (Archaeopteryx, Sapeornis, Confuciusornis), and in the asymmetrical hindwing feathers of Microraptor. However, unlike in crown birds, barb angles were small in the trailing vanes of Mesozoic flight feathers. Our results suggest that small barb angles in cutting-edge vanes are an important aerodynamic adaptation that arose by the late Jurassic, prior to the refinement of numerous features associated with powered flapping flight, whereas large trailing vane barb angles arose crownward of Confuciusornis. This demonstrates a previously unrecognized transitional morphology in the evolution of asymmetrical flight feathers at a critical interval in the refinement of avian flying potential.
