Producer-scrounger foraging games in starlings: A test of rate-maximizing and risk-sensitive models

Social foragers obtain food through two tactics: 'producer' searches for its food, and 'scrounger' exploits food discovered by producer. In the recent literature, two alternative producer-scrounger (P-S) models have been proposed, one rate-maximizing, the other risk-sensitive. Their predictions differ about the effect of food clump density on the equilibrium proportional use of scrounger in a group. The rate-maximizing model predicts no effect, whereas the risk-sensitive model predicts that the proportional use of scrounger should increase with food clump density. These predictions were tested using wild-caught European starlings, Sturnus vulgaris, in an indoor aviary. Increased food clump density resulted in significant increases in the proportional use of scrounger. Proportional use of scrounger was negatively related to food intake variance as assumed by the risk-sensitive model. Thus, scrounger is a risk-averse foraging tactic, and starlings used it in a risk-sensitive fashion. The risk-sensitive model was further challenged by testing its prediction concerning daily food requirement. When producer has a low competitive efficiency, as was found in these starlings, the risk-sensitive model predicts weakly positive effects, if any, of increased requirement on proportional use of scrounger. Results were consistent with this prediction. A significant number of starlings showed consistently positive effects of requirement on proportional use of scrounger, but the magnitude of this increase was non-significant. The results of both experiments taken together show that the proportional use of scrounger tactics within foraging groups may be a risk-sensitive, rather than a rate-maximizing, foraging tactic. (C) 1996 The Association for the Study of Animal Behaviour