Fig. 5. Necessary time invested by an imperial cormorant to cater for oxygen acquisition at the surface as a function of dive duration, starting from a body oxygen concentration of zero. We assume that the bird must acquire oxygen for two processes; (i) diving and (ii) as a reserve, and that the acquisition of the oxygen for the two processes occurs in distinct phases. This condition is given by the output of the model (see text), which projects that birds do not actually normally use their reserves and therefore must acquire oxygen for the dive at a slower rate than they would otherwise do if they had no reserves. For comparison the figure details two options: oxygen is acquired for the projected dive first (solid line, `dives first') followed by acquisition of the reserves (solid line, `reserves second' – cf. Fig. 4B) and (the most likely situation) that oxygen is acquired for the reserves first (broken line, `reserves first') followed by oxygen for the dive (broken line, `dives second'). Since, however, the reserves are not normally used, the drop in efficiency for cormorants diving with reserves, compared with those diving without (top dotted line), is given by the time necessary to build up necessary oxygen levels from a body oxygen concentration of zero (the solid `dives first' line) divided by the time necessary to build up necessary oxygen levels from a body oxygen concentration equivalent to that used in reserves (the broken `dives second' line).