Temporal evolution of the nozzle centre velocity <i>U*</i> (position <i>x/D</i> = 1, <i>y/D</i> = 0, <i>U</i>_max≅1 m/s) and the nozzle diameter <i>D*</i> (<i>D</i>_min≅1 cm) in the claw closure cycle.

<p>The blue curve represents a 2^nd order polynomial fit to the nozzle diameter measurements (f(x) = p1*x^2 + p2*x + p3, Coefficients: p1 = 0.4153, p2 = −0.865, p3 = 0.4606).</p

Forces acting at the falcon during diving at maximum speed and zero acceleration.

<p>For the given flight path angle θ only one angle of attack α leads to the fulfilled condition of equilibrium.</p

Drag coefficient for flow across a sphere as a function of the Reynolds number.

<p><b>A:</b> data from Krause <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086506#pone.0086506-Krause1" target="_blank">[25]</a>. <b>B:</b> measured data by the force balances with typical values documented in the literature as well as the abrupt decrease of the drag at a Reynolds number of about 3.5*10⁵.</p

Three-dimensional orientation of the camera set-up based on the spatial calibration.

<p>The reconstructed 3-D flight path of the falcon is color-coded with the flight velocity magnitude (red-colored: higher velocities). Maximum velocity during the dive was 22.5 m s⁻¹.</p

Reference areas of the falcon model.

<p>Reference areas of the falcon model.</p

Velocity magnitude (A) and acceleration (B) of the falcon during the time pathway flight.

<p>Spline interpolation of the data with the aid of a moving 3^rd-order-polynomial approximation.</p

Temporal evolution of the axial velocity profile at the exit of the channel. Blue: acceleration phase, red: deceleration phase.

<p>Temporal evolution of the axial velocity profile at the exit of the channel. Blue: acceleration phase, red: deceleration phase.</p

Force balances error.

<p>Force balances error.</p

Experimental set-up in front of the dam wall (Olef-Talsperre, Hellenthal, Germany).

<p>The two cameras of the stereo system were positioned opposite, separated by a river leaving the dam wall. The features, displayed on the wall enabled us to calibrate the stereo camera system and to exactly determine the position of the diving falcon in the images of both cameras.</p

CAD-model of the enlarged claw model with the movable plunger in maximum vertical opening position and the socket fixed at the floor with two transparent side-walls made from Perspex which allowed optical access into the cavity.

<p>At the pivot point an axis is included to the two bodies, so the plunger can rotate to the closed position. To accelerate the plunger to the required angular velocity a spring is stretched between the stopper and the plunger.</p