RATE CURRENCIES AND THE FORAGING STARLING - THE FALLACY OF THE AVERAGES REVISITED

In classical optimal foraging models long-term rate of energy intake ( the ratio of expected amount of food over expected time) is assumed to be the maximized currency because doing this is consistent with minim izing the loss of alternative opportunities. Here this possibility and various alternatives are examined quantitatively using European starl ings (Sturnus vulgaris) in the laboratory. The birds chose between two cues. One signaled an option that led to either a fixed delay to food (''single standard,'' Experiment 1) or to one of two equally probable delays to food (''double standard,'' Experiments 2 and 3). The other cue signaled an ''adjusting option'' consisting of a single delay to f ood. This option adjusted according to the previous choices made by th e birds, improving when the standard had been preferred and worsening when the adjusting option had been preferred. Adjustments were made ei ther by changing the delay to food or the amount of food. The rational e underlying this procedure was that the parameter values at which the adjusting option stabilizes should reflect the subjective value of th e standard. This was validated in Experiment 1. In Experiments 2 and 3 the adjusting option fluctuated around parameter values that are inte rpreted as fielding subjective equivalents of the double standard. The results contradict the predictions based on minimizing the lost oppor tunity. First, the birds did not include all the time intervals in the ir assignment of value to the two options, and second, the birds used the expected ratio of amount over time rather than the ratio of expect ed amount over expected time as their rate currency.