Energy budgets and risk-sensitive foraging in starlings

The effect of energy budget on risk-sensitive foraging was assessedin a laboratory experiment using starlings (Sturnus vulgaris).Subjects chose between two options offering the same mean amountof food per trial, but differing in variance: a "fixed" optiongave 5 units food in every trial, and a "variable" option gave2 or 11 units food with probabilities 2/3 and 1/3, respectively.We manipulated energy budgets by controlling the cumulativeamount of food received by each bird at the end of a day. Inone treatment (positive budget) individuals were allowed toeat at the level of their own ad-libitum daily consumption,while for the other (negative budget), food was rationed toprovoke a steady drop in body weight during the experimentalperiod. No subject was allowed to drop below 80% of its ad libitumbody weight. Contrary to predictions from the "energy budgetrule" and contrary to reported results of some other studies, starlingssignificantly preferred the "fixed" option irrespective of energybudget conditions. Our results support the view that persistentrisk aversion for food amounts and risk proneness for food delaysare the norm, and shifts in risk attitude according to energybudget are exceptions. Several algorithms, which may have evolvedto maximize energetic pay off between variable food sources,can produce this trend as a side effect. We discuss two of thesealgorithms: (1) maximization of local (per trial) rate as opposedto global rate of gains, with longer handling time for largerrewards, and (2) choosing larger rewards and smaller delayssubject to Weber's law in the memory for the parameters of eachfood supply.