STARLINGS PREFERENCES FOR PREDICTABLE AND UNPREDICTABLE DELAYS TO FOOD

Risk-sensitive foraging theory is based on the premise that unpredicta ble runs of good or bad luck can cause a variable food source to diffe r in fitness value from a fixed food source yielding the same average rate of gain but no unpredictability. Thus, risk-sensitive predictions are dependent on the food intake from variable sources being not only variable but also unpredictable or 'risky' in outcome. This study tes ted whether unpredictability is a component of the value that foraging starlings, Sturnus vulgaris, attribute to food sources that are varia ble in the delay to obtain food. Two groups of birds chose between a f ixed and a variable delay option; the variable option was unpredictabl e in the risky group and predictable in the risk-free group in the ove rall rate of intake it yielded. In both groups the fixed option was ad justed by titration to quantify the magnitude of preference for predic table and unpredictable variance. On negative energy budgets both grou ps were significantly risk-prone, with the risky group being significa ntly more risk-prone than the risk-free group. Switching the birds to positive budgets by doubling the size of each food reward had no signi ficant effect on preference, and similar trends to those found with ne gative budgets were observed. These results are not readily explained by risk-sensitive foraging theory, but may be explained by the algorit hm used by the birds to attribute value to average expected rewards. ( C) 1997 The Association for the Study of Animal Behaviour.