PRODUCER-SCROUNGER FORAGING GAMES IN STARLINGS - A TEST OF RATE-MAXIMIZING AND RISK-SENSITIVE MODELS

Social foragers obtain food through two tactics: 'producer' searches f or 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. Th eir predictions differ about the effect of food clump density on the e quilibrium proportional use of scrounger in a group. The rate-maximizi ng model predicts no effect, whereas the risk-sensitive model predicts that the proportional use of scrounger should increase with food clum p density. These predictions were tested using wild-caught European st arlings, Sturnus vulgaris, in an indoor aviary. Increased food clump d ensity resulted in significant increases in the proportional use of sc rounger. Proportional use of scrounger was negatively related to food intake variance as assumed by the risk-sensitive model. Thus, scrounge r is a risk-averse foraging tactic, and starlings used it in a risk-se nsitive fashion. The risk-sensitive model was further challenged by te sting 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 inc reased requirement on proportional use of scrounger. Results were cons istent with this prediction. A significant number of starlings showed consistently positive effects of requirement on proportional use of sc rounger, but the magnitude of this increase was non-significant. The r esults of both experiments taken together show that the proportional u se of scrounger tactics within foraging groups may be a risk-sensitive , rather than a rate-maximizing, foraging tactic. (C) 1996 The Associa tion for the Study of Animal Behaviour