Contrasting effects of cooperative group size and number of helpers on maternal investment in eggs and nestlings

Abstract

Females are predicted to adjust their reproductive investment to optimize the trade-off between current and future reproduction. In many cooperatively breeding birds, females have been shown to reduce their investment both pre- and posthatching in response to the presence of food-provisioning helpers. However, in species where not all group members help during the posthatching stage, it is currently unclear to which social cues (i.e. group size versus number of helpers) females should tune their investment. Here, we used the cooperatively breeding placid greenbul, Phyllastrephus placidus, as a model species to examine whether and how group size and number of helpers affect female investment in eggs and food provisioning. We found that females used a contrasting strategy pre- and posthatching in response to different social cues: they laid larger eggs in larger groups while reducing their feeding rate when assisted by more helpers. We also found that fledging success increased with group size and that nestlings raised in groups with helpers fledged with longer wings but found no relation between condition of the young and number of helpers. Since the perceived contrasting investment strategies may have several underlying causes, we conclude that predicting the influence of social conditions on maternal investment may be more challenging than previously believed.