PREFERENCES FOR FIXED AND VARIABLE FOOD SOURCES - VARIABILITY IN AMOUNT AND DELAY

Much research has focused on the effects of environmental variability on foraging decisions. However, the general pattern of preference for variability ill delay to reward and aversion to variability in amount of reward remains unexplained at either a mechanistic or a functional level. Starlings' preferences between a fixed and a variable option we re studied in two treatments, A and D. The fixed option was the same i n both treatments (20-s fixed-interval delay, five units food). In Tre atment A the variable option gave two equiprobable amounts of food (20 -s delay, three or seven units) and in D it gave two equiprobable dela ys to food (2.5-s or 60.5-s delays, five units). In both treatments th e programmed ratio [amount/(intertrial interval + latency + delay)] in the fixed option equaled the arithmetic mean of the two possible rati os in the variable option (ITI = 40 s, latency = 1 s). The variable op tion was strongly preferred in Treatment D and was weakly avoided in T reatment A. These results are discussed in the light of two theoretica l models, a form of constrained rate maximization and a version of sca lar expectancy theory. The latter accommodates more of the data and is based on independently verifiable assumptions, including Weber's law.