Aerodynamic trick for visual stabilization during downstroke in a hovering bird.

We provide physical insight into how a small hovering bird attains stabilized vision during downstroke. A passerine generates a lift force greater than its body weight during downstroke, leading to a substantial swing of the bird body, but the bird's eyes are nearly stable. Employing digital particle-image velocimetry, we demonstrate that a hovering passerine generates a lift force acting dorsal to the center of mass, concurrently resulting in rotational and translational displacements of the bird's body. The most notable finding is that the rotational and translational displacements at the bird's eyes almost cancel each other; the displacement of the eye is ~8% that of the trailing tip of the tail. This aerodynamic trick enables a bird to attain stabilized vision beneficial for the inspection of the environment.