Risk sensitivity in starlings: variability in food amount and in food delay

JUAN C. REBOREDA; KACELNIK, A

Behavioral Ecology

Año: 1991 vol. 2 p. 301 - 308

1045-2249

Starlings' preferences for constant versus variable food sources were studied in the laboratory. The constant alternative gave a fixed amount of food after a fixed delay. The variable alternative offered either a varying amount of food after a fixed delay (treatment A) or a fixed amount of food after a variable delay (treatment B). In both treatments the ratio of amount of food over trial length (the sum of intertrial interval plus delay and handling times) of the constant alternative equaled the average of the two ratios of the variable alternative. The variable ratios were 30% higher and 30% smaller than the fixed ratio. In free-choice trials (both options available in each trial), the subjects were risk-averse or indifferent in treatment A and indifferent or risk prone in treatment B. In no-choice trials (only one source available per trial), the latency to respond was longer in the variable than in the constant source in treatment A and the opposite in treatment B. The greater preference for variability in time than for variability in reward amount is not consistent with either maximizing the ratio of expected energy over expected time or the expected ratio of energy over time for individual trials. There was a negative correlation between individual intake rate and degree of risk proneness for both kinds of variability. We present a model of choice based on an information-processing theory for temporal memory that accounts for the different effects of variability in delay and in amount but cannot explain the effects of intake rate.