ACCURACY OF MEMORY FOR AMOUNT IN THE FORAGING STARLING, STURNUS-VULGARIS

Attempts to include psychological constraints in models of foraging be haviour differ in their assumptions concerning the accuracy of estimat ion of environmental parameters. Psychologists model estimation error as increasing linearly with the magnitude of a stimulus (Weber's Law), whereas behavioural ecologists either ignore error or assume it to be independent of stimulus magnitude. Studies on the estimation of time intervals have confirmed Weber's Law, but there are few data on the ac curacy of estimation of amounts of food. Since the currency of most fo raging models is the amount of food acquired per unit of time spent fo raging, information on estimation of amount is required. Here, a titra tion method was used in which starlings chose between two cues. One co lour signalled a standard food reward, and the other a reward that adj usted in magnitude according to the birds' choices: it increased when the standard was preferred and decreased when the adjusting option was preferred. There were two standards of 3 and 9 units of food, each of which was delivered at two rates to control for possible effects of r ate of reinforcement on discrimination. The observed value of the adju sting option oscillated around a mean value slightly larger than that of the standard. The amplitude and period of these oscillations were l arger when the standard was larger, independent of the rate of reinfor cement. Also, molecular analysis showed that the probability of choosi ng the currently larger alternative increased as the relative differen ce between the adjusting option and standard increased. These results are consistent with Weber's Law applying to starlings' memories for am ounts of food. (C) 1995 The Association for the Study of Animal Behavi our