Pigeons steer like helicopters and generate down- and upstroke lift during low speed turns

Abstract

Turning is crucial for animals, particularly during predator-prey interactions and to avoid obstacles. For flying animals, turning consists of changes in (i) flight trajectory, or path of travel, and (ii) body orientation, or 3D angular position. Changes in flight trajectory can only be achieved by modulating aerodynamic forces relative to gravity. How birds coordinate aerodynamic force production relative to changes in body orientation during turns is key to understanding the control strategies used in avian maneuvering flight. We hypothesized that pigeons produce aerodynamic forces in a uniform direction relative to their bodies, requiring changes in body orientation to redirect those forces to turn. Using detailed 3D kinematics and body mass distributions, we examined net aerodynamic forces and body orientations in slowly flying pigeons (Columba livia) executing level 90°turns. The net aerodynamic force averaged over the downstroke was maintained in a fixed direction relative to the body throughout the turn, even though the body orientation of the birds varied substantially. Early in the turn, changes in body orientation primarily redirected the downstroke aerodynamic force, affecting the bird's flight trajectory. Subsequently, the pigeon mainly reacquired the body orientation used in forward flight without affecting its flight trajectory. Surprisingly, the pigeon's upstroke generated aerodynamic forces that were approximately 50% of those generated during the downstroke, nearly matching the relative upstroke forces produced by hummingbirds. Thus, pigeons achieve low speed turns much like helicopters, by using whole-body rotations to alter the direction of aerodynamic force production to change their flight trajectory.biomechanics | tip reversal | aerodynamics | flapping flight | locomotion M aneuverability is critical to the movement of animals in their natural environment. Turning represents a basic maneuver that is particularly relevant to predator-prey interactions and obstacle avoidance. To begin to understand the mechanisms by which birds achieve and control aerial turns, we examine the role of body rotations in relation to aerodynamic force production to alter the flight trajectory, or path of travel, during turns. More specifically, we ask whether body rotations serve to redirect aerodynamic forces during low speed 90°level turns in pigeons.The three-dimensional (3D) nature of flight requires analyses of aerodynamic force production in relation to body motions not only in a global reference frame but also in a local, body reference frame (Fig. 1). The global frame allows for application of Newton's laws of motion, which for a flying bird means that the resultant of aerodynamic and gravitational forces can be estimated from accelerations of the whole-body center of mass (CM). However, the bird's torso moves relative to the CM, primarily due to the time-varying wing configurations during the wingbeat cycle. Therefore, localization of the CM cannot rely solely on the torso but requires detailed assessmen...

Kinematics and aerodynamics of avian upstrokes during slow flight

“…In sum, these wing motions and the resulting induction of velocity into the air would likely be beneficial to a bird seeking to move forward. This observation is corroborated by our A B 25% 35% 45% 55% 65% 10% 30% 40% 50% previous model of the tip-reversal upstroke (Crandell and Tobalske, 2011) and kinematic measurements of whole-body acceleration that are consistent with an aerodynamically active upstroke in pigeons, a species that uses tip-reversal (Ros et al, 2011). Recent measurements in parrotlets (Forpus coelestis), a species that exhibits tip-reversal in slow flight, indicates minimal weight support; thrust was not measured (Lentink et al, 2015).…”

“…In vivo accelerometers mounted to the trunk of a cockatiel (Nymphicus hollandicus) also indicate that the tipreversal upstroke produces 14% of the net force of downstroke (Hedrick et al, 2004). Lastly, kinematic analyses of pigeons suggest the upstroke produces 50% of the net force of the downstroke during slow-speed maneuvering (Ros et al, 2011). However, the aerodynamic mechanism remains unknown.…”

Transition from leg to wing forces during take-off in birds

“…The wings started to unfold at the same moment as the acceleration profiles of ground reaction forces and kinematics start to diverge (Fig.2A,C). We suggest that forces produced by the wings (Crandell and Tobalske, 2011;Ros et al, 2011) may be responsible for a part of the observed disparity between ground reaction forces and observed kinematics in diamond dove. However, it seems unlikely that zebra finch is generating an aerodynamically active upstroke, as the species uses a flexed-wing upstroke (Tobalske et al, 1999), and no aerodynamic activity was visible in the wake.…”

“…Diamond doves use a tip-reversal upstroke, and we hypothesize that this style of upstroke is aerodynamically active (Crandell and Tobalske, 2011;Ros et al, 2011). Tip-reversal may provide an earlier onset of useful aerodynamic force production during the first wingbeat in diamond dove.…”

“…Indeed, it has been found that wings of birds using a tip-reversal upstroke (such as pigeons, rock dove Columba livia) are capable of producing substantial aerodynamic forces in the upstroke posture (Crandell and Tobalske, 2011). Moreover, pigeons can generate aerodynamic forces during tip-reversal upstroke in low speed turns (Ros et al, 2011).…”