Fig. 5. The predicted net energy gain (R, in W) calculated with observed ground speed (V_g), air speed (V_a) and flight strategy combinations. Different symbols (+ flapping, soaring) represent the predicted R for a measured combination of V_g, V_a and flight strategy for black-headed gulls (A) and lesser black-backed gulls (B). Parameter estimates for R calculations 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 (see Fig. 4), R was not plotted against both V_a and V_g.