FORAGING STOCHASTICITY AND LIPID-ACCUMULATION BY NESTLING PETRELS

1. Procellariiform birds accumulate large fat reserves during the nest ling period; much of this is metabolized prior to fledging. We suggest that these energy reserves result from chronic overfeeding, which pro vides insurance against stochastic variation in food provisioning. 2. We develop a simulation model based on food provisioning and physiolog ical measurements of Leach's storm-petrels and assess fat accumulation as a function of meal size. The simulations reveal that a level of fo od provisioning sufficient for accumulating, on average, the fat reser ves of a typical fledgling results in the starvation of many chicks be cause of the chance occurrence of long periods of below-average feedin g. Fledging success can be elevated by increasing meal size, but this results in overfeeding on average and the accumulation of unnecessaril y large fat reserves that must be reduced prior to flight. 3. A second simulation model addresses foraging at sea by adults. This can produc e the pattern of food provisioning observed at nests by assuming a ran dom distribution of daily foraging success and that adults base their decision to continue foraging or return to the nest to feed their chic k on the state of their own energy reserves. 4. Thus, these models rel ate the accumulation ot fat reserves in petrel chicks to stochastic va riation in foraging experienced by individual birds rather than tempor al variation in the feeding conditions for the population as a whole.