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.