Fig. 6. The predicted differences in net energy gain between soaring () and flapping (+) flight (R_s-R_f) calculated with observed combinations of air speed (V_a, m s^-1), ground speed (V_g, m s^-1) and flight strategy, for black-headed gulls (A) and lesser black-backed gulls (B). Parameter estimates for R are as follows: E_n=20 W, D=10000 m, t_p=1800 s. As observed V_a and V_g values are highly confounded (as shown in Fig. 4), R_s-R_f was not plotted against both V_a and V_g.