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