Group size effect caused by food competition in nutmeg mannikins (Lonchura punctulata)

When foraging group sizes increase, animals generally decrease the time devoted to antipredator detection and increase their foraging rate, the commonly reported group size effect. The increased foraging rate is thought to follow from increased safety from predators because as group size increases, more eyes are available to detect predators and the risk of being a predator's victim is diluted. This increased safety then allows higher feeding rates because individuals can reallocate time spent in vigilance to foraging. However, increased foraging rates can also be due to increased competition for resources as the number of companions increases. We tested whether increased feeding rates are the product of competition or antipredation when group size increases in nutmeg mannikins (Lonchura punctulata). We used edited video playbacks to change group size and type of competitor: vigilant only, feeding only, and controls. We found that the increased feeding rate associated with an increased group size only resulted when the companions were feeding. Video playbacks of nonforaging companions neither decreased an individual's use of vigilance while handling food nor did it release the full increase of feeding rate. Focal birds lowered their scanning time while feeding as the frequency of pecking by simulated nonvigilant companions increased. We conclude that the group size effect reported in nutmeg mannikins is not a product of safety benefits of group living but may also arise from the costs imposed by competition for resources. Copyright 2009, Oxford University Press.