Patterns in the diving behaviour of the pochard, Aythya ferina: a test of an optimality model

Abstract Abstract. Foraging divers face a trade-off between obtaining oxygen at the surface and food underwater. Houston & Carbone (1992, Behav. Ecol. 3, 255-265) developed models to predict the allocation of time over the dive cycle for aerobic divers that maximize gross and net rates of energy intake under a range of conditions. To test some of these predictions diving experiments on the pochard were conducted. The costs and benefits of foraging were manipulated by varying water depth, food concentration and depth of the substrate covering the food. Each stage of the dive cycle was positively correlated with water depth, and the trends were qualitatively similar to the predictions of the models. Food concentration did not significantly affect foraging time: only small effects were predicted. Foraging time (predicted to decrease independently of recovery time, as energy costs during foraging increase) decreased in relation to increasing substrate depth (used to increase foraging costs), while recovery time was not significantly affected. An additional experiment was conducted to determine whether the birds could assess foraging areas that differ in water depth and food concentration. When offered two options, the birds changed their mean preference from a higher to a lower food concentration as the depth of the higher concentration option increased. However, they switched their preference earlier than expected based on estimates of the net rates of gain obtained from these options. This suggests that factors other than energetics influence the choice of foraging areas.