Wild Carib grackles play a producer--scrounger game

Producer--scrounger (PS) game-theoretical foraging models make predictions about the decision of group-feeding animals either to look for food (produce) or for opportunities to exploit the discoveries of other foragers (scrounge). We report the most complete demonstration to date of the applicability of the PS foraging game in a free-living animal, the Carib grackle (Quiscalus lugubris) of Barbados. As assumed by PS games, the payoffs obtained by scroungers were negatively frequency dependent. Experimentally, increasing the cost of scrounging led to a decrease in the observed proportion of scroungers, whereas raising the cost of producing increased the proportion of scroungers. Observations of marked birds revealed that group-level changes could be brought about by individual flexibility in tactic use. Despite consistent individual differences in tactic use, most birds used both tactics and could alter their use of producing and scrounging when conditions changed. We found no difference in the payoffs obtained by producers and scroungers, suggesting a symmetrical game equilibrium. Our results call for testing the PS foraging game in a broader range of biological systems that include different types of scrounging behavior (e.g., scramble, stealthful, or aggressive scrounging) as well as the exploitation of different phases of food production (e.g., searching, handling). Copyright 2007, Oxford University Press.