Where was WAIR in avian flight evolution?

Abstract

Abstract The popular hypothesis that the flapping flight of birds evolved through the stage of wing-assisted incline running (WAIR) is tested through calculations of external forces acting on the wings in the downstroke and upstroke during WAIR activity of an adult chukar partridge (Alectoris chukar). Contrary to previous studies of WAIR, it is found that the total vertical impulse provided by the wings is negative (i.e. downward directed). Thus, the wings cancel out about half of the positive (upward-directed) impulse provided by the legs. The function of the wings in WAIR is not to lift the body, but to push it towards the steep slope. It is like a wing, or aerofoil, of a race car, which is tilted at a negative angle of attack to press the car to the ground for greater friction and adhesion of the wheels and to prevent the race car from becoming airborne. The calculated force pattern suggests that the dominant external forces acting on the wings in the sagittal plane during WAIR are not the virtual forces of inertia but the aerodynamic forces. More specifically, it is only the aerodynamic lift that can be responsible for the wing force component acting at right angles to the wingbeat plane in both the downstroke and the upstroke during WAIR. The lift towards the slope during the WAIR upstroke is ensured by separation of the primary flight feathers and their setting at a negative angle of attack. Analysis of the force pattern in the anatomical coordinate space of a bird’s body shows that the main wing muscles in the WAIR downstroke are the humeral retractor muscles and in the WAIR upstroke the humeral elevator muscles. The humeral protractor muscles are not required at all during WAIR. Therefore, the morphologically necessary protractor state of the supracoracoideus muscle in a ‘semi-flying’ ancestor, which is logically explained by the classical hypothesis of a gliding ancestor, does not fit the WAIR hypothesis of flapping flight origin. Wing-assisted incline running should be regarded as a crown locomotor specialization of birds and is not an appropriate model for locomotion in avian ancestors.