Energy budgets and risk-sensitive foraging in starlings

The effect of energy budget on rish-sensitive foraging runs assessed in a l aboratory experiment using starlings (Sturnus vulgaris). Subjects chose bet ween two options offering the same mean amount of food per trial but differ ing in variance: a "fixed" option gave 5 units food in every trial, and a " variable" option gave 2 or II units food with probabilities 2/3 and 1/3, re spectively. WE manipulated energy budgets by controlling the cumulative amo unt of food received by each bird at the End of a day. In one treatment (po sitive budget) individuals were allowed to eat at the level of their own ad -libitum daily consumption, while for the other (negative budget), food was rationed to provoke a steady drop, in body weight during the experimental period. No subject was allowed to drop below 80% of its ad libitum body wei ght. Contrary to predictions from the "energy budget rule" and contrary to reported results of some of ha studies, starlings significantly preferred t he "fixed" option irrespective of energy budget conditions. Our results sup port the view that persistent risk aversion for food amounts and risk prone ness for food delays are the norm, and shifts In risk attitude according to energy budget are exceptions. Several algorithms, which may have evolved t o maximize energetic pay off between variable food sources, can produce thi s trend as a side effect. We discuss two of these algorithms: (I),maximizat ion of local (per trial) rate as opposed to global rate of gains, with long er handling time for larger rewards, and (2) choosing larger rewards and sm aller delays subject to Weber's law in the memory for the parameters of eac h food supply.