  Wing flapping is one of the most widespread propulsion methods found in
nature; however, the current understanding of the aerodynamics in bird wakes is
incomplete. The role of the unsteady motion in the flow and its contribution to
the aerodynamics is still an open question. In the current study, the wake of a
freely flying European starling has been investigated using long-duration
high-speed Particle Image Velocimetry (PIV) in the near wake. Kinematic
analysis of the wings and body of the bird has been performed using additional
high-speed cameras that recorded the bird movement simultaneously with the PIV
measurements. The wake evolution of four complete wingbeats has been
characterized through reconstruction of the time resolved data, and the
aerodynamics in the wake have been analyzed in terms of the streamwise forces
acting on the bird. The profile drag from classical aerodynamics was found to
be positive during most of the wingbeat cycle, yet kinematic images show that
the bird does not decelerate. It is shown that unsteady aerodynamics are
necessary to satisfy the drag/thrust balance by approximating the unsteady drag
term. These findings may shed light on the flight efficiency of birds by
providing a partial answer to how they minimize drag during flapping flight.
