Group Foraging by a Kleptoparasitic Fish: A Strong Inference Test of Social Foraging Models

Animals that obtain food by using the investment of other foragers (kleptoparasites) often do so in groups. We tested whether group formation by a kleptoparasitic fish, the western buffalo bream (Kyphosus cornelii), fit the predictions of five social foraging models. Two aggregation economy models assumed that there was some benefit to grouping shared by group members, such as reduced predation risk or increased ability to gain access to resources. These models and a third, the dispersion (ideal free) economy model, assumed that kleptoparasites had perfect information regarding the quality of opportunities for kleptoparasitism. The other two models did not make this assumption. These producer-scrounger models assumed that some kleptoparasites (producers) discovered opportunities, while others used producers to reduce the costs of foraging. These last two models differed in whether foragers could estimate the state of current opportunities for kleptoparasitism. We compared typical group size, and the influence of group size on intake rate and the success of kleptoparasitic attempts, with the predictions of these models. We found that typical group size was larger during periods when opportunities for kleptoparasitism were poor than when good and that there was no influence of group size on the likelihood that the group was successful at kleptoparasitizing. Individual intake rate declined with group size for small group sizes, but increased with group size in large groups. However, large groups were rare. For small groups, only a producer-scrounger model allowing foragers to update their information could not be rejected. For large groups, neither that model nor the aggregation economy with foraging benefits model could be rejected. We compare these results with those of other studies of kleptoparasitic groups.