Proportions were tested by a 2 test. Two periods could be distinguished in the evolution of the angle during the descending phase of dives. Discussion During the descent phase of feeding dives, the angle was steeper if the bird encountered prey in the previous dive. An abrupt decrease in the angle would modify the orientation of the vector of the positive lift force, acting against negative buoyancy at these depths, and thus contribute to deceleration. In this case, the birds may pass through the depth zone where the patch is located and then invert their angle quickly to approach the patch from underneath.
Dive angle is a key parameter in determining the depths at which penguins invest foraging effort.
This latter strategy is a consequence of minimising time spent in unprofitable zones. Penguin prey, however, are typically pelagic, and may occur at any depth. Thus, steep dive angles down to specific preferred depths tend to make the birds pass very quickly through the intermediate water layers and thus reduce the probability of prey encounter in these zones. This overall strategy should be tempered according to perceived circumstances. In particular, it is to be expected that where birds have located aggregating prey at depth, it might be advantageous for birds to descend quickly and repeatedly to that depth during subsequent dives because of the greater likelihood that prey will be re-located.