Fig. 9. The forces produced normal to the swimming direction as a function of the position inside the obstacle course. The forces are produced by the body, tail and neck moving at an angle of incidence to the flow. The columns are means of the forces from eight birds passing through the obstacle course at the highest difficulty level (0.36 m between barriers, see Fig. 1). The red line is the sum of the three forces. Positive values are directed dorsally, and negative values are directed ventrally. Vertical arrows denote the position of the barriers. The horizontal axis is the same as in Figs 5, 6, 7, 8 to allow equivalent evaluation.