Fig. 1. Still frames of a guinea fowl during an unexpected perturbation. A 0.6 m long force plate placed at the midpoint of an 8 m long runway rested 8.5 cm below the runway surface. White tissue paper pulled tightly across the resulting gap and secured with white masking tape created the appearance of a uniform surface. The velocity and position of the bird's COM through time (moving from frame A to frame B) were calculated through integration of the measured ground reaction forces and used to calculate total COM energy, as described in Materials and methods.