Energy consumption for wing actuation of hummingbirds during the escape maneuver*

In this study, we consider the rapid escape maneuver of hummingbirds who were initially hovering but then startled by the frontal approach of a looming object. To escape from the perceived threat, the hummingbirds substantially changed their wingbeat frequency, wing trajectory, and other kinematic parameters. Using wing kinematics reconstructed from high-speed videos and computational fluid dynamics modeling, we calculated the time-dependent aerodynamic torques and power output separately for wing stroke, pitch, and deviation, to understand the requirements for wing actuation in this process. The results show that the overall power output of the maneuver is more than twice as hovering.  Wing pitching requires positive power input and is thus active.  For wing stroking, downstroke is much more powerful than upstroke. However, wing deviation during upstroke requires significantly more power than downstroke for the wings to change their trajectories and move caudally, which as a result, redirects the aerodynamic force vector to produce the needed escape acceleration.