Increasing foraging group size increases scrounger use and reduces searching efficiency in nutmeg mannikins ( Lonchura punctulata )

Abstract

Social foragers can benefit from others' success by joining and sharing their food discoveries. In a producer-scrounger (PS) system, foragers can either search for food themselves (play producer) or search for joining opportunities (play scrounger), but not both at the same time. Empirical evidence is accumulating to show that the joining decision of ground-feeding birds like nutmeg mannikins (Lonchura punctulata) can be modeled by a PS game. However some predictions remain to be tested. For instance, foragers are predicted to increase their use of the scrounger tactic as group size increases. Also, one consequence of the incompatibility between producer and scrounger tactics is that the per capita searching efficiency should decrease as the use of scrounger increases. I tested these predictions in an indoor aviary using four flocks of nutmeg mannikins. I manipulated the stable equilibrium frequency (SEF) of the scrounger tactic by varying group size and the finder's share. As predicted by PS games, birds increased their use of scrounger as group size increased. Also, the per capita interval between patch discoveries increased and the per capita finding rate decreased as conditions called for a higher SEF of scrounger. I discuss why the decreased searching efficiency observed likely follows from the incompatibility between producer and scrounger tactics rather than from artifacts of the conditions used or from any form of interference.