M. Bateson et A. Kacelnik, RATE CURRENCIES AND THE FORAGING STARLING - THE FALLACY OF THE AVERAGES REVISITED, Behavioral ecology, 7(3), 1996, pp. 341-352
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.