Fig. 11. Relationship between foraging efficiency (dimensionless) and prey capture speed for a Magellanic penguin foraging according to the conditions set out in the text. The upper line (closed circles) shows the efficiency for an unequipped bird while the lines delineated by squares and diamonds show the efficiency of birds transporting external antennae (200 mmx3 mm) at cruising speeds of 1 m s^–1 and 1.77 m s^–1, respectively. The formula used for the antenna-derived drag was F_d=0.913v²–0.91v^1.5+ 0.183v^0.5+0.014 and is the best-fit curve (r²=0.99997, F=10946, P<0.0001) from the data corresponding to the relevant antenna (see Fig. 8A). Note that the model assumes that birds encounter a prey patch once every 36.3 min, travelling at a mean speed of 1.77 m s^–1, which corresponds to a patch separation of 3.86 km. Thus, swimming at 1 m s^–1, patches with the same spatial distribution are encountered less often (only once every 64.25 min), although the overall foraging efficiency rises. Arrows show the approximate scenarios expected for Adélie and Magellanic penguins due to their different prey capture speeds (see text).