Great Tits and Giving-Up Times: Decision Rules for Leaving Patches

Abstract

AbstractIn this paper I examine the hypothesis that animals employ simple decision rules in exploiting patchily distributed food. This idea arises from the supposition that animals probably cannot compute the optimal patch residence time in the same manner that is calculated in optimal foraging models. Instead they may only approximate the optimal solution using a simple, robust rule-of-thumb. I considered three rules-of-thumb that great tits foraging in a simple experimental habitat may use: a number expectation, a time expectation, and a giving-up time. The experimental habitat consisted of an operant patch in which great tits had to search for food by hopping on a perch. The probability of reward declined for successive patch hops according to a predetermined stochastic schedule. In order to maximize food intake, the great tits had to occasionally leave the patch and fly across the experimental room to reset the reward schedule by hopping on a second perch. Three different lines of investigation were followed to test which of the three departure rules were employed: (a) a statistical analysis showed that there was a strong tendency for great tits to leave the patch only after several successive hops had gone unrewarded; (b) a computer simulation showed that a simple giving-up rule could produce a distribution of patch residence times similar to that observed; (c) an experiment in which the reward schedule was manipulated, successfully altered the patch residence times in accord with predictions made on the basis of a giving-up time rule. Thus, all three tests produced evidence in favour of the giving-up time rule. Although the great tits used a giving-up time rule, a residence time would have resulted in a higher rate of intake. One potential explanation of this apparent error is that the natural food of great tits has a clumped distribution, to which a giving-up time is better suited than a residence time rule. Finally, I point out that the observed giving-up time was much more variable than might be expected if it were adjusted solely in response to the habitat rate of intake. I suggest some hypotheses to explain this large variation.